Electron correlation, basis sets, and the methylene singlet-triplet gap

The effect of basis set and electron correlation on the singlet-triplet splitting (ΔE_ST) of CH₂ is examined using the generalized valence bond (GVB) approach. For a standard double zeta plus polarization basis, the GVB based calculation (with only 20-25 spin eigenfunctions) approaches the full CI result (∼220 000 spin eigenfunctions) of Bauschlicher and Taylor to within 0.5 kcal/mol for this basis, but both differ substantially from experiment (errors of 2.4 and 2.9 kcal/mol for GVB and full CI, respectively). We have studied the convergence of ΔE _ST with basis set and find that an extremely extended basis (triple zeta sp, diffuse sp, triple zeta d, double zeta/) for GVB yields ΔE _ST=9.03 kcal/mol, in excellent agreement with the experimental value of 9.09±0.20 kcal/mol.