A state-specific multi-reference coupled cluster formalism with molecular applications

Abstract

We present in this paper a state-specific coupled cluster method based on a reference function composed of determinants spanning a complete active space (CAS). The method treats all the reference determinants on the same footing and is hence expected to provide a uniform description over a wide range of molecular geometries. The combining coefficients are determined by diagonalizing an effective operator in the CAS and are thus completely flexible, and not constrained to pre-assigned values. The method uses a separate cluster operator for exciting to virtual functions from each reference determinant. The linear dependence implicit in this choice of cluster operators is eliminated by invoking suitable sufficiency conditions. The choice is dictated to ensure size-extensivity. The use of a CAS also guarantees size-consistency. Illustrative applications to the H₈ model system and ground state potential curve for Li₂ indicate both the accuracy of the method and its ability to bypass intruders.