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  • LUNAR AND PLANETARY EXPLORATION  (1,410)
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  • 1
    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 connectivities of submolecules [H. Joela, Theor. Chim. Acta 39, 241 (1975)] corresponding to Kekulé structures of nine nonalternate hydrocarbons and four nonbenzenoid hydrocarbons containing four-membered rings are correlated with their Kekulé indices. In the latter class of compounds it was observed that the corresponding submolecules contain cut vertices and bridges in contrast to submolecules of benzenoid hydrocarbons which are devoid of such bridges. It was observed, furthermore, that the branching index goes up with the number of bridges in the submolecule. The results present an application to the abstract relation [D. Cvetković, I. Gutman, and N. Trinajstić, J. Chem. Phys. 61, 2700 (1974)] between resonance and MO theories.
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  • 2
    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 well known that the CNDO and INDO methods fail in the field of theoretical conformational analysis of conjugated compounds. The NDDO approximation, however, can be applied very successfully to this problem. In this paper, a comparative analysis of these three approximations is performed to clarify the reasons for the failure of the former two methods. It is shown that the defects are inherent in their formalisms, and there is no reasonable possibility to compensate them (e.g., by reparametrization) at the level of the CNDO and INDO approximations. The central inadequacy is the oversimplified treatment of the two-center exchange interaction. In the case of molecules containing heteroatoms with lone-pair electrons the known defects concerning the description of Coulombic interactions of anisotropic charge distributions can additionally influence the results. To overcome these shortcomings, at least an extension of the formalism to the level of the NDDO method is necessary.
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  • 3
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    International Journal of Quantum Chemistry 21 (1982), S. 899-903 
    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: To calculate atom-atom interactions at high temperatures, one needs the electron densities of the interacting atoms. The present paper outlines an approach by which the temperature-dependent electron densities of compressed atoms may be obtained. The approach suggested makes use of the temperature-dependent Thomas-Fermi (TF) equation, and a variational principle by which this equation may be solved approximately.
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  • 4
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    International Journal of Quantum Chemistry 21 (1982), S. 905-915 
    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 performance of a recently proposed scaled one-electron Hamiltonian (SOEH) model is tested against parallel sets of restricted open-shell calculations by the method of Roothaan. It is found that the energy calculated by SOEH model, in general, lies slightly higher than the energy computed by the restricted open-shell method of Roothaan lending credibility to the application of variational argument to the scaled pseudoenergy functional (Eav) for deriving the SOEH model. The numerical stability of the converged SOEH energy with respect to changes in trial vectors indicates the reliability of the method. The SOEH model is shown to perform well in the calculation of geometries of radicals and ions. The convergence behavior of the SOEH model is compared with that of the restricted open-shell method of Roothaan.
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  • 5
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    International Journal of Quantum Chemistry 21 (1982), S. 943-943 
    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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  • 6
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    International Journal of Quantum Chemistry 22 (1982), S. 183-190 
    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: Operator Ω and reaction operator T of Löwdin are studied with a slightly different approach, which leads to a clearer understanding of their nature and brings out the relevance of the conditions for the invertibility of K studied in the preceding work in this series.
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  • 7
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    International Journal of Quantum Chemistry 22 (1982) 
    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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  • 8
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    International Journal of Quantum Chemistry 22 (1982), S. 241-251 
    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 roles of kinetic and potential energy operators in the formation of a chemical bond are investigated using the virial and electrostatic theorems. It is shown that an integrated form of the virial theorem throws a new light on a paradox involving changes in the mean kinetic energy on bond formation. Detailed calculations on the ground state of H2+ lead to a surprisingly simple electrostatic approximation to the bond. The entire potential curve is given to good accuracy (ca. 80% bond energy) by the electrostatic forces between the two protons and a single, nodeless, spherically symmetric charge cloud representing the electron, provided that, if the bond is stretched beyond about 3.6 a.u. the charge cloud switches suddenly from a central location to a position near one proton.
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  • 9
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    International Journal of Quantum Chemistry 22 (1982), S. 289-292 
    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 utilization of an average occupation number for open shell orbitals, having different occupation numbers in a degenerate wave function, tantamounts to disregard first order contributions in a CI-type expansion of the wave function. These contributions are taken into account in a SCF process that derives the differently occupied open shell orbitals as eigenfunctions of different Fock-type operators and accounts for the total symmetry.
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  • 10
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    International Journal of Quantum Chemistry 22 (1982), S. 351-365 
    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 singlet instabilities of the RHF ground state in infinite polyenes have been studied in the framework of a semiempirical PPP Hamiltonian, accounting for long-range Coulomb interactions until convergence of the ground-state energy per electron value. The symmetry-adapted RHF solution (SAS) has been shown to be unstable to the formation of bond-order alternation waves (BAW's) and charge-density waves (CDW's). The CDW solutions have been shown to be higher in energy than the corresponding BAW solutions and to represent saddle points of the energy hypersurface, unstable to the formation of BAW's for physically realistic range of variation of the semiempirical parameters. Analytical formulas for the SAS ground-state energy per electron have been derived in case of a Coulomb law and a Mataga-Nishimoto formula for the two-center Coulomb integrals.
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  • 11
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    International Journal of Quantum Chemistry 22 (1982), S. 415-420 
    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: Calculations with pseudopotentials of double-zeta quality have been performed on ethylene, acetylene, and water molecules. A description of the carbon-carbon double and triple bonds is presented in the framework of the pseudo-FSGO method. A possible model of the oxygen lone pairs has been established and its functioning has been proved by calculations on the water molecule.
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  • 12
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    International Journal of Quantum Chemistry 22 (1982), S. 435-440 
    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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  • 13
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    International Journal of Quantum Chemistry 22 (1982), S. 441-441 
    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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  • 14
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    International Journal of Quantum Chemistry 22 (1982), S. 459-484 
    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 generalization of the Hartree-Fock molecular orbital (MO) theory for treating diradical intermediates was explained pictorially by drawing molecular orbitals of diradical species such as ring-opened trimethylene. The generalized MO theory applied to elucidate electronic mechanisms of concerted, ionic, radical, and ion-radical reactions of organic reactants in the ground state. Generalized MO computations revealed the most essential characteristics of these reactions and mutal relationships between the worlds of Woodward-Hoffmann and Hughes-Ingold. Generalized MO studies supported our orbital symmetry, stability and pairing rules for concerted, ionic and radical reactions in the ground state, respectively. An extension of MO treatments to excited states reactions was briefly pointed out in relation to the density and spin correlation functions by the multireference CI wave functions.
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  • 15
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    International Journal of Quantum Chemistry 22 (1982), S. 547-556 
    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 nonplanarity of a peptide unit has been studied by the ab initio method (GAUSSIAN 76) considering both the pyramidal structure of nitrogen and the variation of the ω angle for the N-methyl-acetamide and N-ethyl-acetamide in their cis and trans conformations. Several semiempirical methods have also been used for the sake of completeness of this work. All methods except CNDO and PCILO support a nonpyramidal structure for the nitrogen. The distortions of the planarity of the peptide unit show for both cis and trans conformations of the model compounds a significant asymmetric shift of the minimum on the energy curve for the ω rotation.
