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  • Articles  (1,576)
  • Theoretical, Physical and Computational Chemistry  (1,576)
  • 1995-1999  (1,576)
  • 101
    Electronic Resource
    Electronic Resource
    Studies on the hydrogenation steps of the nitrogen molecule at the Azotobacter vinelandii nitrogenase site (1998)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 70 (1998), S. 1159-1168 
    Details
    ISSN: 0020-7608
    Keywords: nitrogenase ; nitrogen fixation ; INDO ; DFT ; PM3tm ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We follow the initial activation of the nitrogen molecule at the FeMo cofactor of nitrogenase and subsequently model the hydrogenation of N2 up to the fourth protonation step using the intermediate neglect of differential overlap quantum-chemical model. The results obtained favor a reaction mechanism going through hydrazido intermediates on the 4-Fe surfaces, externally to the FeMo cofactor. Calculations using density functional theory on smaller model systems also support the suggested mechanism over other possible schemes that involve early release of the first molecule of ammonia as a product of the enzymatic reaction. We also demonstrate that dielectric stabilization due to the protein around the cofactor could lower markedly the barrier for the product release as an ammonium ion.   © 1998 John Wiley & Sons, Inc. Int J Quant Chem 70: 1159-1168, 1998
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  • 102
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    Phytochrome structure: A new methodological approach (1998)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 70 (1998), S. 1145-1157 
    Details
    ISSN: 0020-7608
    Keywords: phytochrome ; absorption spectra ; semiempirical ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Higher plants use the protein phytochrome as a photosensor. In physiological temperatures phytochrome exists in two forms: Pr and Pfr. The chromophore of phytochrome is an open-chain tetrapyrrole. On the pathway from Pr to Pfr four intermediates (Lumi-R, Meta-Ra, Meta-Rb, and Meta-Rc) can be distinguished, while only two (Lumi-F and Meta-F) can be seen on the way back from Pfr to Pr. We have used the x-ray structure of the C-Phycocyanin protein Fremyella diplosiphon bacteria as a template to build a model (∼200 atoms) that includes only the chromophore and five amino acids of the phytochrome (Arg316-Cys321-His322-Leu323-Gln324) around it. Using the existing experimental evidences, we have proposed a three-dimensional (3D) structure for Pr, Pfr, and intermediates and a mechanism for the photoisomerization as well. Structures were fully optimized using AM1 (Unichem package on a Cray J90-NACAD). Using the INDO/S method of Zerner and co-workers, we calculated the absorption spectra of the model compounds and compared them with the experimental data. The oscillator strength ratio is an indicator of the chomophore conformation in biliproteins. The calculated spectra reproduces well the spectra of the phytochrome (Pr, Pfr, and intermediates) except for the lower energy band. This result is attributed to the small number of amino acids in the models. The calculated ratios (fVIS/fUV-fosc of visible band over fosc of UV band and f2/f1-fosc of second absorption band over fosc of first absorption band) for the models match very well the experimental ratios obtained for the phytochrome (Pr, Pfr, and intermediates). This supports the proposed mechanism for the photoisomerization process.   © 1998 John Wiley & Sons, Inc. Int J Quant Chem 70: 1145-1157, 1998
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  • 103
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    RHF conformational analysis of the auxin phytohormones n-ethyl-indole-3-acetic acid (n=4, 5, 6) (1998)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 70 (1998), S. 1169-1175 
    Details
    ISSN: 0020-7608
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: RHF/6-31G* investigations of 4-, 5-, and 6-ethyl(Et)-indole-3-acetic acid (IAA) yielded 11 symmetry-unique local minima with syn-periplanar orientation of the —COOH group for each of these compounds. The global minima are of C1 symmetry in all cases. Comparison with earlier results shows that ethylation or chlorination in position 5 or 6 introduces only minor changes on the orientation of the acetic acid side group, with no effect on the reaction paths related to this group. For 4-Et-IAA, the deviations from unsubstituted IAA are larger but preserve the pattern of reaction paths that is present in unsubstituted IAA, which is in contrast to 4-Cl-IAA, where local minima and reaction paths are completely different.   © 1998 John Wiley & Sons, Inc. Int J Quant Chem 70: 1169-1175, 1998
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  • 104
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    Parametric transform and moment indices in the molecular dynamics of n-alkanes (1998)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 70 (1998), S. 1185-1194 
    Details
    ISSN: 0020-7608
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The integrated molecular transform (FTm) is a unitary numerical index of structure that is capable of uniquely representing different molecular structure conformations with the exception of enantiomers. Other molecular indices have been derived from FTm as well as from the normalized molecular moment (Mn), for example, the analogous electronic and charge transforms (FTe and FTc) and moments (Me and Mc). In this study, each of these indices was calculated for up to 10 sampled conformations of each of the C1-C10 normal alkanes as they were subjected to a standard annealing process. Statistical analyses of the resulting data in the individual series and subsequent box plots, permitting facile examination of those results, indicated that the respective transform indices (FTm, FTe, FTc) are unique, that is, with no statistically significantly overlap across the series. For the Mn and Me indices, the numerical values for methane overlapped those of ethane in the first instance and both ethane and propane in the second. The Mc index values overlapped in several instances in the series. Inasmuch as the noted molecular indices are based only on parameters of structural origin, these results have profound implications for the correlation and estimation of properties derived not only from a general structure representation, but also for those properties which may be dependent on specific molecular conformations. This includes the potential for indices of molecular flexibility and conformationally dependent atomic electron densities.   © 1998 John Wiley & Sons, Inc. Int J Quant Chem 70: 1185-1194, 1998
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  • 105
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    Optimal molecular connectivity descriptors for nitrogen-containing molecules (1998)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 70 (1998), S. 1209-1215 
    Details
    ISSN: 0020-7608
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We report on optimal molecular connectivity descriptors for nitrogen atoms in amines for use in structure-property correlations. The descriptors represent generalized molecular connectivity indices with adjusted diagonal entries in the adjacency matrices of the corresponding molecular graphs, such that the standard error in a regression for boiling points in a set of amines is minimized. Advantages of the so-optimized descriptors for multivariate regression analysis in structure-property-activity studies are discussed.   © 1998 John Wiley & Sons, Inc. Int J Quant Chem 70: 1209-1215, 1998
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  • 106
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    Unambiguous exchange-correlation energy density for Hooke's atom (1998)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 70 (1998), S. 583-589 
    Details
    ISSN: 0020-7608
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Recently, we used Helmholtz's theorem to construct an unambiguous exchange-correlation energy density for use in density functional theory. This energy density requires only knowledge of the density dependence of the exchange-correlation energy functional, EXC, for its calculation. We calculate this energy density for Hooke's atom in three different regimes: the high-density (or weakly correlated) limit; a moderate density, comparable to that of the He atom; and a low density, in which the system is strongly correlated. We compare the exact unambiguous energy density with approximate energy densities found from approximate energy functionals. The exchange-correlation energy can be deduced directly from the density in the highly correlated limit and a new formula for the high-density limit of the correlation energy is given.   © 1998 John Wiley & Sons, Inc. Int J Quant Chem 70: 583-589, 1998
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  • 107
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    Nonconventional partitioning of the many-body Hamiltonian for studying correlation effects (1998)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 70 (1998), S. 571-581 
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    ISSN: 0020-7608
    Keywords: partitioning ; level shift ; quasi-degenerate PT ; Dyson equation ; geminals ; excited states ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: For the treatment of electron correlation, one most often uses the Møller-Plesset (MP) partition which defines the zero-order Hamiltonian through the spectral resolution of the Fockian. We investigate how the MP partitioning can be improved while still using the Hartree-Fock (HF) reference state; and how the HF wave function can be substituted by a correlated one preserving the formal simplicity of the HF-based approach. To improve the MPn result, we introduce a fine tuning of energy denominators replacing the HF orbital energies with the ionization potentials obtained from the second-order Dyson equation. As this equation usually tends to close the gaps, a slight decrease of the denominators is expected, inducing an improvement of low-order correlation energies. We keep the simplicity of the MP partitioning and handle Dyson corrections as simple level shifts. Substituting doubly filled HF orbitals by strongly orthogonal geminals, one introduces a correlated reference state which is variational, size-consistent, and properly describes single-bond dissociation. This wave function, the antisymmetrized product of strongly orthogonal geminals (APSG), offers a good starting point for further corrections. We show that the use of an APSG reference state in the equation-of-motion technique leads to Tamm-Dankoff approach (TDA) equations which account for correlation effects in electronic excitation energies.   © 1998 John Wiley & Sons, Inc. Int J Quant Chem 70: 571-581, 1998
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  • 108
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    3,5-contracted Schrödinger equation: Determining quantum energies and reduced density matrices without wave functions (1998)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 70 (1998), S. 557-570 
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    ISSN: 0020-7608
    Keywords: electron correlation ; reduced density matrices ; N-representability ; cumulants ; particle-hole duality ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Through the 3,5-contracted Schrödinger equation (3,5-CSchE) quantum energies and 3-particle reduced density matrices (3-RDMs) are determined directly without wave functions. Since the 3,5-CSchE involves the 5-RDM, its solution is indeterminate without N-representability conditions. However, the indeterminacy of the 3,5-CSchE may be removed through a reconstruction strategy for building the 4- and 5-RDMs from the 3-RDM. We present a systematic procedure for obtaining corrections for Valdemoro's reconstruction functionals from two complementary approaches, the particle-hole duality and the theory of cumulants. With the cumulants we are able to demonstrate that we have obtained all terms in the reconstruction functionals which may be written as antisymmetric products of the lower rdms. The cumulants allow us to understand the reconstruction functionals in terms of a renormalized many-body perturbation theory. The reconstruction functionals also lead to a natural generalization of Wick's theorem for evaluating expectation values of fermionic annihilation and creation operators with respect to correlated reference states. Previous work [Phys. Rev. A 57, 4219 (1998)] has explored the determination of correlation energy and 2-RDMs through the 2,4-CSchE, also known as the density equation. Because the reconstruction functionals employed with the 3,5-CSchE depend only on the antisymmetric products of lower RDMs in constrast to those used with the 2,4-CSchE, the 3,5-CSchE method presented here does not require the solution of systems of linear equations during reconstruction or the storage of the reconstructed RDMs. Application of the 3,5-CSchE technique to a quasi-spin model generates ground-state energies and 2-RDMs similar in accuracy to single-double configuration interaction (SDCI). We employ a simple iterative procedure for the solution of the 3,5-CSchE without traditional diagonalization. The CSchE techniques offer an approximate solution of the N-representability problem and a new approach to electron correlation.   © 1998 John Wiley & Sons, Inc. Int J Quant Chem 70: 557-570, 1998
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  • 109
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    More about the leaky aquifer function (1998)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 70 (1998), S. 623-626 
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    ISSN: 0020-7608
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The leaky aquifer function W(x, y) is an incomplete Bessel function which has had application in hydrology and more recently in electronic-structure calculations. This article presents an expansion which improves the efficiency of the calculation of W in the only part of its range not treated adequately by previously published methods, namely, the regime where x and y are both larger than unity but one is much larger than the other.   © 1998 John Wiley & Sons, Inc. Int J Quant Chem 70: 623-626, 1998
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  • 110
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    Excitation energies in Brillouin-Wigner-based multireference perturbation theory (1998)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 70 (1998), S. 613-622 
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    ISSN: 0020-7608
    Keywords: excitation energies ; dynamic correlation effects ; perturbation theory ; MBPT ; multireference perturbation theory ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We present the formalism for the treatment of several states of the same symmetry in basis-set reduction (BSR), a form of second-order Brillouin-Wigner multireference perturbation theory, which incorporates the first-order correction of the primary-space wave function with respect to its orthogonal complement. We benchmark this method for some valence and some Rydberg excitations of four small molecules (O2, CO, ethene, and ozone). In direct comparison with the underlying MR-SDCI benchmark results, we find an average accuracy of 0.2 eV or better for the excitation energies of the molecules considered and demonstrate the stability of the method with increasing size of the basis set and primary space. We argue that the configuration-based approach in BSR allows an accurate description of dynamical correlation effects with minimal primary space wave functions, containing far fewer configurations than are required for a CASSCF-based perturbative treatment of the molecules.   © 1998 John Wiley & Sons, Inc. Int J Quant Chem 70: 613-622, 1998
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  • 111
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    Approximate Brueckner orbitals and shakeup operators in electron propagator calculations: Applications to F- And OH- (1998)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 70 (1998), S. 651-658 
