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 the single-reference MMCC theory and its performance in calculations of potential energy surfaces involving bond breaking, we introduce the method of moments of the generalized Bloch equation and the method of moments of the state-universal coupled-cluster equations (MM-SUCC). The main idea of the MM-SUCC theory is that of the noniterative energy corrections that, when added to the ground- and excited-state energies obtained in approximate SUCC calculations, 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. All rights reserved.