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  • 16
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    International Journal of Quantum Chemistry 22 (1982), S. 293-298 
    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 calculations of long-range interaction energy are often based on multipole expansion. The truncated multipole expansion and interaction energy calculated with it are noninvariant with respect to an arbitrary choice of local coordinate systems. In this paper we show that truncated multipole expansion of form Σk = 1n CkR-k is “numerically” independent on a choice of local coordinate systems, if convergence conditions are satisfied.
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  • 17
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    International Journal of Quantum Chemistry 22 (1982), S. 307-330 
    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: Euler transformation for accelerating convergence of a series is considered in the context of handling divergent (asymptotically convergent) perturbation series. A generalized (parametrized) version of this transformation is developed, based on the conjecture of Dalgarno and Stewart, which works better. Viewed from this standpoint, the Padé approximants follow as a special case of the parametrized Euler transformation (PET), as is the case with the μ transformation procedure of Feenberg in a perturbative context. The PET is shown to serve as a more general method of handling a divergent series and is able to appreciate the construction and convergence behavior of specific sequences of Padé approximants. The role of parametrization in the context of the Z-1 perturbation theory of atoms is also noted and the workability of the adopted strategy is demonstrated by choosing some specific test cases.
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  • 18
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    International Journal of Quantum Chemistry 22 (1982), S. 385-395 
    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 method is formulated for enumerating and constructing isomerization reactions of molecules exhibiting large amplitude nonrigid motions. This method not only enumerates the isomers of nonrigid molecules and the corresponding rigid molecules but also the symmetry species spanned by the equivalent structures whose representative is an isomer. Consequently, using the method of correlating the symmetry species of a group to the symmetry species of its subgroup the splitting patterns of isomers of nonrigid molecule to those of rigid molecule are obtained. This provides an elegant method for both enumerating and constructing reaction graphs. The method is illustrated with examples.
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  • 19
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    International Journal of Quantum Chemistry 22 (1982), S. 429-431 
    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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  • 20
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    International Journal of Quantum Chemistry 22 (1982), S. 443-443 
    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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  • 21
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    International Journal of Quantum Chemistry 22 (1982), S. 537-545 
    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: Enzymes are large aperiodic structures and this hinders both ab initio molecular orbital and Bloch-type band theory of calculations. A frontier orbital perturbation theory of catalysis is applied to enzymes. Reasons are given for proposing that the induced-fit conformational changes, essential to enzymatic catalysis, leads to an increase in the electronic eigenvalue density at the active site, enhancing the necessary catalytic orbital perturbation.
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  • 22
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    International Journal of Quantum Chemistry 22 (1982), S. 575-582 
    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 hypothesis that an electrostatic effect of the buried aspartate side chain may contribute considerably to the extreme catalytic power of serine proteinases is presented. This statement is based on an analysis of environmental effects, due to amino acid residues in the vicinity of the Ser-221…His-64 couple and to hydration in subtilisin BPN'.
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  • 23
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    International Journal of Quantum Chemistry 22 (1982), S. 661-662 
    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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  • 24
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    International Journal of Quantum Chemistry 22 (1982), S. 681-690 
    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 influence of a small deformation of CĈO angle in phenol (tilt), into the rotational far-infrared (FIR) spectrum is analyzed using several approaches. In all of them, the CNDO/2 method is used to determine the potential energy functions. In a first step, the C—O bond and the rotation axis are both supposed to coincide with the C2 symmetry axis of the phenyl group. With this assumption the torsional frequencies are calculated in both the symmetric and asymmetric rotor approximations. In a second step, the tilt of the C—OH bond is determined theoretically and found to be -3°, measured from the C2 symmetry axis, the C—OH bond crossing this axis, Using this second geometry, and taking as the rotation axis the C2 axis, the torsional frequencies are again determined in both approximations. An improvement of the calculated transition energies is encountered at each stage of the calculation, when compared with experimental data. Finally the importance of the introduction of a tilt into the FIR torsional frequency calculations is discussed.
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  • 25
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    International Journal of Quantum Chemistry 22 (1982), S. 775-782 
    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 considerations of Walsh rules are extended to rationalize the loss of planarity in the 1,3nπ* states of simple carbonyl and thiocarbonyl molecules. The role of Fermi correlation in shaping the differences between conformations in the singlet and the triplet state is emphasized. The role played by the π* orbital is also considered.
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  • 26
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    International Journal of Quantum Chemistry 22 (1982) 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
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    Topics: Chemistry and Pharmacology
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  • 27
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    International Journal of Quantum Chemistry 22 (1982), S. 871-888 
    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 presence of external electric and magnetic fields, the Schrödinger equation for many-electron systems is transformed into a continuity equation and an Euler-type equation of motion in configuration space. Then, using the natural-orbital Hamiltonian, as defined by Adams, the two fluid-dynamical equations are derived in the three-dimensional space. This generates a “classical” view of such quantum systems, corresponding to an MCSCF wave function: The many-electron Schrödinger fluid consists of individual fluid components, each corresponding to a natural orbital and having its own charge density and current density. The local observables, viz., the net charge density and net current density, are obtained by merely summing over the natural orbitals, with the occupation numbers as weight factors; but, the net velocity field cannot be so obtained. Further, although each fluid component moves irrotationally in the absence of a magnetic field, the net velocity field is not irrotaional. The irrotational character of each velocity component is destroyed by rotation of the nuclear framework of the system while electron spin introduces an additional term, the spin magnetization moment, into each component current density. The physical significance of the fluid-dynamical equations as well as their advantages and disadvantages are discussed.
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    International Journal of Quantum Chemistry 22 (1982), S. 971-988 
    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 diabatic representation is introduced to describe electronic states around adiabatic pseudocrossings, as an alternative to presently available representations. A transformation from the adiabatic to the new diabatic representation is chosen to assure well behaved diabatic potentials for arbitrary momentum coupling strengths. Parameters of the transformation are determined by minimizing momentum couplings in the pseudocrossing region. The problem of two electronic states in one relative position variable is treated in detail and is studied with a numerical model. A comparison of results of our procedure and of other available ones is given. The present developments also provide a criterion for neglecting momentum couplings based on their strength and on the range of collision energies.
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    International Journal of Quantum Chemistry 22 (1982), S. 939-970 
    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 magnitude of reorganization energies in the photoelectron (PE) spectra of various transition metal compounds with Mn, Fe, and Ni as 3d center is studied by means of a variable INDO Hamiltonian. The Koopmans defects are analyzed as a function of the one-electron resonance integral βμνAB and as function of the one- and two-center electron-electron interaction integrals. βμνAB has the property of an inverse coupling constant; reorganization effects are enlarged with reduced βμνAB values. In the limit of very small resonance integrals a reduction of the calculated Koopmans defects due to modified localization properties of the orbital wave function is encountered. The two-center electron-electron interaction integrals γμνAB have been calculated via an exponential formula with a variable range parameter. In the limit of long-range potentials with flattened γ;μνAB gradients a significant reduction of relaxation and correlation is diagnozed; large defects are predicted in the short-range limit with steep gradients in the repulsion potential. The one-center Coulomb and exchange integrals (γμνAA, KμνAA) have been modified by a multiplicative factor. With enlarged one-center integrals enhanced Koopmans defects are encountered. The reorganization energies are determined by means of a Green's function approach with a renormalized approximation for the self-energy part.
