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

A correction to the multi-reference configuration interaction method with singles and doubles (MR-CISD) is proposed. Assuming that a reasonable choice for the reference space permits good evaluation of the nondynamic correlation within the MR-CISD framework, the correction is designed to improve the description of the dynamic correlation effects. It is based on the single-reference cluster analysis of the wavefunction and intends to approximate the contribution from those triple and quadruple excitations which are lacking in the MR-CISD expansion. The correction is applied to several model systems, showing its ability to improve significantly the MR-CISD energies and, because of its simple noniterative character, can be seen as an attractive alternative to other methods designated for ground state energy calculations in the presence of quasi-degeneracy.