A matrix coupled-cluster correction to the multi-reference configuration interaction method

A matrix coupled-cluster (CC) correction to the multi-reference configuration interaction method with singles and doubles (MR-CISD) is proposed. The MR-CISD method can be expressed in terms of the wave operator and effective Hamiltonian, and the correction is designed to modify the MR-CISD effective Hamiltonian matrix. That allows us to improve the description of dynamic correlation for several MR-CISD states at a time that can be especially convenient when excited states are considered. The correction provides an estimate of the effect of excitations that are not present in the MR-CISD expansion in a coupled-cluster manner. The performance of the method is investigated in preliminary calculations for two model systems.