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    International Journal of Quantum Chemistry 22 (1982), S. 1153-1175 
    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 the optimal wave functions obtained by means of the superposition of correlated configurations method, elaborated previously, static and dynamic polarizabilities and rigorous lower bounds to them have been calculated for the ground states of three-electron atoms in the lithium isoelectronic sequence (Li I-C IV). The results can be treated with considerable confidence, especially for higher members of the sequence. Furthermore, the calculated rigorous lower bounds for static polarizabilities enable us to rule out several theoretical and experimental values obtained previously. The results obtained by us are then employed to evaluate the leading unknown terms in the Z-1-type expansion for the polarizability, extending thereby our treatment to much higher members of the isoelectronic sequence.
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    International Journal of Quantum Chemistry 22 (1982), S. 1209-1220 
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Two small groups (E, σν) and (E, C2) are proposed to describe the orbital symmmetry of the whole reaction paths with respect to suprafacial and antarafacial sigmatropic rearrangements. Furthermore, the symmetries of the two small groups are extended to the systems as a many body treatment performed by Matsen.
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    International Journal of Quantum Chemistry 22 (1982), S. 1271-1279 
    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: There is considerable evidence indicating that the carcinogenic action of vinyl chloride involves metabolic conversion to the epoxide (chlorooxirane) as the initial step. In order to learn more about its subsequent behavior, we have computed structures, energies and other properties for two different protonated forms of the epoxide, and also for two possible rearrangement products, chloroacetaldehyde and acetyl chloride. An ab initio SCF-MO procedure (GAUSSIAN 70) was used. Oxygen protonation is found to weaken both C—O bonds, the effect being greater for the bond involving the carbon bearing the chlorine. Chlorine protonation leads to a marked weakening of the C—Cl bond; this suggests a possible loss of HCl, leaving behind a carbonium ion (and possible alkylating agent or rearrangement precursor). Thus, while C—O bond breaking is doubtless an important reaction pathway for chlorooxirane, our results indicate that attention should also be focused upon the C—Cl bond; its rupture may conceivably be a key step in the biological action of vinyl chloride.
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    International Journal of Quantum Chemistry 31 (1987), S. 1-1 
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
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    International Journal of Quantum Chemistry 31 (1987), S. 33-44 
    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 theory of MCSCF and CI energy derivatives with respect to geometrical variations is briefly reviewed with special attention given to the MCSCF and MRCI energy gradients. A computational procedure is proposed for MRCI energy gradients that does not require the solution to any “coupled-perturbed MCSCF” equations, it does not require any expensive direct-CI matrix-vector products involving derivative integrals, and it does not require any derivative integrals to be transformed from the AO basis to the MO basis. An additional feature is that it does not require any changes to existing MCSCF gradient evaluation programs in order to compute MRCI gradients. The only difference in the two cases is the exact nature of the data passed to the gradient evaluation program from the previous steps in the computational procedure. The additional effort required to compute the entire MRCI energy gradient vector is approximately that required for one additional iteration of the MRCI diagonalization procedure and for one additional MCSCF iteration. For large scale MRCI wave functions, the MRCI energy gradient evaluation should only require about 10% of the effort of computing the wave function itself. This computational procedure removes a major computational botleneck of potential energy surface evaluation.
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    International Journal of Quantum Chemistry 31 (1987), S. 65-71 
    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: Preliminary results are presented for transition probabilities in the H + H2 system derived from an adiabatic representation in terms of surface functions on hyperspheres. Special attention is given to the resonance structure for transition probabilities in the first vibrational level.
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    International Journal of Quantum Chemistry 31 (1987), S. 99-112 
    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: Low-lying excitation energies from the ground state of Be were calculated using a basis set of 61 Cartesian Gaussian functions. Three approximations were employed: the time-dependent Hartree-Fock (TDHF), second-order equations-of-motion (EOM), and multiconfigurational time-dependent Hartree-Fock (MCTDHF). The TDHF excitation energies are 0.5-1.1 eV lower than experiment, and the EOM values are 0.3-1.2 eV lower than experiment, whereas the MCTDHF excitation energies deviate on the absolute average from experiment by only 0.03 eV. We found that in an MCTDHF calculation, any proper MCSCF stationary point is a good reference (i.e., initial) state, not just the ground state. Experimental values for oscillator strength are accurately known only for the 2s2X1S → 2s2p1P0 transition. The TDHF value and the MCTDHF value agree with experiment, but the EOM value does not. The agreement of the TDHF value with experiment seems to be coincidental, because for higher lying transitions the TDHF values differ by approximately a factor of two or more from the more accurate MCTDHF. Frequency independent polarizabilities, α(0), were also calculated with the TDHF, HRPA, and MCTDHF and frequency dependent polarizabilities, β(ω), were calculated with the MCTDHF. No experimental data for Be polarizabilities exist, but we expect the MCTDHF values to be among the most accurate calculations available.
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    International Journal of Quantum Chemistry 31 (1987), S. 941-950 
    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 version of the CNDO/1 method has been outlined that includes dominant relativistic effects. The method utilizes a nonempirical parametrization based on atomic Dirac-Fock calculations. The goal of this QR-CNDO/1 method lies in its applicability to molecules consisting of arbitrary atoms (z = 1-118). Applications to molecular geometries and ionization potentials for 50 molecules are presented.
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    International Journal of Quantum Chemistry 32 (1987) 
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
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  • 39
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    International Journal of Quantum Chemistry 32 (1987), S. 13-18 
    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: According to the formulas obtained in the preceding paper, it may be used for all types of the hybridization with any set of azimuthal quantum numbers l, l = 0 through l = 5, and a complete theoretical data of bond angles and bond strengths are shown in this paper.
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  • 40
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    International Journal of Quantum Chemistry 32 (1987), S. 71-86 
    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 unitary-group formulation of quantum chemistry, the spin-projected, configuration-state spaces of quantum chemistry are realized by the irreducible representation spaces (IRS) of the freeon unitary group U(n), where n is the number of freeon orbitals. The Pauli-allowed IRS are labeled by the partitions [λ] = [2(N/2)-s, 12S], where N and S are the particle number and the spin, respectively. The generator-state approach (GSA) to the unitary-group formulation consists of (1) the construction of the overcomplete, nonorthonormal generator basis for each IRS; (2) the Lie-algebraic computation of matrix elements over generator states; (3) the Moshinsky-Nagel construction of the complete, orthonormal Gel'fand basis in terms of the generator basis; and (4) the computation of matrix elements over Gel'fand states in terms of matrix elements over generator states.
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  • 41
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    International Journal of Quantum Chemistry 32 (1987) 
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
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  • 42
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    International Journal of Quantum Chemistry 32 (1987), S. 181-191 
    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 has been shown that an easier procedure to the study of molecular electrostatic potentials than the prevalent monopole isopotential approach is to use a dipole as the mapping device: allow the dipole to move on a geometrical surface enclosing the molecule such that the distance between the surface and the nearest atoms of the molecule (closest distance of approach) is fixed, say at 2 Å, calculate the electrostatic interaction energy between the molecule and the dipole at different points of the surface, and let the dipole orient itself along the minimum energy direction and treat these minimum energies as the desired electrostatic potentials. Thus the favorable binding sites of a given molecule for other species along with their relative orientations can be obtained. This procedure has been applied to the nucleic acid bases using the necessary input data from two independent sources. Thus success of the procedure in predicting important features of molecular electrostatic potentials has been demonstrated.
