The coupled-cluster correction to multi-reference configuration interaction method: application to excited states

The multi-reference configuration interaction method applies equally well to ground and excited states. However, a disadvantage of the MR-CI scheme is that its accuracy can be seriously affected by inadequate description of dynamical electron correlation effects. This problem can be alleviated to a large extent through an a posteriori correction based on the coupled-cluster (CC) expansion. Since our correction is derived within the single-reference CC formulation, its application to excited states is less obvious although possible and fully justified. In this Letter, we explore this possibility and study the performance of the correction for excited states.