AN AB-INITIO DETERMINATION OF (1)A(1)-B-3(1) ENERGY-GAP IN CH2 USING ORTHOGONALLY SPIN-ADAPTED STATE-UNIVERSAL AND STATE-SPECIFIC COUPLED-CLUSTER METHODS

The orthogonally spin-adapted open-shell state-universal (SU) and state-specific (SS) single and double coupled-cluster methods (CCSD) are applied to the singlet-triplet separation in methylene. Calculations have been performed using atomic-natural-orbital basis [5s4p3d2f1g/3s2p1d] and a few smaller basis sets. The best SS/SU CCSD result for the vibrationless (1)A(1)-B-3(1) separation (T-e(nr,BO)) using the nonrelativistic and Born-Oppenheimer approximations is 3315 cm(-1). Accounting for the zero-point energies, relativistic and diagonal. Born-Oppenheimer corrections yield T-0=3184 or 3133 cm(-1), in excellent agreement with the experimental values of 3147 +/- 5, 3156 +/- 5 and 3165 +/- 20 cm(-1).