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

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.