    Details
    ISSN: 0020-7608
    Keywords: electron propagator ; propagator theory ; Brueckner orbitals ; anions ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Vertical electron detachment energies of F- and OH- have presented difficulties for perturbative electron propagator methods. A recently derived, nondiagonal, renormalized approximation and two additional improvements are studied here. These improvements are replacement of the Hartree-Fock reference determinant by a determinant of approximate Brueckner orbitals generated by a coupled-cluster doubles calculation and retention of correlation terms in the 2hp-2hp block of the superoperator Hamiltonian matrix. Agreement with experiment is significantly better with these methods. When Hartree-Fock orbitals are used, electron detachment energies are underestimated, but approximate Brueckner orbitals lead to overestimates.   © 1998 John Wiley & Sons, Inc. Int J Quant Chem 70: 651-658, 1998
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  • 112
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    Excited states in density functional theory (1998)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 70 (1998), S. 681-691 
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    ISSN: 0020-7608
    Keywords: excited states ; Kato's theorem ; adiabatic connection ; optimized potential method ; KLI approximation ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Density functional theory for a single excited state is presented using Kato's theorem and the concept of adiabatic connection. The degenerate case is also detailed. The optimized potential method is generalized. The generalized Krieger, Li, and Iafrate (KLI) approximation is derived.   © 1998 John Wiley & Sons, Inc. Int J Quant Chem 70: 681-691, 1998
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  • 113
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    A density functional study of small nickel clusters (1998)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 70 (1998), S. 693-701 
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    ISSN: 0020-7608
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Small nickel clusters up to the tetramer are investigated within the framework of the local spin density functional theory. Several competitive states are studied for the dimer. Both the geometry and the spin state are optimized for several starting symmetries in the case of the trimer and the tetramer. Moreover, all those calculations are followed by a vibrational analysis in order to discriminate between real minima and saddle points on the potential energy surface. It is found that Jahn-Teller deformations play an important role in determining transition-metal cluster geometries. Equilibrium geometries, electronic configurations, binding energies, magnetic moments, and harmonic frequencies are reported in this work.   © 1998 John Wiley & Sons, Inc. Int J Quant Chem 70: 693-701, 1998
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  • 114
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    Excited electronic states of carotenoids: Time-dependent density-matrix-response algorithm (1998)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 70 (1998), S. 711-727 
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    ISSN: 0020-7608
    Keywords: density matrix response functions ; N-scaling ; time-dependent Hartree-Fock ; nonlinear response ; bosonization ; carotenoids ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The response of the single-electron density matrix of a many-electron system to an external field is calculated using the time-dependent Hartree-Fock (TDHF) technique. A procedure for inverting the resulting nonlinear response functions to obtain an effective quantum multilevel system that has the same response is developed. The number of effective states is gradually increased as higher-order nonlinearities are computed. The complete set of intrastate and interstate density matrices and excited-state energies can be calculated. A favorable N-scaling of computational effort with size can be obtained making use of the localization of the optical transitions in real space.   © 1998 John Wiley & Sons, Inc. Int J Quant Chem 70: 711-727, 1998
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  • 115
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    Effect of heavy atom on the second hyperpolarizability of tetrahydrofuran homologs investigated by ab initio molecular orbital method (1998)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 70 (1998), S. 737-743 
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    ISSN: 0020-7608
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Static second hyperpolarizabilities were calculated for molecules including tetrahydrofuran homologs by ab initio molecular orbital methods at the Hartree-Fock and various correlation levels. Substitution of the heteroatom with a heavier atom is found to be effective for increase of the second hyperpolarizability of these homologs. Comparison between the present results and previous results for furan homologs shows that π-conjugation is not effective for the increase of second hyperpolarizability of the molecules when the heteroatom is heavier.   © 1998 John Wiley & Sons, Inc. Int J Quant Chem 70: 737-743, 1998
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  • 116
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    Structure dependence of the low-lying excited states and the first dipole hyperpolarizability of phenol blue (1998)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 70 (1998), S. 745-750 
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    ISSN: 0020-7608
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: (CH3)2NC6H4NC6H4O is a merocyanine dye that exists in resonance between the two extreme conformations of a keto and an eno form. The keto form is more stable in the gas phase and the eno form is believed to be more stable in solution. We show that the keto form can prevail in nonpolar solvents, but in polar solvents like chloroform, the eno form should be dominant. Analysis is made of the solvent effects in the low-lying absorption transition and its consequence in the first dipole hyperpolarizability. We show by explicit calculations that the hyperpolarizability of the eno form is one order of magnitude greater than in the keto form. We then contend that this structure dependence may also lead to very large changes of the hyperpolarizability of similar molecules and polymers in solvents.   © 1998 John Wiley & Sons, Inc. Int J Quant Chem 70: 745-750, 1998
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  • 117
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    Theory and calculations of electric field effects on hyperfine interactions (1998)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 70 (1998), S. 771-778 
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    ISSN: 0020-7608
    Keywords: electric field effects ; hyperfine interactions ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A recently published theory of electric field effects on electron spin resonance hyperfine couplings [Phys. Ref. Lett. 79, 379, (1997)] is extended to nuclear quadrupole coupling. With the use of the ab initio coupled Hartree-Fock approach and augmented Sadlej basis set, results are obtained for the first-order corrections to the isotropic and anisotropic parts of hyperfine coupling tensor and the electronic part of the electric field gradient tensor for 17O and 2H nuclei in OH radical. The first-order coefficients exhibit a number of interesting features. The calculated results, in general, appear to be insensitive to the bond-centered polarization functions used in the calculation.   © 1998 John Wiley & Sons, Inc. Int J Quant Chem 70: 771-778, 1998
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  • 118
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    Differential equations for ground-state electron density and Slater sum in atoms and molecules with and without external fields (1998)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 70 (1998), S. 779-788 
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    ISSN: 0020-7608
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The ground-state density amplitude {ρ(r)}1/2 for atoms and molecules satisfies a Schrödinger equation in which the customary one-body potential energy V(r) of density functional theory is supplemented by the addition of the Pauli potential Vp(r). Since neither the exchange-correlation potential Vxc or Vp are presently known as functionals of the electron density ρ(r), approximations are currently unavoidable. Here, widespread use is made of semiclassical approximations, within a self-consistent field framework both with and without magnetic fields. The importance of low-order gradient quantities ∇2ρ/ρ and (∇ρ/ρ)2 is one focal point, while a generalized low-density approximation is another. New relativistic differential equations are given. Then, the arguments are generalized to embrace the so-called Slater sum P(r, β) : β=(kβT)-1, of statistical mechanics, generated by the one-body potential V(r). This is a generalized partition function, and differential equations are set up for this quantity P(r, β) with and without external fields. Finally, some potentially fruitful directions for treating cylindrically symmetric inhomogeneous electron liquids are outlined, following the very recent work of Amovilli and March. These include modeling the Slater sum along the electric field direction for the Stark effect in a hydrogenlike atom.   © 1998 John Wiley & Sons, Inc. Int J Quant Chem 70: 779-788, 1998
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    Some aspects of data processing for an optical absorption experiment in a pulsed 1000-Tesla magnet (1998)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 70 (1998), S. 797-804 
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    ISSN: 0020-7608
    Keywords: pulsed magnetic field ; Zeeman effect ; quadruple metal bonds ; octachlorodirhenate ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A procedure is given for the analysis of optical absorption data acquired in the hostile environment of a pulsed 1000-Tesla magnet.   © 1998 John Wiley & Sons, Inc. Int J Quant Chem 70: 797-804, 1998
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    Relativistic effects on the structural phase stability of molybdenum (1998)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 70 (1998), S. 825-830 
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    ISSN: 0020-7608
    Keywords: molybdenum ; phase stability ; relativity ; Douglas-Kroll ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The body-centered cubic-face-centered cubic (bcc-fcc) structural phase stability of molybdenum (Mo) is studied as a function of volume with both nonrelativistic and scalar-relativistic linear combinations of Gaussian-type orbitals-fitting functions (LCGTO-FF) calculations. It is demonstrated that relativity has a significant, albeit small effect, on the bcc-fcc structural energy difference, which increases with pressure. The scalar-relativistic structural energy difference curve is shown to be in excellent agreement with an earlier scalar-relativistic calculation using the full-potential linear muffin-tin orbital (FP-LMTO) method, clearly demonstrating the ability of the scalar-relativistic LCGTO-FF method to resolve an extremely subtle relativistic effect. It is argued that relativity will tend to delay pressure-induced structural phase transitions that are triggered by electron band reordering.   © 1998 John Wiley & Sons, Inc. Int J Quant Chem 70: 825-830, 1998
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    Water cluster approach to study hydrogen-bonded pattern in liquid water: Ab initio orientational defects in water hexamers and octamers (1998)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 70 (1998), S. 831-853 
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    ISSN: 0020-7608
    Keywords: Liquid water ; H-bond pattern ; orientational defect ; water cluster ; “dangling” bond ; ab initio HF/6-311(d, p) calculation ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Fifteen different structures of the water hexamer found ab initio within the 6-311G(d, p) basis set in the interval of 1.75 kcal/mol above the global minimum represent an unprecedented wide range of conformational plasticity of liquid water. The present work also provides the first ab initio demonstration of the existence of pentacoordinated water clusters of an orientational defect type and elaborates their properties.   © 1998 John Wiley & Sons, Inc. Int J Quant Chem 70: 831-853, 1998
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    Intramolecular hydrogen bonding in resonance-stabilized systems (1998)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 70 (1998), S. 863-875 
    Details
    ISSN: 0020-7608
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The 3-substituted-2-methoxybenzoic acid system exhibits resonance-stabilized intramolecular hydrogen bonding between the 2-methoxy oxygen and the adjacent carboxylic acid. This intramolecular hydrogen bond can be disrupted by adding another substituent with variable size on the neighboring 3-position of the ring. To relieve steric strain, the system must sacrifice hydrogen bonding and/or resonance stabilization. Full-energy optimizations have been done at HF/D95V (valence double-zeta Dunning-Huzinaga), HF/6-31G* (Pople), HF/D95 (full double-zeta Dunning-Huzinaga), HF/D95V(d, p), and HF/6-31+G(d, p). Further single-point calculations were done at MP2/D95V, MP2/6-31G*, MP2/D95, MP2/D95V(d, p), and MP2/6-31+G(d, p). The thermal populations of various conformational states including the hydrogen-bonding conformation are presented. The computational results were compared with the experimental thermal population of hydrogen bonding determined by nuclear magnetic resonance (NMR) and infrared (IR) spectroscopies. Results indicate that polarization of the second-row elements in intramolecular hydrogen bonding and perturbation-theory calculations that correct for electron correlations are very important for intramolecular hydrogen bonding. Adding polarization and diffuse functions to the hydrogens, while useful, are quite costly for these systems and do not seem to be as important.   © 1998 John Wiley & Sons, Inc. Int J Quant Chem 70: 863-875, 1998
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    The interface of electronic structure and dynamics for reactions in solution (1998)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 70 (1998), S. 887-896 
    Details
    ISSN: 0020-7608
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We compare two systematic approaches to the calculation of reaction rates in liquid solutions: the separable equilibrium solvation (SES) approximation and the equilibrium solvation path (ESP) approximation. These approaches are tested for two reactions, ClCH3+NH3→Cl-+H3CNH3+ (R1) and NH4+…N′H3→NH3…N′H4+ (R2), both in aqueous solution. The first reaction illustrates the importance of variational optimization of the transition state, and the second illustrates the importance of tunneling. Free energies of solvation are calculated by the Solvation Model 5. All calculations are carried out by the new AMSOLRATE program, which is an interface of the AMSOL and POLYRATE programs.   © 1998 John Wiley & Sons, Inc. Int J Quant Chem 70: 887-896, 1998
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    Single-electron-capture cross sections by alpha-particles from ground state K(4s) and Rb(5s): A molecular-state approach (1998)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 70 (1998), S. 909-917 
    Details
    ISSN: 0020-7608
