VALENCE-BOND CORRECTED SINGLE REFERENCE COUPLED-CLUSTER APPROACH .1. GENERAL FORMALISM

An extension of the single reference coupled cluster method truncated to 1- and 2-body cluster components (CCSD) to quasidegenerate systems, where 3- and 4-body connected cluster components play an important ro le, is proposed. The basic idea is to extract the information concerni ng the 3- and 4-body clusters from some independent source, similarly as was implicitly done in the so-called ACPQ or ACC(S)D methods, and c orrect accordingly the absolute term in the CCSD equations. As a sourc e of these approximate 3- and 4-body clusters, simple valence bond (VB ) type wave functions are employed, since they are capable of describi ng electronic structure of various molecular systems for a wide range of nuclear conformations including their dissociation. The cluster ana lysis of these VB wave functions, that provides the desired informatio n concerning the connected 3- and 4-body cluster components, is outlin ed and the explicit form of required correction terms to the CCSD equa tions is given.