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    Topics: Chemistry and Pharmacology
    Notes: A systematic examination of the components of the interaction energy, obtained with the Kitaura and Morokuma method, for nine H-bonded dimers without and with counterpoise corrections (CP) is presented. The nine dimers Hn A … HBHm correspond to all the possible combinations of HF, H2O, and NH3 as electron donors and electron acceptors. The interaction energy and the corresponding components have been computed over a sizable interval of intermolecular distances with five basis sets (STO-3G, MINI-1, 3-21G, 4-31G, 6-31G**) selected among those most extensively used to study interactions in larger systems. The CP corrections to the ΔE components have been obtained with a method, implemented in our group, which permits assignment to the pertinent components of ΔE of a physically reasonable portion of the CP correction even though different CP corrections are adopted. We examine here three versions of the CP correction, namely, the full CP correction (i.e., the original version of Boys and Bernardi) and CP corrections limited to the virtual space of the partner or to the electron donor only. The resulting data are employed to assess the basis set dependence of several models of hydrogen bonding (the electrostatic model, the semiclassical model, etc.) both with and without CP corrections.
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
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    Topics: Chemistry and Pharmacology
    Notes: A detailed analysis of the ΔE(R) curves for nine hydrogen bonded dimers involving HF, H2O, and NH3 as partners, computed with five basis sets (STO-3G, MINI-1, 3-21G, 4-31G, and 6-31G**) and subjected to counterpoise (CP) corrections with three different methods is reported. Using several criteria and tests, the positive effect of full CP corrections for the description obtained with all the basis sets (with the exception of the STO-3G one) is pointed out. The CP correction at the 6-31G** levels is still sizable and improves the results with respect to the estimated Hartree-Fock limits of ΔE(Req) and Req. The results obtained with the application of the full CP correction to the MINI-I energies are of relatively good quality. The STO-3G energies at the SCF level are, on the contrary, overcorrected by the full CP correction and slightly improved by “limited” CP corrections: this basis set, however, is not recommended for calculations of geometries and stabilization energies of H-bonded dimers of this size.
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    International Journal of Quantum Chemistry 32 (1987), S. 295-312 
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: An accurate analytical electron density for the beryllium atom is obtained by using a fast and systematic method recently developed and tested for the neon atom. Asymptotic conditions both at the nucleus and at large distances are obeyed. A point-by-point comparison between our density and the one obtained from an almost “exact” configuration interaction wave function shows that differences are less than 0.5% for r between 0 and 5 bohrs and less than 1 % up to 9 bohrs. The accuracy of the density is also assessed by comparing results of density moments and x-ray scattering factors.
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    International Journal of Quantum Chemistry 32 (1987), S. 361-375 
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We discuss a procedure for calculating numerical Hartree-Fock orbitals that can be applied to polyatomic systems. This approach is formulated in momentum space to avoid Coulomb singularities and uses fast Fourier transforms to solve integral convolutions. Results for a number of simple systems are presented.
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    International Journal of Quantum Chemistry 31 (1987), S. 405-415 
    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: Binding of Zn2+ to imidazole (Im) and methyl imidazole (MeIm) is studied by ab initio methods as a model for the effect of cation binding on tautomeric energies. Gradient energy optimized conformations were obtained for all neutral and ionic structures, including the deprotonated molecules and the ylides. The N3—H tautomer of MeIm is calculated to be more stable than N1—H by about 1 kcal/mole. However, binding of a Zn2+ cation to the available nitrogen site is found to reverse the order of binding, leaving N1—H more stable by 1 kcal/mole. Binding of Zn2+ produces a significant perturbation in the electronic structure, a smaller shift in the equilibrium conformation of the imidazole ring, and only a small absolute shift in the relative tautomer energies. Methyl substitution at C5 has a small effect on both conformation and energetics.A high-energy ylide tautomer is produced by moving the proton bound to C2 to the N1 atom. The binding of Zn2+ to the C2 site is substantially stronger than to the N1 site, yielding nearly isoenergetic ZnIm2+ conformations for binding to either N or C atoms. For the deprotonated salts the lowest energy conformation has the C2—N3 bond bridged by Zn2+.
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    International Journal of Quantum Chemistry 31 (1987), S. 425-427 
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    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 made clear that two different statements in the literature concerning energy derivatives are completely compatible by deriving them as two different interpretations of the same equation. Some other aspects of these results are also discussed.
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    International Journal of Quantum Chemistry 31 (1987), S. 435-443 
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Accurate methods for computing energies and electronic properties of atoms and molecules have been derived from direct treatment of localized pairs of electrons. The conceptual development and implementation of such methods is reviewed.
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    International Journal of Quantum Chemistry 32 (1987), S. 473-489 
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The rigorous derivation of the energy density functional is proposed within the framework of the spinfree, or spin-restricted formulation of the energy density functional theory. It is shown particularly that the kinetic energy density functional is given by a sum of the Weizsacker term and the so-called “modified” Thomas-Fermi one. The variational principle is formulated for the energy density functional theory in terms of the Euler-Lagrange equation, and the virial theorem is proposed.
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    International Journal of Quantum Chemistry 32 (1987), S. 591-594 
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The time-dependent Schrouml;dinger equation for the oscillator H = α(t)p2 + β(t)x2 + γ(t)x is exactly solved. The time evolution operator is easily obtained by means of an operator algebra and the quantum-mechanical equations of motion. The problem is reduced to solving the classical equations of motion. The method is shown to apply to multidimensional oscillators.
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    International Journal of Quantum Chemistry 31 (1987), S. 445-453 
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A modified Bohr orbit procedure is used to calculate the energies for the 1S ground state and the 2P, 3D, and 4F excited states of the helium atom. The energies are calculated from \documentclass{article}\pagestyle{empty}\begin{document}$ {{\int\limits_0^\pi {E\left(\Phi \right)P\left(\Phi \right)d\Phi } } \mathord{\left/ {\vphantom {{\int\limits_0^\pi {E\left(\Phi \right)P\left(\Phi \right)d\Phi }} {\int\limits_0^\pi {P\left(\Phi \right)d\Phi } }}} \right. \kern-\nulldelimiterspace} {\int\limits_0^\pi {P\left(\Phi \right)d\Phi } }} $\end{document}, in which E(φ) is the Bohr orbit energy for angle φ between the position vectors \documentclass{article}\pagestyle{empty}\begin{document}$ \vec r_1 $\end{document} r1 and \documentclass{article}\pagestyle{empty}\begin{document}$ \vec r_2 $\end{document} r2, and P(φ) is a probability function for this angle. Numerical procedures are used to evaluate the integrals. Energies that range between -2.9082 and -2.9054 au are calculated for the 1S state (cf. -2.9037 au, exact). The Bohr energies for the excited states are -2.1318, -2.1240, -2.0562, -2.0555, -2.0314, and -2.0312 au, which are generally close to the exact energies of -2.1332, -2.1239, -2.0557, -2.0557, -2.0313, and -2.0313 au for the 23P, 21P, 33D, 31D, 43F and 41F states. Some relationships that exist between the Bohr theory and the Schrödinger local energies are discussed. Approximate Bohr orbit estimates for the energies of the 2P states of He, Li+,…,Ne8+ are reported.The invariance of the two-electron Bohr hamiltonian with respect to the interchange of the electron coordinates leads to two classical probability functions when the orbit quantum numbers for the two electrons differ.
