The coupled-cluster corrections to multi-reference configuration interaction method: the HF and F2 bond breaking description*

Abstract

Potential energy curves may be difficult to describe by multi-reference (MR) coupled-cluster or perturbation methods because of the intruder state problem. To avoid intruder states one may have to change the reference space while going from one internuclear distance to another which can make the description inconsistent and cause discontinuities on the potential energy curve. Conversely to this, the MR configuration interaction method with singles and doubles (MR-CISD) does not require the reference space that is specially tailored for particular geometries to avoid intruders. A disadvantage of the MR-CISD scheme is the relatively poor description of the dynamic correlation which does not benefit from the efficiency of the cluster expansion. However, if the coupled-cluster corrections to MR-CISD are a posteriori used to improve that description then a very accurate potential energy curve can be obtained. In this preliminary study the corrections are applied in model calculations to investigate the HF and F2 bond breaking cases. Also the problem of size-inextensivity of the MR-CISD method is addressed. A numerical test shows that the corrections are capable of significant reduction of the MR-CISD inextensivity error.