Ab initio study of CaO. The importance of atomic correlation and a bondlength question

Abstract

Extensively correlated ab initio potential energy and dipole moment curves are calculated for the X 1Σ+, a 3Π, A 1Σ+, and 3Σ+ states of CaO over the range 3.0 au ⩽ R ⩽ 6.0 au. The calculations are the first which correctly predict that X 1Σ+ is the ground state of CaO. Vertical spectra, adiabatic spectra, spectroscopic constants, curve crossings, and dipole moments are determined. The 3Σ+ results are apparently the first accurate values available. The dipole moment function is a linear function of bondlength for the Π states and 3Σ+. Two extrema occur in the dipole moment function of X 1Σ+. Combined with experimental results, this suggests that μ(X 1Σ+) exhibits a maximum near Re in the heavy alkaline earth monoxides. The μ(A 1Σ+) increases sharply near Re. The X 1Σ+ calculations require special attention near Re: Localized ionic entities are involved which have different MO configurations and different correlation requirements. A second-order theory which correctly describes atomic correlation is necessary. The ab initio bondlengths are systematically larger than the experimental bondlengths by amounts that are greater than the expected accuracy limits of the calculations. This overestimation is also observed in KF, KCl and KOH, but ab initio bondlengths of the lighter metal alkali halides and alkaline earth monoxides agree closely with experiment. It appears that the core electrons must be responsible, possibly because of explicit core correlation requirements and/or relativistic corrections.