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    International Journal of Quantum Chemistry 31 (1987), S. 463-470 
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Valence-bond calculations are reported for the isoelectronic series of molecules and ions: N2, CO, BF, NO+ and CN-. The most important structures are N≡N, C=O, Bπ—F, N+=O and C=N. Hybridization of the 2s and 2p orbitals is important. Only two or three structures are required to obtain an energy lower than that obtained with the molecular orbital approximation. Structures in which the electronegative element loses a σ-orbital or gains a π-orbital are favored. π-bonds tend to be favored over σ-bonds. The bond in NO+ resembles that in CO, whereas that in CN- resembles the bonding in N2.
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    International Journal of Quantum Chemistry 31 (1987), S. 471-487 
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Ab initio SCF, CI, CEPA, and MCSCF techniques are compared and contrasted in a theoretical study of the 1s core ionized Li2 molecule from the point of view of core hole localization. In agreement with earlier studies of symmetric core ionized molecules, SCF theory is found to give a physically reasonable description of core ionization only when symmetry breaking is allowed. This results in a dramatic lowering of the energy of the core ionized state and hence of the ionization potential. By extension, CI wave functions that are developed in terms of a broken symmetry SCF reference plus its single and double excitations are found to perform significantly better than those constructed from symmetry adapted SCF orbitals. Alternatively, if the full point group symmetry is to be retained, a multiconfigurational treatment is called for and, in agreement with the conclusions of an analogous study on O2 [H. Ågren, P.S. Bagus, and B.O. Roos, Chem. Phys. Letters 82, 505(1981)], it is found that a modestly sized MCSCF wave function is capable of accounting for the energetic effects of symmetry breaking. Potential energy curves for Li2 and several states of core ionized Li2 have been calculated, allowing predictions of the adiabatic ionization potential and vibrational, satellite, and Auger structures in the photoelectron spectrum of Li2 to be made.
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    International Journal of Quantum Chemistry 31 (1987), S. 489-505 
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A practical algorithm is described for generating a set of symmetry- and spin-adapted antisymmetrized products of molecular orbitals (SAAPs) which form an orthogonal basis for a full active configuration space. The spin-adaptation is completely general. The space-symmetry adaptation is accomplished for the groups C∞v and D∞v.
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    International Journal of Quantum Chemistry 32 (1987), S. 685-697 
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A complete description of the rotational isomerism of monothioformic acid is presented. Ab initio results concerning the barrier heights and the energy difference between the stable isomers are in good agreement with the experimental data. A reduced potential function deduced from our calculations has been used to characterize the kinetics and thermodynamics of the rotational isomerization. The reported ab initio values of equilibrium constants are in good agreement with the experimental ones. From the analysis of the available experimental data, we have found that the free energy and enthalpy behavior are quite similar and that no noticeable entropy contributions are involved in the internal rotation, in agreement with previous studies on unimolecular isomerization processes.
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    International Journal of Quantum Chemistry 32 (1987), S. 723-723 
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
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    International Journal of Quantum Chemistry 32 (1987), S. 1-1 
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
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    International Journal of Quantum Chemistry 32 (1987), S. 85-103 
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The equilibrium geometry, stabilization energy, and electric polarizability of formic acid, formamide, and the three possible cyclic hydrogen-bonded pairs are obtained by ab initio calculations using the STO-3G, 4-31G, and 6-31G** bases. These three properties are found to be very much dependent on the basis set extension. The polarizability of the dimers is found to be basically additive in contribution from the monomeric moieties.
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    International Journal of Quantum Chemistry 32 (1987), S. 133-147 
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    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 course of conformational motions of molecules the changes in shapes of electronic charge distributions follow that of the nuclear framework. However, this coupling between the changes in the nuclear geometry and electron density may depend on the actual nuclear displacement; the coupling may be weak or strong for a given conformational motion. It is of some interest to analyze how faithfully the charge density variations follow the nuclear displacements in a family of conformational rearrangements. In certain cases small conformational changes may induce large changes in the shape of charge density distributions, while in other cases large and qualitatively important conformational changes may involve qualitatively inessential distortions in the shape of electron distributions. In this article we describe a new classification of conformations based on those domains of nuclear configuration space within which the „shape groups“ (symmetry independent homology groups) of the electric charge density remain invariant. Such an analysis might be valuable when seeking correlations between molecular structure and certain biological or biochemical activities.
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    International Journal of Quantum Chemistry 32 (1987), S. 211-219 
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Practical aspects of the calculation of the proton transfer process in a model of the active site of the thiol protease papain are explored with basis sets of different sizes. Results from ab initio calculations with the STO-3G, 4-31G, 6-31G, 6-31G* basis set, and a 6-31G basis set augmented with polarization functions on the sulfur atom are compared for their performance in describing the proton transfer energy. The nature of the convergence of the calculated properties of the potential curve for proton transfer with the increase in basis set indicates the need for a split-valence basis set and for polarization functions on the sulfur in order to achieve an appropriate description of this system. Correlation corrections to the calculated energies are shown to contribute significantly to the characteristics of the proton transfer energy curve.
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    International Journal of Quantum Chemistry 32 (1987), S. 281-288 
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The scope of multistep modeling (MSM) is expanded by adding a least-squares minimization step in the procedure to fit backbone reconstruction consistent with a set of C-alpha coordinates. The analytical solution of Phi and Psi angles, that fits a C-alpha x-ray coordinate [1] is used for tyr-tRNA synthetase. Phi and Psi angles for the region where the above mentioned method fails, are obtained by minimizing the difference in C-alpha distances between the computed model and the crystal structure in a least-squares sense. We present a stepwise application of this part of MSM to the determination of the complete backbone geometry of the 321 N terminal residues of tyrosine tRNA synthetase to a root mean square deviation of 0.47 Å from the crystallographic C-alpha coordinates.
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    International Journal of Quantum Chemistry 32 (1987), S. 325-330 
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We have examined an application of the weighted identification number in the QSAR study of the toxicity of aliphatic ethers on mice. The results obtained are superior to those achieved by the connectivity index.
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    International Journal of Quantum Chemistry 32 (1987) 
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    International Journal of Quantum Chemistry 32 (1987), S. 13-14 
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
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    International Journal of Quantum Chemistry 32 (1987), S. 89-97 
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Ab initio self-consistent-field electronic state calculations have been carried out for an interstitial-iron-substitutional-boron impurity pair complex in silicon. The calculations do not provide support for the currently accepted microscopic ionic model for the pair. Rather, we have shown that the covalent effects play a basic role in determining the physical properties of the complex.
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    International Journal of Quantum Chemistry 32 (1987), S. 115-129 
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
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    Topics: Chemistry and Pharmacology
    Notes: By analyzing the convergence properties of the lattice sums in the exchange part of the restricted Hartree-Fock orbital energy, we isolate the source of the nonanalytic behavior of a partially occupied band at the Fermi energy. This analysis shows how an extended system behaves qualitatively differently from a finite system but also provides a possibility of following the development of nonanalyticity as the size of the system grows.
