Coupled-cluster corrected MR-CISD method with noniterative evaluation of connected triples

The single-reference coupled-cluster (CC) analysis of the multi-reference configuration interaction singles and doubles (MR-CISD) wave function shows how the MR-CISD results can be used to mimic more effective exponential cluster expansion. The CC corrections have been constructed relying on this analysis and used to improve the accuracy of the MR-CISD energy and reduce the inextensivity error of the MR-CISD method. Disconnected contributions that approximate those components which are not present in the MR-CISD wave function generate the correction terms. Although the linear Cl expansion is usually improved through the inclusion of disconnected contributions the coupled-cluster methodology introduces a variety of corrections that, contrary to the Cl case, account for higher excitation rank connected components. In this article, we present a combination of both approaches and illustrate its performance on some numerical examples. (c) 2008 Wiley Periodicals, Inc.