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  • 1
    Electronic Resource
    Electronic Resource
    Method of moments approach and coupled cluster theory (1991)
    Springer
    Theoretical chemistry accounts 80 (1991), S. 223-243 
    Details
    ISSN: 1432-2234
    Keywords: Coupled cluster approach ; Method of moments approach ; Many-electron correlation problem ; Intruder states ; Quasi-degeneracy
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Summary The single reference coupled cluster (CC) approach to the many-electron correlation problem is examined from the viewpoint of the method of moments (MM). This yields generally an inconsistent (overcomplete) set of equations for cluster amplitudes, which can be solved either in the least squares sense or by selective projection process restricting the number of equations to that of the unknowns. These resulting generalized MM-CC equations always contain the standard CC equations as a special case. Since, in the MM-CC formalism, the Schrödinger equation will be approximately satisfied on a subspace spanned by non-canonical configurations, this procedure may be helpful in extending the standard single reference CC theory to quasi-degenerate situations. To examine the potential usefulness of this idea, we explore the linear version of the CC approach for systems with a quasi-degenerate reference, in which case the standard linear theory is plagued with singularities due to the intruder states. Implications of this analysis for the structure of the wavefunction are also briefly discussed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01117411
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  • 2
    Electronic Resource
    Electronic Resource
    Unitary-group-based open-shell coupled-cluster method with corrections for connected triexcited clusters. I. Theory (1998)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 70 (1998), S. 65-75 
    Details
    ISSN: 0020-7608
    Keywords: Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Unitary-group-based perturbation theory for both low and high spin open-shell systems is employed to obtain perturbative corrections for the contribution of the connected triexcited clusters as well as of pseudo-doubles (three-body doubles), within the framework of the unitary-group-based coupled-cluster method with interacting singles and doubles [UGA-CCSD(is)]. A set of triple excitation operators, generating orthonormal triply excited configurations, which can be employed in both noniterative and iterative coupled-cluster calculations involving connected triexcited clusters, is constructed and various schemes for perturbative triple corrections are introduced.   © 1998 John Wiley & Sons, Inc. Int J Quant Chem 70: 65-75, 1998
    Additional Material: 1 Tab.
    Type of Medium: Electronic Resource
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    Paper (German National Licenses)
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