Basis set and correlation effects on computed lithium ion affinities

Ab initio calculations have been performed to investigate the basis set and correlation energy dependence of computed lithium ion affinities of a series of first- and second-row neutral bases (NH3, H2O, HF, HCN, CO, PH3, H2S, HCl) and of the corresponding anions which result from the loss of H+. The basis set dependence was evaluated at fourth-order many-body Moller-Plesset perturbation theory [MBPT(4) = MP4], using the Dunning correlation-consistent polarized split-valence basis sets (cc-pVXZ, where X = D for double, T for triple, and Q for quadruple split) and these sets augmented with diffuse functions on atoms other than H and Li (aug'-cc-pVXZ). The presence of diffuse functions in the basis set lowers computed Li+ affinities and reduces the basis set superposition error. Computed aug'-cc-pVXZ Li+ affinities converge with increasing basis set size, with satisfactory convergence occurring at aug'-cc-pVTZ. Comparison with CCSD(T) results indicates that the Moller-Plesset expansion appears to be converging except for the Li+ affinities of the anionic first-row bases NH2-, OH-, and F-.