A-POSTERIORI CORRECTIONS TO THE CONFIGURATION-INTERACTION METHOD WITHSINGLES AND DOUBLES

The configuration interaction with singles and doubles (CISD) method i s plagued with the size-extensivity error. We propose a correction to the CISD energy that aims at a full evaluation of the multiple substit ution effects the lack of which in the limited Ct expansion is respons ible for the CI inextensivity problem. The correction is designed to r eproduce the corresponding coupled-cluster (CCSD) energy in a post CI calculation while the previously discussed corrections of this type ha ve usually targeted one of the approximated versions of CCSD, By apply ing the correction the agreement up to the fifth order of the perturba tion expansion with the CCSD energy is obtained. Some illustrative app lications are presented where the results given by our correction can be compared with those of CCSD and a variety of other corrections.