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  • Articles  (2,549)
  • Theoretical, Physical and Computational Chemistry  (1,576)
  • Computational Chemistry and Molecular Modeling  (973)
  • 1995-1999  (2,549)
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  • Articles  (2,549)
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  • 1
    Electronic Resource
    Electronic Resource
    Chemical bonding in the hydrogen molecule (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 421-431 
    Details
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The chemical bond in the hydrogen molecule is examined using the electron density and the generalized overlap amplitudes. Logarithmic derivatives of the electron density provide a clear picture of its behavior in the bonding region as well as in the outer region. The GOA expansion of the density is used to examine the dependence of the rate of decay of the density on the GOA ionization potentials. The increase in the electron density at the nuclei and in the bonding region coincides with the higher ionization potential of H2 over the H atom. The density in the bonding region along the internuclear axis does not decay exponentially, but its shape is very nearly an inverted Gaussian. © 1996 John Wiley & Sons, Inc.
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  • 2
    Electronic Resource
    Electronic Resource
    Theoretical characterization of the ground state of the alkaline-earth monocarbides: Ordering of the two lower-lying states of the BeC, MgC, and CaC molecules (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 433-438 
    Details
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The two lower-lying electronic states (3Σ- and 5Σ-) of the BeC, MgC, and CaC molecules were investigated using restricted Hartree-Fock (RHF), generalized valence bond (GVB), and configuration interaction (CI) calculations to establish the relative ordering of those states as a function of the size of the alkaline-earth element. It is shown that as a result of the competition between bonding effects, which predominate for the 3Σ- states, and exchange effects, which stabilize the 5Σ- states, the ordering of these states can be reversed as we move from the Be to the Ca atom. For both the BeC and MgC molecules, the ground state was found to be a triplet X3Σ- state, but for the CaC molecule, the high-spin X5Σ- becomes more stable. © 1996 John Wiley & Sons, Inc.
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  • 3
    Electronic Resource
    Electronic Resource
    Nodal structure of the electronic Wigner function of many-electron atoms and moleculesDedicated to Professor R. McWeeny in recognition of his outstanding contributions to quantum chemistry. (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 439-452 
    Details
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: On the example of several atomic and small molecular systems, the regular behavior of nodal patterns in the electronic one-particle reduced Wigner function is demonstrated. An expression found earlier relates the nodal pattern solely to the dot-product of the position and the momentum vector, if both arguments are large. An argument analogous to the “bond-oscillatory principle” for momentum densities links the nuclear framework in a molecule to an additional oscillatory term in momenta parallel to bonds. It is shown that these are visible in the Wigner function in terms of characteristic nodes. © 1996 John Wiley & Sons, Inc.
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  • 4
    Electronic Resource
    Electronic Resource
    Interatomic potential for the X1Σ+g state of Be2 (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 453-466 
    Details
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: An extended geminal model has been applied to determine the interatomic potential for the X1Σ+g state Be2. By adopting a [11s, 9p, 6d, 4f, 2g] contracted Gaussian-type basis, the following potential minimum parameters are obtained: Re = 4.67 a.u. (4.63 a.u.) and De = 3.70 mH (3.82 ± 0.05 mH), experimental values in parentheses. A calculation with a nuclei-centered [9s, 7p, 4d, 2f, 1g] GTO basis plus two sets of bond-type function, each set comprising diffuse (2s, 2p, 2d, 2f, 1g) GTOs, yielded -3.79 mH as the value of the potential at R = 4.63 a.u. On the basis of an error analysis the best theoretical estimate of the binding energy is determined to be 3.83 ± 0.08 mH. The calculated value for the fundamental vibrational frequency is v0→1 = 224.7 cm-1 (exp. = 224 ± 3 cm). © 1996 John Wiley & Sons, Inc.
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  • 5
    Electronic Resource
    Electronic Resource
    On the lowest electronic states of the C2F radical (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 467-473 
    Details
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The adiabatic energy surfaces of the lowest three electronic states [2(2A′ and 2A′)] and 2Σ+[2A′] of the C2F radical were investigated by the Hartree-Fock multiconfiguration self-consistent field (HF - MCSCF) ab initio method using a large set of atomic natural orbitals (ANO) and an extended configuration space, and the results were shown to be in agreement with the predictions of valence theory for this radical. The electronic ground state was found to have a bent equilibrium structure, hence contradicting the Walsh rule which predicts for the isoelectronic molecules a 2 linear state. The three states were found to be nearly degenerate and the potential energy surfaces of the two lowest electronic states exhibit an avoided crossing at an energy ∼2000 cm-1 above the ground-state minimum, lower than the highest vibrational fundamental. The strong adiabatic interaction which is responsible for the ordering of the electronic states and their equilibrium geometry involves not only the bending coordinate as normally found for Renner-Teller pairs of states, but also the C - C stretching coordinate, due to the near degeneracy of the 2Σ+ and the 2 lowest electronic states at linear geometries. © 1996 John Wiley & Sons, Inc.
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  • 6
    Electronic Resource
    Electronic Resource
    Generalized oscillator strengths of polyatomic molecules. II. NH3 (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 475-486 
    Details
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Generalized oscillator strengths for a number of singlet transitions of the NH3 molecule, evaluated according to the random-phase approximation approach, are presented and discussed so as to provide characterization of some portions of the Bethe surface of the molecule. © 1996 John Wiley & Sons, Inc.
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  • 7
    Electronic Resource
    Electronic Resource
    MCSCF calculation of the frequency-dependent hyperpolarizability of the lithium atomDedicated to Roy McWeeny. (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 487-492 
    Details
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The second electric dipole hyperpolarizability of the lithium atom is calculated using a series of multiconfiguration SCF wave functions with an increasing number of active space orbitals. In agreement with other recent calculations, we find a very large correlation correction to the hyperpolarizability. We analyze the frequency dependence of the dc Kerr hyperpolarizability γK(ο) = γ (- ο ο, 0, 0) and observe very significant dispersion effects. © 1996 John Wiley & Sons, Inc.
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  • 8
    Electronic Resource
    Electronic Resource
    A study of the molecular orbital localization into an extended Hartree - Fock approach: Application to the BeH2 ground state (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 493-504 
    Details
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: In this article, we present a study of the localization and properties of the molecular orbitals (MOs) of polyatomic systems by using a comprehensive version of the G1 model. In this version, the wave function is written as a DODS product of univocally determined spin orbitals (MOs), “projected” on the singlet ground state. A procedure for determining the MOs is given and applied to the BeH2 ground state. Equivalent split shell and localized MOs are found. The Be orbitals are seen to exhibit sp hybridization and the localized valence MOs are found to produce - 13.7 kcal/mol localization energy. Multistructural calculations are carried out and show that the present approach is able to describe localized and well-oriented bonds whenever the molecule under study presents only a single well-defined nonresonant chemical structure. © 1996 John Wiley & Sons, Inc.
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  • 9
    Electronic Resource
    Electronic Resource
    Potential energy surfaces of pseudoaromatic molecules: An MMVB and CASSCF study of pentalene (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 505-512 
    Details
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The S0 and S1 potential energy surfaces of pentalene were studied using MMVB - a hybrid force-field/parametrized valence bond (VB) method designed to simulate CASSCF calculations for ground and covalent excited states. The results were calibrated against full CASSCF calculations. Four distinct critical points were optimized: on S0, a C2h minimum (with alternating single and double bonds) and a D2h transition structure; and on S1, a D2h minimum and an adjacent S1/S0 conical intersection. A VB exchange density matrix (which is independent of the choice of the spin-coupled basis) was used to rationalize the S0 and S1 surface topologies. Craig defined pseudoaromatic molecules to be those with nontotally symmetric electronic ground states. For pentalene, this is true for both CASSCF and MMVB calculations: the CASSCF S0 transition structure is an open-shell B1x singlet, and the VB ground state is dominated by a spin-coupling which transforms as B1g. A C2v minimum and a D2h transition structure were located on the CASSCF S2 potential energy surface. This state cannot be represented by MMVB because of the importance of ionic configurations. The characters of the S1 S2 states of pentalene are shown to be reverse of the S1 and S2 states of benzene. © 1996 John Wiley & Sons, Inc.
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  • 10
    Electronic Resource
    Electronic Resource
    Bond length alternation in cyclic polyenes. VII. Valence bond theory approachThis paper is dedicated to Professor Roy McWeeny with affection and admiration on the occasion of his 70th birthday. (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 513-527 
    Details
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The problem of bond length alternation in linear extended φ-electron systems with conjugated double bonds is examined using the valence bond theory applied to a simple model of cyclic polyenes CNHN with N = 4v and N = 4v + 2 sites as described by the Pariser-Parr-Pople Hamiltonian. Overlap enhanced atomic orbitals are employed in order to achieve the optimal treatment with only two Kekulé structures. The predicted bond length alternation and its magnitude are in good agreement with earlier molecular orbital based calculations and with experiment. Special attention is given to the discussion of the origin of bond length alternation in long polyenic chains and to the role of the resonance energy leading to stabilization of undistorted, symmetric structures for small aromatic (N = 4v + 2) cycles. © 1996 John Wiley & Sons, Inc.
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  • 11
    Electronic Resource
    Electronic Resource
    Calculation of 13C shielding of the isotopomers CH3Cl, CH2DCl, CHD2Cl, and CD3Cl (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 529-534 
    Details
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The 13C shielding of the isotopomers CH3Cl, CH2 DCl, CHD2Cl, and CD3Cl has been calculated for a range of temperatures from an self-consistent field (SCF) shielding surface computed by Buckingham and Olegario. It is found that each successive deuterium substitution increases the shielding by about 0.19 ppm and that a very slight nonadditivity occurs. The principal factor which governs the nuclear motion correction for each isotopomer is the stretching of the bonds with both first- and second-order terms being significant. Angle bending contributions are very small at first order but quite substantial at second order. Not only should the 13C-isotope shifts in this experimentally uninvestigated series be easily measured but the temperature dependence of the shielding in any one isotopomer should be observable provided that careful measurements are made. The 13C-shielding difference between CH3 35Cl and CH3 37Cl has also been calculated and is found to agree well with experiment. © 1996 John Wiley & Sons, Inc.
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  • 12
    Electronic Resource
    Electronic Resource
    Exchange - correlation potential and excited-state density functional theory (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 535-543 
    Details
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The exchange-correlation potential associated with excited-state density functional theory is studied numerically using accurate electron densities for ground and excited states of He, Be, Ne, Mg, and Ar. The long-range and short-range behaviors of this potential are discussed. First-excitation energies are estimated using various prescriptions. © 1996 John Wiley & Sons, Inc.
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  • 13
    Electronic Resource
    Electronic Resource
    Modern valence-bond description of the electronic structure of benzocyclobutadiene (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 545-552 
    Details
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The ground state of benzocyclobutadiene, a bicyclic molecule with 8π electrons containing a benzene and a cyclobutadiene ring, is studied by means of modern valence bond (VB) theory in its spin-coupled (SC) form and the complete-active-space self-consistent field (CAS SCF) approach. The CAS SCF wave function is used to optimize the geometry, and SC theory - to obtain a well-correlated and easy to visualize and understand model of the active space hosting the π electrons. It is shown that the π system of the ground state of benzocyclobutadiene can be described with sufficient accuracy as a combination of the π systems of a distorted benzene ring and an isolated double bond. Each of the eight SC orbitals is found to be well-localized about one carbon atom only, with small distortions toward its nearest neighbors. The analysis of the optimal active-space spin-coupling pattern within the SC wave function for benzocyclobutadiene shows unambiguously that this molecule inherits neither the aromatic nor the antiaromatic character of its cyclic components, and it is most appropriate to regard it as nonaromatic. © 1996 John Wiley & Sons, Inc.
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  • 14
    Electronic Resource
    Electronic Resource
    Momentum-space electron densities - localized orbitals in hydrocarbons, boranes, and transition metal complexes (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 579-592 
    Details
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Position and momentum space plots are presented for localized molecular orbitals in hydrocarbons, boranes, a carborane, and two octahedral transition metal complexes. The p-space representation proves to be valuable for visualizing such orbitals since it highlights the differences in their character from one molecule to another. Factors influencing the form of the orbitals in p space, including the oscillatory behavior caused by contributions to an orbital from more than one center, are examined in detail. © 1996 John Wiley & Sons, Inc.
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  • 15
    Electronic Resource
    Electronic Resource
    Roy McWeeny (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 1-2 
    Details
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/qua.560600102
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  • 16
    Electronic Resource
    Electronic Resource
    Applications of the group-function theory to the field of materials science (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 617-634 
    Details
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The group-function theory, as proposed by McWeeny for the study of weak intermolecular interactions and developed by Huzinaga in the context of valence-electron methods, is shown to be applicable to the ab initio study of tunable solid-state laser materials made of defective ionic crystals. The applicability of the theory relies on the existence of local electronic states (to which the demonstrated/potential laser activity is ascribed), which are essentially localized in a small cluster of atoms including the defect and whose electron correlation interactions with the surrounding crystal components are negligible. According to the group-function formalism, it is possible (a) to neglect electron correlation effects beyond the defect cluster and (b) to define a quantum mechanical embedding potential which embodies the rest of the so-called host effects. Computationally, the theory becomes applicable as the embedding potential is approximated through ab initio model potentials (AIMP). The results of AIMP embedded-cluster calculations demonstrate that it is possible to calculate the local structure and spectroscopy of the active defect at an ab initio level, the attainable accuracy being comparable to the usual one in molecular ab initio studies in the gas phase. Also, in this article, we present a systematic study of the local distortions produced upon doping divalent first-series transition-metal ions in rock-salt oxides, MO:Me2+ (M=Mg, Ca, Sr; Me=Sc-Zn) and Tl+ in KMgF3 and KF hosts. This study leads to the calculation of the local structures of the defects in these materials, which have not been measured. The results suggest that the use of the mismatch of the empirical ionic radii of the impurity and the substituted ion in order to predict local distortions in doped ionic crystals is not significant when it is smaller than 0.1 Å, and when it is larger, it should be weighted by a reduction factor depending on the host. For the first-series divalent transition-metal ion impurities, this factor is shown to be 0.15 for SrO, 0.25 for CaO, and around 0.50 for MgO. © 1996 John Wiley & Sons, Inc.
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  • 17
    Electronic Resource
    Electronic Resource
    Accurate lower and upper bounds of the energy spectrum for the asymmetrical two-well potentials (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 641-648 
    Details
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Trigonometric basis sets are used in a Rayleigh-Ritz variational method for computing two-sided eigenvalue bounds of the Schrödinger equation in one dimension. The method is based on truncating the infinite interval and solving an eigenvalue problem which obeys the von Neumann and the Dirichlet boundary conditions, respectively. Highly accurate numerical results are presented for the asymmetrical two-well oscillators. © 1996 John Wiley & Sons, Inc.
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  • 18
    Electronic Resource
    Electronic Resource
    On the evaluation of overlap integrals with the same screening parameters of Slater-type orbitals using binomial coefficients (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 637-640 
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    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A general formula has been established for the overlap integrals with the same screening parameters of Slater-type orbitals in terms of bionomial coefficients. The final results are especially useful for the calculation of these integrals for large quantum numbers, which occur in the multicenter integrals. © 1996 John Wiley & Sons, Inc.
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  • 19
    Electronic Resource
    Electronic Resource
    Study of methanetriol decomposition mechanisms (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 649-655 
    Details
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Using ab initio HF/6-31G** and MP2/6-31G** calculations alternative reactions HC(OH)3 → HCO2H + H2O and HC(OH)3 + H2O → HCO2H + 2H2O are investigated and the results are compared with relevant PM3, HF/3-21G, and HF/6-31G data. Reactant and product complexes as well as transition states are located on corresponding potential energy surfaces. © 1996 John Wiley & Sons, Inc.
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  • 20
    Electronic Resource
    Electronic Resource
    Importance of angular and radial electron correlations in first born (e, 2e) spectroscopies of heliumlike targets (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 657-665 
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    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The ground-state electronic structure of heliumlike systems can be described by several simple wave functions expressed in terms of the three distances between particles (one nucleus and two electrons). The dependence of (e, 2e) ionization cross sections on the description of radial and angular correlations between target electrons is analyzed. The cases of plane wave and Coulomb wave representations of ejected electron trajectories are examined together with the feasibility of analytic calculations. The relative sensitivity of (e, 2e) cross sections to the description of correlations between K-shell electrons is also investigated as a function of nuclear charge. Existing experimental data on helium are used to support a comparison with calculated (e, 2e) cross sections. © 1996 John Wiley & Sons, Inc.
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  • 21
    Electronic Resource
    Electronic Resource
    Absorption maxima study of chromophores of indigoid dyes (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 681-687 
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    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Molecular structure of 21 chromophores of indigoid dyes were studied by an ab initio MP2/6-31 + G*/ /HF/6-31 + G* method. Bond lengths and bond angles were affected by π-electron conjugation. The difference between molecular structures of chromophores and indigoid dyes indicated that benzene rings and five-membered rings in indigoid dyes are structurally important. Absorption maxima of chromophores were successfully calculated by the CI-singles-MP2/6-31 + G* theory. Like indigoid dyes, absorption maxima of the chromophores are affected by the positions of the donor and acceptor groups on the trimethine group. Bathochromic shifts of the absorption maxima were observed with the best donor group of (SINGLE BOND)NH among (SINGLE BOND)NH, (SINGLE BOND)O, and (SINGLE BOND)S groups. Appropriate substitution of longer-chain polymethines brought about chromophoric systems having hypsochromic shifts. From these calculations, the absorption maxima of some indigoid dyes could be explained by their chromophores qualitatively. © 1996 John Wiley & Sons, Inc.
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  • 22
    Electronic Resource
    Electronic Resource
    Ensemble-representable densities for atoms and molecules. II. Application to CoCl4 2- (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 667-680 
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    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: In the previous article we introduced a method to obtain an ensemble density describing a molecule in a crystal from diffraction experiment structure factors. Here the method is applied to the CoCl42- molecular ion in a Cs3CoCl5 crystal for which accurate magnetic structure factors are known. First, the approximations involved in the interpretation of polarized neutron experiment are reviewed with special emphasis on the collinearity approximation which has been avoided in this work. Second, the derivation of magnetic structure factors corresponding to theoretical ensemble densities is explained (the spin and the exact orbital contributions have been included). Third, the fitting procedure is presented and results at different levels of approximation are discussed. The main conclusions are: (1) A density built by using several molecular wave functions can give a very good agreement with the experimental data. (2) The ensemble representability constraint is necessary to retrieve physical information from the optimized parameters. (3) Taking into account the proper paramagnetic contribution to the magnetization improves significantly the agreement between theory and experiment. (4) Neglecting the diamagnetic contribution and the fact that the magnetization may be locally noncollinear to the applied external field is fully justified for the system under study. © 1996 John Wiley & Sons, Inc.
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  • 23
    Electronic Resource
    Electronic Resource
    Masthead (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996) 
    Details
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/qua.560600201
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  • 24
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    Near-dissipationless coherent excitations in biosystemsPresented at the 1995 Sanibel Symposium. (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 689-697 
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    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We consider the question of long-range propagation of informational signals in biosystems. This is done for the case of a modeled biosystem of the type considered by H. Fröhlich and A. S. Davydov but under general nonequilibrium thermodynamic conditions. For that purpose we resort to the so-called informational statistical thermodynamics. It is shown that Fröhlich's effect may follow in this system. It consists in the emergence of a self-organized dissipative homogeneous and stationary structure (in Prigogine's sense), resembling a nonequilibrium Bose-Einstein condensation in the low-lying-in-frequency modes of polar vibrations. It follows once a critical level of pumping of metabolic energy is achieved. Furthermore, it is shown that signals in this system propagate in the form of Davydov solitons, which are strongly thermally damped in near equilibrium at physiological conditions. However, the situation drastically changes in nonequilibrium conditions, leading to Fröhlich's condensation, when the lifetime of a Davydov-like soliton is enormously enhanced, allowing for very long range propagation of signals. © 1996 John Wiley & Sons, Inc.
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    Dynamics during spectroscopic transitions - basic concepts, E. Lippert and J. D. Macomber, Editors, Springer-Verlag, Berlin, Heidelberg, 1995, 580 pp., 214 figures, 7 tables. Hardcover ISBN 3-540-58703-9 (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 709-709 
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    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/qua.560600202
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    An optimized approach to compute hybridization displacement charge and a study of its effects on electrostatic potentials of some biologically important molecules (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 699-708 
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    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Electrostatic potential maps of certain biologically important molecules (guanine, adenine, cytosine, thymine, and two conformers of glycine) were studied employing a “hybridization displacement charge (HDC)” correction to Löwdin charge distributions and using the conventional Mulliken charge distributions of the molecules. In these calculations, the magnitude of each of the point charges was distributed in 3-dimension in a spherically symmetric manner. The method of computing HDC using MNDO wave functions was optimized by introducing two adjustable parameters. It has been clearly demonstrated that HDC-corrected Löwdin charge distributions of the molecules mentioned above are able to reproduce most of the important features of their ab initio MEP maps, while the conventional Mulliken charge distributions fail in many respects. Thus, HDC-corrected Löwdin charge distributions are shown to be quite reliable for the study of molecular electrostatic properties, particularly for molecules with more than one competing electrophilic site. © 1996 John Wiley & Sons, Inc.
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    Statistical mechanics and therniodynamics, Claude Garrod, Oxford University Press, New York, 1995, 622 pp, one 3.25 in. diskette. Hardcover, ISBN 0-19-508523-X (IBM software), ISBN 0-19-509775-0 (Macintosh software) (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 711-711 
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    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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    URL: http://dx.doi.org/10.1002/qua.560600203
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    Secular polynomials for chemical graphs of alkanes in terms of atoms and bonds and their spectral properties (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 743-752 
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    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Secular polynomials (SPs) have been constructed and studied for the adjacency matrices A(GaCh) and A(GbCh) corresponding to chemical graphs of alkanes in terms of atoms (GaCh) and in terms of bonds (GbCh). The three second-class Chebyshev polynomials Up(Q), Up - 1(Q), and Up - 2(Q), with respect to the variable Q proportional to the SP of an isolated CH2-like subgraph, have been shown to appear within both SPs P[A(GaCh)] and P[A(GbCh)] and to play the role of algebraic analogues of a (CH2)p-like subgraph. Common noncanonical algebraic expressions for both SPs reflecting the regular internal structure of alkanes have been constructed on this basis. Spectral properties of both graphs GaCh and GbCh have been shown to be determined by those of Up(Q), e.g., the band limits of spectra proved to be related to the orthogonality interval Q = [-1;1] for polynomials Up(Q). Since the adjacency matrices (AMs) A(GbCh), but not A(GaCh), are proportional to definite model Hamiltonian matrices, the obtained results serve to interpret the one-electron spectra of alkanes in terms of peculiarities of usual chemical structure. © 1996 John Wiley & Sons, Inc.
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    The calculation of NMR shielding tensors based on density functional theory and the frozen-core approximation (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 753-766 
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    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The application of the frozen-core approximation to the calculation of the shielding tensor of nuclear magnetic resonance (NMR) spectroscopy is discussed and an implementation is presented. A complete formulation of the shielding calculation within the frozen-core approximation is given, both in general terms and for the special case of density functional theory (DFT) and “gauge including atomic orbitals” (GIAOs). The practical implementation is validated by a detailed discussion of the consequences of the approximation. The general conclusion is drawn that the frozen-core approximation is a useful tool for shielding calculations - if the valence space is increased to contain at least the ns, np, (n - 1)p, (n - 1)d (fourth period and higher) shells, where n is the number of the given period in the periodic table of elements. The new method is applied to 77Se shieldings and chemical shifts for a small number of compounds. The agreement between theory and experiment is good for relative shifts, whereas calculated absolute shieldings are generally too small by about 300-400 ppm. This difference is attributed to the relativistic contraction of the core density at the selenium atom that had been explicitly incorporated into the experimental absolute shielding scale. © 1996 John Wiley & Sons, Inc.
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    Masthead (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996) 
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    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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    URL: http://dx.doi.org/10.1002/qua.560600301
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    A comparison of Hartree - Fock, MP2, and DFT results for the HCN dimer and crystal (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 767-778 
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    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A number of hydrogen-bond related quantities - geometries, interaction energies, dipole moments, dipole moment derivatives, and harmonic vibrational frequencies - were calculated at the Hartree - Fock, MP2, and different DFT levels for the HCN dimer and the periodic HCN crystal. The crystal calculations were performed with the Hartree - Fock program CRYSTAL92, which routinely allows an a posteriori electron-correlation correction of the Hartree - Fock obtained lattice energy using different correlation-only functionals. Here, we have gone beyond this procedure by also calculating the electron-correlation energy correction during the structure optimization, i.e., after each CRYSTAL92 Hartree - Fock energy evaluation, the a posteriori density functional scheme was applied. In a similar manner, we optimized the crystal structure at the MP2 level, i.e., for each Hartree - Fock CRYSTAL92 energy evaluation, an MP2 correction was performed by summing the MP2 pair contributions from all HCN molecules within a specified cutoff distance. The crystal cell parameters are best reproduced at the Hartree - Fock and the nongradient-corrected HF + LDA and HF + VWN levels. The BSSE-corrected MP2 method and the HF + P91, HF + LDA, and HF + VWN methods give lattice energies in close agreement with the ZPE-corrected experimental lattice energy. The (HCN)2 dimer properties are best reproduced at the MP2 level, at the gradient-corrected DFT levels, and with the B3LYP and BHHLYP methods. © 1996 John Wiley & Sons, Inc.
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    Quantum mechanical electronic structure calculations with chemical accuracy, edited by S. R. Langhofl, Kluwer Academic Publishers, Dordrecht, 1995, 447 pp., $208, ISBN 0-7923-3264-4 (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 791-791 
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    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
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    URL: http://dx.doi.org/10.1002/qua.560600303
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    Virtual orbitals (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 793-801 
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    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: It is shown that the conventional concepts based on the use of the HOMO and LUMO are founded on very questionable assumptions. In particular, it is shown both theoretically and computationally that the LUMO of almost all neutral atoms and molecules is a continuum function with energy zero. The LUMO of any SCF calculation is an artifact of the basis-expansion method. A proposal is made for the identification of the pair donor and pair acceptor orbitals which play the true role of the HOMO and LUMO. © 1996 John Wiley & Sons, Inc.
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    Valence energy in variational Monte Carlo: CuH dissociation energy (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 803-808 
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    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We show how to estimate the dissociation energy of CuH using the variational Monte Carlo method. The techniques involved are (i) an all-electron approach, (ii) a diffusion-only Metroplis algorithm which is well-suited for sampling the nodal regions properly, and (iii) a core-valence partitioning scheme such that the dissociation energy is estimated from the valence energies of CuH and Cu only. This approach avoids several of the approximations inherent in pseudopotential methods. Using relatively crude wave functions, we obtain an estimate of the dissociation energy and dipole moment with an accuracy on par with much more elaborate calculations in the literature. © 1996 John Wiley & Sons, Inc.
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    Basis sets for ab initio periodic Hartree - Fock studies of zeolite/adsorbate interactions: He, Ne, and Ar in silica sodalite (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 809-820 
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    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Silica sodalite is an ideal model system to establish base-line computer requirements of ab initio periodic Hartree-Fock (PHF) calculations of zeolites. In this article, we investigate the effect of various basis sets on the structural and electronic properties of bulk silica sodalite. We also study the interaction of He, Ne, and Ar with the sodalite cage. Our work shows that basis-set superposition errors (BSSE) in calculations using STO-3G and 6-21G(*) basis sets are as large as the interaction energies, leading to poor confidence in the results. To cure this problem, we present high-quality basis sets for Si, O. He, Ne, and Ar, optimized for use with PHF methods, and demonstrate that the new basis set greatly reduces BSSE. The theoretical barriers for transfer of the rare gases between sodalite cages are 5.6, 13.2, and 62.1 kcal/mol for He, Ne, and Ar. © 1996 John Wiley & Sons, Inc.
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    Note on the generation of Gaussian bases for pseudopotential calculations (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 821-824 
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    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We present a technique to generate Cartesian Gaussian bases for electronic configuration and cross-section calculations on molecules. The technique is specially useful for pseudopotential work, when the bases cannot be tabulated because they depend on the specific choice of the pseudopotential. © 1996 John Wiley & Sons, Inc.
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    Factorization of interaction graphs with n-fold symmetry: Some applications in PPP calculation and molecular vibrational analysis (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 825-832 
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    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The G and F matrices in the molecular vibration problem, the secular matrix in Hückel calculation including some nonneighbor interactions, and the Fock matrices at any stage of iteration in the Pariser-Parr-Pople (PPP) calculations on cis- and trans-butadiene, benzene, and s-triaminobenzene systems have been shown to be factorizable by representing them graphically and then applying the generalized technique of splitting of graphs with n-fold symmetry. © 1996 John Wiley & Sons, Inc.
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    Symmetry reduction of the matrix elements of a two-particle operator (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 833-841 
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    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Local coordinate systems are chosen for each quadruple of atoms relative to a four-center integral, in order to avoid linear combinations of orbitals when symmetry operations perform on an orbital. This choice can utilize the complete molecular symmetry to attain the optimal number of symmetry-unique integrals and to construct two-particle matrix elements by multiplying symmetry-unique integrals, called the “standard four-center integrals,” by the corresponding coefficients, called the “C coefficients.” A simple algorithm to use the complete molecular symmetry to reduce calculations of molecular matrix elements is outlined for general highly symmetric molecules. A tetrahedral molecule is analyzed. © 1996 John Wiley & Sons, Inc.
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    On the use of multipole expansion of the Coulomb potential in quantum chemistry (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 843-852 
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    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Some features of the multipole expansion of the Coulomb potential V for a system of point charges are studied. It is shown that multipole expansion is convergent both locally in L2(R3) and weakly on some classes of functions. One-particle Hamiltonians Hn = H0 + Vn, where H0 is the kinetic energy operator and Vn is the n-th partial sum of the multipole expansion of V, are discussed, and the convergence of their eigenvalues to those of H = H0 + V with increasing n is proved. It is also shown that the discrete spectrum eigenfunctions of Hn converge to those of H both in L2(R3) (together with their first and second derivatives) and uniformly on R3. © 1996 John Wiley & Sons, Inc.
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    The π, π* state in formaldehyde and thioformaldehyde (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 1661-1671 
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    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: For formaldehyde, the C — O stretch potential of 1(π, π*) crosses all 1A1 Rydberg potentials, such as n, 3py, n, 3dyz, etc., thereby transferring the intensity of the unassigned 1(π, π*) ← X??? system to these Rydberg states. For thioformaldehyde, the situation is similar but a shift in the potentials allows for direct observation of 1(π, π*). In its 1(π, π*) state, H2CO is planar, having a low barrier of about 0.2 eV toward the nonplanar 1(σ, π*) state. For H2CS, the planar conformation of 1(π, π*) is a saddle point, with 1(π, π*) being the global minimum on the 21A′ surface. The triplet π, π* states of H2CO and H2CS are nonplanar, having inversion barriers of 0.1 and 0.05 eV, respectively. For both H2CO and H2CS, the π, π* configuration also crosses the ground-state configuration, which explains predissociation and radiationless transitions of some Rydberg states. © 1996 John Wiley & Sons, Inc.
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    Condensed-phase effects on the conformational equilibrium of ethylene glycol (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 1651-1660 
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    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Density functional calculations for ethylene glycol (CH2OHCH2OH) in the gas and in a dielectric medium are reported. The condensed-phase calculations are based on the self-consistent reaction field approach and the environment has the dielectric constant of liquid methanol. NPT Monte Carlo simulations of ethylene glycol (ETG) in liquid methanol are also reported. The simulations were carried out for three conformers of ETG (tGg′, gGg′, and tTt). Comparison between SCRF results for the conformational equilibrium in the gas and in the dielectric suggests that the tGg′ conformer is slightly stabilized relative to the gGg′ conformer in the solvent. However, the energy difference between them is less the 1.0 kJ/mol, which indicates that frequent interconversions between the tGg′ and gGg′ conformers are expected in the condensed phase. The all-trans conformer (tTt) is higher than the most stable conformer in the gas by 14 kJ/mol. Monte Carlo simulations predict that the tGg′ and gGg′ conformers have very similar energies in the solvent. However, the simulations also show, in agreement with experimental data, that the tTt conformer is stabilized in liquid methanol, relative to the gas phase. The microscopic mechanism leading to the stabilization of the tTt conformer in the liquid is related to the differential hydrogen-bonding formation between the ETG conformers and the methanol molecules. © 1996 John Wiley & Sons, Inc.
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    SCMEH-MO calculations on lanthanide systems. III. Ln(CO)6, Ln(OC)6 (Ln = Nd, Sm) (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 1673-1677 
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    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The SCMEH-MO method with average relativistic and spin-orbit effects has recently been applied to study the electronic structure and bonding in samarium pentamethylcyclopentadienyls. In this report the same approach has been utilized in studying the electronic structures of Nd and Sm hexacarbonyls. In contrast to the stable transition metal d-block carbonyls, these lanthanide carbonyls are found to be quite unstable. These findings are based on calculated electronic structures and bond energies. © 1996 John Wiley & Sons, Inc.
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    The electronic g-tensor of MgF: A comparison of ROHF and MRD—CI level results (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 1679-1684 
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    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Despite their importance in the characterization of molecular magnetism, electronic g-tensors have received rather little theoretical attention. In this work, however, the ground-state g-tensor of MgF is computed at both the ROHF and multireference CI levels. The calculations are expanded complete to second order in appropriate Breit-Pauli terms and the contributions of all important magnetically coupled excited states are accounted for. Both the CI- and ROHF-level Δg∥-values (-59 and -55 ppm, respectively) are in agreement with experiment (-319 ± 500 ppm) within the range of experimental uncertainty. For Δg⊥, however, the ROHF treatment yields a value (-659 ppm) in substantial disagreement with experiment (-1319 ± 500 ppm). Fortunately, this discrepancy is alleviated by CI level treatment (Δg⊥ = -1447 ppm). © 1996 John Wiley & Sons, Inc.
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    Wave-function splitting technique for calculating above-threshold ionization electron spectra (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 1685-1689 
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    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: In intense laser fields, atoms and molecules can absorb many more photons than required for ionization. This phenomenon is called above-threshold ionization (ATI) and it shows up in kinetic energy spectra of ionized electrons. The presence of very high energy photoelectrons necessitates the use of extremely large grids in numerical simulations based on the full time-dependent Schrödinger equation. We show that by using a wave function splitting scheme one can circumvent the problem of large grids and thus obtain accurate multiphoton photoelectron spectra. This scheme will be very useful in studies of ATI spectra generated by molecules in intense laser fields. We illustrate this method for a one-dimensional model of the H atom in intense laser fields. © 1996 John Wiley & Sons, Inc.
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    Kernel projector matrices for leu1—zervamicin (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 1691-1700 
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    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: By performing ab initio calculations on fragments of leu1(SINGLEBOND)zervamicin, it is demonstrated that accurate wave functions can be obtained for this large structure by combining the results from the fragments. Input information consists of atomic coordinates as obtained, for example, from a crystal structure determination. The fragments are composed of a kernel of atoms surrounded by a chosen neighborhood of atoms. The entire molecule is divided into individual kernels and their neighborhoods are added for the individual fragment calculations on the basis of the distances of other atoms from the atoms in a kernel. The hydrated leu1(SINGLEBOND)zervamicin is composed of 295 atoms which could be handled in full by Gaussian 94. The results of the fragment calculations were combined to provide an electron density distribution for the molecule. This distribution was compared with one that represents the distribution obtained from a calculation on the entire molecule at once. The clear implication of this study is that it is feasible to perform ab initio calculations on structures by the use of fragments. The time involved increases essentially linearly with complexity. © 1996 John Wiley & Sons, Inc.
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    Fission of metal clusters: A comparison of jellium model calculations and shell correction method calculations (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 1701-1707 
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    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We investigate the fission process Ag2+23 → Ag+12 + Ag+11 in order to compare total energies that calculated by the shell correction method and jellium models. A cavity potential and a Woods-Saxon-type potential are used as the shell potential for the shell correction method, with which the single-particle energy levels are calculated. Shell corrections are obtained by using the Balian-Bloch formula and by smoothing the discrete energy levels in the shell potentials. The jellium model calculations are performed in the framework of the local spin density functional approximation. The conventional jellium model and stabilized jellium model are used. Although the qualitative agreement between the shell correction method calculations and the stabilized jellium calculations is very good, an improvement of the liquid drop energy will be necessary for the satisfactory quantitative agreement. © 1996 John Wiley & Sons, Inc.
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    Combined effect of the screening of a donor ion and the conduction band nonparabolicity on the binding energy of a donor at the center of a spherical quantum dot (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 1719-1722 
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    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We carried out variational model calculations for the assessment of the combined effect of the nonparabolicity of the electron effective mass and the screening of the donor ion by the valence electrons of GaAs for a donor placed at the center of a spherical quantum dot. We considered finite confining potentials between the GaAs QD and the surrounding Ga1-xAlxAs matrix. We found that the combined effect becomes more pronounced as the radius of the quantum dot decreases. © 1996 John Wiley & Sons, Inc. Int J Quant Chem 60: 507-510, 1996
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    Symmetric group approach to relativistic CI. I. General formalism (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 61 (1997), S. 1-10 
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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: General formulas for matrix elements of spin-dependent operators in a basis of spin-adapted antisymmetrized products of orthonormal orbitals are derived. The resulting formalism may be applied to construction of the Hamiltonian matrices both for Pauli and for projected no-pair relativistic configuration interaction methods. From a formal point of view, it is a generalization of the symmetric group approach to the CI method for the case of spin-dependent Hamiltonians. © 1997 John Wiley & Sons, Inc.
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    Symmetric group approach to relativistic CI. II. Reduction of matrices in the spin space (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 61 (1997), S. 11-20 
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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: An algorithm for the evaluation of matrix representations of products of permutation operators and of one- and two-electron spin-dependent operators in a spin-adapted basis of the N-electron spin space is presented. In particular, the case of the basis functions in which p' electrons are described by products of singlet-coupled pairs is considered in detail. The N-electron spin integrals are, in this case, reduced explicitly to the (N - p') electron ones. © 1997 John Wiley & Sons, Inc.
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    Symmetric group approach to relativistic CI. III. Matrix elements for spin-dependent operators (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 61 (1997), S. 21-34 
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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: Matrix element formulas for spin-dependent operators in a basis of spin-adapted antisymmetrized products of orthonormal orbitals (configuration-state functions) are presented. The coefficients of one- and two-electron integrals depend only on the form of the spin-dependence of the operator and on the mutual arrangement of the orbitals which are singly occupied in the pertinent pair of the configuration-state functions. The formulas are useful in implementations of relativistic two-component CI approaches, such as the Pauli CI and projected no-pair CI method. They may be readily used within existing nonrelativistic SGA - CI codes. As examples of applications of the new formulae, spin-own-orbit and scalar spin - spin matrix elements were derived. © 1997 John Wiley & Sons, Inc.
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    Using full-CI algorithms in Bethe-Goldstone-type expansions of the correlation energy (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 61 (1997), S. 35-43 
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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 the early 1960s, Nesbet proposed to develop correlation energy in terms of two-, three-, four-, etc., electron contributions. This expansion was, in principle, applicable to a large number of electrons without a size-extensivity error. The now available full-CI algorithms may be used to obtain those expansions in terms of either occupied spin - orbitals or, more efficiently, in terms of sets of occupied or virtual molecular orbitals. Tests on the NH3 molecule with a DZP basis-set problem show the slow convergence of this approach. © 1997 John Wiley & Sons, Inc.
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    Unexpected CASSCF bistability phenomenon (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 61 (1997), S. 45-54 
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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: One exhibits a problem in which two strongly nonorthogonal complete active-space self-consistent field (CASSCF) solutions are obtained. The problem concerns a molecular frame which presents two stable geometries at the CASSCF level, quinoidal and diradical forms, in disagreement with experiment which indicates a unique minimum corresponding to an intermediate geometry. Those two stable solutions are obtained in a significant domain of intermediate geometries and are related with the CASSCF wave functions of the two stable structures. Obtaining a reliable potential surface starting from CASSCF solutions (even from larger CAS) appears as a very difficult task. © 1997 John Wiley & Sons, Inc.
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    Traces of spin-adapted reduced density matrices (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 61 (1997), S. 55-61 
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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 traces of the p-order reduced density matrices (p-RDM) split into independent contributions associated to the subsets of p-electron eigenstates of the Ŝ2 and Ŝz operators. Here, we report the partial traces for the blocks of the low-order RDMs corresponding to pure spin states of an N-electron system. A systematic method for calculating those of higher order RDMs is described and some useful relations are also given. All these relations which must be fulfilled independently by a RDM can be considered as N- and S-representability conditions © 1997 John Wiley & Sons, Inc.
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    Average energy of an N-electron system in a finite-dimensional and spin-adapted model space (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 61 (1997), S. 63-65 
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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: An expression for the average energy of an N-electron system in a finite-dimensional, antisymmetric, and spin-adapted model space (as, e.g., a full-configuration interaction space) is derived using elementary properties of the Hamiltonian in the Fock space. © 1997 John Wiley & Sons, Inc.
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    Nonempirical wave functions for very large molecules. II. The PRDDO/M/FCP method (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 61 (1997), S. 67-76 
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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 variation of the frozen-core potential (FCP) method is developed and implemented within the modified version of the method of partial retention of diatomic differential overlap (PRDDO/M). The explicit treatment of core electrons is replaced with a potential based on the actual core-valence integrals rather than upon an arbitrary model potential. The core-valence orthogonality requirement is replaced by an energy shift operator. PRDDO/M/FCP calculations exhibit good agreement with ab initio calculations with the same basis set, while reducing the computational cost significantly. © 1997 John Wiley & Sons, Inc.
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    Maximum-entropy analysis of momentum densities in diatomic molecules (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 61 (1997), S. 77-83 
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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 one-particle density in momentum space γ(p) is studied for diatomic molecules by using the maximum-entropy technique. The knowledge of one or more momentum expectation values 〈pn〉 provides approximations on the density γ(p) for any value of the momentum, which are convergent when increasing the number of known moments. Other unknown expectation values are estimated in terms of the constructed maximum-entropy densities. A numerical study of the quality of the approximations is carried out by means of experimental and theoretical data for the momentum expectation values involved. Experimental errors are also taken into account to have an idea of the sensibility of the results to the information from which they are obtained. © 1997 John Wiley & Sons, Inc.
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    Electron affinities of metals computed by density functional theory and ab initio methods (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 61 (1997), S. 93-100 
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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: An extensive computational study of the meal electron affinity was performed using the ab initio and density functional theory (DFT) methods. HF, MP2, MP3, MP4, QCISD, and QCISD(T) was used as computational methods, while the hybrid, local, and nonlocal DFT methods with the LYP, P86, PW91, and VWN correlation functionals were used. Two basis sets, one small and applicable to almost all metals (LanL2DZ) and one large [6-311 + + G(3df, 3 pd)] used only for small metals, were employed. The computed results were compared with the experimental data and the capabilities of the DFT methods to perform this study were discussed. © 1997 John Wiley & Sons, Inc.
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    The relative performance of the local density approximation and gradient-corrected density functional theory for computing metal-ligand distances in Werner-type and organometallic complexes (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 61 (1997), S. 85-91 
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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: Optimized metal-ligand (M-L) bond lengths for 17 classical Werner-type transition-metal complexes were calculated using the local density approximation (LDA) and a gradient-corrected (GC) extension. GCs lengthen the bonds by between 0.02 and 0.09 Å relative to the LDA results. The latter range from 0.02 Å shorter than observed to 0.05 Å longer, while the GC data range from exact agreement with experiment to some 0.12 Å too long. The LDA rms deviation is 0.025 Å compared to the GC error of 0.070 Å. In contrast, data from the literature for organometallic species show that the LDA gives systematically too short M-L distances and GCs lead to a better agreement with experiment. The relative performance of LDA and GC functionals reflects the qualitatively different chemistries of organometallic and Werner-type complexes. The magnitude of the GC bond-length expansion for the latter correlates with the ionicity of the M-L interaction. © 1997 John Wiley & Sons, Inc.
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    CFPHGLF calculation of 3S excited states for heliumlike three-body systems (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 61 (1997), S. 101-105 
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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 correlation-function potential-harmonic and generalized Laguerre function method (CFPHGLF) proposed recently by us is used to directly solve the Schrödinger equations of low-lying triplet states n3S (n = 2-5) for a set of heliumlike systems, including He, Li+, and Be2+. The eigenenergies converge fast and steadily with potential harmonics (PH) and generalized Laguerre functions (GLF). With 10 PH, the percentage errors in the convergent ionization energies for 23S, 33S, 43S, and 53S states of the helium atom are 0.548, 0.291, 0.247, and 0.265% relative to the Hylleraas CI variational values. Somewhat better precision is achieved for Li+ and Be2+ systems. © 1997 John Wiley & Sons, Inc.
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    Theoretical studies on the structures and electronic spectra of C70CH2 (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 61 (1997), S. 815-822 
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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: Intermediate neglect of differential overlap (INDO) calculations were used to study the structure of C70CH2. It was found that the CH2 group is mainly added to the CI(SINGLE BOND)CII (CI represents the first kind of carbon atom and so on) or the CIII(SINGLE BOND)CIII bond in C70 and a cyclopropane feature with Cs symmetry is formed in the structure, which is in agreement with Smith et al.'s experiment. The electronic spectra of eight isomers of C70CH2 were calculated based on the optimized geometries. It was shown that the UV/vis spectra of C70CH2 and C70 resemble each other in many ways with the exception of the absorptions beyond 500 nm. The red-shift of the absorptions and NMR spectra of C70CH2 are discussed. © 1997 John Wiley & Sons, Inc.
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    Peptide models. XIV. Ab initio study on the role of side-chain backbone interaction stabilizing the building unit of right- and left-handed helices in peptides and proteins (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 61 (1997), S. 797-814 
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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: Previous ab initio computations revealed that the conformational building unit of the right-handed helix (φ ≈ -54°, ψ ≈ -45°) is not an energy minimum on two-dimensional-type Ramachandran potential energy surfaces (E = E{φ, ψ}). Theoretical investigations were performed on several single-amino-acid diamides such as For-Gly-NH2, For-L-Ala-NH2, Ac-L-Ala-NHMe, and For-L-Val-NH2 containing amino acid residues (e.g., Ala) which can often be found in helices as shown by X-ray data analysis of globular proteins. The current ab initio [self-consistent field (SCF)] results (based on four different basis sets [3-21G, 4-21G, 4-21G*, and 6-31G*]) presented point toward an intrinsic (i.e., non-environmental-assisted) stability of the right-handed helical subconformation of a simple amino acid diamide if the residue contains a polar side chain. Such is the case for a serine derivative when its (SINGLE BOND)CH2OH side chain is favorably oriented. For the For-L-Ser-NH2 model compound two slightly different right-handed helical backbone conformations were determined. Depending on the relative orientation of the side chain, the conformational monomer of the 310 helix (a sharper helical structure with an [i, i + 3]-type H-bond network) as well as the building block of the “standard” α-helix (the regular helical structure with an [i, i + 4]-type H-bond network) were determined computationally by geometry optimization. © 1997 John Wiley & Sons, Inc.
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    Cloppa RPA-AM1 analysis of the anisotropy of NMR 1J(XY) coupling tensors in Me3XY compounds (X = 13C, 29Si, 119Sn, 207Pb; Y = 19F, 35Cl) (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 61 (1997), S. 823-833 
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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 this work, the CLOPPA approach is extended to allow the bond-contribution analysis of the anisotropic part, ΔJ, of the indirect nuclear spin-spin coupling tensor, J. As an example this method is applied to study the role played by the X-Y bond, its antibonding orbital, and the Y lone pairs in determining the Fermi contact-spin dipolar cross contribution to Δ1J(XY) in compounds of type Me3X(SINGLE BOND)Y (X = C, Si, Sn, Pb; Y = F, Cl). That contribution is found to be the main one, except for × = Pb, in this series of compounds. Calculations are carried out using semiempirical AM1 ground-state wave functions. © 1997 John Wiley & Sons, Inc.
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    Density-gradient analysis for density functional theory: Application to atoms (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 61 (1997), S. 835-845 
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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 present an analysis of local or semilocal density functionals for the exchange-correlation energy by decomposing them into their gradients rs (local Seitz radius), ζ (relative spin polarization), and s (reduced density gradient). We explain the numerical method pertaining to this kind of analysis and present results for a few atoms and ions. The atomic shell structure is prominent, and only the ranges 0 〈 rs 〈 10 and 0 〈 s 〈 3 are important. The low-density and large-gradient domains, where the approximations for the exchange-correlation energy are least trustworthy, have very little weight. © 1997 John Wiley & Sons, Inc.
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    The effect of an electric field on the vibrational frequency of CN (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 61 (1997), S. 859-863 
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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: Ab initio calculations are used to compute the change in the vibrational frequency of CN with an applied electric field aligned parallel to the bond axis. The effect of the field on the CN- vibrational frequency is of the same magnitude as for CN, but in the opposite direction. These results are compared to previous results for CO. © 1997 John Wiley & Sons, Inc.
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    Conductivity in polyacetylene. IV. Ab initio calculations for a two-site model for electron transfer between allyl anion and allyl (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 61 (1997), S. 847-857 
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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 exchange between allyl molecules is studied using quantum chemical ab initio methods in order to model interchain electron transfer (ET) in polyacetylene. The reaction path, reorganization energy for ET (λ), and the electronic factor (Δ) are calculated using the UMP2, CASSCF, CASPT2, and CASSI methods. Changes in bond lengths are very small and their contribution to the internal reorganization energy almost negligible. Only changes in the bond angle leave significant contributions to the reorganization energy. The calculated potential energy surfaces can be represented as interacting parabolic-well potentials only in the case of CASSI. The UMP2, CASSCF, and CASPT2 methods give the localization of the electron even in the symmetric case and incorrect energy surfaces, with a cusp at the symmetry point. The electronic factor Δ is calculated using the two different methods, Koopmans' theorem and CASSI, with the same basis set. They are quantitatively very close for all intermolecular distances under study. However, Δ is sensitive to the basis set and cannot be calculated accurately in the present case. © 1997 John Wiley & Sons, Inc.
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    Theoretical investigation of the rates of electron transfer processes Q-I + QII → QI + Q-II and Q-I + Q-II → QI + Q2-II in photosynthesis (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 61 (1997), S. 865-879 
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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 theoretical investigation on the rates of electron-transfer processes Q-I + QII → Q-I + Q-II and Q-I + Q-II → QI + Q2-II was carried out by using the Marcus theory of long-range electron transfer in solution. The molecular reorganizational parameter λ, the free-energy change ΔG0 for the overall reaction, and the electronic matrix element HDA for these two processes were calculated from the INDO-optimized geometries of molecules QI, QII, and histidine. QI and QII are plastoquinones (PQ) which are hydrogen-bonded to a histidine each, and the two histidines may or may not be coordinated to a Fe2+ ion. The plastoquinone representing QI is additionally flanked by two peptide fragments. Each of the species (Pep)2QI · His and His · QII has been considered to be immersed in a dielectric continuum that represents the surrounding molecules and protein folds. INDO calculations confirm the standard reduction potential for the first process (calculated 0.127 V; observed 0.13 V) and predict a midpoint potential of 0.174 V for the second process at 300 K at pH 7 (experimental value remains uncertain but is known to be close to 0.13 V). The plastoquinone fragment carries almost all the net charge (about 95.7%) in [PQ · His]- and the net charge in [PQH · His]-. The electron is transferred effectively from the plastoquinone part of [(Pep)2QI · His]- to the plastoquinone moiety of QII · His in the first step and to the plastoquinone fragment of HisH+ · Q-II in the second step. Therefore, we made use of the formula for the rate of through-space electron transfer from QI to QII (and to Q-II). The plastoquinones are, of course, electronically coupled to histidines, and the transfer is, in reality, through the molecular bridge consisting of histidines and also Fe2+. The through-bridge effect is inherent in our calculation of ΔG0, HDA, and the reorganization parameter λ. We investigated the correlation between half-times for the transfer and (D-1op- D-1s), where Dop and Ds are, respectively, optical and static dielectric constants of the condensed phase in the vicinity of the plastoquinones. We found that with reasonable values of Dop (2.6) and Ds (8.5) the experimental rates are adequately explained in terms of transfers from the plastoquinone moiety of QI to that of QII. The t1/2 values calculated for the two processes are 247 and 472 μs in the absence of Fe2+ and 134 and 181 μs in the presence of Fe2+. These are in good agreement with the observed values which are ≈ 100 and ≈ 200 μs when Fe2+ is present in the matrix and which are known to be almost twice as large when the Fe2+ is evicted from the matrix. The present work also shows that the Marcus-Hush theory of long-range electron transfers can be successfully applied to the investigation of processes occurring in a semirigid condensed phase like the thylakoid membrane region. © 1997 John Wiley & Sons, Inc.
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    Design of an exchange functional with correct asymptotics (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 61 (1997), S. 891-898 
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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: Knowledge of asymptotic conditions on exchange allows for a better design of exchange energy expressions in density functional theory. By working inside an exchange-only framework, the fulfillment of such conditions by some of the most widely used exchange functionals is discussed. In turn, we propose a model expression which partially meets the energetics and asymptotics of both the exchange energy density and potential. Improvement upon the local spin density approximation without the use of generalized gradient corrections is also presented. Hartree-Fock orbitals are employed to build electron densities. © 1997 John Wiley & Sons, Inc.
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    Addendum to construction of linearly independent relativistic symmetry orbitals for finite double-point groups including time-reversal symmetry (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 61 (1997), S. 929-933 
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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 a previous article [J. Meyer, Int. J. Quantum Chem. 33, 445 (1988)], the formalism for the construction of linearly independent relativistic symmetry-adapted molecular basis functions under consideration of time-reversal symmetry was published. We present here an addition and correction of the application of time reversal on the construction. This new theory is now applicable to all double-point groups, including the icosahedral groups not considered so far. © 1997 John Wiley & Sons, Inc.
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    Correlation entropy of the H2 molecule (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 61 (1997), S. 935-941 
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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: Correlation of a quantum many-body state makes the one-particle density matrix nonidempotent. Therefore, the Shannon entropy of the natural occupation numbers measures the correlation strength on the one-particle level. Here, it is shown how this general idea of a correlation entropy must be adapted for two-electron systems in view of conservation laws which mix Slater determinants even in the noninteracting limit. Results are presented for the correlation entropy s of H2 as a function of the nucleus-nucleus separation R. In the ground state, the entropy of the spatial factor of the wave function maximizes 1.7 bohr beyond the Coulson-Fischer separation. The role of the correlation entropy in density functional theory is also discussed. © 1997 John Wiley & Sons, Inc.
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    Optimized Gaussian basis sets for use with relativistic effective (core) potentials: K, Ca, Ga—Kr (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 61 (1997), S. 943-952 
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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: Correlation-consistent valence basis sets were developed for the third-row main block elements (K, Ca, Ga - Kr) for use with relativistic effective core potentials. These basis sets are somewhat larger than double-zeta in size, with polarization functions, and are balanced for use in both Hartree-Fock and correlation calculations. Spin-orbit splittings for atoms and molecules are calculated and compared to experiment. These calculations use the approximate spin-orbit operator from the relativistic effective core potentials. The use of these results in the calculation of accurate thermochemical data is discussed. © 1997 John Wiley & Sons, Inc. This article is a US Government work and, as such, is in the public domain in the United States of America.
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    An ab initio study of the structure, dissociation energy, and heat of formation of Na2S (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 61 (1997), S. 953-957 
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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 equilibrium geometries and fundamental frequencies of Na2S are calculated at HF, MP2(FC, FU), and MP3 with the 6-31G(d) basis set and at HF and MP2(FC, FU) with the 6-31G(d) basis set, respectively. The total energy at MP2(FU)/6-31G(d)-optimized geometry is computed at MP4 with 6-311G(d, p), 6-311 + G(d, p), and 6-311G(2df, p), at QCISD(T)/6-311G(d, p), and at MP2/6-311G(3df, 2p) levels, respectively. The dissociation energy, the atomization energy, and the heat of formation for Na2S are evaluated using the G1 and G2 models. The calculated results indicated that Na2S in its ground state was a bent structure (C2v). Electron correlation corrections on the bending angle are very significant. The equilibrium geometrical parameters are Re(Na-S) = 2.45 Å and ∠Na-S-Na = 111.13° at the MP2(FU)/6-31G(d) level. The theoretically estimated dissociation energy, total atomization energy, and heat of formation are 67.07, 117.55, and 0.35 kcal mol-1, respectively, at 298.15 K. © 1997 John Wiley & Sons, Inc.
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    Theoretical studies on local anesthetics: Procaine, lidocaine, tetracaine, bupivacaine, and dibucaine - neutral and monoprotonated (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 61 (1997), S. 959-980 
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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 molecular basis of the pharmacological action of tertiary amine local anesthetics (LA) is still unclear. However, there is experimental evidence that the LA penetrates into the axon as a neutral form and acts in the charged form from the intracellular phase. In this work we report quantum chemical semiempirical results for the neutral and monoprotonated forms of procaine, lidocaine, tetracaine, bupivacaine, and dibucaine. All geometries have been fully optimized with the AM1 Hamiltonian. Solvent effects were included at the self-consistent reaction field (SCRF) approximation. We have found that the most stable conformers of positively charged LA agents are all characterized by intramolecular H-bond formation involving the protonated amine groups. INDO/S-CIS calculations have revealed that the gas-phase absorption spectra of LAs result from the superposition of the spectra of the ring π system and the carbonyl, in a large extent perturbed by intramolecular charge transfer (CT). For all studied LAs, the benzene B2u bands are very weak and the benzene B1u bands gain intensity due to symmetry breaking. Formation of intramolecular H bond strongly affects the carbonyl CT bands. On the other hand, the spectrum of dibucaine is dominated by the quinoline π system. © 1997 John Wiley & Sons, Inc.
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    Structure and energy of the positively ionized water clusters (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 61 (1997), S. 981-990 
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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: Geometry optimization of small (H2O)n+ clusters (n ≤ 4) at the UHF/4-31 + + G** level indicates that the cations consist of two fragments: the OH radical and the H2n-1 O+n-1 ion. The latter can be considered as a thermodynamically stable combination of a distorted H3O+ ion and (n-2) H2O molecules. The H bond between the fragments becomes weaker with increasing cluster size. Extrapolation of the adiabatic ionization potentials calculated for the (H2O)n oligomers (n ≤ 4) at the MP2 level to an infinite cluster size provides the value of approximately 8.7 eV, which can be presumably necessary for the ionization of liquid water in a vacuum. © 1997 John Wiley & Sons, Inc.
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    Calculations of the structure and electronic properties of extended polar hydrocarbons (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 61 (1997), S. 991-996 
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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 structure and nonlinear optical potential of a series of polar unsaturated hydrocarbons containing the electron-donating heptafulvene ring and electron-attracting pentafulvene ring have been explored in terms of their first hyperpolarizabilities using a semiempirical sum-over-states and time-dependent coupled Hartree-Fock approach. Both methods predict that the hyperpolarizabilities of these simple hydrocarbons are substantial and exceed those found for many extended conventional donor-acceptor systems. © 1997 John Wiley & Sons, Inc.
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    Time-evolution operator for a forced parametric oscillator (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 62 (1997), S. 125-135 
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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 apply an algebraic technique to describe the evolution of a parametric harmonic oscillator forced by a constant quartic potential. As the first step, we make use of iterative Bogolubov transformations (IBT) to incorporate information from the anharmonic part of the interaction in a nonperturbative form, yielding a unitary time-evolution operator. Later on, we make use of first-order perturbation theory to deal with that part of the interaction which was not incorporated previously. We show numerically that the resulting time-evolution operator is closer to unitarity than is the one obtained if no IBT is applied. The quantum fluctuations of position and momentum are evaluated for “the ground” state. Squeezing and correlation effects are observed. © 1997 John Wiley & Sons, Inc.
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    Crystalline and local vibrations of paired bases in poly(dG)-poly(dC) interacting with the h-b-1 hydrogen bond (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 62 (1997), S. 115-124 
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    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Some types of atomic vibrations in a chain of the DNA type constructed of G-C pairs were studied. These are the atomic vibrations of the lateral groups of guanine and N(3)H(1)H(2) of cytosine connected by the hydrogen bond h-b-1 and the vibrations of the centers of masses of bases in the direction parallel to bonds h-b-i, i = 1-3. The vibrations mix partially due to the dependence of the energy of the bond h-b-1 on its length and split into two bands because of the interaction between neighboring base pairs. It was shown that the excitation of the bond h-b-1 results in the splitting off of the two local vibrations and in a small deformation of the chain in the vicinity of the pair with the localized hydrogen bond. The law of the dispersion of band vibrations, values of the split-off frequencies, and degree of poly(dG)-poly(dC) chain deformation were determined. © 1997 John Wiley & Sons, Inc.
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    Single-reference CCSD approach employing three- and four-body CAS SCF corrections: A preliminary study of a simple model (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 62 (1997), S. 137-151 
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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 idea of correcting the single-reference coupled cluster (CC) method truncated at the pair cluster level (CCSD) by means of three- and four-body corrections coming from some external source [J. Paldus, J. Čížek, and M. Takahashi, Phys. Rev. A 30, 2193 (1984); J. Paldus and J. Planelles, Theor. Chim. Acta 89, 13 (1994)] is explored at the ab initio level using the CAS SCF wave functions as a source of the triply and quadruply excited cluster amplitudes. The method (referred to as CCSD-CAS) is applied to three simple systems based on minimum basis set and double-zeta models of the BH molecule, in which we continuously vary, respectively, (i) the two electron repulsion, (ii) the electronic charge, and (iii) the internuclear separation in order to explore the performance of the method in quasi-degenerate situations. Both the energies and the higher than pair cluster amplitudes are compared with the corresponding exact full configuration interaction (FCI) results. The relative importance of the three- and four-body cluster components is also examined. In all cases considered, the CCSD-CAS method provides the best result. © 1997 John Wiley & Sons, Inc.
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    Hydrogen-bonding effects, electrostatic potential, and the antitumor activity of flavone acetic acid and related compounds. III. Ab initio studies on the conformation space (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 62 (1997), S. 97-113 
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    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Restricted geometry optimizations at the ab initio SCF level with the 3-21G basis set were employed to investigate the conformation space of flavone acetic acid (FAA) and its related compounds. All the conformations are produced from a conformation which is, according to our previous work, probably the active conformation in terms of antitumor activity shown by these compounds. Detailed studies on FAA were carried out while only brief discussions are made on the analogs. The main results obtained are that (1) FAA is a very flexible molecule, e.g., with the energy barrier up to about 3 kcal mol-1 from the reference conformation, the important torsional angle τ1 can change from 27.0° to 117.0°, τ2 from -168.0° to 2.0°, and τ3 from -50.0° to 30.0°; (2) the hydrogen-bonding effect plays an important role in determining lower-energy conformations; (3) among all the FAA conformations considered, some are active and some are inactive; (4) it seems that the analogs will have similar behavior to FAA when the torsional angle τ3 is restricted to the values which are around the equilibrium values; and (5) the hypothesis put forward previously has been further developed in this work. Now, we postulate that efficient charge transfers will lower the energy and that proper charge transfers will activate the molecule. There are mainly two different types of charge transfer corresponding to two different types of conformation, which are specified in this article. © 1997 John Wiley & Sons, Inc.
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    On the restricted step method coupled with the augmented Hessian for the search of stationary points of any continuous function (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 62 (1997), S. 153-165 
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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 any optimization using the augmented Hessian technique, the step is not restricted to any length. Since the restriction of the step at each iteration is very important in order to achieve good convergence, we present a coupled method such that the augmented Hessian automatically gives both the adequate length of the step and the correct Hessian structure. The method is showed for the minima and saddle points of any order. © 1997 John Wiley & Sons, Inc.
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    Statistical electron densities (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 64 (1997), S. 85-93 
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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: It is known that in numerous interesting systems one-electron states appear with a multifractal internal structure. Physical intuition suggests, however, that electron densities should be smooth both at atomic distances and close to the macroscopic limit. Multifractal behavior is expected at intermediate length scales, with observable nontrivial statistical properties in considerably, but far from macroscopically sized clusters. We have demonstrated that differences of generalized Rényi entropies serve as relevant quantities for the global characterization of the statistical nature of such electron densities. Asymptotic expansion formulas are elaborated for these values as functions of the length scale of observation. The transition from deterministic electron densities to statistical ones along various lengths of resolution is traced both theoretically and by numerical calculations.    © 1997 John Wiley & Sons, Inc. Int J Quant Chem 64: 85-93, 1997
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    Adiabaticity and gauge transformations for an oscillator coupled to a single-mode field (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 64 (1997), S. 111-119 
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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 components of the Floquet wave function of an oscillator in various representations are correlated. The transformation from length to velocity gauge can be viewed as a change from a diabatic to an adiabatic representation in the field variable. This is shown in three different ways.   © 1997 John Wiley & Sons, Inc. Int J Quant Chem 64: 111-119, 1997
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    The optimized effective potential method of density functional theory: Applications to atomic and molecular systems (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 64 (1997), S. 95-110 
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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: Using the optimized effective potential method in conjunction with the semianalytical approximation due to Krieger, Li, and Iafrate, we performed fully self-consistent exact exchange-only density functional calculations for diatomic molecules with a fully numerical basis-set-free molecular code. The results are very similar to the ones obtained with the Hartree-Fock approach. Furthermore, we present results for ground states of positive atomic ions including correlation contributions in the approximation of Colle and Salvetti. It is found that the scheme performs significantly better than do conventional Kohn-Sham calculations.   © 1997 John Wiley & Sons, Inc. Int J Quant Chem 64: 95-110, 1997
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    Energies of dipole-bound anionic states (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 64 (1997), S. 183-191 
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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: Dipole-bound anionic states of CH3CN, C3H2, and (HF)2 were studied using highly correlated electronic structure methods and extended one-electron basis sets. The electron detachment energies were calculated using the coupled cluster method with single, double, and noniterative triple excitations. Geometrical relaxation of the molecular framework upon electron attachment and the difference in the harmonic zero-point vibrational energies between the neutral and the dipole-bound anionic species were calculated at the MP2 level of theory. We demonstrate that the dispersion interaction between the loosely bound electron and the electrons of the neutral molecule is an important component of the electron binding energy, comparable in magnitude to the electrostatic electron-dipole stabilization. The geometrical relaxation upon electron attachment and the change in the zero-point vibrational energy is important for the weakly bound HF dimer. The predicted values of the vertical electron detachment energies for the dipole bound states of CH3CN and C3H2 of 112 and 188 cm-1, respectively, are in excellent agreement with the recent experimental results of 93 and 171±50 cm-1, respectively. For (HF)2-, the predicted value of adiabatic electron detachment energy is 396 cm-1, whereas the experimental vertical detachment energy is 508±24 cm-1. The possibility of formation of the neutral dimer in an excited vibrational state is considered.   © 1997 John Wiley & Sons, Inc. Int J Quant Chem 64: 183-191, 1997
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    Accurate density-functional calculations on large systems (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 64 (1997), S. 193-203 
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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: Efforts to compute accurate all-electron density-functional energies for large molecules and clusters using Gaussian basis sets are reviewed and their use in fullerene science described. The foundation of this effort, variational fitting, is described first. When discovered experimentally, C60 was naturally assumed to be particularly stable, but local-density-functional calculations showed that C60 is quite unstable relative to the higher fullerenes and graphene (a single sheet of graphite). In addition to raising questions about the relative abundance of the various fullerenes, this work conflicted with the then state-of-the-art density-functional calculations on crystalline graphene. Now high accuracy molecular and band structure calculations are in fairly good agreement with each other and experiment. These calculations clearly demonstrate that each of the 12 pentagons, which are necessary to close a fullerene, is best viewed as a rather high-energy, more than 2 eV, defect in a graphene sheet. The effect of the heptagon, the second most common defect in fullerene materials, is described. Most recently, we have developed accurate, variational gradient-corrected forces for use in geometry optimization of clusters and in molecular-dynamics simulations of friction. The gradient-corrected optimized geometry of C60 is given.   © 1997 John Wiley & Sons, Inc. Int J Quant Chem 64: 193-203, 1997
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    Density-functional investigation of some decomposition routes of methyl nitrate (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 64 (1997), S. 205-210 
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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 density-functional methods, B3/LYP/6-31+G(d) and B3/PW91/6-31G(d, p) have been used to investigate some possible decomposition pathways for methyl nitrate. Two likely ones were found to be (a) loss of NO2, followed by eventual rearrangement to H2C(SINGLE BOND)OH, and (b) formation of H2C(DOUBLE BOND)O and HONO. The initial energy requirement for each process is about 40 kcal/mol, and the second is exothermic, ΔH=-16 kcal/mol of H3C(SINGLE BOND)ONO2. The B3/LYP procedure is found to underestimate the H3CO(SINGLE BOND)NO2 and H3C(SINGLE BOND)ONO2 dissociation energies by about 6 kcal/mol.   © 1997 John Wiley & Sons, Inc. Int J Quant Chem 64: 205-210, 1997
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    The role of the Jahn-Teller distortions on the structural, binding, and magnetic properties of small Fen clusters, n≤7 (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 64 (1997), S. 223-230 
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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 clusters of iron atoms, up to heptamers, were studied by density functional theory. The calculations performed were all-electron and both local and gradient-dependent potentials were used. Most of the symmetric or compact structures show degenerate electronic states which are stabilized, through Jahn-Teller distortions, in lower symmetry or more open geometries. As the cluster size increases, the coordination numbers increase, the metal-metal bond strengths are reduced, and the equilibrium bond lengths are enlarged. This behavior accounts for the occurrence of high magnetic moments on the atomic sites, which are coupled ferromagnetically.   © 1997 John Wiley & Sons, Inc. Int J Quant Chem 64: 223-230, 1997
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    Studies of chemical hardness and chemical potential on isomers and hardness profiles of hydrogen-bonded systems (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 64 (1997), S. 231-242 
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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 the present investigation, chemical hardness and electronic chemical potential for isomers of different molecules were studied by taking into account the principle of maximum hardness. Different isomers of the molecules, XC(O)OX (X=F, Cl), C2H3NO, C2H2, and HCNC, were considered for the above investigation. In most of the cases, the chemical hardness was found to be a good indicator for predicting the most stable isomer and not the chemical potential. The hardness profiles of different configurations of the interacting systems, HF(DOTTED BOND)HCN, HF(DOTTED BOND)HCl, and CH3OH(DOTTED BOND)H2O hydrogen-bonded complexes, were studied. The extrema of the hardness profiles does not coincide well with the energy, but in some cases, the hardness profile follows the interaction path. The higher hardness-higher stability concept among different configurations of the hydrogen-bonded complexes are discussed.   © 1997 John Wiley & Sons, Inc. Int J Quant Chem 64: 231-242, 1997
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    Ionized van der Waals systems: Structure and interactions (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 64 (1997), S. 211-222 
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    ISSN: 0020-7608
    Keywords: quantum chemistry ; weak interactions ; cationic van der Waals complexes ; cluster cations ; potential energy surfaces ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Ions of molecular complexes with internal interactions partly of van der Waals type differ significantly from their neutral parent species in binding, structure, and dynamics. Information about such systems is still rather scarce, both experimentally and theoretically. The article discusses some aspects of the present knowledge concerning cationic dimers and trimers of the type RgnM+ with Rg denoting a rare-gas atom, and M, either a (diatomic) molecule or a rare-gas atom, in comparison with the neutral counterparts. In some more detail, potential energy surfaces (PESs) for the six lowest electronic states of ArHCl+ are reported, as obtained from multireference CI calculations with an extended Gaussian AO basis set. From the PESs, structural, energetical, and dynamical features are derived. For the cationic argon trimer Ar3+ in the electronic ground state, a detailed analysis of the PES, the mode couplings, and the classical internal dynamics is carried out, leading to a consistent picture of a floppy molecular aggregate with rapid energy exchange between internal degrees of freedom and early onset of classical chaos with increasing energy.   © 1997 John Wiley & Sons, Inc. Int J Quant Chem 64: 211-222, 1997
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    An LDA calculation of the conformation and electronic structure of polytetrafluoroethylene (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 64 (1997), S. 243-246 
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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 calculation of the structural and electronic properties of polytetrafluoroethylene systems with several different dihedral angles is presented using two different local density approximations, the Gáspár-Kohn-Sham and the Perdew-Zunger. All the geometrical parameters were optimized simultaneously for the stable conformation. The torsional potential curve shows an absolute minimum at the dihedral angle of 163.7°, corresponding to a slightly deformed planar zigzag conformation. Other two distinct local minima for the quasi-stable helical conformation are found at the gauche form. The calculated valence and conduction bands are discussed and compared with other theoretical calculations and experiment.   © 1997 John Wiley & Sons, Inc. Int J Quant Chem 64: 243-246, 1997
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    Computation of geometries and frequencies of singlet and triplet nitromethane with density functional theory using Gaussian-type orbitals (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 64 (1997), S. 263-269 
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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 results of computational studies of the structures, energies, dipole moments, and infrared spectra for singlet and triplet nitromethane are presented. Five different hybrid (B HandH, BHandHLYP, B3LYP, B3P86 and B3PW91), local (SVWN), and nonlocal (BLYP) density functional theory (DFT) methods were used in this study with various sizes of a Gaussian-type basis set. The obtained results were compared to Hartree-Fock (HF), second-order Møller-Plesset (MP2), and multiconfiguration self-consistent-field (MCSCF) ab initio calculations, as well as to the experimental results. The suitability of DFT methods to correctly reproduce experimental results is discussed.   © 1997 John Wiley & Sons, Inc. Int J Quant Chem 64: 263-269, 1997
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    Computation of some ionization potentials for second-row elements by ab initio and density functional theory methods (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 64 (1997), S. 255-261 
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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: Four ionization potentials of elements from the second row of the periodic table were computed with ab initio (HF, MP2, MP3, MP4, QCISD, G1, G2, and G2MP2) and density functional theory (DFT) (B3LY, B3P86, B3PW91, XALPHA, HFS, HFB, BLYP, BP86, BPW91, BVWN, XAPLY, XAP86, XAPW91, XAVWN, SLYP, SP86, SPW91 and SVWN) methods. In all of the calculations, a large 6-311++G(3df, 3pd) Gaussian-type basis set was used. The computed values were compared to experimental results, and the suitability of these ab initio and DTF methods for computing atomic ionization potentials was discussed. It was found that G2 will produce ionization potentials that are indistinguishable from the experimental values. Hybrid B3LYP and nonlocal BLYP will generate very good ionization potentials. For larger atoms, SLYP and SPW91 will compute ionization potentials that are of G2 quality.   © 1997 John Wiley & Sons, Inc. Int J Quant Chem 64: 255-261, 1997
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    Comparative AM1 study of the electronic structure of etiocholanes (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 64 (1997), S. 249-254 
    Details
    ISSN: 0020-7608
