An ab initio determination of 1A 1- 3B 1 energy gap in CH 2 using orthogonally spin-adapted state-universal and state-specific coupled-cluster methods

Abstract

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 [5s4p3d2flg/3s2pld] and a few smaller basis sets. The best SS/SU CCSD result for the vibrationless 1A 1- 3B 1 separation ( T nr,BO e) 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.