Correlation consistent basis sets for explicitly correlated wavefunctions: valence and core–valence basis sets for Li, Be, Na, a

Correlation consistent basis sets for the alkali and alkaline earth metals Li, Be, Na, and Mg have been optimized for use with explicitly correlated F12 methods. These include orbital basis sets for valence-only (denoted cc-pVnZ-F12) and core–valence (cc-pCVnZ-F12) correlation, along with compact complementary auxiliary basis sets for use in the resolution of the identity approximation. Additional auxiliary basis sets that can be employed in the density fitting of two-electron integrals in both explicitly correlated methods and in more conventional correlated methods, such as density fitted second-order Møller–Plesset perturbation theory (DF-MP2), have also been developed by adding additional diffuse or core–valence functions to the cc-pVnZ/MP2FIT sets of Hättig. Explicitly correlated calculations with the approximate CCSD(T)-F12b method have been carried out with these basis sets on a series of sixteen test molecules to demonstrate their accuracy for optimized geometries, harmonic vibrational frequencies, and atomization energies. Results comparable to conventional CCSD(T) quintuple-ζ, which are near the complete basis set limits for these molecules, are obtained using CCSD(T)-F12b with just triple-ζ quality basis sets. The effects on the spectroscopic constants from correlating the outer core electrons are accurately recovered with just the cc-pCVDZ-F12 basis sets