Analysis of the multireference state-universal coupled-cluster Ansatz

An algorithm for the multireference (MR) cluster analysis of configuration interaction (CI) wave functions, based on the state-universal (SU) coupled-cluster (CC) Ansatz of Jeziorski and Monkhorst, is formulated. It is illustrated on two often-studied model systems that consist of two (the so-called H4 model) and four (the H8 model), slightly stretched, hydrogen molecules and enable one to vary the degree of quasidegeneracy via a single geometric parameter. The role played by higher-than-two-body clusters is examined and their importance in the MR effective Hamiltonian formalism, particularly in the presence of intruder states, is explored in detail. It is pointed out that in contrast with the single-reference CC methods, the MR SU CC approach with singles and doubles (CCSD) cannot yield exact energies even when exact one- and two-body clusters are available. It is shown that this limitation, which is particularly crucial in the presence of intruder states, can be remedied by employing a small subset of higher-than-pair-cluster amplitudes, extracted via cluster analysis from the MR CISD wave functions that are based on the same model space as the SU CCSD method, not unlike in the reduced MR CCSD method. (C) 2003 American Institute of Physics.