    Keywords: charge transfer ; α-particles ; alkali atoms ; MO expansion ; close coupling ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Cross sections for single-electron capture by α-particles from ground state K and Rb were calculated in the low-to-intermediate energy region by employing the molecular expansion method in the framework of impact parameter formulation. The colliding partners are treated as a pseudo-one-electron system and the technique of the pseudopotential is used to account for their mutual interactions. The molecular wave function of the quasimolecule formed during the collision is expanded in terms of basis sets of atomic orbitals on two centers. The resulting coupled equations are solved semiclassically where a straight-line trajectory describes the relative motion of the two nuclei. The effect of electron translation is also suitably incorporated. The calculated cross sections, both total and partial, are presented and compared with the available experimental measurements.   © 1998 John Wiley & Sons, Inc. Int J Quant Chem 70: 909-917, 1998
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    Extension of the LAP functional to include parallel spin correlation (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 64 (1997), S. 427-446 
    Details
    ISSN: 0020-7608
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The kinetic energy density-dependent correlation functional LAP1 is extended to include parallel-spin correlation beyond the exchange level. Two exchange-correlation schemes are considered, combining the new correlation functional (LAP3) with the GGA exchange of Becke and the GGA exchange of Perdew. Extensive tests on molecules and hydrogen-bonded systems are presented and discussed elucidating the role of parallel-spin correlation in different cases. Its inclusion in the LAP functional leads, on average, to a slight improvement of the calculated binding energies and equilibrium geometries of molecules. Particularly high sensitivity of the energy results on the relative share of parallel-spin correlation is observed for aromatic molecules and for systems involving weak hydrogen bonds.   © 1997 John Wiley & Sons, Inc. Int J Quant Chem 64: 427-446, 1997
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    On the electronic structure of Cu(H2O)62+ (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 64 (1997), S. 453-458 
    Details
    ISSN: 0020-7608
    Keywords: hexisa aqua copper cation ; charge transfer excited states ; 3d→3d excited states ; basis sets ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The electronic structure of the ground state and the doublet excited states due to 3d→3d transitions and charge-transfer transitions from ligand to copper of Cu(H2O)62+ are investigated by ab initio calculations. The excited states corresponding to the 3d→3d transitions are calculated to be 1.1-1.4eV above the ground state and this is in good agreement with experiment. The charge-transfer excited states are not so easily determined as are the 3d→3d states, and they are described as an electron transfer from an orbital localized on each water. We have focused our attention on the description of these excited states. The charge-transfer excitations are predicted to start around 6.5 eV for Cu(H2O)62+.   © 1997 John Wiley & Sons, Inc. Int J Quant Chem 64: 453-458, 1997
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    HNNH3, a new possible isomer of N2H4: An ab initio study (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 64 (1997), S. 447-452 
    Details
    ISSN: 0020-7608
    Keywords: N2H4 ; isomer ; ab initio calculation ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Ab initio studies applying the 3-21G, 6-31G, and 6-31G** basis sets and also including the MP2 correction were carried out on H2NNH2, HNNH3, and the transition state of the reaction H2NNH2(DOUBLE BOND)HNNH3. First, the geometries of molecules were optimized using the theoretical methods mentioned in the restricted Hartree-Fock (RHF) scheme. The energies of the molecules corresponding to RHF/6-31G** geometries were subsequently calculated including electron-correlation effects at the level of the second-order Møller-Plesset (MP2) perturbation theory. The vibrational frequencies, net charges, and dipole moments were obtained from the theoretical calculations. The results of our calculations indicate unambiguously that H2NNH2 is thermodynamically more stable than is HNNH3. On the other hand, an isolated HNNH3 molecule once created would be stable since barriers for its unimolecular isomerization and decomposition are relatively high. But HNNH3 is unlikely to be isolated in measurable amounts because of bimolecular tautomerization. Nevertheless, HNNH3 can be considered as an intermediate in chemical processes involving N2H4.   © 1997 John Wiley & Sons, Inc. Int J Quant Chem 64: 447-452, 1997
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    Polyiodide chains in crystalline organic iodides: Ab initio Hartree-Fock crystal orbital study (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 64 (1997), S. 473-479 
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    ISSN: 0020-7608
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Hartree-Fock crystal orbital calculations of two crystalline organic iodides, tetrathiafulvalenium triiodide (TTF∗I3) and dipyridinium decaiodide (NHC5H5)2∗I10, were carried out. The former crystal contains no true polyiodide chains, whereas such chains are present in the latter crystal. In such a way, the effect of the polyiodide chain formation on the electronic structure of crystalline organic iodides was studied. The present calculations show that crystalline organic iodides with polyiodide chains could, in principle, be quasi-one-dimensional semiconductors. A polyiodide chain could be a carrier of semiconductivity only when it is formed by fully charged iodide anions (no charge transfer from the chain).   © 1997 John Wiley & Sons, Inc. Int J Quant Chem 64: 473-479, 1997
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    Studies on the band structures of some low-temperature superconductors (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 64 (1997), S. 459-472 
    Details
    ISSN: 0020-7608
    Keywords: low-temperature superconductors ; tight-binding energy-band calculations ; superconducting transition temperatures ; chemical bondings ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: In the present article, the electronic energy-band structures of 20 series of low-temperature superconductors were investigated employing the tight-binding method within the extended Hückel approximation. The energy bands, the densities of states, and the crystal orbital overlap populations of them are discussed. It is found that, for the crystals in the same series with the same structure type and the component elements in the same group of the periodic table, their overall energy-band structures are similar to each other. The analysis of their chemical bondings shows that the stronger bonding is generally correlated with the lower superconducting transition temperature to some extent. The higher Tc's of 4d metals or compounds relative to those of the corresponding 5d crystals are mainly due to the difference of the electronegativities of metal atoms, while the steric effects play a more important role in the higher Tc of 4d crystals than do those of the corresponding 3d crystals.   © 1997 John Wiley & Sons, Inc. Int J Quant Chem 64: 459-472, 1997
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    Introduction to the workshop on properties of molecules in strong magnetic fields (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 64 (1997), S. 495-496 
    Details
    ISSN: 0020-7608
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: No abstract.
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    List of participants (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 64 (1997), S. 497-499 
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    ISSN: 0020-7608
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: No abstract.
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    Effect of different subsets on convergence patterns of hyperspherical harmonic expansion for the S states of the helium atom (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 64 (1997), S. 661-668 
    Details
    ISSN: 0020-7608
    Keywords: Hyperspherical harmonics ; radial correlation ; angular correlation ; l limits ; 1S and 3S states of the helium atom ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Effects of different subsets on convergence patterns of hyperspherical harmonic (HH) expansions for the low-lying 1S and 3S states of the helium atom have been investigated with the correlation-function-hyperspherical-harmonic-generalized-Laguerre-function (CFHHGLF) method by successively introducing HH subsets with the fixed three-dimensional angular momentums (l) into the atomic wave functions. The eigenenergies given by the HH subsets of l=0, 1, 2, and 3 are in good agreement with the best s-, sp-, spd-, and spdf limits of variational configuration interaction (CI) calculations, respectively. The final eigenenergies of the ground state as well as the examined low-lying excited 1S and 3S states are quite close to the exact Hylleraas CI (HCI) values at the sixth decimal place. Moreover, l=0 and l≠0 expansion results also tell us that it is not necessary to take into account too many HHs at the given l, especially for higher l, and that the more the absolute electron correlation energies the bigger l it takes to obtain precise eigenenergies.   © 1997 John Wiley & Sons, Inc. Int J Quant Chem 64: 661-668, 1997
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    Role of oxygen content in electronic structures of Ba0.6K0.4BiO3-δ (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 64 (1997), S. 703-710 
    Details
    ISSN: 0020-7608
    Keywords: Superconductor ; Ba0.6K0.4BiO3-δ ; oxygen content ; band structure ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The band structure calculations on the K-doped superconductor Ba0.6K0.4BiO3-δ were carried out, and the effect of the oxygen content on its electronic structures was studied in the present work. The results show that the variation in the oxygen content caused by the partial substitution of K for Ba has a great effect on its electronic structure. The oxygen vacancies cause the saddle point singularity to be displaced gradually as the oxygen content is decreased, whereas the band structure is only translated a little and remains unchanged in shape, which reflects the non-rigid-bandlike behavior of Ba0.6K0.4BiO3-δ in the presence of the oxygen vacancies. When the saddle point singularity is displaced and lies at the Fermi level Ef, the total density of states at Ef, N(Ef), has the highest value which is twice as large as that of the Ba0.6K0.4BiO3 compound without the oxygen vacancies. These results reveal the important role of the oxygen content in the Ba-K-Bi-O superconducting system.   © 1997 John Wiley & Sons, Inc. Int J Quant Chem 64: 703-710, 1997
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    The quantum theory of radiation (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 64 (1997), S. 735-735 
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    ISSN: 0020-7608
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: No abstract.
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    Symmetric group approach to relativistic CI. IV. Representations of one-electron spin operators and their products in a symmetric group-adapted basis of N-electron spin functions (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 64 (1997), S. 737-737 
    Details
    ISSN: 0020-7608
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: No abstract.
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    Theoretical studies of inclusion complexes of β-cyclodextrin with methylated benzoic acids (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 64 (1997), S. 711-719 
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    ISSN: 0020-7608
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: AM1 semiempirical molecular orbital calculations have been performed on the inclusion complexes of β-cyclodextrin (β-CD) with methylated benzoic acids in two orientations, the “head-first” and “tail-first” positions. In the former, the CO2H group points toward the primary hydroxyls of the CD. In the latter, it points away from them. Out of 30 possible inclusion complexes, AM1 results predict only three clearly stable inclusion complexes. These are β-CD with 4-methyl benzoic acid in the head-first position, β-CD with 2,4-dimethyl benzoic acid in the head-first position, and β-CD with 3,5-dimethyl benzoic acid in the tail-first position. The orientations of the stable inclusion complexes correlate with the total number of intramolecular hydrogen bonds and intermolecular hydrogen bonds. The stability of a complex also correlates with the closeness of the host and guest geometries in the complex to their isolated molecule geometries.   © 1997 John Wiley & Sons, Inc. Int J Quant Chem 64: 711-719, 1997
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    Vibronic instability of molecular configurations in the Hartree-Fock-Roothaan approximation (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 65 (1997), S. 37-48 
    Details
    ISSN: 0020-7608
    Keywords: pseudo-Jahn-Teller effect ; vibronic coupling ; molecular geometry ; energy surfaces ; random-phase approximation ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The traditional description of pseudo-Jahn-Teller molecular configuration instability based on Bader's formula for the curvature of the adiabatic potential is reconsidered in order to make it consistent with straightforward calculations of the ground-state energy surface within Hartree-Fock-Roothaan approximation. The proposed approach employs floating molecular orbitals constructed by “frozen LCAO” coefficients (computed in the reference geometry) and floating atomic orbitals, which allows one to exclude the vibronic mixing with the excited states irrelevant from chemical point of view. The relaxation (vibronic) contribution to the instability, expressed in terms of one-electron quantities, involves two sets of orbital vibronic constants. These are, respectively, defined as matrix element derivatives of correct and “frozen LCAO” Fock operators between occupied and unoccupied molecular orbitals. The canonical form of the relaxation contribution can be achieved when the vibronic interaction is presented as a mixing between the ground electronic state and the excited states calculated within the random-phase approximation   © 1997 John Wiley & Sons, Inc. Int J Quant Chem 65: 37-48, 1997
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    The freeon theory of magnetism. I. The Heisenberg interaction (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 65 (1997), S. 287-297 
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    ISSN: 0020-7608
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The freeon theory of magnetism is a viable alternative to the well-known Heisenberg theory. In particular, the freeon-exchange theory provides a deeper insight into the nature of the magnetic interaction than does the spin-exchange theory. In addition, it avoids the superfluous MS quantum number in zero-field splitting, the overcounting of spin states, the spin frustration, and the spin paradigm. The freeon-exchange theory employs the algebra of the symmetric group and/or the unitary group in place of the spin algebra. The basis vectors of freeon theory are the Gel'fand states which are uniquely labeled by Gel'fand diagrams; the latter provide both the electron configuration and the spin quantum number. Both the spin and the freeon formulations support the Landé interval rule. In this article, we apply freeon theory to transition-metal dimers, trimers, and tetramers; these are examples of molecular magnets which have applications to microcircuitry. The freeon theory follows the permutation-group principle laid down by Herman Weyl over one-half a century ago.   © 1997 John Wiley & Sons, Inc. Int J Quant Chem 65: 287-297, 1997
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    Comparison of electron propagator methods for calculating electron detachment energies of anions (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 65 (1997), S. 463-469 
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    ISSN: 0020-7608
