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

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Abstract
The effect of basis set and electron correlation on the singlet–triplet splitting (ΔEST) 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 ΔEST with basis set and find that an extremely extended basis (triple zeta sp, diffuse sp, triple zeta d, double zeta f ) for GVB yields ΔEST=9.03 kcal/mol, in excellent agreement with the experimental value of 9.09±0.20 kcal/mol.