ATOMIZATION ENERGIES OF SO AND SO2 - BASIS-SET EXTRAPOLATION REVISITED

The addition of tight functions to sulfur and extrapolation to the com plete basis set limit are required to obtain accurate atomization ener gies. Six different extrapolation procedures are tried. The best atomi zation energies come from the series of basis sets that yield the most consistent results for all extrapolation techniques. In the variable ct approach, a values larger than 4.5 or smaller than 3 appear to sugg est that the extrapolation may not be reliable. It does not appear pos sible to determine a reliable basis set series using only the triple a nd quadruple-zeta based sets. The scalar relativistic effects reduce t he atomization of SO and SO2 by 0.34 and 0.81 kcal/mol, respectively, and clearly must be accounted for if a highly accurate atomization ene rgy is to be computed. The magnitude of the core-valence (CV) contribu tion to the atomization is affected by missing diffuse valence functio ns. The CV contribution is much more stable if basis set superposition errors are accounted for. A similar study of SF, SF+, and SF6 shows t hat the best family of basis sets varies with the nature of the S bond ing.