PERFORMANCE OF VALENCE-UNIVERSAL MULTIREFERENCE COUPLED-CLUSTER THEORY FOR QUASI-DEGENERATE STATES - THE H8 AND DZP H4 MODELS

Valence-universal coupled-cluster (VU-CC) methods corresponding to the approximations S similar or equal to S-1 + S-2 (VU-CCSD) and S simila r or equal to S-1 + S-2 + S-3 (VU-CCSDT) for the cluster operator S ha ve been applied to two model systems for which the degree of quasi-deg eneracy of the two lowest, totally symmetric, singlet states may be va ried over a wide range by changing their geometry. The performance of the VU-CC methods is investigated with the aim of verifying the findin gs of recent extensive reliability studies of these methods [Jankowski et al., J. Chem. Phys. 97, 7600 (1992); Ibid. 101, 3085 (1994)] based on calculations for the less complex minimum basis set H4 model. The present results confirm these findings. In particular, the VU-CCSD ene rgies are less accurate than those obtained with other single- and mul tireference methods including up two-body amplitudes. It is also confi rmed that the impact of the S-3 clusters is significant and the accura cy of description is rather insensitive to the kind of molecular orbit als employed. (C) 1995 John Wiley and Sons, Inc.