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

A correction to the multi-reference configuration interaction method with s ingles and doubles (MR-CISD) is proposed. Assuming that a reasonable choice for the reference space permits good evaluation of the nondynamic correlat ion 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 t he contribution from those triple and quadruple excitations which are lacki ng in the MR-CISD expansion. The correction is applied to several model sys tems, showing its ability to improve significantly the MR-CISD energies and , because of its simple noniterative character, can be seen as an attractiv e alternative to other methods designated for ground state energy calculati ons in the presence of quasi-degeneracy. (C) 1999 Elsevier Science B.V. All rights reserved.