Evaluation of alkali metal cation affinities and basicities using extrapolation to the complete basis set limit.

The complete basis set (CBS) extrapolation is used at Hartree-Fock and second-order Møller-Plesset perturbation theory levels and with the def2-xZVP x-ζ basis set (x = 2-4). This approach leads to general, robust, and well-calibrated methods, especially when Hartree-Fock energy (E(HF)) and correlation energies (E(CE)) are extrapolated separately. Indeed, the absolute deviations between theoretical and experimental data are usually smaller than the reported experimental errors. We also point out the need to change usual parameters utilized in CBS methods when calculations involve atoms from third and subsequent rows. The best CBS scheme studied in the current work for obtaining energies for the estimation of alkali metal cation affinities and basicities is E(CBS)[∞] = 1.10529·E(HF)[4] - 0.10529·E(HF)[2] + 0.92703·E(CE)[4] - 0.07297·E(CE)[2], where E(HF)[2], E(HF)[4], E(CE)[2], and E(CE)[4] are the Hartree-Fock energy (E(HF)) and MP2 correlation energies (E(CE)) obtained with def2-QZVP (x = 4) and def2-SV(p) (x = 2) basis sets.