Extension of the method of moments of coupled-cluster equations to a multireference wave operator formalism

The recently proposed method of moments of coupled-cluster equations (MMCC) is extended to a multireference wave operator formalism. After reviewing t he single-reference MMCC theory and its performance in calculations of pote ntial energy surfaces involving bond breaking, we introduce the method of m oments of the generalized Bloch equation and the method of moments of the s tate-universal coupled-cluster equations (MM-SUCC). The main idea of the MM -SUCC theory is that of the noniterative energy corrections that, when adde d to the ground- and excited-state energies obtained in approximate SUCC ca lculations, recover the exact energies. Approximate variants of the MM-SUCC formalism that may lead to significant improvements in the results of the standard SUCC calculations are discussed. (C) 2001 Elsevier Science B.V. Al l rights reserved.