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Full CI calculations are presented for the 1A1–3B1 separation in CH2 as a calibration for other levels of theory. Compared to the full CI result, the SCF/SDCI separation, after applying the Davidson correction for higher excitations, is found to be in error by only 0.38 kcal/mol, while the error in a CASSCF/MRSDCI treatment is less than 0.01 kcal/mol.
Bauschlicher et al. (1986) studied this question.