Repulsive ∑ and low-lying (⩾ 1.9 eV) ