APPLICABILITY OF VALENCE-UNIVERSAL MULTIREFERENCE COUPLED-CLUSTER THEORIES TO QUASI-DEGENERATE ELECTRONIC STATES .2. MODELS INVOLVING 3-BODY AMPLITUDES

The reliability study of the valence-universal coupled-cluster (VU-CC) methods [Jankowski et al., J. Chem. Phys. 97, 7600 (1992)] is continu ed by considering approximations involving one-, two-, and three-elect ron components of the cluster operator S, S similar or equal to S-1 S-2 + S-3. The explicit equations determining the cluster amplitudes f or the quadratic VU-CCSDT approximation were derived and applied to th e low lying quasidegenerate states of the model system consisting of t wo slightly stretched, interacting hydrogen molecules. The degree of q uasidegeneracy of the two lowest, totally symmetric, singlet levels of this system may be varied over a wide range by changing its geometry. Along with the quadratic VU-CCSDT theory its linear version (VU-LCCSD T) is also examined. It is found that the effect of the S-3 clusters i s significant and is mainly due to the spectator part of this operator . The inclusion of this operator increases the number of physically me aningful multiple solutions of the equations for the cluster amplitude s. It also considerably improves the accuracy of the resulting energie s, both in the strongly and weakly quasidegenerate regions, but not in the intermediately quasidegenerate region. It is shown that this beha vior is a manifestation of an avoided crossing of the lowest excited s tates of the parent system in the valence-universal genealogy.