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Electron propagator methods for calculating vertical electron detachment energies of closed-shell anions are compared with respect to accuracy and efficiency. Examination of self-energy terms and numerical tests on typical anions show that the partial third-order (P3) quasi-particle approximation is superior to the third-order and outer valence Green's function methods in both respects.   © 1997 John Wiley & Sons, Inc. Int J Quant Chem 65: 463-469, 1997
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    Maximum principles in DFT from reciprocal variational problem (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 65 (1997), S. 499-501 
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    ISSN: 0020-7608
    Keywords: density-functional theory ; variational principle ; reciprocal problem ; maximum principle ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Formalism of density-functional theory (DFT) is based on the calculus of variation. In the Hohenberg and Kohn theorem, a variational equation minimizing electronic energy with respect to an electron density is constructed. The calculus of variation allows one to formulate a problem which is reciprocal to an original one. Also, we may consider the problem of finding the electron density determining a given energy E=E[ρ] for a maximum number N=N[ρ] of the electrons forming the system. In this work, the reciprocal variational problem is discussed. Mathematical considerations are followed by a presentation of an application of the reciprocal problem (maximum entropy principle). Other possibilities of the applications are sketched.   © 1997 John Wiley & Sons, Inc. Int J Quant Chem 65: 499-501, 1997
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    Real-space multigrid methods for large-scale electronic structure problems (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 65 (1997), S. 531-543 
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    ISSN: 0020-7608
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We describe the development and applications of a new electronic structure method that uses a real-space grid as a basis. Multigrid techniques provide preconditioning and convergence acceleration at all length scales and therefore lead to particularly efficient algorithms. The salient points of our implementation include: (i) new compact discretization schemes in real space for systems with cubic, orthorhombic, and hexagonal symmetry and (ii) new multilevel algorithms for the iterative solution of Kohn-Sham and Poisson equations. The accuracy of the discretizations was tested by direct comparison with plane-wave calculations, when possible, and the results were in excellent agreement in all cases. These techniques are very suitable for use on massively parallel computers and in O(N) methods. Tests on the Cray-T3D have shown nearly linear scaling of the execution time up to the maximum number of processors (512). The above methodology was tested on a large number of systems, such as the C60 molecule, diamond, Si and GaN supercells, and quantum molecular dynamics simulations for Si. Large-scale applications include a simulation of surface melting of Si and investigations of electronic and structural properties of surfaces, interfaces, and biomolecules.   © 1997 John Wiley & Sons, Inc. Int J Quant Chem 65: 531-543, 1997
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    Correlated relativistic calculation of the giant resonance in the Gd3+ absorption spectrum (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 65 (1997), S. 575-584 
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    ISSN: 0020-7608
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The aim of the present study is to extend the multiconfiguration Dirac-Fock approach to include spin polarization in order to study magnetically ordered systems. Routines handling time-dependent ionization phenomena adapted to multilevel and multichannel autoionization processes have been developed and used to describe the giant resonance in the Gd3+ absorption spectrum. In our calculation of the complete energy structure of 4d94f8 configuration it is discovered that the majority of the oscillator strength associated with the 4d-4f transitions is limited to a relatively narrow spectral width, creating a giant resonance. Our calculations take account of the most important correlation effects, and the agreement with the experimentally observed giant resonance in gadolinium is quite good.   © 1997 John Wiley & Sons, Inc. Int J Quant Chem 65: 575-584, 1997
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    Low-energy electron scattering from a model H2 potential using finite elements in two dimensions (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 65 (1997), S. 591-600 
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    ISSN: 0020-7608
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The Schrödinger equation for the scattering of an electron by a hydrogen molecule is solved by the finite element method, in spherical coordinates, using fifth-order Hermite interpolating polynomials. The computational method is quite similar to the work of Shertzer and Botero [Phys. Rev. A 49, 3673 (1994), and references therein]. However, to study large systems, an effective one-particle dynamical equation is defined, unlike the procedure of Shertzer and Botero. To illustrate the basic computational procedure, a model electron-H2 interaction potential (static+exchange+polarization) is constructed and the K-matrix is calculated. A novel feature of the present method is the procedure for extracting the partial-wave amplitudes at a value of r, the size of which is fixed by the range of nonlocal potentials in the problem, and then propagating the scattering amplitudes out to an effective infinity where the converged K-matrix is determined.   © 1997 John Wiley & Sons, Inc. Int J Quant Chem 65: 591-600, 1997
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    Systematic study of the lowest energy states of Aun (n=1-4) using DFT (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 65 (1997), S. 749-758 
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    ISSN: 0020-7608
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A density functional theory study of Au, Au2, Au3, and Au4 is performed focusing on the study of gold tips or contacts of interest in investigations of molecular-scale electronics. The ground state for the four systems corresponds to the one with the lowest multiplicity, and the ordering of energies follows the multiplicity in all cases. It is found that the tetrahedral geometry of Au4 is not the preferred one for the lowest spin states with neutral charge, but is the preferred one for charged systems. The ionization potential and electron affinities for the four systems were also obtained. A qualitative interpretation of the Kohn-Sham highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) orbitals is given based on the present calculations.   © 1997 John Wiley & Sons, Inc. Int J Quant Chem 65: 749-758, 1997
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    Ab initio prediction of the geometry and IR frequencies of the mono- and dihydrated complexes of the oxo-amino-tautomers of guanine (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 65 (1997), S. 759-765 
    Details
    ISSN: 0020-7608
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The results of ab initio post-Hartree-Fock study of the molecular structures, relative stabilities, and Hartree-Fock level calculations of IR frequencies and intensities for mono- and dihydrated oxo-amino-tautomers of guanine are reported. The geometries of the local minima were optimized without symmetry restrictions at the second-order Møller-Plesset perturbation theory. The standard 6-31 G(d) basis set was used. Local minima were verified by calculations of the matrix of energy second derivatives [Hessian at the HF/6-31 G(d) level]. The single-point calculations were also performed at the MP4(SDQ)/6-31G(d)//MP2/6-31G(d) and MP2/6-31++G(d, p)//MP2/6-31G(d) levels of theory. The total energies were corrected for the zero-point energy contributions scaled by a factor of 0.9. The structural parameters of mono- and dihydrated complexes were analyzed and related to the characteristics of hydrogen bonds. We found that the interaction of guanine tautomers with two water molecules changes the order of the gas-phase stability: 7GUA≈9GUA into the order which corresponds to the stability of the guanine tautomers in the polar solvent: 9GUA≫7GUA. The predicted IR HF/6-31G(d) level frequencies and intensities are presented.   © 1997 John Wiley & Sons, Inc. Int J Quant Chem 65: 759-765, 1997
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    Quantum chemical study of the molecular dynamics of hydrated Li+ And Be2+ cations (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 65 (1997), S. 803-816 
    Details
    ISSN: 0020-7608
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Molecular dynamics simulations of clusters of Li+ and Be2+ cations with up to 12 water molecules were performed calculating the particle trajectories using Hartree-Fock-derived forces. It was found that independent of the starting configuration tetrahedral clusters are obtained in less than 1 ps. Only for Li+ clusters, transition states with five and three water molecules are found. We discuss the structures of the clusters and the vibrational dynamics of the water molecules and compare them with other ab initio simulations, with simulations using analytical potential functions, and with static calculations. Water-dissociation (hydrogen transfer) reactions which take place at elevated temperatures were investigated.   © 1997 John Wiley & Sons, Inc. Int J Quant Chem 65: 803-816, 1997
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    Vibrational analysis of TeCl4. II. A Hartree-Fock, MP2, and density functional study (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 65 (1997), S. 817-826 
    Details
    ISSN: 0020-7608
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The geometry and vibrational spectrum of TeCl4 was calculated with various quantum chemical methods [Hartree-Fock, second-order Møller-Plesset (MP2) and generalized gradient approximation density functional theory (GGA-DFT)]. Five different basis-set combinations were tested: the relativistic effective core potentials with double-zeta split valence basis (RECP) of Hay & Wadt (HW) and Stevens et al. (CEP); the above RECP basis sets extended with polarization functions for Te and using a 6-31G* basis for Cl (HW* and CEP*); a medium-size all-electron basis set (ALL). The quality of the calculated data was assessed by comparison with recent experimental results. The Hartree-Fock method combined with the HW and CEP basis sets provided a very good approximation of the experimental vibrational spectra. The quality of the results is comparable to those of the best methods (MP2, B3-P, B3-PW with HW* and CEP* and B3-LYP, B3-P, B3-PW with the ALL basis set). However, the HW and CEP basis sets provided very poor geometry and vibrational frequencies when they were used in combination with any correlated method in this work. Similarly, the DFT methods using Becke's 1988 exchange functional (B-LYP, B-P, B-PW) without the inclusion of the exact exchange let to very poor results with the basis sets used in this study.   © 1997 John Wiley & Sons, Inc. Int J Quant Chem 65: 817-826, 1997
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    Theoretical calculation of carbon clusters (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 65 (1997), S. 867-875 
    Details
    ISSN: 0020-7608
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Structural parameters and energy have been calculated for C1-3 and C9 clusters using density functional theory through the deMon program. The C1-3 clusters were fully optimized using deMon; different basis sets were tested in order to choose the suitable one to be used in the C9 clusters. In the case of C2 the results were compared with experimental values. DZVP2 basis was selected because it always gave the closest value to the experimental data. The C9 carbon clusters were designed in relation to C(100) diamond surface. The energies of the nonrelaxed and relaxed surfaces with and without hydrogen were calculated. In the same way CH3-relaxed and CH2-relaxed species were calculated. The diamond growth mechanism proposed in the literature was evaluated by an energy analysis. The C(SINGLE BOND)CH3 distance is reported.   © 1997 John Wiley & Sons, Inc. Int J Quant Chem 65: 867-875, 1997
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    Dimerization of dexanabinol by hydrogen bonding accounts for its hydrophobic character (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 65 (1997), S. 1057-1064 
    Details
    ISSN: 0020-7608
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Dexanabinol, a dihydroxylated synthetic cannabinoid, is a member of the nonpsychotropic (+) 3S, 4S enantiomeric series. Experimental evidence suggests that dexanabinol might form aggregates (e.g., dimers) in which the two OH (a phenol and an allylic alcohol) groups are involved in hydrogen bonding. The extremely low solubility of dexanabinol in water implies that this interaction may not involve solvent molecules. A theoretical study of this phenomenon in the framework of the PM3 molecular approximation is described. Simple molecular models (phenol and 6-cyclohexene-1-methanol) were initially examined followed by extension of the calculations to dexanabinol. The results indicate that dimers of dexanabinol resulting from hydrogen bonding are more stable than the isolated molecules with the differences attributed to hydrogen bonding energies. It is suggested that the phenolic hydroxy group of one molecule forms a hydrogen bond with the allylic OH group of the second molecule and vice versa, resulting in dimers which contain two hydrogen bonds. The hydrogen bonds are more stable (6.14 kcal/mol) and the complex formed is more favored energetically when the phenol groups act as hydrogen bond donors and the allylic OH groups as acceptors. These interactions are also energetically more favored than those between dexanabinol and water (3.70 kcal/mol). The dexanabinol dimer manifested a lower dipole moment as compared to the monomer (1.211 vs. 2.221 debye) as well as a much larger log P (11.16 vs. 5.90), indicating strong hydrophobic character. The optimized structure shows that the OH groups involved in hydrogen bonds are oriented to the interior of the dimers, while the lipophilic side chains are oriented toward the exterior. These properties of the dimer may explain the low water solubility of dexanabinol.   © 1997 John Wiley & Sons, Inc. Int J Quant Chem 65: 1057-1064, 1997
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    Evaluation of strain in heterosiliranes: Systematics, surprises, and problems (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 58 (1996), S. 707-715 
    Details
    ISSN: 0020-7608
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A series of different isodesmic and homodesmic reactions defining strain energies in 3-membered rings are described and discussed. One isodesmic and one homodesmic reaction are applied numerically with the purpose of estimating strain in heterosiliranes including second-row and third-row heteroatoms. All molecules involved are optimized using SCF/6-31G**, and energies are calculated using MP2/6-31G**//SCF/6-31G** calculations. The results are discussed with reference to limitations of the chosen models. © 1996 John Wiley & Sons, Inc.
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    Bond alternation in ring-shaped molecules: The stability of the Peierls instabilityDedicated to Jens Peder Dahl on his 60th birthday. (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 58 (1996), S. 699-706 
    Details
    ISSN: 0020-7608
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We investigate the phenomenon of bond alternation in ring molecules of the type (CH)2n, which occurs due to the Peierls instability. We prove that the energy-minimizing configuration of bond lengths always has period two when n is odd. When n is even, a new instability may destroy the periodicity two as long as n is not too large. We also analyze the corresponding problem for the Heisenberg antiferromagnetic “spin-Peierls” system and prove that instabilities other than period two never occur there. © 1996 John Wiley & Sons, Inc.