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    International Journal of Quantum Chemistry 32 (1987), S. 163-170 
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
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    Topics: Chemistry and Pharmacology
    Notes: The aromatic (A) and quinoid (Q) forms of polythiophene (PT) have entirely different energy gaps: ∼0.5 eV for the quinoid form and ∼2 eV for the aromatic form, respectively. The energy gaps and stability of derivatives of PT are studied by total MNDO geometry optimization using energy band theory for the total energy calculations followed by a Hückel energy band calculations to approximate the energy band structure. Addition of fused rings to PT reverses the order of stability of the aromatic and quinoid isomers and the ordering in the size of the energy gaps. Small energy gap polymers are suggested on the basis of the calculations.
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    International Journal of Quantum Chemistry 32 (1987), S. 191-198 
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
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    Topics: Chemistry and Pharmacology
    Notes: Chemical reactions and conformational changes of N-atom systems can be described as displacements in a (3N-6)-dimensional metric configuration space M provided with a global metric. Although space M has a metric, it is not in general a vector space; it is a topological space. In contrast to the commonly used internal configuration spaces based on bond length/bond angle internal coordinates, and having no global metrics, within space M each internal configuration of the nuclei of the molecule corresponds to one and only one point of the space. This property of M is advantageous when analyzing chemical reactions. The global metric of M ensures that differences between any two internal configurations can be interpreted as a distance in this space that allows one to provide M with coordinate systems by turning M into a manifold with boundary. Certain formal reaction paths show some counterintuitive behavior within this space: they may undergo a formal reflection at some points of M. A condition, the tangent criterion, is used for the diagnosis of such reaction paths and for the determination of special nuclear configurations where such reflections occur.
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    International Journal of Quantum Chemistry 31 (1987), S. 649-662 
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Energies and geometries for a number of small hydrogen bonded dimers have been calculated by semiempirical method based on a perturbation approach. Results have been compared with experimental and the best theoretical data. A quite good description of equilibrium configurations has been obtained in every case when local multipoles from sufficiently accurate wave functions have been used. Hydrogen bond lengths have not been predicted with sufficient precision. Results indicate, however, that it should be possible to achieve improvement in the framework of the applied calculation scheme by modification of the parameter values.
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    International Journal of Quantum Chemistry 32 (1987), S. 283-295 
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We examine the photoaddition of hydroxylazoaromatic compounds and related thione analogs with olefins. By examining the properties of the lowest lying singlet and triplet states, we conclude that the product distribution is best described by the unpaired spin density of the triplet state. This suggests that absorption of light by the aromatic is followed by conversion into the triplet, and this triplet is the active precursor. Since unrestricted molecular orbital calculations are easily performed on the lowest triplet of these aromatics, prediction of possible products is straightforward.
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    International Journal of Quantum Chemistry 32 (1987), S. 267-282 
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
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    Topics: Chemistry and Pharmacology
    Notes: Hydrogen addition to acetylene has been studied by molecular orbital methods with the unrestricted Hartree-Fock (HF)approach and with Møller-Plesset (MP) perturbation theory up to fourth order using the 6-31G*, 6-31G**, and 6-311G** basis sets. For each basis set a number of points along an approximate reaction path were calculated by fixing the attacking H—C distance and optimizing all of the remaining parameters. At these geometries, single point MP4 calculations with and without spin projection were carried out, yielding the position, height, and shape of the addition barrier at the various levels of calculation with each of the three basis sets. The results of this approach were confirmed by a grid search of the region near the transition state using the 6-31G* basis set. The unprojected MP2, MP3, and MP4 barriers are too narrow and are 10-20 kcal/mol too high. With spin projection, the barriers are much lower and broader. At the PMP4SDTQ/6-311G** level, with zero point energy calculated at UHF/6-31G*, the vibrationally adiabatic barrier at 0 K is 4.0 kcal/mol, and the attacking H—C distance is 2.0 Å.
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    International Journal of Quantum Chemistry 32 (1987), S. 313-319 
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
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    Topics: Chemistry and Pharmacology
    Notes: We explore the implications of static disorder on electron transfer (ET) in a microscopically inhomogeneous polar medium, where the environment of each solute is characterized by a single dielectric relaxation time, τ, with the distribution function of τ being related to the complex dielectric susceptibility. For the interesting situation of solvent-controlled ET, the decay of the population probability is nonexponential, which can be expressed in terms of a cumulant expansion. The microscopic decay function can be related to the macroscopic step-response function of the dielectric medium.
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    International Journal of Quantum Chemistry 32 (1987), S. 355-362 
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
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    Topics: Chemistry and Pharmacology
    Notes: A self-consistent model capable of describing the interaction of a metal electrode/liquid electrolyte interface is presented. As in a real system, a capacity maximum shifted towards positive charges is obtained from the model. The metal is represented by the planar uniform background model (jellium). The solvent is described by a polar liquid whose response to charge is accounted for in terms of the local approximation. The interaction between the metal and ions is modelled by a charged plane placed near the metal surface. Charge neutrality of the whole system is preserved. The present model is a good approximation for highly concentrated solutions.
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    International Journal of Quantum Chemistry 31 (1987), S. 733-737 
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
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    Topics: Chemistry and Pharmacology
    Notes: In this paper we outline briefly how microlocal analysis can be applied to give a general approach to the mathematical theory of resonances in the semiclassical limit. We also describe recent results about the asymptotic behavior of resonances generated by closed trajectories and stationary points in the classical flow.
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    International Journal of Quantum Chemistry 31 (1987), S. 739-746 
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
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    Topics: Chemistry and Pharmacology
    Notes: In this paper we make explicit constructions of a family Aθ of intertwining operators between the Schrödinger operator Hv = -Δ + v(x) and H0 = -Δ. The wave operators and the scattering matrix are expressed in terms of the Aθ, and some methods of inverse scattering are described.
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    International Journal of Quantum Chemistry 31 (1987), S. 707-731 
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
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    Topics: Chemistry and Pharmacology
    Notes: Examples from music and nuclear, as well as atomic and molecular, physics are given to introduce and illustrate the resonance concept. Some fundamental concepts of scattering theory such as the differential and the total cross section are presented. The concept of the collision complex is illustrated with a light particle scattering reaction in nuclear physics. The concept of channels is introduced, and this formalism (which is so far empirical in nature) is dressed in the language of quantum mechanics. Finally I show that our descriptions of phenomena in nuclear physics can also be used in atomic and molecular physics.
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    Notes: The analytic energy gradient method that we have recently implemented for CCSD wave functions is applied to fully optimize the pyramidal C3v and planar D3h structures of ammonia. Using a double-zeta plus polarization basis set, results for harmonic vibrational frequencies, infrared intensities, and dipole moments have been obtained. Comparison with different levels of truncated configuration interaction suggests that CCSD values are of better than CISD quality and generally closer to CISDTQ results.
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    International Journal of Quantum Chemistry 32 (1987), S. 539-545 
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
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    Topics: Chemistry and Pharmacology
    Notes: Effective core potentials are utilized in conjunction with polarization propagator calculations of excited state properties. The propagator method employed is based on an antisymmetrized geminal power wave function as the reference state. Calculations are presented on the low-lying excited states of HF and NaH. All-electron and valence-electron results are compared for HF. The choice of basis set is found to be very important for obtaining good results.
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    International Journal of Quantum Chemistry 32 (1987), S. 569-572 
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
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    Topics: Chemistry and Pharmacology
    Notes: The nonadiabatic energy correction term of the hydrogen molecule at large separation is discussed to determine whether or not the nuclear motion induces the inter-atomic interaction potential. Nonadiabatic vibrational motion of nuclei induces the internal charge polarization of atoms to give the novel correction term to the London R-6 force. The correction term is (4/M) (1/R3), which is important at large R despite the overwhelming magnitude of nuclear mass.
