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  • Computational Chemistry and Molecular Modeling  (16)
  • Wiley-Blackwell  (16)
  • Molecular Diversity Preservation International
  • Springer
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  • Wiley-Blackwell  (16)
  • Molecular Diversity Preservation International
  • Springer
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  • 1
    Electronic Resource
    Electronic Resource
    Molecular formation and electron correlation in HeH+ (1970)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 4 (1970), S. 431-450 
    Details
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A one-centre CI wave function for HeH+ reported by Stuart and Matsen for 0.1 ≤ R ≤ 5.0 has been analysed in detail from the viewpoint of molecular formation. Further, by means of a natural orbital analysis, it was possible to obtain some measure of the electron correlation contained within such wave functions for various R values. These effects were illustrated by means of a series of difference maps for the electron density. One- and two-particle expectation values were obtained as a function of R. Thus, it was possible to study several aspects of the influence of the proton on the electron charge cloud as we pass from He through to the united atom Li+. The occupation numbers within the natural expansions were compared with those which arise from a similar analysis of a two-centre wave function for HeH+. The “character” of such wave functions for HeH+, and also for He and Li+, were analysed and compared.
    Additional Material: 5 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/qua.560040409
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  • 2
    Electronic Resource
    Electronic Resource
    EOM/Propagator ionization potentials: Effect of an extended operator manifold on the predicted valence-shell ionization spectra of ethane, ethylene, and acetylene (1985)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 27 (1985), S. 145-157 
    Details
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The orthogonalized direct diagonalization (ODD) method is used to predict the valence-shell ionization spectra for the series of hydrocarbons ethane (C2H6), ethylene (C2H4), and acetylene (C2H2) within an EOM/propagator formalism. Both (a) third-order and (b) higher-order (with h5 terms in the excitation operator manifold) calculations are presented in order to illustrate the effect of the extended operator manifold on the predicted valence-shell spectra. It is shown that the major effect of the h5 manifold is concentrated in its “occupied” part. Along with a general shifting of the main lines for outer valence ionizations to lower values (together with a slight reduction in pole strength), it is found that the effect of the h5 manifold in the inner valence region is dependent on the system being studied.
    Additional Material: 3 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/qua.560270205
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  • 3
    Electronic Resource
    Electronic Resource
    Calculations on the electronic structure and spectroscopy of C60 and C70 cage structures (1991)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 40 (1991), S. 557-568 
    Details
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A study of the electronic structure and spectroscopy of models of C60 and C70 cage structures has been carried out using the intermediate neglect of differential overlap (INDO) model Hamiltonian. The geometry for these cages was obtained using gradient-driven methods, and at least in the case of C60, where information is available, is in good accord with the calculations of others. Using a small active space of only single excitations, both configuration interaction (CI) and random phase approximations (RPA) yield spectra in excellent agreement with that obtained from recent experiments. The oscillator strength of the band calculated at ∼47000 cm-1, however, dramatically diminishes as the active space is increased, even though the sum rule increases. We discuss this interesting observation as well as the calculated structure and spectrum of C70.
    Additional Material: 8 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/qua.560400849
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  • 4
    Electronic Resource
    Electronic Resource
    Calculation of 13C-NMR chemical shift using the intermediate neglect of differential overlap model (1992)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 43 (1992), S. 327-342 
    Details
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The local origin/local orbital (LORG) method of Hansen and Bouman has been implemented with the intermediate neglect of differential overlap Hamiltonian for spectroscopy (INDO/S). The method is shown capable of demonstrating the inductive effects associated with electron-withdrawing substituents through the diamagnetic shielding term. In addition, the method is capable of differentiating chemical shift in differing bond environments. The calculated paramagnetic contribution, however, is deficient for substituents that saturate the minimal basis such as oxygen and fluorine, which severely limits the general utility of the procedure. Through the utilization of reduced linear equations for the paramagnetic term, the method is amenable to any molecule for which a self-consistent field can be performed and therefore can potentially be used to study very large systems. At present, however, the LORG method when used with the rapid INDO/S model Hamiltonian does not reliably reproduce the paramagnetic contribution to the shielding.
    Additional Material: 3 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/qua.560430304
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  • 5
    Electronic Resource
    Electronic Resource
    On the calculation of oscillator strength for electronic transitions using “effective core” methods (1992)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 44 (1992), S. 475-486 
    Details
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: We examine the effect on calculated oscillator strengths for electronic transitions caused by reintroducing the nodes of valence orbitals in effective core methods through a simple Schmidt orthogonalization. This refinement is then tested within the Intermediate Neglect of Differential Overlap (INDO) model, a valence orbital only model, in both configuration interaction and Random-Phase Approximation (RPA) calculations. It is shown that the differences in oscillator strengths calculated using the dipole-length and dipole-velocity formulations are reduced somewhat for π → * transitions and significantly for n → π* transitions. The oscillator strengths calculated from the dipole-length formalism by the RPA model are in best accord with experiment. © 1992 John Wiley & Sons, Inc.
    Additional Material: 5 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/qua.560440843
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  • 6
    Electronic Resource
    Electronic Resource
    A density functional study of chemical reactions (1995)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 56 (1995), S. 733-746 
    Details
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Density functional theory (DFT) was used to study reactions involving small molecules. Relative energies of isomers and transition structures of diazene, formaldehyde, and methylenimine were determined using various DFT functionals and results were compared with MP2 and MP4 calculations. DFT reaction barriers were found to be consistently lower. For some reactions, such as OH + H2→ H2O + H, gradient-corrected functionals predict very low or nonexistent barriers. The hybrid Hartree-Fock-DFT adiabatic connection method (ACM) often provides much better results in such cases. The performance of several density functionals, including ACM, was tested in calculations on over 100 atomization, hydrogenation, bond dissociation, and isodesmic reactions. The ACM functional provides consistently better geometries and reaction energetics than does any other functional studied. In cases where both HF and gradient-corrected DFT methods underestimate bond distances, the ACM geometries may be inferior to those predicted by gradient-corrected DFT methods. © 1995 John Wiley & Sons, Inc.
