Theoretical study of the diatomic alkali and alkaline-earth oxides

A b initio calculations at the self-consistent field and singles plus doubles configuration-interaction level are used to determine accurate spectroscopic parameters (De,re,ωe) for the 2Π and 2∑+ states of the alkali oxides and the a 3Π (or A 1Π) states of the alkaline-earth oxides. Numerical Hartree–Fock (NHF) calculations performed on KO demonstrate that the extended Slater basis sets employed are near the Hartree–Fock limit. When the dissociation is referenced to the ionic limits, the differential correlation contribution is found to increase the D0 by a relatively small amount (0.0–0.2 eV). An accurate description of the alkali and alkaline-earth oxides requires correlating both the oxygen and metal electrons. The theoretical dissociation energies (D0) permit a critical assessment of the experimental literature and allow us to recommend D0 values that are accurate to 0.1 eV for all systems considered. There is a strong correlation between the dissociation energy (to ions) and re, because the bonding is predominantly electrostatic in origin. Theoretical 2Π–2∑+ energy separations are presented for the alkali oxides. An extensive study of the 2Π–2∑+ energy separation in KO reveals a 2∑+ ground state at all levels of theory. Basis set studies in combination with NHF calculations indicate different basis set requirements for the 2Π and 2∑+ states. In the NHF limit the 2∑+ state of KO is lower by about 250 cm−1. The separation is almost unaffected when the 15 valence electrons are correlated at the singles plus doubles level using an extended Slater basis.