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    Transfer matrices, band structures, and localized orbitals in quasi-one-dimensional systemsDedicated to Jens Peder Dahl on the occasion of his 60th birthday. (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 58 (1996), S. 717-731 
    Details
    ISSN: 0020-7608
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We present a scheme that - within certain approximations - connects the single-particle energies of defect-induced localized orbitals in quasi-one-dimensional systems to the band structures of related periodic structures. The mathematical foundations for the scheme are based on a transfer-matrix formulation of the Schrödinger equation. In contrast to most earlier approaches based on transfer matrices, the present formulation is directly related to parameter-free methods for electronic-structure calculations with more or less well-converged basis sets. Thereby, the transfer matrices get a dimension that in the general case is larger than two and it is, in addition, shown that a complete description of the system requires the introduction of a complementary set of matrices. However, in the ultimate formulation of the scheme, neither set of matrices needs to be defined. The scheme is illustrated through three examples, for which the band structures of the periodic structures have been obtained using a first-principles, density-functional, full-potential LMTO method for helical polymers. The three examples include trans-polyacetylene and polycarbonitrile as examples of conjugated polymers as well as the hydrogen-bonded polymer hydrogen fluoride. In both cases, we study solitonic defects. As the last example, we study selenium helices with special emphasis on defects involving local distortions of the dihedral angle. We finally discuss the approximations and limitations of the approach and will give some simple estimates of their implications. © 1996 John Wiley & Sons, Inc.
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    Density functional calculations of dioxygen binding in mono- and dinuclear copper complexes (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 65 (1997), S. 1077-1086 
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    ISSN: 0020-7608
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We studied the electronic structure, electronic excitation energies, charge distribution, and magnetic properties of three dinuclear copper complexes, [{[L-Cu]2O2}2+, L=1,4,7-triaza-cyclo nonone] (1), [{L-Cu]2O2}2+, L=hydrotis-(pyrazolyl)-borate (2) and {[(NH3)3-Cu]2O2}2+ (3) in two core isomers. The theoretical model complex 3 is sufficient to describe the qualitative electronic structure and related properties of 1 and 2. The main features of the electronic structure are similar between the three systems; the electronic excitation energies between copper- and oxygen-based orbitals are insensitive to ligand effects. In addition, we studied the energetics of mononuclear complexes to determine the mechanism of the formation of 1. In the suggested mechanism the end-on mononuclear complex forms first followed by an isomerization between the trans-μ-1,2-O2, μ-η2:ν2-O2 and (μ-O)2 dinculear isomers.   © 1997 John Wiley & Sons, Inc. Int J Quant Chem 65: 1077-1086, 1997
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    Local dielectric constants and Poisson-Boltzmann calculations of DNA counterion distributions (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 65 (1997), S. 1087-1093 
    Details
    ISSN: 0020-7608
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Recent work by the authors on the calculation of local solvent dielectric constants around polyelectrolytes using the Poisson-Boltzmann approach is analyzed in terms of the effect on surface potentials and counterion concentrations. Polyelectrolyte surface geometry, local electric fields, and counterion distributions contribute to the self-consistent prediction of local solvent dielectric constants. For an all-atom cell model of DNA with added monovalent salt varying from 0 to 0.5M, the Poisson-Boltzmann-determined electrostatic potential increases (negatively) by 50-100% upon the inclusion of local dielectric constants. This, in turn, implies that hydronium ion concentrations in the major and minor grooves increase by about 0.65 and 0.35 pH units, respectively. While counterion concentrations in the major groove change only slightly, those in the minor groove increase by 60-90%. It is also noted that while the local dielectric constant in the major groove monotonically increases away from the surface toward the bulk value of water the dielectric constant in the minor groove has a minimum about 2 Å from the surface due primarily to the local electric field. Certain other properties, such as ionic and dipole first passage times, are affected little by local dielectric constants (less than about 3%).   © 1997 John Wiley & Sons, Inc. Int J Quant Chem 65: 1087-1093, 1997
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    Kramers' unrestricted Hartree-Fock and second-order Møller-Plesset perturbation methods using relativistic effective core potentials with spin-orbit operators: Test calculations for HI and CH3I (1998)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 66 (1998), S. 91-98 
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    ISSN: 0020-7608
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The Kramers' restricted Hartree-Fock (KRHF) and second-order Møller-Plesset perturbation (KRMP2) methods using relativistic effective core potentials (RECP) with spin-orbit operators and two-component spinors are extended to the unrestricted forms, KUHF and KUMP2. As in the conventional unrestricted methods, the KUHF and KUMP2 methods are capable of qualitatively describing the bond breaking for a single bond. As a result, it is possible to estimate spin-orbit effects along the dissociation curve at the HF and MP2 levels of theory as is demonstrated by the test calculations on the ground states of HI and CH3I. Since the energy lowering due to spin-orbit interactions is larger for the I atom than for the closed-shell molecules, dissociation energies are reduced and bond lengths are slightly elongated by the inclusion of the spin-orbit interactions.   © 1998 John Wiley & Sons, Inc. Int J Quant Chem 66: 91-98, 1998
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    The conscious mind: In search of a fundamental theory (1998)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 66 (1998), S. 107-108 
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    ISSN: 0020-7608
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: No abstract.
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    Indexing scheme for classes of SN; partitions of N (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 64 (1997), S. 421-426 
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    ISSN: 0020-7608
    Keywords: Symmetric group ; partitions ; graphical representations ; classes of SN ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The classes of the symmetric group SN are identified by partitions of N. In this work an indexing scheme is presented which provides a dense enumeration of the classes of SN. The method is based on a graphical representation of partitions of N, which also enables the determination of the class corresponding to a given number.   © 1997 John Wiley & Sons, Inc. Int J Quant Chem 64: 421-426, 1997
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    The effects of pseudomagnetic fields in molecular spectra and scattering (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 64 (1997), S. 581-597 
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    ISSN: 0020-7608
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Pseudomagnetic fields appear in the Born-Oppenheimer method for molecules when conical intersections or electronic angular momenta are taken into account. These fields are not real magnetic fields but they have the same mathematical properties and can lead to real observable effects in the dynamics of molecules. A general vector potential (gauge theory) approach for including these field effects in the Born-Oppenheimer method is introduced and applied to H+O2 scattering and the vibrational spectrum of Na3(X) for zero total angular momentum (J=0). The scattering results for HO2 show significant shifts in the resonance energies and lifetimes due to a “magnetic solenoid”-type field originating from the C2v conical intersection in HO2. Significant changes in the state-to-state transition probabilities are also observed. The nondegenerate A1 and A2 vibrational spectra of Na3(X) show significant shifts in the energy levels due to a “magnetic solenoid”-type field originating from the D3h conical intersection in Na3. These two examples show that the effects of pseudomagnetic fields can be significant and in many cases they must be included to obtain agreement between theory and experiment. The newly developed gauge theory techniques for treating pseudomagnetic fields are also relevant for including the effects of real magnetic fields.   © 1997 John Wiley & Sons, Inc. Int J Quant Chem 64: 581-597, 1997
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    Chemical systems for exploration of high magnetic field effects (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 64 (1997), S. 607-611 
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    ISSN: 0020-7608
    Keywords: State inversion ; Zeeman effect ; quadruple metal bonds ; octachlorodirhenate ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A review is given of molecules that may exhibit observable structural or electronic changes at accessible pulsed magnetic fields. There is an emphasis on structural changes that modulate the ultraviolet-visible spectrum. Transition-metal dimers with quadruple bonds are recommended for testing in magnetic fields on the order of 1000 Tesla.   © 1997 John Wiley & Sons, Inc. Int J Quant Chem 64: 607-611, 1997
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    Generalized double exponential potential functions for diatomic molecules (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 65 (1997), S. 1-8 
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    ISSN: 0020-7608
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Two variants of the double exponential potential function and their virial modifications are proposed and tested. The first in reduced variables is F(t)=e-mt{[m(m2-1)-1/2-1]exp[-(m2-1)1/2t]-[m(m2-1)-1/2+1]exp[(m2-1)1/2t]} where t=κs=κ(R-Re)/Re, κ is a scaling constant, and m is a parameter. The second is G(t)=e-mt{e-mt-exp[(m2-1)1/2t]+exp[-(m2-1)1/2t]}. For m〈1, F(t) and G(t) are expressible in terms of trigonometric functions. A new procedure [multiplication by es/(1+s)] is illustrated that modifies potential functions so that they necessarily satisfy the molecular virial theorem. The generalized double exponential functions generate scaled first and second Dunham coefficients that well describe the experimental results for both ground and excited states.   © 1997 John Wiley & Sons, Inc. Int J Quant Chem 65: 1-8, 1997
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    N-representability of diagonal elements of second-order reduced density matrices (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 65 (1997), S. 127-142 
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    ISSN: 0020-7608
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The problem of pure-state N-representability of the two-particle spin-dependent density function ρ(x1, x2) is considered for an N-electron system, and a procedure for finding an N-representable ρ(x1, x2) is advanced. The problem is formulated in the framework of a family of N × N matrices formed from integrals of auxiliary two-particle functions θn(x1, x2) converging at n → ∞ to ρ(x1, x2)/[N(N-1)]. The simple requirement of positive definiteness of these matrices is shown to play a decisive role in finding an N-representable ρ(x1, x2). The results obtained may open new possibilities for using ρ(x1, x2) in the density-functional theory.   © 1997 John Wiley & Sons, Inc. Int J Quant Chem 65: 127-142, 1997
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    Nonsingular two/one-component relativistic Hamiltonians accurate through arbitrary high order in α2Presented at the 10th Seminar on Computational Methods of Quantum Chemistry, Strasbourg, September 3-6, 1996 (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 65 (1997), S. 225-239 
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    ISSN: 0020-7608
    Keywords: relativistic Hamiltonians ; two-component methods in relativity ; perturbation methods in relativity ; Douglas-Kroll approximation ; regular approximations in relativity ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A series of nonsingular two-component relativistic Hamiltonians is derived from the Dirac Hamiltonian by first performing the free-particle Foldy-Wouthuysen transformation and then a block-diagonalizing transformation. The latter is defined in terms of operators which can be determined iteratively through arbitrary order in α, leading to transformed Hamiltonians with the two-component block accurate through α2k, k=1, 2, 3,… . These Hamiltonians give relativistic energies which differ from Dirac's energies only in terms higher than α2k. Their relation to other nonsingular methods of relativistic quantum chemistry (the Douglas-Kroll method, the regular Hamiltonian schemes) is discussed. By removing the spin-dependent operators, the derived Hamiltonians can be written in spin-free one-component form. The computational effort involved is essentially the same as in the case of the Douglas-Kroll scheme and amounts to relatively easy modification of the core Hamiltonian.   © 1997 John Wiley & Sons, Inc. Int J Quant Chem 65: 225-239, 1997
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    Physicochemical aspects of an industrial processOrginally intended for presentation at the 1997 Sanibel Symposium. (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 65 (1997), S. 277-285 
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    ISSN: 0020-7608
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We briefly describe a nonclassical nonlinear thermodynamic theory with statistical foundations. This statistical approach is based on a generalization to arbitrary nonequilibrium conditions of the Gibbs ensemble algorithm, the so-called nonequilibrium statistical operator method, which can be considered to be encompassed within the scope of Jaynes' predictive statistical physics. This emerging formalism is used to provide a study of the technoindustrial process of thermal stereolithography (or infrared-laser-induced rapid prototyping). The experimental results are interpreted and the technical requirements fundamental for the success of the process are pointed out.   © 1997 John Wiley & Sons, Inc. Int J Quant Chem 65: 277-285, 1997
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    Real-space multigrid solution of electrostatics problems and the Kohn-Sham equations (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 65 (1997), S. 477-486 
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    ISSN: 0020-7608
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A multigrid method for numerically solving electrostatics and quantum chemical problems in real space is discussed. Multigrid techniques are used to solve both the linear Poisson equation and the nonlinear Kohn-Sham and Poisson-Boltzmann equations. The electrostatic potential, Laplacian, charge densities (electrons and nuclei), Kohn-Sham DFT orbitals, and the self-consistent field potential are all represented discretely on the Cartesian grid. High-order finite differences are utilized to obtain physically reasonable results on modestly sized grids. The method is summarized and numerical results for all-electron atomic and molecular structure are presented. The strengths and weaknesses of the method are discussed with suggested directions for future developments, including a new high-order conservative differencing scheme for accurate composite grid computations which preserves the linear scaling property of the multigrid method.   © 1997 John Wiley & Sons, Inc. Int J Quant Chem 65: 477-486, 1997
