Performance of valence‐universal multireference coupled‐cluster theory for quasi‐degenerate states: The H 8 and DZP H 4 models

Valence‐universal coupled‐cluster ( VU – CC ) methods corresponding to the approximations S ≃ S 1 + S 2 ( VU – CCSD ) and S ≃ S 1 + S 2 + S 3 ( VU – CCSDT ) for the cluster operator S have been applied to two model systems for which the degree of quasi‐degeneracy of the two lowest, totally symmetric, singlet states may be varied over a wide range by changing their geometry. The performance of the VU – CC methods is investigated with the aim of verifying the findings 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 H 4 model. The present results confirm these findings. In particular, the VU – CCSD energies are less accurate than those obtained with other single‐ and multireference methods including up two‐body amplitudes. It is also confirmed that the impact of the S 3 clusters is significant and the accuracy of description is rather insensitive to the kind of molecular orbitals employed. © 1995 John Wiley & Sons, Inc.