A self-consistent quasidegenerate coupled-cluster theory

A new multireference coupled-cluster theory is suggested which extends the idea of a self-consistent primary space introduced earlier (J. Chem. Phys. 108 (1998) 8317) in the context of quasidegenerate perturbation theory. The suggested approach, which we refer to as the self-consistent quasidegenerate coupled-cluster (SC-QDCC) method, is developed within a Hilbert space framework. The method is both stable to intruder states and reduces to a correct single-reference coupled-cluster theory; moreover, it is appropriate for both strongly and weakly quasidegenerate systems. The theory uses canonical normalization and produces hermitian effective Hamiltonians.