New type of noniterative energy corrections for excited electronic states:Extension of the method of moments of coupled-cluster equations to the equation-of-motion coupled-cluster formalism

The recently proposed method of moments of coupled-cluster equations (MMCC) is extended to excited states via the equation-of-motion coupled-cluster ( EOMCC) formalism. The main idea of the new MMCC theory is that of the nonit erative energy corrections which, when added to the excited-state energies obtained in standard approximate EOMCC calculations, recover the exact ener gies. The MMCC corrections are expressed in terms of the generalized moment s of the EOMCC equations. Approximate variants of the excited-state MMCC fo rmalism, including the MMCC(2,3) approach, are introduced. In the MMCC(2,3) method, very simple energy corrections, expressed in terms of matrix eleme nts of the triples-reference, triples-singles, and triples-doubles blocks o f the EOMCCSD (EOMCC singles and doubles) similarity-transformed Hamiltonia n, are added to the excited-state energies obtained in EOMCCSD calculations . The performance of the MMCC(2,3) approach is illustrated by the results o f pilot calculations for the potential energy curves of ground and excited states of CH+. (C) 2001 American Institute of Physics.