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  • 1
    Electronic Resource
    Electronic Resource
    Bond length alternation in cyclic polyenes. IV. Finite-order purturbation theory approach (1984)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 25 (1984), S. 423-443 
    Details
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The problem of bond length alternation in linear extended systems with conjugated double bonds is examined on a simple cyclic polyene model using finite-order many-body perturbation theory. Three different partitionings of the model Hamiltonian are employed, namely the Hückel, Møller-Plesset, and Epstein-Nesbet partitionings. The dependence of correlation energy on bond length alternation is examined in each case, showing an almost constant behavior of Møller-Plesset and Epstein-Nesbet perturbation energies in contrast to a strong dependence on distortion, favoring undistorted structures, for the Hückel perturbation and UHF correlation energies. The origin of an unphysical character of the Hückel perturbation theory and the inappropriateness of the UHF approach for the problem considered are pointed out. The second- and third-order Møller-Plesset and also the second-order Epstein-Nesbet perturbation theories yield results which are similar to those obtained with the RHF method and which clearly favor the bond length alternating structures, leading to the bond length distortion of about 0.045 Å and to the stabilization energy per site (relative to the equidistant geometry) of about 0.03 eV.
    Additional Material: 9 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/qua.560250212
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    Paper (German National Licenses)
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  • 2
    Electronic Resource
    Electronic Resource
    Perturbation theory and electron correlation in extended systems: Cyclic polyene model (1983)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 24 (1983), S. 707-727 
    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 applicability of the finite-order many-body perturbation theory to the electron correlation problem in extended one-dimensional systems is examined. The cyclic polyenes CNHN, N = 4ν + 2, ν = 1, 2, …, with the DNh geometry as described by both the Pariser-Parr-Pople and Hubbard Hamiltonians, are employed to model the metallic-like one-dimensional systems. The second-order perturbation theory contributions to the correlation energy are obtained with three different partitionings of the Hamiltonian (Hückel, M⊘ller-Plesset, and Epstein-Nesbet). The third- and fourth-order contributions are also calculated in special cases. A comparison with other methods is given wherever available. For the Hubbard Hamiltonian the asymptotic behavior of the perturbation theory expansion is examined numerically. It is shown that the finite-order perturbation expansion can provide reliable results for the correlation energy of one-dimensional systems even in the correlation region which corresponds to the spectroscopically determined physical value of the coupling constant.
    Additional Material: 10 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/qua.560240614
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    Paper (German National Licenses)
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  • 3
    Electronic Resource
    Electronic Resource
    Degeneracy and coupled-cluster approaches (1984)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 26 (1984), S. 237-244 
    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: Problems which arise in the application of closed-shell coupled-cluster approaches to quasidegenerate or almost degenerate situations are discussed and the basic classification of quasidegeneracy types is outlined. Recent coupled-cluster results obtained for the cyclic polyene model, particularly in the strongly correlated limit, are briefly discussed and the unexpected features of approximate and localized coupled-pair approaches are pointed out.
    Additional Material: 1 Tab.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/qua.560260824
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    Paper (German National Licenses)
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  • 4
    Electronic Resource
    Electronic Resource
    Bond length alternation in cyclic polyenes. V. Local finite-order perturbation theory approach (1984)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 26 (1984), S. 349-371 
    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 finite-order many-body perturbation theory using the localized Wannier orbital basis is applied to the problem of bond length alternation in the Pariser-Parr-Pople model of cyclic polyenes CN HN, N = 4v + 2, which may be regarded as a simplified model of polyacetylene. Both the Møller-Plesset and the Epstein-Nesbet-type partitionings of the model Hamiltonian are employed. The localized orbital basis enables an efficient truncation of the perturbation theory summations over the intermediate states as well as an elimination of energetically unimportant diagrams, thus enabling one to obtain the fourth-order Møller-Plesset-perturbation energies with a relatively small computational effort even for large polyenes. The results obtained with the second-, third-, and fourth-order Møller-Plesset and with the third-order Epstein-Nesbet perturbation theories yield very similar bond length distortions (about 0.05 Å) and stabilization energies per site (about 0.04 eV) as obtained earlier with the RHF, one-parameter AMO, and delocalized orbital perturbation theories. The effects of truncation and diagram elimination in the fourth-order Møller-Plesset perturbation theory and the abnormal behavior of the second-order Epstein-Nesbet perturbation theory results in the localized Wannier basis near the instability threshold of the RHF solutions are discussed.
    Additional Material: 9 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/qua.560260305
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    Paper (German National Licenses)
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