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    International Journal of Quantum Chemistry 31 (1987), S. 755-794 
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
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    Topics: Chemistry and Pharmacology
    Notes: As an extension of the recent atomic work of Krylstedt et al. [1, 2], we propose the synthesis of the static exchange plus polarization model within the multiple scattering Xα formalism and the exterior complex rotation method for the study of shape resonances occurring in molecular collisional systems. The bound state multiple scattering Xα method is reviewed with special attention to the form of the various wave functions for the different molecular regions. In connection with the inclusion of continuum electron exchange into the scattering model, we analyze a possible solution to the problem of asymptotical behavior caused by the local density-based free electron gas approximation used in this case. We also propose a method to fix the cutoff parameter, inherent in the polarization potential, for symmetry-related molecular participants in the scattering process, thereby obtaining a possible predictive power for the one-parameter scattering model.Titchmarsh-Weyl's theory is used to formally connect the above formulation to scattering theory. The theoretical difficulties in obtaining a unique meromorphic continuation of the S matrix are investigated in connection with the requirements on the actual potential to be exterior dilation analytic. Furthermore, the occurrence of asymptotic quantum numbers is noted and discussed in conjunction with muffin tin-like approximations and related exterior complex rotated schemes.It is found that the electron exchange part of the nonstatic one-body potential exhibits a functional form that is not exterior dilation analytic, albeit the general electron and nuclear many-body problem involving Coulomb forces are dilation analytic. Although the immediate consequences indicate a rotation angle (θ) dependence on the S matrix continuation, a uniquely defined assignment of the cutoff parameter r0 = r0(θ) makes the present nonstatic model “exterior dilation analytic” in the sense that it mimics the dilation analytic structure of the full many-body problem at the same time obeying “asymptotic” spherical symmetry via the associated constant of evolution. However, in the static exchange approximation the above difficulties are shown to be circumvented via a certain reformulation, leading to a regular analytic asymptotic form for the interaction potential.
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    International Journal of Quantum Chemistry 31 (1987) 
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    International Journal of Quantum Chemistry 31 (1987), S. 841-845 
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
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    Topics: Chemistry and Pharmacology
    Notes: In the present article we show how the bound states of the Coulomb potential may be associated with resonances that occur when this potential is perturbed by a barrier potential. The main idea is to trace the bound states on successive switching on of the barrier perturbation. It is found that those bound states that are localized inside the barrier are highly sensitive to variation with respect to the barrier height, whereas those that are localized outside are less sensitive. However, there are certain intervals for the barrier height when the role of being “a state localized inside the barrier” is shifted from one bound state to another. The result can be pictured as a “relay race,” where the “deliveries of the baton” are carried out over corresponding avoided crossings. The baton is ultimately handed over to a shape-type resonance state.
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    International Journal of Quantum Chemistry 31 (1987), S. 847-853 
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Methyl substitutents located peri to bay-region benzo rings of polycyclic aromatic hydrocarbons generally reduce or eliminate carcinogenic activity, most likely because such substituents force the hydroxyl groups of dihydrodiol intermediates to adopt preferentially pseudodiaxial, rather than pseudodiequatorial, conformations. Empirical force field calculations have been employed to examine the influences of peri and other positional substituents on hydroxyl conformation, taking naphthalene, anthracene, and phenanthrene as model compounds. For the unsubstituted naphthalene and anthracene dihydrodiols, the pseudodiequatorial conformation is preferred. This preference is reduced by adjacent methyl substitution and reversed by peri methyl substitution. Because of steric crowding the phenanthrene bay-region dihydrodiol preferentially adopts a pseudodiaxial conformation. The results are discussed in relation to recent experimental evidence.
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    International Journal of Quantum Chemistry 31 (1987), S. 855-869 
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The electronic absorption spectra of some phenylethylamine drugs, namely, d-pseudoepherine, l-pseudoephedrine, l-ephedrine, dl-ephedrine dl-norephedrine, phenylethylamine, methoxyphenamine, and l-noradrenaline were investigated in polar and nonpolar solvents. The observed transitions were interpreted, and the role of σ-π interaction was explored. Molecular orbital calculations were performed on representatives of the above group of compounds, namely, d-pseudoephedrine, l-ephedrine, and l-noradrenaline using the INDO procedures and adopting the best conformer of the molecule. The transition energy, band intensity, and dipole moments were calculated and corresponded satisfactorily with the experimental values.
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    International Journal of Quantum Chemistry 31 (1987), S. 893-901 
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
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    Topics: Chemistry and Pharmacology
    Notes: The analytic expressions of the integral prototypes involving both Slater and s-type Gaussian functions, explicity derived in Ref. 1, are generalized to the case of higher order modified Gaussian functions [2].
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    Notes: By expanding the wave function of a system of N particles in terms of products of functions of one and (N-1) particles, the one-particle, nonlocal operator F̂EKT (extended Koopmans' theorem) is determined. It is shown that although this operator is nonhermitian, its eigenvalues and eigenfunctions represent the ionization energies and occupied orbitals, respectively. The eigenfunctions of F̂EKT are the one-particle functions that enter into the expansion of the wave function of the system as partners of the (N-1)-particle wave functions. The eingenvalues are also one-particle energies that, multipled by the orbital occupancy probalities, enter the expression for the total N-particle energy of the system.
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    International Journal of Quantum Chemistry 31 (1987), S. 927-939 
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We apply completely transferable, strictly localized molecular orbitals for the calculation of molecular electrostatic fields. This approach, derived from our previous bond fragment method for the calculation of molecular electrostatic potentials, reduces computational efforts drastically. The fields around small molecules containing first- and second-row atoms are systematically overestimated as compared with ab initio calculations with a minimal STO-3G basis set. However, deviations can be corrected by a simple multiplicative factor, which means that the overall shape of the potential and field around the molecule is correctly reproduced. Our approximate field can be used to determine possible hydration sites around molecules as proposed earlier by Peinel and coworkers. Application of the method is illustrated on the formamide molecule.
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    International Journal of Quantum Chemistry 32 (1987), S. 399-399 
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
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    International Journal of Quantum Chemistry 32 (1987), S. 403-403 
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
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    International Journal of Quantum Chemistry 32 (1987), S. 427-434 
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
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    Topics: Chemistry and Pharmacology
    Notes: The quality measure δ of Sordo and Pueyo is computed for 94 orbital basis sets for the atomic helium Hartree-Fock problem and compared with various information theoretic quality measures reported previously by us for this data base. δ is in error often enough that it is unlikely to be a useful general purpose measure.
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    International Journal of Quantum Chemistry 32 (1987), S. 457-466 
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
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    Topics: Chemistry and Pharmacology
    Notes: The Sturmian transformation is reviewed and some of its properties are discussed, in particular, its use in perturbative approaches, its connection with variational schemes, and its use for the study of resonances. One potential problem (Doolen) having no bound states is described exactly.
