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  • Computational Chemistry and Molecular Modeling  (24)
  • Wiley-Blackwell  (24)
  • 2020-2020
  • 1980-1984  (24)
  • 1983  (24)
  • 11
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 24 (1983), S. 651-685 
    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 molecular density required to give the correct values for one-electron properties is rarely given by wave functions obtained from variation methods based on the total energy or the eigenvalues. Perhaps if we knew how the density should be shaped in any particular volume to fit a particular property, the whole molecular density might then be properly described to fit the whole volume. The secant-parametrization procedure is used to constrain minimum basis set RHF wave functions for N2, FH, CO, and LiH to determine the effects of different constraints on RHF wave functions, and to see how constraints improve the quality of small basis set RHF wave functions. One-electron property expectation values, energies, and unweighted and property weighted populations and electron density difference profiles are used to analyze the constrained wavefunctions. With the information from the constrained wave functions it should be possible to select a LCAO-CI basis and states to give the correct density for all properties. This should map onto the constrained wave function in the region of the constraint and at the same time minimize the energy of the total molecular wave function. Such a density would be suitable for the density analyses favored by Bader and Nguyen-Dang [Adv. Quantum Chem. 14, 113 (1981)], Mezey [Theor. Chim. Acta 54, 95 (1980); 58, 309 (1981); 59, 321 (1981)], and March [Theoretical Chemistry (Royal Society of Chemistry, London, 1981), Vol. 4, p. 158], and show the real atom needed to generate the molecule.
    Additional Material: 3 Ill.
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  • 12
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 23 (1983), S. 567-575 
    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: Different methods proposed for taking into account the momentum transfer in electron capture collisions are reviewed. Exact one-electron two-Coulomb-center molecular wave functions are considered for studying fully stripped ions-hydrogen atom collisions. The semiclassical impact parameter treatment is used, since it is well adapted to the keV/amu energy range investigated.
    Additional Material: 4 Ill.
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  • 13
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 23 (1983), S. 437-446 
    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: Ab initio electronic structure calculations are reported for five electronic states of the methylene amidogen radical. Structure parameters for the ground electronic state are predicted by RHF and D-MBPT (4) calculations. Vertical excitation energies were determined using four different theoretical chemical models: complete active space (CAS) MCSCF, CAS/MCSCF plus singles and doubles Cl, fourth-order many-body perturbation theory SDQ-MBPT(4), and coupled-cluster theory.
    Additional Material: 1 Ill.
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  • 14
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 23 (1983), S. 739-751 
    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: Proton transfers in a number of systems are investigated using ab initio molecular orbital methods. Calculations are carried out with several different basis sets ranging in size from 4-31G to 6-311G**. Electron correlation is included using Møller-Plesset (MP) perturbation theory to second and third orders. Enlargements of the basis set invariably lead to higher energy barriers to proton transfer, while substantial reductions result from inclusion of correlation effects. Application to (HOHOH)- of third-order MP theory with a triple-valence basis set augmented by polarization functions on oxygens and the central proton, denoted MP3/6-311G*(*), leads to excellent agreement with the results of Roos et al. whose calculations involved an extensive CI treatment with a large basis set. For equivalent hydrogen bond lengths, the transfer barrier in the cation (H2OHOH2)+ is nearly identical to that for the (HOHOH)- anion while the barrier in (H3NHNH3)+ is somewhat smaller. The reduction of the SCF barrier height resulting from inclusion of correlation is greater for (O2H3)- than for the above cations. The lowest energy structure of (O2H5)+ contains a symmetric hydrogen bond in which the proton is located midway between the two oxygens whereas asymmetric H bonds are found in the equilibrium geometries of (N2H7)+ and (S2H5)+. The difference in energy between the symmetric and asymmetric configurations of (O2H3)- is extremely small.
    Additional Material: 2 Ill.
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  • 15
    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 coupled-cluster single and double excitation model (CCSD) is applied to an energy path for the insertion of Be into H2 and compared to the full configuration interaction (FCI) and full valence-multiconfiguration self-consistent field (FV-MCSCF) results. This model problem is a severe test of a single-reference-function correlated method since two configurations are heavily weighted in the FCI description. CCSD is demonstrated to describe the FCI results using a single reference function which, however, changes orbital characteristics along the sampling path. In this case CCSD gives excellent agreement with the FCI results.
    Additional Material: 3 Ill.
