MULTICONFIGURATIONAL SPIN-ADAPTED SINGLE-REFERENCE COUPLED-CLUSTER FORMALISM

We exploit the unitary group formalism in formulating a multiconfigura tional single-reference coupled cluster method for cases involving one or two electrons in open shells. The linear version of CCSD theory fo r the simple open shell case and for low lying singlet states of close d shell systems are considered in detail. The entire formalism is rela ted to the unitary group based CISD method, and explicit expressions f or size-extensivity corrections, leading to the L-CCSD formalism, are given. An illustrative example of the minimum basis set model of the B eH radical is also presented. (C) 1993 John Wiley & Sons, Inc.