Ja n 20 01 Assessment of W 1 and W 2 theories for the computation of electron affinities , ionization potentials , heats of formation , and proton affinities

The performance of two recent ab initio computational thermochemistry schemes, W1 and W2 theory [J., is assessed for an enlarged sample of thermochemical data consisting of the ionization potentials and electron affinities in the G2-1 and G2-2 sets, as well as the heats of formation in the G2-1 and a subset of the G2-2 set. We find W1 theory to be several times more accurate for ionization potentials and electron affinities than commonly used (and less expensive) computational thermochemistry schemes such as G2, G3, and CBS-QB3: W2 theory represents a slight improvement for electron affinities but no significant one for ionization potentials. The use of a two-point A + B/L 5 rather than a three-point A + B/C L extrapolation for the SCF component greatly enhances the numerical stability of the W1 method for systems with slow basis set convergence. Inclusion of first-order spin-orbit coupling is essential for accurate ionization potentials and electron affinities involving degenerate electronic states: inner-shell correlation is somewhat more important for ion-ization potentials than for electron affinities, while scalar relativistic effects are required for the highest accuracy. The mean deviation from experiment for 1 the G2-1 heats of formation is within the average experimental uncertainty. W1 theory appears to be a valuable tool for obtaining benchmark quality proton affinities.