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    Transannular interactions in S82+ and Se82+: Reality or artifact? (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 65 (1997), S. 609-616 
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    ISSN: 0020-7608
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Electronic structure calculations, carried out at the HF/6-311G*, MP2/6-311G*, and BLYP/6-311G* levels of theory, reveal an unexpectedly large influence of electron correlation on bonding in the octasulfur dication S82+. Whereas a structure with a strong transannular bond linking two sulfur atoms with localized positive charges is predicted within the Hartree-Fock (HF) approximation, second-order Møller-Plesset (MP2) and Becke-Lee-Yang-Parr density functional (BLYP) calculations produce delocalized dications in which the transannular interactions are completely missing. Rigorous analysis of the computed electronic wave functions explains this phenomenon, which is also observed in the HF/TZVP and MP2/TZVP optimized geometries of Se82+. In agreement with the experimental data, the endo-exo conformers are found to possess the lowest energies for the octachalcogen dications, in contrast to the corresponding neutral species in which steric interactions determine the relative energies, favoring the exo-exo ring conformations. None of the three theoretical approaches is capable of reproducing the experimental solid-state geometries of S82+ and Se82+, which are almost certainly distorted to a large degree by the crystal packing and counterion effects. The results of the present study underscore the need for a concerted experimental and theoretical effort to provide the definitive answer to the question of transannular interactions in chalcogen compounds.   © 1997 John Wiley & Sons, Inc. Int J Quant Chem 65: 609-616, 1997
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    Ab initio calculations of the magnetic exchange coupling in sulfur-bridged binuclear Ni(II) complexes (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 65 (1997), S. 633-641 
    Details
    ISSN: 0020-7608
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The geometry dependence of the exchange integrals and the total energies of sulfur-bridged complexes of the types L5NiSNiL5, L4NiS2NiL4, and L3NiS3NiL3 is studied by quantum chemical ab initio methods. The linear monobridged complex is antiferromagnetic, the bi- and triply-bridged complexes are ferromagnetic for NiSNi angles between 85° and 100° and antiferromagnetic for smaller and larger angles. The superexchange mechanism is analyzed, and a comparison with oxygen-bridged complexes and experimental data is performed.   © 1997 John Wiley & Sons, Inc. Int J Quant Chem 65: 633-641, 1997
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    Reaction path for the insertion reaction of SnCl2 into the Pt(SINGLE BOND)Cl bond: An ab initio study (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 65 (1997), S. 643-650 
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    ISSN: 0020-7608
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The reaction pathway for the insertion reaction of SnCl2 into the Pt(SINGLE BOND)Cl bond on the cis-Pt(Cl)2(PH3)2 compound was investigated at the ab initio MO level of theory. The optimized structure obtained for the transition state indicates that this reaction proceeds through a three-center transition state, and the formed intermediate cis-Pt(Cl)(PH3)2(SnCl3) easily isomerizes to the trans-Pt(Cl)(PH3)2(SnCl3) compound. The nature of the bonds was investigated with the charge decomposition analysis (CDA) method and this method indicates that the SnCl3 group is a stronger trans director than is the PH3 group.   © 1997 John Wiley & Sons, Inc. Int J Quant Chem 65: 643-650, 1997
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    Free and hindered rotations in endohedral C60 fullerene complexes (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 65 (1997), S. 655-663 
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    ISSN: 0020-7608
    Keywords: Fullerenes ; endohedral complexes ; rotational spectroscopy ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We propose a two-parameter model Hamiltonian to analyze the low-energy dynamics of endohedral C60 fullerene complexes such as Li+@C60, Na+@C60, CO@C60, LiF@C60, and LiH@C60. The simplicity of the model is a direct consequence of very strong constraints imposed by both high symmetry and close confinement conditions, on the spherical anisotropy of the guest-cage interaction. In the parameter space region expanded by the selected systems, the guest low-energy dynamics undergo transitions from almost free rotations to more or less hindered rotations. These dynamics produce very particular features in the low-temperature rotational spectra, which can be taken, therefore, as fingerprints of a particular guest-cage interaction.   © 1997 John Wiley & Sons, Inc. Int J Quant Chem 65: 655-663, 1997
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    Electron correlation effects upon the static (hyper)polarizabilities of push-pull conjugated polyenes and polyynes (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 65 (1997), S. 679-688 
    Details
    ISSN: 0020-7608
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Ab initio calculations, including the electron correlation effects via the Møller-Plesset partitioning, of the static longitudinal polarizability (αL), first and second hyperpolarizabilities (βL and γL) of small push-pull molecules have been carried out with the 6-31G and 6-31G* basis sets. Two types of conjugated segment (polyacetylene and polyyne) were used as linker between the nitro and amino fragments, chosen as acceptor and donor groups. For small chains, the triply bonded structures lead to larger γL than the doubly bonded structures, although the αL and βL values order in the opposite direction. The second-order Møller-Plesset (MP2) corrections are essential to obtain correct estimates of the hyperpolarizabilities and are generally much larger than the MP3 and MP4 contributions. Geometry optimizations were carried out with both basis sets within the Hartree-Fock and MP2 schemes; the MP2 geometries lead to smaller bond length alternations and thus larger (hyper)polarizabilities. For αL a careful choice of the geometry is more crucial than the use of polarization functions during the electronic state calculations; however, for the hyperpolarizabilities, both the geometry and the basis set are of importance.   © 1997 John Wiley & Sons, Inc. Int J Quant Chem 65: 679-688, 1997
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    Vibrational versus electronic first hyperpolarizabilities of mono- and disubstituted benzenes: An ab initio coupled Hartree-Fock investigation (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 65 (1997), S. 689-696 
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    ISSN: 0020-7608
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Static electronic and vibrational longitudinal first hyperpolarizabilities (βLe(0) and βLv(0)) of a series of mono- and disubstituted benzenes were calculated at the Hartree-Fock 6-31G level by using coupled Hartree-Fock and the double harmonic oscillator approximations, respectively. Although the βLv(0)/βLe(0) ratio is slightly larger than unity and rather constant with respect to the substituent(s), it turns out, for the monosubstituted compounds, that the variations of βLe(0) upon substitution can be most accounted for by the mesomeric effects, whereas for βLv(0), the inductive effects are also of importance.   © 1997 John Wiley & Sons, Inc. Int J Quant Chem 65: 689-696, 1997
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    C(SINGLE BOND)H(DOTTED BOND)O and N(SINGLE BOND)H(DOTTED BOND)O hydrogen bonds in liquid amides investigated by Monte Carlo simulation (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 65 (1997), S. 709-717 
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    ISSN: 0020-7608
    Keywords: Monte Carlo simulation ; amides ; hydrogen bond ; radial distribution functions ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Monte Carlo simulations of liquid formamide, N-methylformamide (MF), and N,N-dimethylformamide (DMF) have been performed in the isothermal and isobaric ensemble at 298 K and 1 atm, aiming to investigate the C(SINGLE BOND)H(DOTTED BOND)O and N(SINGLE BOND〉H(DOTTED BOND)O hydrogen bonds. The interaction energy was calculated using the classical 6-12 Lennard-Jones pairwise potential plus a Coulomb term on a rigid six-site molecular model with the potential parameters being optimized in this work. Theoretical values obtained for heat of vaporization and liquid densities are in good agreement with the experimental data. The radial distribution function [RDF, g(r)] obtained compare well with R-X diffraction data available. The RDF and molecular mechanics (MM2) minimization show that the C(SINGLE BOND)H(DOTTED BOND)O interaction has a significant role in the structure of the three liquids. These results are supported by ab initio calculations. This interaction is particularly important in the structure of MF. The intensity of the N(SINGLE BOND)H(DOTTED BOND)O hydrogen bond is greater in the MF than formamide. This could explain some anomalous properties verified in MF.   © 1997 John Wiley & Sons, Inc. Int J Quant Chem 65: 709-717, 1997
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    A semiempirical study on the ring-opening process for the cyclopropanone, 2,2-dimethylcyclopropanone, trans-2,3-di-tert-butylcyclopropanone, and spiro(bicyclo[2.2.1]heptane-2.1′-cyclopropan)-2′-one systems in solution (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 65 (1997), S. 729-738 
    Details
    ISSN: 0020-7608
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The molecular mechanisms for the ring openings of cyclopropanone, 2,2-dimethylcyclopropanone, trans-2,3-di-tert-butylcyclopropanone, and spiro(bicyclo[2.2.1]heptane-2.1′-cyclopropan)-2′-one systems were studied at the PM3 semiempirical level in the gas phase and including solvent effects. The behavior of the solvent polarity was considered by using the SCRF polarizable continuum method. Six solvents were selected: hexane, ether, tetrahydrofuran, pyridine, acetone, and acetonitrile. An extensive exploration of the potential energy surface using analytical gradient techniques allows the characterization of stationary points associated to the stereomutation conversion. Along a disrotatory ring-opening mechanism, cyclopropanone, 2,2-dimethylcyclopropanone and trans-2,3-di-tert-butylcyclopropanone are intraconverted via an oxyallyl intermediate. The epimeric forms of the spiro(bicyclo[2.2.1]heptane-2.1′-cyclopropan)-2′-one intraconvert along two competitive pathways correspond to two-step processes by a disrotatory ring-opening mechanism. Two oxyallyl intermediates and four transition structures were obtained and the corresponding transition vectors are associated to the carbon-carbon bond-breaking process and the dihedral angle measuring the conrotatory movement of the plane defined by the three carbon atoms of the cyclopropanone ring. The oxyallyl intermediates and the transition structures for the four model systems present similar structures and energies and they are located on a rather flat region. The analysis of the theoretical results shows that the solvent reaction field decreases the energy barriers for the ring-opening processes and a stabilization of the oxyallyl intermediates takes place. The calculated relative barrier heights are in good agreement with the experimental data available, and the trends in the kinetics can be explained primarily by steric interactions. Nevertheless, for the spiro(bicyclo[2.2.1]heptane-2.1′-cyclopropan)-2′-one system, it is necessary to include a specific interaction of a discrete molecule of the nucleophilic solvent on the quantum mechanical representation to explain the experimental behavior.   © 1997 John Wiley & Sons, Inc. Int J Quant Chem 65: 729-738, 1997
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    A theoretical study of the electronic structure of transition-element carbides MnC (M=Fe, Ni, Cu, n=1, 5; and M=Ti, n=1, 7) and their interactions with an O atom by DFT methods (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 65 (1997), S. 919-928 
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    ISSN: 0020-7608
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Electronic structures and properties of carbides MC, M5C (M=Fe, Ni, Cu), and TiC and Ti7C were studied using density functional methods (DF), particularly the local density approach (LDA) with the Vosko-Wilk-Nusair (VWN) correlation functional, the generalized gradient approximation (GGA) of Becke and Perdew (using Becke's 1988 exchange functional and Perdew's 1986 correlation functional - BP86), and the ADF program of Baerends et al. In the first part of this report, we studied equilibrium geometries and dissociation energies for the process MC→M+C involving the doublet ground state for M=Cu, singlet and triplet states for M=Ni, and triplet and quintuplet states for M=Fe, Ti. Charge distributions by population analysis, dipole moments, and vibrational frequencies were also evaluated. All calculations were done using triple-zeta basis sets, with frozen-core orbitals and the GGA corrections. In the second part of this report, we consider the doublet states of Cu5C and singlet and triplet states of Ni5C, taking planar and nonplanar models for the M5 clusters. The triplet and quintuplet states of Fe5C were studied with a nonplanar model for the Fe5 cluster, whereas in the case of the triplet and quintuplet states of Ti7C, a Ti7 planar model was chosen. Optimization of the position of the C atom in the cluster, dissociation energies, distribution of charges in the molecule, and dipolar moments were also analyzed, and comparisons with the results obtained for the corresponding carbides (MC) were made. These results, together with the ones of the corresponding carbonyls - that have previously been done by the same methodology - provide us with an interesting comparison of the M(SINGLE BOND)C bond dissociation energy in carbides and carbonyls. Calculations of the C(SINGLE BOND)O bond dissociation energies for the MCO and MnCO compounds were also performed.   © 1997 John Wiley & Sons, Inc. Int J Quant Chem 65: 919-928, 1997
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    A semiempirical study on leupeptin: An inhibitor of cysteine proteases (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 65 (1997), S. 1125-1134 
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    ISSN: 0020-7608
    Keywords: leupeptin ; papain ; semiempirical ; cysteine protease ; active center ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: In this work, we modeled leupeptin (Ac.Leu.Leu.Arg.CHO), a natural inhibitor of proteases, and the active site of papain, a cysteine protease, using as a template the crystal structure of a leupeptin-papain complex recently obtained by Schroder and co-workers [FEBS Lett. 135(1), 38 (1993)] and including 11 amino acids relevant to the proteolytic activity of the enzyme. Our results show that the AM1 fully optimized leupeptin is more stable than is the leupeptin crystal structure by about 6.0 kcal/mol. Our results show also that in the modeled active center of papain the S - H⋅⋅⋅N structure is favored. When the aldehyde is included in the calculation, however, proton transfer occurs with a strengthening of the S-⋅⋅⋅HIm+⋅⋅⋅O(DOUBLE BOND)C (Asn175) catalytic triad. The AM1 method reproduces fairly well the interactions between the enzyme and the host molecule.   © 1997 John Wiley & Sons, Inc. Int J Quant Chem 65: 1125-1134, 1997
