APPLICABILITY OF VALENCE-UNIVERSAL MULTIREFERENCE COUPLED-CLUSTER THEORIES TO QUASI-DEGENERATE ELECTRONIC STATES .1. MODELS INVOLVING AT MOST 2-BODY AMPLITUDES

The recently developed algebraic formulation of valence-universal coupled-cluster (VU-CC) theories [Jeziorski and Paldus, J. Chem. Phys. 90, 2714 (1989)] for open-shell systems has been employed in a systematic derivation of explicit equations defining cluster amplitudes assuming Lindgren's normal ordered exponential ansatz for the wave operator. The latter is approximated by its one- and two-electron components. Various aspects of the applicability of this version of the VU-CC theory to quasidegenerate electronic states are studied for a model system consisting of two slightly stretched, interacting hydrogen molecules. A single parameter that determines the geometry of this system makes it possible to vary the extent of quasidegeneracy of the two lowest-energy states over a wide range. Along with the complete theory, the linear version (VU-LCC) is also examined. The results are compared with the full configuration interaction results as well as with those obtained using other approaches. It was found that, at least in the strongly quasidegenerate region, the VU-CC energies are less accurate than those obtained with other multireference CC theories. It is shown that the VU-CC equations for cluster amplitudes possess multiple solutions representing various pairs of states. The individual solutions that arise for a system possessing n-valence electrons may be characterized in terms of a genealogical scheme that involves states of systems with 0,1,...,n valence electrons.