The second-order approximate internally contracted multireference coupled-cluster singles and doubles method icMRCC2

Abstract

The second-order approximate internally contracted multireference coupled-cluster singles and doubles method icMRCC2 is defined and tested. The method is designed to bridge the gap between multireference perturbation theory and single-reference second-order approximate coupled-cluster theory (CC2). By including semi-internal double excitations into the zeroth-order expansion, the new method is able to reliably describe the coupling between excitations within the active space and the entire single-excitation spectrum. This helps, for instance, to provide a balanced treatment of valence and ionic states in polyenes (as explicitly demonstrated for cyclopentadiene) and to arrive at a more complete coverage of the excitation spectrum without the need to include diffuse orbitals into the active space. Good performance is also seen for notoriously difficult molecules such as C2 and CN. Furthermore, the multireference extension removes the main failures of single-reference CC2 theory, such as in the case of ozone.