A STATE-SPECIFIC MULTIREFERENCE COUPLED-CLUSTER FORMALISM WITH MOLECULAR APPLICATIONS

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 o n the same footing and is hence expected to provide a uniform descript ion over a wide range of molecular geometries. The combining coefficie nts are determined by diagonalizing an effective operator in the CAS a nd are thus completely flexible, and not constrained to pre-assigned v alues. The method uses a separate cluster operator for exciting to vir tual 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 siz e-extensivity. The use of a CAS also guarantees size-consistency. Illu strative applications to the H-8 model system and ground state potenti al curve for Li-2 indicate both the accuracy of the method and its abi lity to bypass intruders.