Gas-phase acidities: a comparison of density functional, MP2, MP4, F4, G2(MP2, SVP), G2(MP2) and G2 procedures

A comparison of several methods for calculating gas-phase acidities is reported. Methods examined include density functional theory with the B-LYP, B-P86, B3-LYP and B3-P86 non-local functionals, second-order [MP2] and fourth-order [MP4] Møller-Plesset and fourth-order Feenberg [F4] theories, quadratic configuration interaction (QCISD(T)), and variants of G2 theory including G2(MP2, SVP), G2(MP2) and G2 itself. Excellent results are obtained with the G2(MP2, SVP), G2(MP2) and G2 procedures. MP4 and F4 methods using the 6–311 + G(3df, 2p) basis produce similar excellent agreement with experiment. On the other hand, acidities calculated using both the B-LYP and B-P86 methods with either the 6–311 + G(d, p) or 6–311 + G(3df, 2p) basis sets show large errors. In contrast, the two hybrid density functional methods, B3-P86 and B3-LYP, significantly outperform all the other methods at the 6–311 + G(d, p) basis level, B3-P86 producing the better results. With the 6–311 + G(3df, 2p) basis set, B3-LYP yields excellent acidities, with an accuracy comparable to that achieved at the G2 level, but B3-P86 does not perform as well.