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  • 16
    Electronic Resource
    Electronic Resource
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 23 (1983), S. 1191-1199 
    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: Collective modes of superfluid 3He B have been studied by means of sound attenuation and sound velocity studies. A new sound attenuation peak has been observed in superfluid 3He B at ħω ≈ Δ(T). This peak has been identified with the real squashing mode. Pulse time of flight methods has been used in these studies. At high sound amplitudes, interesting transient effects have been observed which have been tentatively identified with solitonlike behavior.
    Additional Material: 4 Ill.
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  • 17
    Electronic Resource
    Electronic Resource
    New York, NY [u.a.] : Wiley-Blackwell
    Journal of Computational Chemistry 4 (1983), S. 114-122 
    ISSN: 0192-8651
    Keywords: Computational Chemistry and Molecular Modeling ; Biochemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Computer Science
    Notes: The MM2 molecular mechanics force field, as modified for halogens, is able to predict the structures, dipole moments, and energies of the isomers of benzene hexachloride. For benzene tetrachloride and naphthalene tetrachloride the field exaggerates the stability of conformers with axial chlorine, but gives satisfactory results if the field is modified to allow for interaction between electronegative substituents and the π electrons.
    Additional Material: 2 Ill.
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  • 18
    Electronic Resource
    Electronic Resource
    New York, NY [u.a.] : Wiley-Blackwell
    Journal of Computational Chemistry 4 (1983), S. 449-460 
    ISSN: 0192-8651
    Keywords: Computational Chemistry and Molecular Modeling ; Biochemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Computer Science
    Notes: As a model reaction for peptide and bond formation, the SN2 reactions between glycine and ammonia have been studied with and without amine catalysis: \documentclass{article}\pagestyle{empty}\begin{document}$${\rm NH}_3 + {\rm NH}_2 {\rm CH}_2 {\rm COOH} + ({\rm NH}_3 ) \to {\rm NH}_2 {\rm CH}_2 {\rm CONH}_2 + {\rm H}_2 {\rm O} + ({\rm NH}_3 )$$\end{document} using ab initio molecular-orbital methods. For each of the catalyzed and uncatalyzed reactions, two reaction mechanisms have been examined: a two-step and a concerted mechanism. The stationary points of each reaction, including intermediate and transition states, have been identified and free energies calculated for all geometry-optimized reaction species to determine the thermodynamics and kinetics of the reaction. The calculations demonstrate that a second ammonia molecule catalyzes amide bond formation, and that the two-step mechanism is more favorable than the concerted one for the catalyzed reaction, while for the uncatalyzed reaction both mechanisms are competitive.
    Additional Material: 5 Ill.
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  • 19
    Electronic Resource
    Electronic Resource
    New York, NY [u.a.] : Wiley-Blackwell
    Journal of Computational Chemistry 4 (1983), S. 552-555 
    ISSN: 0192-8651
    Keywords: Computational Chemistry and Molecular Modeling ; Biochemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Computer Science
    Notes: A very simple yet effective device for reducing the number of self-consistent-field (SCF) cycles in central finite-difference calculations of molecular properties is presented. The efficiency of this is demonstrated by MINDO/2 calculations of the force constants of a variety of molecules. The device consists of using information about the wavefunction at two values of a perturbation parameter to provide an estimate of the wavefunction at a third. This estimate is then used as an initial guess in an SCF procedure. The underlying theory is discussed and applications to other problems are suggested.
    Additional Material: 1 Tab.
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  • 20
    Electronic Resource
    Electronic Resource
    New York, NY [u.a.] : Wiley-Blackwell
    Journal of Computational Chemistry 4 (1983), S. 594-604 
    ISSN: 0192-8651
    Keywords: Computational Chemistry and Molecular Modeling ; Biochemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Computer Science
    Notes: Proton affinities (PA) of 80 neutral bases were calculated using the semiempirical molecular orbital procedure MNDO. These were compared with the corresponding experimental and, where available, ab initio STO-3G and 4-31G data. For the 12 bases studied which led to ions which were not hyperconjugatively stabilized, the mean absolute error between the calculated and experimental values was 7.2 kcal mol-1. However, a plot of these data revealed a clear tendency of MNDO to underestimate the PAs of the more basic molecules. Where hyperconjugative stabilization of the ion was possible, the calculated PAS were underestimated by a further 7-10 kcal mol-1 for each attached alkyl group. Calculated and observed methyl-and ethyl-cation affinities were compared for 18 bases with qualitatively similar results.
    Additional Material: 2 Ill.
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