A b i n i t i o calculation of reaction energies. III. Basis set dependence of relative energies on the FH2 and H2CO potential energy surfaces

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Abstract
The relative energies of the stationary points on the FH2 and H2CO nuclear potential energy surfaces relevant to the hydrogen atom abstraction, H2 elimination and 1,2‐hydrogen shift reactions have been examined using fourth‐order Mo/ller–Plesset perturbation theory and a variety of basis sets. The theoretical absolute zero activation energy for the F+H2→FH+H reaction is in better agreement with experiment than previous theoretical studies, and part of the disagreement between earlier theoretical calculations and experiment is found to result from the use of assumed rather than calculated zero‐point vibrational energies. The fourth‐order reaction energy for the elimination of hydrogen from formaldehyde is within 2 kcal mol−1 of the experimental value using the largest basis set considered. The qualitative features of the H2CO surface are unchanged by expansion of the basis set beyond the polarized triple‐zeta level, but diffuse functions and several sets of polarization functions are found to be necessary for quantitative accuracy in predicted reaction and activation energies. Basis sets and levels of perturbation theory which represent good compromises between computational efficiency and accuracy are recommended.