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    Theoretical studies of inclusion complexes of α- and β-cyclodextrin with benzoic acid and phenol (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 65 (1997), S. 1135-1152 
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    ISSN: 0020-7608
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A series of semiempirical molecular orbital calculations using the AM1 method were performed on the inclusion complexes of α- and β-cyclodextrin with benzoic acid and phenol in the “head-first” and “tail-first” positions. The AM1 results show that α-cyclodextrin complexes with both guest compounds in the “head first” position are more stable than in the “tail-first” position, while the β-cyclodextrin complex with phenol in the “tail-first” position is more stable, but with benzoic acid, the “head-first” position is more stable. The driving forces for complex formation were investigated based on different intramolecular and intermolecular interactions. In addition, 1SCF AM1 calculations were performed on the β-cyclodextrin complexes with benzoic acid in the “tail-first” and “head-first” positions with the benzoic acid moved stepwise along the Z-axis of the β-cyclodextrin principal axis coordinate system.   © 1997 John Wiley & Sons, Inc. Int J Quant Chem 65: 1135-1152, 1997
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    Construction and applications of symmetrized valence bond wave functions (1998)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 66 (1998), S. 1-7 
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    ISSN: 0020-7608
    Keywords: Symmetry ; projection operator ; VB wave function ; transition energy ; chemical bonds ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A method constructing symmetry-adapted bonded Young tableau bases is proposed, based on the symmetry properties of bonded tableaus and the projection operator associated with a point group. Several examples including the ground states and π excited states of O3-, O3, O3+, and C3- are shown for instruction to construct the symmetrized valence bond (VB) wave function. Excitation energies of transitions from the ground states to π excited states of O3-, C3H5, and C3- are calculated with an optimized symmetrized valence bond wave function in the σ-π separation approximation. Good agreement between the VB and experimental excitation energies is observed. The bonding features of the ground state and the first π excited singlet and triplet states for S3 are discussed according to bonding populations from VB calculations. Both the singlet-biradical and the dipole structures have significant contributions to the ground state X 1A1 of S3, while the excited state 1 1B2 is essentially composed of the dipole structures, and the 1 3B2 excited state is comprised from a triplet-biradical structure.   © 1998 John Wiley & Sons, Inc. Int J Quant Chem 66: 1-7, 1998
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    Comparative theoretical study of transition structures, barrier heights, and reaction energies for the intramolecular tautomerization in acetaldehyde/vinyl alcohol and acetaldimine/vinylamine systems (1998)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 66 (1998), S. 9-24 
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    ISSN: 0020-7608
    Keywords: keto/enol and imine/enamine tautomerizations ; intramolecular hydrogen shift ; transition structures ; comparison of several calculation methods ; asynchronous process ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The transition structures associated with the possible intramolecular tautomerization for acetaldehyde/vinyl alcohol and acetaldimine/vinylamine systems as models of keto/enol and imine/enamine interconversion processes, respectively, were characterized. The relative stabilities of the tautomers and the associated barrier heights were calculated. Ab initio analytical gradients and second derivatives at the HF level of theory and 3-21G, 6-31G, 6-31G**, 6-31++G**, and 6-311++G** basis-set, DFT (BP86/6-311++G** and BLYP/6-311++G**), and semiempirical (AM1 and PM3) procedures were used to identify the stationary points. Correlation effects were estimated using the perturbational approach at MP2/6-31G**, MP2/6-311++G**, and MP2/6-311++G (3df,2p) levels. The geometry, electronic structure, harmonic vibrational frequencies, and transition vector associated with the transition structures as well as the relative stabilities of different isomers and barrier heights were analyzed. The dependence of these properties upon theoretical methods is analyzed and discussed. The transition structures are four-membered rings and the corresponding transition vectors are associated to collective fluctuations. The 1,3 intramolecular hydrogen migration is much more advanced than are the hybridization changes on donor and acceptor centers at the transition structure. The corresponding barrier heights can be related to the change of bond orders and acid/base properties of these centers. A comparison of the results obtained with different methods renders that the nature of the transition structure seems to be a rather robust entity.   © 1998 John Wiley & Sons, Inc. Int J Quant Chem 66: 9-24, 1998
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    Upper- and lower-bound Hylleraas-CI calculations for the nonrelativistic Po states of the 4He isotope (1998)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 66 (1998), S. 25-30 
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    ISSN: 0020-7608
    Keywords: Hylleraas-CI ; P states of 4He ; variance minimization ; upper and lower bounds ; perturbation theory ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Extensive Hylleraas-CI calculations for the lowest Po states of 4He were performed. The dependence of the variational energy values Eκ on the mass parameter κ given by κ=mHe2+/me- is discussed. Furthermore, lower bounds to Eκ were calculated using variance minimization.   © 1998 John Wiley & Sons, Inc. Int J Quant Chem 66: 25-30, 1998
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    Calculation of magnetic properties of HF, H2O, NH3, and CH4 molecules using a longitudinal gauge for the vector potential (1998)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 66 (1998), S. 31-45 
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    ISSN: 0020-7608
    Keywords: magnetic susceptibility ; magnetic shielding tensor ; gauge invariance ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A transformation of the transverse Coulomb vector potential was implemented to calculate molecular magnetic properties via the random-phase approximation (RPA) within the framework of a “longitudinal gauge.” In this gauge, the diamagnetic contribution to magnetic susceptibility is a tensor with equal diagonal components as in atoms, irrespective of molecular symmetry, whereas diagonal and average diamagnetic contributions to the nuclear magnetic shielding are the same as in the Coulomb gauge. Near-Hartree-Fock magnetic susceptibility and nuclear magnetic shielding tensors were evaluated for a set of small molecules, HF, H2O, NH3, and CH4, employing extended Gaussian basis sets. The peculiar features of the longitudinal gauge, and the fulfillment of a series of sum rules involving the virial operator, which must be satisfied to guarantee gauge invariance of total magnetic tensors, were exploited to check the degree of convergence of theoretical values and to estimate the corresponding Hartree-Fock limit for the properties.   © 1998 John Wiley & Sons, Inc. Int J Quant Chem 66: 31-45, 1998
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    A comparative three-dimensional Hartree-Fock crystal orbital study of double-stack organic charge-transfer (semi)conductors: TTF-TCNQ, TTF-DCNQI, and TTF-2,5-Me2-DCNQI (1998)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 66 (1998), S. 47-68 
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    ISSN: 0020-7608
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Three-dimensional crystal structures of the charge-transfer complexes of tetrathiafulvalene (TTF) with tetracyanoquinodimethane (TCNQ), (N,N′-dicyanbenzoquinondiimine (DCNQI), and N,N′-dicyan-2,5-dimethyl-1,4-benzoquinondiimine (2,5-Me2-DCNQI) were studied within the ab initio Hartree-Fock crystal orbital approximation using the CRYSTAL92 routine package. A qualitative agreement with the experimental data was achieved, and a definite border between one-electron and many-electron effects in the specific physical properties of the crystals under study was drawn. The calculations led to the tentative conclusion that the true chemical reaction corresponding to the charge transfer in such systems is a two-step transfer of two electrons from the donor's HOMO to the acceptor's LUMO and not only single-electron transfer, as usually believed. Then, whether the system is conductive or semiconductive depends upon the degree of the charge transfer (one, two, or no electrons, respectively). But the final degree of the charge transfer and the density of states on the Fermi level should be determined by many-electron effects. The theoretical approach used in this work seems to be of crucial importance in designing organic crystals with specific physical properties.   © 1998 John Wiley & Sons, Inc. Int J Quant Chem 66: 47-68, 1998
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    Interpretation of energy bands of polyethylene in terms of effective interactions between first- and second-neighboring CH2 groups (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 64 (1997), S. 481-494 
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    ISSN: 0020-7608
    Keywords: Polyethylene ; energy bands ; dispersion relation ; Hamiltonian matrix eigenfunctions ; non-one-dimensionality ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The alternative way of solving secular problems for the Hamiltonian matrices of regular quasi-one-dimensional systems developed previously [V. Gineityte, Int. J. Quant. Chem. 60(3), 717 (1996)] has been applied to polyethylene. An implicit form of the dispersion relation has been obtained in terms of three local-structure-determined energy-dependent functions δ(ε), τ(ε), and η(ε), describing the effective interactions inside a separate CH2 group and those between first- and second-neighboring CH2 groups, respectively. The actual shapes of dispersion curves proved to be determined by relative mean values of the functions τ(ε) and η(ε) within the ε region under interest. The unusual minimum within the low-energy branch of dispersion curves situated at a low-symmetry point of the first Brillouin zone (k≈0.6π/a) has been established to appear owing to considerable values of effective interactions between the second-neighboring CH2 groups within the respective energy interval. Just the latter type of interactions has been concluded to be responsible also for non-one-dimensionality of the polyethylene chain. The Hamiltonian matrix eigenfunctions of this chain have been expressed as the Bloch sums of eigenvalue-dependent local-structure-determined basis orbitals.   © 1997 John Wiley & Sons, Inc. Int J Quant Chem 64: 481-494, 1997
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    Molecules in strong magnetic fields: Some perspectives and general aspects (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 64 (1997), S. 501-511 
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    ISSN: 0020-7608
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: In view of the coming availability of magnetic field strengths in the regime B≈100 T at the National High Magnetic Field Laboratory in Tallahassee, we present a brief overview of some relevant problems associated with molecular systems in strong magnetic fields. Both perspectives for future research as well as general aspects for molecules in external fields are outlined. Subsequently, two major fundamental concepts for molecules in strong magnetic fields are investigated in some detail. First, the pseudoseparation of the collective and internal motion is performed. Relevant dynamical effects occurring due to the coupling of the center of mass and electronic motion are investigated using the hydrogen atom. Second, we review the performance of the adiabatic separation and approximation for molecular systems in magnetic fields. The screening effect and its dynamical origin are discussed to some extent.   © 1997 John Wiley & Sons, Inc. Int J Quant Chem 64: 501-511, 1997
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    Application of Gaussian-type basis sets to ab initio calculations in strong magnetic fields (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 64 (1997), S. 513-522 
    Details
    ISSN: 0020-7608
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: It is shown that systematically constructed Gaussian-type basis sets applied to simplest one-electron systems in magnetic field varying from 0 to 2×108 T can reliably provide accuracy 10-6 hartrees and better. Important features of the considered type of basis sets are absence of variational parameters and possibility to approach completeness in a controlled systematic manner. Methods of construction of such basis sets, outlined in the study, are of importance of future work on atomic and molecular calculations in strong magnetic fields.   © 1997 John Wiley & Sons, Inc. Int J Quant Chem 64: 513-522, 1997
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    Many-body approaches to atoms and molecules in external magnetic fields (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 64 (1997), S. 523-552 
    Details
    ISSN: 0020-7608
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The proper treatment of many-body effects for fermions has long been a goal of theorists working in atomic and molecular physics. The computational demands of such a treatment, however, when coupled to the added difficulties imposed by the presence of external electromagnetic sources, have resulted in few studies of many-body effects in strong magnetic fields, i.e., in the field regime where perturbation theory is no longer applicable. In this article, we review the fundamental aspects of the problem and describe a variety of theoretical approaches for small atoms and molecules in strong fields, beginning with mean-field theory (Hartree-Fock) and progressing through variational and exact stochastic methods.   © 1997 John Wiley & Sons, Inc. Int J Quant Chem 64: 523-552, 1997
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    Molecular geometry and symmetry from a differential geometry viewpoint (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 64 (1997), S. 669-678 
    Details
    ISSN: 0020-7608
    Keywords: Molecular geometry ; symmetry ; symmorphy ; topology ; electron density ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Relations between an earlier generalization of molecular symmetry called symmorphy and a molecular equivalence based on diffeomorphisms of electron density functional graphs (the so-called DFG equivalence introduced in our previous work) are analyzed. Any two DFG-equivalent electron density functions can be derived from one another by a suitable transformation of the spatial coordinates and the electronic charge density scale; the classes of DFG equivalence are the orbits of a group of linear operators operating in the space of electron density functions. Within the symmorphy framework, the symmetry group is derived from the symmorphy group by taking an intersection of a subgroup of the symmorphy group and the group of isometries for a natural choice of the Riemannian metric tensor. The Riemannian metric properties provide a choice for a suitable reference electron density function for each class of equivalent densities. Such reference densities serve as tools for a systematic classification of the infinite family of electron densities of molecular conformations.   © 1997 John Wiley & Sons, Inc. Int J Quant Chem 64: 669-678, 1997
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    EPR correlations in the molecular domain (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 64 (1997), S. 679-687 
