A posteriori corrections to the configuration interaction method with singles and doubles

The configuration interaction with singles and doubles (CISD) method is plagued with the size-extensivity error. We propose a correction to the CISD energy that aims at a full evaluation of the multiple substitution effects the lack of which in the limited Ct expansion is responsible for the CI inextensivity problem. The correction is designed to reproduce the corresponding coupled-cluster (CCSD) energy in a post CI calculation while the previously discussed corrections of this type have usually targeted one of the approximated versions of CCSD, By applying the correction the agreement up to the fifth order of the perturbation expansion with the CCSD energy is obtained. Some illustrative applications are presented where the results given by our correction can be compared with those of CCSD and a variety of other corrections.