Alkali and alkaline earth metal compounds: core—valence basis sets and importance of subvalence correlation

Abstract

Core-valence basis sets for the alkali and alkaline earth metals Li, Be, Na, Mg, K, and Ca are proposed. The basis sets are validated by calculating spectroscopic constants of a variety of diatomic molecules involving these elements. Neglect of (3s, 3p) correlation in K and Ca compounds will lead to erratic results at best, and chemically nonsensical ones if chalcogens or halogens are present. The addition of low-exponent p functions to the K and Ca basis sets is essential for the smooth convergence of molecular properties. Inclusion of inner-shell correlation is important for accurate spectroscopic constants and binding energies of all the compounds. In basis set extrapolation/convergence calculations, the explicit inclusion of alkali and alkaline earth metal subvalence correlation at all steps is essential for K and Ca, strongly recommended for Na, and optional for Li and Mg, while in Be compounds an additive treatment in a separate ‘core correlation’ step is probably sufficient. Consideration of (ls) inner-shell correlation energy in first-row elements requires the inclusion of (2s, 2p) ‘deep core’ correlation energy in K and Ca for consistency. The latter requires special CCVnZ ‘deep core correlation’ basis sets. For compounds involving Ca bound to electronegative elements, additional d functions in the basis set are strongly recommended. For optimal basis set convergence in such cases, we suggest the sequence CV(D + 3d)Z, CV(T + 2d)Z, CV(Q + d)Z, and CV5Z on calcium.