    Details
    ISSN: 0020-7608
    Keywords: Quantum mechanics ; chemistry ; molecules ; Einstein-Podolsky-Rosen correlations ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: “Quantum mechanics is the first mathematically formulated scientific theory where in a nontrivial way the whole is more than the combination of its parts” (H. Primas, 1982). Nowadays most chemical observations and phenomena are understood on the basis of the molecular point of view. The three-dimensional molecules chemists speak about are micro-entities that as such are subject to a theoretical description in terms of quantum mechanics. Therefore one would expect that also in the molecular domain holistic effects appear which are caused by the well-known Einstein-Podolsky-Rosen (EPR) correlations. In the present study we demonstrate that - the unrestricted validity of quantum mechanics presupposed - such correlation phenomena play an important role in the description of a dissociation reaction A2⇒2A, where an intrinsic (structure establishing) degree of freedom fades into a mere translation of the fragments. Finally we will address the question, why in general EPR correlations are observed quite rarely in the domain of molecules - despite their fundamental importance for the notion of molecule itself.   © 1997 John Wiley & Sons, Inc. Int J Quant Chem 64: 679-687, 1997
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    Electronic absorption spectra of some nicotinamides and nicotinic acids. Molecular orbital treatment (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 64 (1997), S. 689-701 
    Details
    ISSN: 0020-7608
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The electronic absorption spectra of the position isomers nicotinamide and isonicotinamide, nicotinic acid, and isonicotinic acid were investigated, together with the spectra of thionicotinamide, N-methyl nicotinamide and nicotinic acid N oxide. Apparent differences in the spectra of the position isomers were interpreted in terms of the torsion angle between the planes of the molecule, the height of the barrier to internal rotation, and the results of molecular orbital (MO) calculations. The largest perturbation effect was observed in the case of thionicotinamide whereas the smallest effect was observed in the case of nicotinic acid N oxide. MO calculations have indicated the existence of overlapping transitions. The observed transitions proved to be π-π* transitions, none of the n-π* was observed.   © 1997 John Wiley & Sons, Inc. Int J Quant Chem 64: 689-701, 1997
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    Electronic properties of flavins: Implications on the reactivity and absorption properties of flavoproteins (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 64 (1997), S. 721-733 
    Details
    ISSN: 0020-7608
    Keywords: Flavins ; ab initio calculations ; CIS/CNDO ; electronic properties ; reactivity of flavoproteins ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Electronic and structural properties of three protonated forms [HFlox, H2Flox+(N5), and H2Flox+(N1)] of lumiflavin have been determined at the HF/3-21G level of approximation. Larger delocalization and basicity explain the favorable protonation of N1 with respect to N5 whereas N1 protonation considerably activates the electrophilicity of N5 with respect to C4a. The relative position of the first electronic transition evaluated at the CNDO/CI level for the radicalar H2Fl·(N5) and H2Fl·(N1) structures further supports the classification of the flavoenzymes into two classes. Moreover, the π-π* transition has been correctly placed as the lowest energy transition for the neutral oxidized lumiflavin and predicts a blue shift of the low-lying electronic transition upon monoprotonation. Finally, from the analysis of the molecular complex between oxidized lumiflavin (HFlox) and hydroquinone, we have rationalized the complex formation in terms of the complementarity between the molecular electrostatic potentials as well as in terms of the overlap between the frontier orbitals involved in these charge transfer process.   © 1997 John Wiley & Sons, Inc. Int J Quant Chem 64: 721-733, 1997
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    Spontaneous symmetry breaking and electron correlation (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 65 (1997), S. 9-17 
    Details
    ISSN: 0020-7608
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The Dyson equation and Green's function formalism map the problem of N electrons in the Born-Oppenheimer nuclear field into a one-electron Schrödinger equation problem in quantum chemistry. The mass operator contains the information about electron correlation, and the ground-state energy, electron affinity, and ionization energy of a molecular system. This pseudopotential approach is combined with the random-phase approximation (RPA) method to give a model with spontaneous symmetry breaking, where the relevant features are incorporated in a transparent way. An effective potential is developed consisting of a Hartree-Fock solution with auxiliary fields to probe for deformation instabilities. The model is then connected to a closely related perturbative propagator approach and linked with the configuration interaction formalism.   © 1997 John Wiley & Sons, Inc. Int J Quant Chem 65: 9-17, 1997
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    A method to evaluate the Sachs formula (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 65 (1997), S. 19-36 
    Details
    ISSN: 0020-7608
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A rule-based method for evaluating the Sachs formula is introduced. For simple molecules, such as linear chains and monocycles, the Sachs formula can be evaluated by directly using the rules. For complex molecular systems, the Sachs formula can be evaluated by first breaking up the molecule into constituent pieces of sufficient simplicity so that data for them are available in the data base and then constructing the molecule from the known data by adding all pieces together, one at a time, via a series of binary rules. The method changes the way of evaluating the Sachs formula from a trial-and-error-type numerical search to a purely algebraic manipulation and thus tremendously reduces the computing times.   © 1997 John Wiley & Sons, Inc. Int J Quant Chem 65: 19-36, 1997
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    Quantum field lability indexes of ligands for predicting some properties of platinum complexes (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 65 (1997), S. 49-57 
    Details
    ISSN: 0020-7608
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The state of a ligand in a molecule is discussed in terms of the quantum field theory operating with wave functions and occupation numbers in the Fock space. Following the basic concepts of the quantum theory, the state of a ligand is considered as an average weighed value of two ionic states given in the form of effective electron charge on ligand. A shift in the ligand state between the ionic states under the influence of an external field is estimated in terms of lability indexes arising as a result of the information statistical theory application to the problem of quantum parameter estimation. The proposed indexes combined with their energetic characteristics are used for predicting cis and trans effects in platinum(II) complexes forming the Werner-Miolatti series.   © 1997 John Wiley & Sons, Inc. Int J Quant Chem 65: 49-57, 1997
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    Hartree-Fock wave functions with a modified GTO basis for atoms (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 65 (1997), S. 59-64 
    Details
    ISSN: 0020-7608
    Keywords: Atomic structure ; Hartree-Fock ; modified Gaussian orbitals ; momentum density ; single-particle density ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We have solved the atomic Hartree-Fock equations by using the algebraic approach, expanding the single-particle radial wave function in terms of a modified Gaussian type orbitals (GTOs) basis. Several atomic properties such as Kato's cusp condition for the electron density or the correct asymptotic behavior of the electron momentum density distribution are accurately verified. Additionally the energy of the atomic ground state can be obtained by using a smaller number of basis functions than in standard GTO expansions. This study has been performed for several atoms of the first three rows.   © 1997 John Wiley & Sons, Inc. Int J Quant Chem 65: 59-64, 1997
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    Electronic structure and properties of the carbonyls TiCO and Ti7CO and carbenes TiCH2 and Ti7CH2 by density functional methodsDedicated to Prof. Geerd G. H. Dierksen on his 60th birthday. (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 65 (1997), S. 65-73 
    Details
    ISSN: 0020-7608
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Density functional theory (DFT) calculations of the electronic structure and properties of clusters of one and seven Ti atoms interacting with CO and CH2 are reported because of their importance in heterogeneous catalysis (especially in Fischer-Tropsch synthesis). The local spin-density approximation (LSDA), i.e., Slater's exchange functional coupled to the Vosko-Wilk-Nussair correlation functional, as well as the generalized-gradient approximation (GGA), using Becke's 1988 exchange functional and Perdew's 1986 correlation functional (BP method) were applied to the study of the clusters using a triple-zeta plus polarization (TZP) basis set. Triplet and quintuplet states of the clusters were studied, and the dissociation energies are provided. The DFT results are compared with other values available in the literature and found to be of reasonable accuracy.   © 1997 John Wiley & Sons, Inc. Int J Quant Chem 65: 65-73, 1997
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    Semiclassical calculation of rotational excitation of atom-symmetric top scattering (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 65 (1997), S. 89-96 
    Details
    ISSN: 0020-7608
    Keywords: Semiclassical approach ; rotational excitation ; A-A variables ; Monte Carlo calculation ; total cross section ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A semiclassical approach is applied to calculating the state-to-state cross sections of rotational excitation of ammonia molecule scattering with the argon atom. Semiclassical theory is more useful in studying heavy molecular scattering. Ammonia molecule is regarded as a rigid symmetric top and argon as a structureless atom. The inversion motion of ammonia is not taken into account. We make use of classical action-angle variables to describe the degrees of freedom of the system. The ab initio potential is used, and at two different energies the computing of total cross sections are performed by using the Monte Carlo procedure. Good agreements with close-coupling calculations and crossed molecular beam experiment are obtained. The semiclassical approach thus appears to be valid for ammonia-argon scattering system.   © 1997 John Wiley & Sons, Inc. Int J Quant Chem 65: 89-96, 1997
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    Relevant space within the spin-adapted reduced Hamiltonian theory. I. Study of the BH molecule (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 65 (1997), S. 97-105 
    Details
    ISSN: 0020-7608
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The algorithm for evaluating the elements of the spin-adapted reduced Hamiltonian (2-SRH) involves the whole basis set of molecular orbitals. However, under a specific condition, its eigenvectors are very sparse. These two properties lead us here to propose a projection of the 2-SRH matrix, which is equivalent to an effective truncation which facilitates considerably the applicability of the SRH method. The criteria for performing this truncation, amounting to the determination of a relevant space are discussed. The results obtained for the BH molecule are analyzed here.   © 1997 John Wiley & Sons, Inc. Int J Quant Chem 65: 97-105, 1997
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    Relevant space within the spin-adapted reduced Hamiltonian theory. II. Study of the π cloud in benzene and naphthalene (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 65 (1997), S. 107-119 
    Details
    ISSN: 0020-7608
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The properties of the spin-adapted reduced Hamiltonian (SRH) matrices and of their eigenvectors permit in many cases a projection of the two-electron matrices, which amounts to an effective truncation of the basis at the stage of the calculations which are time and as memory consuming. Besides this effective truncation of the basis, another simplification can be introduced by segregating an n-electron cloud from the N electrons of the system. Thus, the energy and the electron distribution of a smaller electronic cloud, for instance, the π or the σ cloud in aromatic systems, can be calculated; their separability being subsequently analyzed. Different relevant spaces have been examined in the study of the π-electron cloud in benzene and naphthalene.   © 1997 John Wiley & Sons, Inc. Int J Quant Chem 65: 107-119, 1997
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    Hydrogen-type orbitals in terms of Gaussian functions (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 65 (1997), S. 143-150 
    Details
    ISSN: 0020-7608
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We express the hydrogen-type orbitals as integral transforms of Gaussian functions. The topographic features of the analytical expressions for the weight functions provide ways for the design of basis sets.   © 1997 John Wiley & Sons, Inc. Int J Quant Chem 65: 143-150, 1997
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    Relativistic multiconfiguration Hartree-SFock by means of direct perturbation theory (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 65 (1997), S. 151-158 
    Details
    ISSN: 0020-7608
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A relativistic direct perturbation theory approach has been implemented at the multiconfiguration Hartree-Fock level into the numerical program package LUCAS. The method has been applied to the closed-shell Be, Zn, Cd, and Hg atoms and to the rare gases Ne to Rn. The scalar relativistic valence-correlation correction to the rare gases is found to be very small, while for Zn, Cd, and Hg the first-order relativistic corrections to the valence-correlation energy are calculated to be -4.6, -6.3, and -17.4 mH, respectively.   © 1997 John Wiley & Sons, Inc. Int J Quant Chem 65: 151-158, 1997
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    Electronic structure and spectra of ruthenium binuclear complexes: Localized versus delocalized model (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 65 (1997), S. 183-193 
    Details
    ISSN: 0020-7608
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Calculations of spectral characteristics of Ru binuclear (II, II) and (III, III) complexes were performed by the restricted CI method. Both delocalized and localized basis sets were used and led to equivalent results. The post-Hartree-Fock nature of (III, III) compound wave functions were demonstrated. The advantage of a localized model in describing the properties of large molecular systems built from a number of clearly distinguished fragments is discussed.   © 1997 John Wiley & Sons, Inc. Int J Quant Chem 65: 183-193, 1997
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    Erratum: Simple modification of the Lee-Yang-Parr correlation functional to satisfy exact nonuniform scaling requirements (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 65 (1997), S. 195-195 
    Details
    ISSN: 0020-7608
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: No abstract.
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