Electronic Structure and Low-Lying Triplet States of CaO

Precise symmetry-restricted Hartree–Fock calculations based on the method of Roothaan have been carried out to provide information not previously available on the existence of low-lying triplet states of the CaO molecule and new information on its normal electronic structure. It is shown that in the Hartree–Fock approximation the lowest states in order of increasing energy are 3Π, 1Π, 3Σ+, and 1Σ+, with 3Π being the Hartree–Fock ground state. In their general features, these states in both structure and order are similar to those calculated earlier by others for BeO and MgO in the same series. In contrast with the conclusions of earlier studies, however, an open-shell configuration of σσ structure is shown to be the true variational reference state for the lowest 1Σ+ term. This may be the case also for the other alkaline-earth monoxides. On the grounds of straightforward arguments, it is then demonstrated that strong configuration interaction is necessarily an essential feature of the lowest experimental 1Σ+ state. It is suggested that this configuration interaction is likely to depress 1Σ+ below 3Π, giving an X 1Σ+ ground state in agreement with experimental evidence.