Journal
CHEMICAL PHYSICS LETTERS
Volume 324, Issue 5-6, Pages 389-402Publisher
ELSEVIER SCIENCE BV
DOI: 10.1016/S0009-2614(00)00644-8
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Coupled cluster (CC) theory carried out with a spin-unrestricted Hartree-Fock (UHF) reference wave function suffers less from spin contamination because infinite order electron correlation effects covered by the CC method in question reduce spin contamination. For example, UHF-CCSD, contrary to UHF, is not contaminated by a S + 1 state. However, the value of [(S) over cap(2)](CCSD), is strongly influenced by the response of the UHF-CCSD wave function on the spin contamination present at UHF and, therefore, adopts a relatively large value. A reliable diagnostic tool correctly reflecting the influence of spin contamination on the UHF-CCSD energy is obtained by just considering the energy related part of ([(S) over cap(2)])(CCSD) (C) 2000 Published by Elsevier Science B.V.
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