COMPARISON OF THE OPEN-SHELL STATE-UNIVERSAL AND STATE-SELECTIVE COUPLED-CLUSTER THEORIES - H4 AND H8 MODELS

The spin nonadapted or partially spin adapted spin orbital multirefere nce (MR) state-universal (SU) or Hilbert space coupled cluster (CC) me thod is analyzed from the viewpoint of possible spin symmetry breaking or spin contamination and contrasted with the recently introduced sta te specific (SS), automatically spin adapted, spin-free unitary group based CC method. It is shown that the partial spin-adaptation of the t wo-determinantal MR SUCC approach can only be effective if the complem entary singlet and tripler states have a similar structure in the sens e that their weights at corresponding singlet and triplet configuratio ns, having the same orbital occupancies, are identical or nearly ident ical. The performance of both SS and SU CC methods, and of correspondi ng CI method (all truncated at the double excitation level) for the lo west lying open-shell singlet and triplet states of the H4 and H8 mode l systems was examined, showing the superiority of the unitary group b ased CC approach in all studied aspects. (C) 1995 American Institute o f Physics.