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    International Journal of Quantum Chemistry 32 (1987), S. 27-35 
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
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    Topics: Chemistry and Pharmacology
    Notes: Basis set and correlation effects on computed hydrogen bond energies of the negative ion complexes AHn · AHn-1-1, for AHn = NH3, OH2, and FH, have been evaluated. The addition of diffuse functions on nonhydrogen atoms to valence double- and triple-split plus polarization basis sets [6-31G(d, p) and 6-311G(d, p)] significantly decreases binding energies by 9-19 kcal/mol, depending on the particular complex and the level of theory. Adding diffuse functions to hydrogens has a negligible effect, while replacing the single set of polarization functions on each atom by two sets alters energies by 1 kcal/mol or less. Electron correlation increases the hydrogen bond energies of these complexes and has a greater effect for basis sets without diffuse functions. Since the Hartee-Fock energies computed with these basis sets are already too large, correlation energy calculations should not be performed in these cases. For basis sets including diffuse functions, the correlation energy contribution to the binding energies of these complexes is significant, with the Møller-Plesset second-order term being the largest term and having a stabilizing effect of from 3-6 kcal/mol. The third and fourth order terms are smaller, and may be of opposite sign. As a result, the MP2 and MP4 energies differ by no more than 1 kcal/mol, with the MP2 stabilization energy being greater except for N2H5-1. The computed standard solvation enthalpy of OH-1 by H2O based on either MP4/6-311 + G(2d, 2p) or MP2/6-31 + G(d, p) electronic energies is -26.8 kcal/mol, in excellent agreement with a recent gas-phase experimental measurement.
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    International Journal of Quantum Chemistry 32 (1987), S. 105-110 
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A new definition of molecular similarity in terms of electron density is proposed and a method for calculating similarity based on molecular electrostatic potential and molecular electric field introduced. It is applied to some simple isosteres.
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    International Journal of Quantum Chemistry 32 (1987), S. 127-132 
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    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Most of the important conformations of biomolecules possess only trivial symmetry. Consequently, symmetry groups have no roles in the characterization of the shapes of such conformations. However, an alternative group theoretical model, based on homology groups of algebraic topology, provides a detailed description of shapes for all conformations. These shape groups are useful for precise comparison of molecular shapes and are proposed as a computational tool for QSAR. A new computational method for the determination of various shape groups is described which is suitable for the simultaneous analysis of a pair or a family of molecular properties. In this note a general method is described and applied to the shape of electronic charge distribution along van der Waals surfaces.
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  • 96
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 32 (1987), S. 189-209 
    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: Studies on the proton transfers in water clusters (H2O)n (n = 2, 3, 4, 5, 6), and in DNA base pairs, adenine-thymine base pair, and guanine-cytosine base pair, have been done by using the potential functions of polarization model for water and ab initio SCF method with STO-3G basis set for base pairs, respectively.
    Additional Material: 12 Ill.
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  • 97
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 32 (1987), S. 261-280 
    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: Briefly noting earlier studies on the polypentapeptide of elastin, (Val1-Pro2-Gly3-Val4-Gly5)n, and on elastin, it is emphasized that entropic elastomeric force can be exhibited by nonrandom, anisotropic polypeptide systems and therefore that entropic elastomeric force does not necessarily result from isotropic random chain networks as required by the classical theory of rubber elasticity, nor does it result from solvent entropy effects as deduced from the slow loss of elastomeric force on thermal denaturation. Instead entropic protein elasticity can be the result of internal chain dynamics, specifically of librational processes that become damped on chain extension. This new mechanism of entropic protein elasticity allows for an understanding not only of elastin but also of the passive tension of striated muscle, of the voltage-dependent interconversion between open and closed conductance states in the sodium channel of squid nerve, and of protein elastic forces producing strain in a substrate bond during enzyme catalysis. Because entropic elastomeric force develops as a result of an inverse temperature transition, it becomes possible to shift the temperature of the transition to higher or lower temperatures by decreasing or increasing, respectively, the hydrophobicity of the elastomeric polypeptide chain. In warm-blooded animals this allows for biochemical modulation of the relaxation or development of entropic elastomeric force by an enzymatically modulated decrease or increase of the hydrophobicity, as for example, by phosphorylation or dephosphorylation of the elastomeric polypeptide chain. This understanding provides a mechanism for modulating protein function, whether for example enzymatic or channel, a mechanism for the remarkable reversible structural processes that attend parturition, and a mechanism for the connective tissue anomalies of wound repair and enviromentally induced lung disease.
    Additional Material: 9 Ill.
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  • 98
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 32 (1987), S. 297-315 
    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 have recently discovered a new entirely unexpected, and highly selective protein - ligand interaction. This new kind of molecular interaction was recognized by chromatography of proteins on divinylsulfonated agarose gels which had been deactivated using mercaptoethanol. The essential structure of the interacting immobilized ligand is quite simple and nonionic. It can be generally represented by: \documentclass{article}\pagestyle{empty}\begin{document}$$\hbox{agarose-O-CH}_2 \hbox{CH}_2\hbox{-SO}_2\hbox{-CH}_2\hbox{-X-}$$\end{document} where X was first a thioether but can also be N, O(S 〉 N ≫ O) or possibly any atom with at least one lone electron pair. We have provisionally termed peptides and proteins interacting with this ligand „thiophilic,“ in recognition of their affinity for the definitive thioether sulfone constituents. The thiophilic adsorption process is promoted by water-structuring or „antichaotropic“ salts such as sulfates or phosphates and would appear to be entropically driven. The thermodynamics of such a process are discussed relative to protein recognition of the immobilized thioether-sulfone ligand. We do not yet know the precise mechanism for the interaction but we believe that salt allows the protein into close contact with the immobilized thioether-sulfone group where short-range forces are likely to be important. Evidence suggests that aromatic side chains on the protein-binding site may be involved and we therefore expect that some kind of electron-donor-acceptor or proton-acceptor mechanism is likely involved. Two important applications of thiophilic adsorption are the selective immobilization of functional antibodies as well as purification of immunoglobulins from serem, ascites fluid, and hybridoma cell culture media. Monoclonal antibodies can be purified in one step under extremely mild (structure-stabilizing) conditions. We therefore consider the further characterization of thiophilic adsorption of major significance in the fields of immunology and biotechnology and hope that this presentation will inspire attempts to explain the interaction in terms of quantum chemistry.
    Additional Material: 9 Ill.
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  • 99
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 32 (1987), S. 347-349 
    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
    Additional Material: 2 Tab.
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  • 100
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 32 (1987), S. 15-29 
    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 attempt is made to analyze the present state of first-principles methods used in solid state theory to elucidate where these methods derive their strength and why they may be expected to gain even more predictive power in the near future. The latter will depend on the success of recently contemplated improvements. As regards the ground state properties of solids, there are two - in principle -  rigorous ways of treating the associated N-electron problem. One is based on the Hohenberg-KohnSham (HKS) theory, which maps the electronic pair correlation onto a strictly local, energy-independent potential that occurs in the associated one-particle equations. The alternative approach, which will not be discussed in this paper, is connected with determining quasi-particle amplitudes from the Dyson equation, where the information on the electronic pair correlation is absorbed into a nonlocal energy-dependent self-energy operator.Distinctly different from HKS theory, which only applies to ground states, the quasi-particle scheme lends itself to describe excitations (e.g., interband transitions) just as well. We shall furthermore discuss a generalization of the HKS theory that explains why naive extensions of this approach to excited states have been relatively successful in a variety of cases.
    Additional Material: 2 Ill.
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