    Additional Material: 5 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/qua.560560609
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  • 7
    Electronic Resource
    Electronic Resource
    Geometry optimization in cartesian coordinates: The end of the Z-matrix? (1991)
    New York, NY [u.a.] : Wiley-Blackwell
    Journal of Computational Chemistry 12 (1991), S. 606-610 
    Details
    ISSN: 0192-8651
    Keywords: Computational Chemistry and Molecular Modeling ; Biochemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Computer Science
    Notes: Geometry optimization directly in Cartesian coordinates using the EF and GDIIS algorithms with standard Hessian updating techniques is compared and contrasted with optimization in internal coordinates utilizing the well known Z-matrix formalism. Results on a test set of 20 molecules show that, with an appropriate initial Hessian, optimization in Cartesians is just as efficient as optimization in internals, thus rendering it unnecessary to construct a Z-matrix in situations where Cartesians are readily available, for example from structural databases or graphical model builders.
    Additional Material: 1 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/jcc.540120510
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  • 8
    Electronic Resource
    Electronic Resource
    An algorithm for the location of branching points on reaction paths (1988)
    New York, NY [u.a.] : Wiley-Blackwell
    Journal of Computational Chemistry 9 (1988), S. 465-475 
    Details
    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 branching point is a point on a reaction path leading from reactants to products (via a transition state) at which it is energetically favorable for the system to break symmetry. Such a point can be defined in terms of normal modes along the reaction path and corresponds to zero curvature (a zero Hessian eigenvalue) along a symmetry-breaking mode. An effective method for the location of such points is presented and realized in an efficient, practical algorithm designed for use in the ab initio program package Gaussian 82.
    Additional Material: 6 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/jcc.540090505
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  • 9
    Electronic Resource
    Electronic Resource
    Hybrid Monte Carlo: An efficient algorithm for condensed matter simulation (1994)
    New York, NY [u.a.] : Wiley-Blackwell
    Journal of Computational Chemistry 15 (1994), S. 838-846 
    Details
    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 detailed comparison has been made of the performance of molecular dynamics and hybrid Monte Carlo simulation algorithms for calculating thermodynamic properties of 2D Lennard-Jonesium. The hybrid Monte Carlo simulation required an order of magnitude fewer steps than the molecular dynamics simulation to calculate reproducible values of the specific heat. The ergodicity of the two algorithms was compared via the use of intermediate scattering functions. For classical systems the intermediate scattering functions should be real; however, a simple analysis demonstrates that this function will have a significant imaginary component when ergodicity breaks down. For q vectors near the zone boundary, the scattering functions are real for both algorithms. However, for q vectors near the zone center (i.e., harmonic, weakly coupled modes), the scattering function calculated via molecular dynamics had a significantly larger imaginary component than that calculated using hybrid Monte Carlo. Therefore, the hybrid Monte Carlo algorithm is more ergodic and samples phase space more efficiently than molecular dynamics for simulations of 2D Lennard-Jonesium. © 1994 by John Wiley & Sons, Inc.
    Additional Material: 7 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/jcc.540150805
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  • 10
    Electronic Resource
    Electronic Resource
    The evaluation of molecular electron affinitiesDedicated to Professor John A. Pople on the occasion of his 60th birthday. (1986)
    New York, NY [u.a.] : Wiley-Blackwell
    Journal of Computational Chemistry 7 (1986), S. 349-358 
    Details
    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 performance of a variety of levels of theory in evaluating molecular electron affinities (EAs) has been systematically examined. Calculations have been carried out for six different basis sets and for nine theoretical procedures including unrestricted (UHF) and restricted (RHF) Hartree-Fock theory, Møler-Plesset perturbation theory (UMP2, UMP3, UMP4), configuration interaction (UCISD, RCISD, RCISD(Q)) and equations-of-motion (EOM) approaches. Electron affinities were evaluated for CH3, NH2, OH, F, C2H, CN, BO, N3, OCN, and NO2. Very poor results are generally obtained unless diffuse functions are included in the basis set and electron correlation is incorporated. Even with the largest basis set used in the present study (6-311 + + G(2d, 2p)), there are still residual errors greater than 0.2 eV (UMP4) or 0.6 eV (CISD) in the absolute EAs. However, better results are obtained under certain circumstances for relative EAs. The results appear to be significantly affected by spin contamination in the UHF wave-functions. For those systems for which spin contamination is small, best absolute values of the EAs generally come from the EOM and UMP2 calculations, whereas the most constant errors (thereby allowing systematic correction) are found at the UMP4, CISD, and RCISD(Q) levels. For the systems for which spin contamination is larger, best results are obtained with the CI-based procedures (CISD and RCISD(Q)). The errors in calculated EAs for the molecules with differing electronic characteristics can vary quite widely. Caution must therefore be exercised before applying schemes which rely on a constancy of errors to estimate electron affinities. The UMP procedures appear particularly suspect in this regard if spin contamination is significant. The RCISD(Q) approach is recommended under such circumstances.
    Additional Material: 16 Tab.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/jcc.540070312
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