Accurate ab initio calculations on the singlet--triplet separation in methylene

Configuration interaction (CI) calculations which include all single and double excitations have been performed for the 3B1 and 1A1 states of methylene. The basis sets used were of better than triple-zeta quality and included two sets of polarization functions. The separation is computed to be 10.6 kcal/mol. A trend is shown to exist from the carbon atom through CH to CH2, which suggests that the actual splitting is less than 10 kcal/mol. This is in disagreement with the interpretation of the latest experimental data, but agrees with earlier experimental work