Multiconfigurational spin-adapted single-reference coupled cluster formalism

Authors

  • X. Li,

    1. Quantum Theory Group, Department of Applied Mathematics, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1
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  • J. Paldus

    1. Quantum Theory Group, Department of Applied Mathematics, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1
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    • Also at Department of Chemistry and Guelph-Waterloo Center for Graduate Work in Chemistry, Waterloo Campus, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1.


Abstract

We exploit the unitary group formalism in formulating a multiconfigurational single-reference coupled cluster method for cases involving one or two electrons in open shells. The linear version of CCSD theory for the simple open shell case and for low lying singlet states of closed shell systems are considered in detail. The entire formalism is related to the unitary group based CISD method, and explicit expressions for size-extensivity corrections, leading to the L-CCSD formalism, are given. An illustrative example of the minimum basis set model of the BeH radical is also presented. © 1993 John Wiley & Sons, Inc.

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