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Etiocholanes are metabolites of androstenedione (4-androsten-3,17-dione) and testosterone (17β-hydroxy-4-androsten-3-one). These compounds are produced by the action of 5-reductases which break the π double bond of the androstenes at C4 and C5. The most important etiocholanes are 5β-androstanedione (etiocholanedione), 3α-hydroxy-5β-androstan-17-one (etiocholanolone), 3β-hydroxy-5β-androstan-17-one (epietiocholanolone), and 17β-hydroxy-5β-androstan-3-one (5β-dihydrotestosterone; 5βDHT). Among their most important biological effects are depression of the central nervous system, relaxation of smooth muscle, and stimulation of sexual behavior. The intent of the present study is a determination of the effects of carbonyl and hydroxyl groups at C3 and C17 on the electronic structure of these androgens, in an attempt to gain some insight into their biological action. All calculations were of the semiempirical AM1 type. The decrease of enthalpy caused by an hydroxyl group was found to be twice that of a carbonyl group. The effect of two groups was found to be additive. Carbonyl groups on the other hand led to an increase of dipole moment and of the electrostatic charges on neighboring carbons that were higher than those produced by hydroxyl groups. Carbonyl groups at C17 pushed the highest occupied molecular orbital (HOMO) to higher energy whereas a carbonyl at C3 had little or no effect. However, in mono and di-keto structures, both HOMO and lowest unoccupied molecular orbital (LUMO) were located as expected closer to the carbonyl group. Etiocholanedione, a diketo structure, showed degeneracy of frontier orbitals with its related HOMO-1 and LUMO+1. Structures with mono-hydroxyl or di-hydroxyl functional groups showed the lowest HOMO values; the highest LUMO values and quasi degeneracy of HOMO-1 and LUMO+1. The HOMO and LUMO of etiocholane and for the mono and di-hydroxyl structures were observed diffused throughout the molecules in a “sausagelike” or “ribbonlike” fashion. These results might explain some metabolic steps. Likewise, the difference of intermolecular forces, i.e., dipole moments and charges displayed by the carbonyl and hydroxyl groups, might help to elucidate some biological effects.   © 1997 John Wiley & Sons, Inc. Int J Quant Chem 64: 249-254, 1997
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    Coupling-constant dependence of atomization energies (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 64 (1997), S. 285-295 
    Details
    ISSN: 0020-7608
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The local spin-density (LSD) functional and Perdew-Wang 91 (PW91) generalized gradient approximations to atomization energies of molecules are investigated. We discuss the coupling-constant dependence of the atomization energy and why exchange errors of the functionals are greater than exchange-correlation errors. This fact helps to justify hybrid schemes which mix some exact exchange with density functional approximations for exchange and correlation. It is shown that the biggest errors in the atomization energies occur when there is a strong interaction between different electron pairs, which vanishes upon atomization. We argue that the amount of exchange character of a molecular property, such as the atomization energy, depends on the property itself. We define an exact mixing coefficient b, which measures this exchange character, and show that both LSD and PW91 typically overestimate this quantity. Thus, nonempirical hybrid schemes which approximate this quantity by its LSD or PW91 value typically do not improve the exchange-correlation energy.   © 1997 John Wiley & Sons, Inc. Int J Quant Chem 64: 285-295, 1997
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    QR-SCMEH-MO calculations on lanthanide systems. IV. The [SmCp*]4 cluster (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 64 (1997), S. 297-300 
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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: Additional calculations via the quasi-relativistic self-consistent-modified extended Hückel-MO method (QR-SCMEH-MO) were conducted on the model cluster [SmCp*]48+ and the real [SmCp*]4(N2H2)2(N2H3) 4(NH3)2 cluster. It is shown that this is no trivial problem, owing to correlation arising from the electron repulsion among open shells, plus the close proximity of open- and closed-shell configurations. An acceptable result is shown to be sensitive to spin-orbit splitting and electron-pairing interactions.   © 1997 John Wiley & Sons, Inc. Int J Quant Chem 64: 297-300, 1997
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    The distribution of polynuclear aromatic hydrocarbons between aquatic plants and sediments (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 64 (1997), S. 271-283 
    Details
    ISSN: 0020-7608
    Keywords: acropetal ; adsorption ; aquatic plants ; polynuclear aromatic hydrocarbons ; sediments ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Saturation phenomena for the uptake of polynuclear aromatic hydrocarbons (PAHs) by rooted aquatic plants were observed in both field and laboratory experiments. From these data, a Langmuir adsorption model was derived and was shown to describe the assimilation of sediment PAHs by aquatic plants at low-sediment PAH concentrations. The low-concentration restrictions were relaxed by using an empirical Freundlich model. This permitted the specification of equilibrium constants, relative rate constants for adsorption and desorption, and saturation levels, and it provided a realistic description of the acropetal mobilization of sediment PAHs by rooted aquatic macrophytes. The growth of aquatic plants was found to be inhibited by sediment PAHs, but at concentrations several orders of magnitude above the corresponding threshold effect levels for aquatic animals. The effects of the acropetal transport of sediment PAHs into the above-soil portions of aquatic plants merits further investigation because of the possibility of an enhanced availability of these carcinogens to associated aquatic food webs. In addition, these results may suggest pathways for bioremediation of the impact of PAHs on associated biota.   © 1997 John Wiley & Sons, Inc. Int J Quant Chem 64: 271-283, 1997
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    Geometry optimization of polymers by the elongation method (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 64 (1997), S. 301-323 
    Details
    ISSN: 0020-7608
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Theoretical studies on the electronic and the geometrical structures for various molecules by the molecular orbital or the density functional theory have recently been developed and applied widely under the progress of computer technologies. At present, it is possible to carry out a theoretical investigation on electronic properties for small molecules at the Hartree-Fock and the post-Hartree-Fock levels by the improvement of advanced program packages. However, it is difficult to perform the theoretical calculations on electronic structures for large polymers with the aperiodic sequence of molecular segments, because the theoretical treatment of random systems has not yet been established. We recently proposed the elongation method as a useful theoretical approach to obtain the electronic states of any polymers without the periodic geometry of molecular fragments. In the previous works, the reliability of our treatment has been shown by the application to many polymers under single-point calculations with fixed molecular geometry. Thus, as the next step of our study, an attempt for the geometry optimization of large polymers by the elongation method was made in this work. As the first samples of geometry optimization, the periodic polymers of polyethylene, polyacetylene, and polyglycine were examined. Also, as the second samples, the locally aperiodic polymers of polyacetylene with local defects of positively and negatively charged solitons were tested. Total energies, optimized geometries, and electron densities were checked by those obtained from the conventional molecular orbital method.   © 1997 John Wiley & Sons, Inc. Int J Quant Chem 64: 301-323, 1997
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    Molecular orbital approach to the Peierls instability in polyenes and its application to model crystals of charge-transfer complexes (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 64 (1997), S. 325-336 
    Details
    ISSN: 0020-7608
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We have already proposed a convenient method to estimate the magnitude of the Peierls instability in finite one- and two-dimensional (1D and 2D) polyenes from the viewpoint of orbital symmetry [J. Chem. Phys., 103, 9726 (1995)]. This treatment is applied to a model system for charge-transfer complexes, in which the unpaired electron is occupied in a molecule, generating a half-filled energy band by intermolecular interaction. As the first step, a model molecular crystal composed of ethylene anion (or cation) molecules is treated to confirm the applicability of our formulas for 2D polyene. It is clearly shown that molecules in a crystal are structurally distorted to gain the energetic stabilization within the 1D structure, while they are rearranged in a regularly equidistant structure under strong interchain interaction. This treatment would provide insight into the relationship between the crystal structure of charge-transfer complexes and its electronic property.   © 1997 John Wiley & Sons, Inc. Int J Quant Chem 64: 325-336, 1997
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    Heterogeneous reactions over fractal surfaces: A multifractal scaling analysis (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 64 (1997), S. 337-350 
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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: Monte Carlo simulations of Eley-Rideal (ER) mechanisms with fixed and varying sticking probabilities, p, as well as a three-step catalytic reaction (TCR) over fractal surfaces were performed to examine the morphological effect on the above-mentioned reactions. The effect of decay and enhancing profiles on the reaction probability distribution (RPD) for the ER reaction as well as effects of a varying probability of reaction steps on the normalized selectivity distribution (NSD) for the three-step reaction were then analyzed by multifractal scaling techniques. For ER with a fixed sticking probability, a small p value tends to yield homogeneous RPD. For ER with a varying sticking probability, the RPD is revealed to be spatially uniform at fast decay and rather concentrated at a faster enhancing rate. For the three-step reaction, a large dimerization/isomerization ratio increases the position distinction among active sites as the adsorption probability equals 1. At a small adsorption probability, the dimerization/isomerization ratio causes no effect on the NSD. Heterogeneity of surfaces as reflected in the multifractal analysis will also be discussed.   © 1997 John Wiley & Sons, Inc. Int J Quant Chem 64: 337-350, 1997
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    Davydov solitons in proteins Contribution to the Proceedings of the 2nd Congress of the International Society for Theoretical Chemical Physics at New Orleans Louisiana April 9-13 1996. (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 64 (1997), S. 351-377 
    Details
    ISSN: 0020-7608
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The mechanism for energy and signal transport in proteins as suggested by Davydov is discussed. The idea is based on a coupling of amide-I oscillators to acoustic phonons in a hydrogen-bonded chain. Results as obtained with the usually used ansätze are discussed. The quality of these states for an approximate solution of the time-dependent Schrödinger equation is investigated. It is found that the semiclassical ansatz is a poor approximation, while the more sophisticated |D1〉 state seems to represent the exact dynamics quite well. Calculations at a temperature of 300 K for one chain, as well as for three coupled ones (as they are present in an α-helix), are presented and discussed. From the calculations, it is evident that Davydov solitons are stable for reasonable parameter values at 300 K only for special initial excitations close to the terminal sites of the chain. However, for soliton formation, it is not necessary that the initial excitation occurs at the chain end which has its C(DOUBLE BOND)O group directly coupled to the lattice as it is the case for T=0 K. At higher temperatures, solitary waves can be formed from both chain ends. Since the model for temperature effects used was criticized from the theoretical point of view, we suggest an improved theory for temperature effects. Finally, we discuss recent experimental findings which indicate that normal modes describing the N(SINGLE BOND)H stretch and its coupling to the hydrogen bonds should be considered in addition to the amide-I vibration.   © 1997 John Wiley & Sons, Inc. Int J Quant Chem 64: 351-377, 1997
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    Outlines of a general framework of cancer initiation in the cell (1997)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 64 (1997), S. 379-385 
    Details
    ISSN: 0020-7608
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: According to the central dogma of molecular biology, information flows in the living cell from DNA through RNA to proteins. Therefore most investigations of cancer initiation try to explain these effects by carcinogen binding to, or radiation hits on, DNA which lead to the first steps of the malignant transformations. On the other hand, recent detailed theoretical investigations have shown that proteins are good disordered hopping conductors (their conductivity is in the order of good conducting amorphous glasses). Their conductivity can be substantially influenced by binding of chemicals or by the effects of radiation if they cause conformational changes (as recent calculations have shown). These effects can also destroy bonds or generate new bonds in proteins. If the affected proteins are regulatory proteins, they can be inactivated in both ways. Namely, on the basis of Warburg's experiments, one can postulate that if the hindrance of oxygen metabolism leads to fermentation, and with it to the malignant transformation, this means also the hindrance of electron flow in the Szent-Györgyi-Krebs cycle. In other words the hindrance of electron transport in this cycle most probably has the same effect as the lack of oxygen, which in this way most probably leads again to a malignant transformation. Finally the inactivation of regulatory enzymes can influence also the regulation of the expression of oncogenes. If in this way oncogenes become overactivated (or antioncogenes become inactive), the changes started by the inactivation of regulatory enzymes become hereditary. It seems that if we look at the cell as a complicated self-regulatory system, primary changes both at their DNA or regulatory protein molecules caused by external agents can disturb its self-regulation and transform it in this way into another stationary, possibly precancerous, state.   © 1997 John Wiley & Sons, Inc. Int J Quant Chem 64: 379-385, 1997
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