Multireference Brillouin-Wigner coupled cluster singles and doubles study of the singlet-triplet separation in alkylcarbenes

Methylene, methylcarbene, dimethylcarbene, and methylethylcarbene were calculated by the multireference Brillouin-Wigner coupled cluster singles and doubles (MR BWCCSD) method with cc-pVDZ. cc-pVTZ, and also the cc-pVQZ basis set for methylcarbene. The calculations confirmed the trend predicted by previous calculations. In the series of methylene, methylcarbene, dimethylcarbene, and methylethylcarbene, the MR BWCCSD/cc-pVTZ singlet-triplet energy gap decreases as 10.18, 4.75,-0.82, -1.56 kcal/mol, respectively, where the minus sign means that the singlet is the ground state.