A STUDY OF (1)A(1)-(3)B(1) SEPARATION IN CH2 USING ORTHOGONALLY SPIN-ADAPTED STATE-UNIVERSAL AND STATE-SPECIFIC COUPLED-CLUSTER METHODS

The orthogonally spin-adapted (OSA) state-universal coupled-cluster method involving singly and doubly excited clusters (CCSD) and the recently proposed OSA open-shell state-specific CCSD method are applied to the problem of singlet-triplet separation in methylene. Both double zeta (DZ) and DZ plus polarization bases are utilized. Comparison with other correlation treatments, including limited and full configuration interaction and various many-body approaches based on multi-configurational self-consistent-field reference, indicates that our method represents a highly efficient theoretical tool that can lead to reliable predictions of the 1A1-B-3(1) splitting in CH2 when larger bases are employed.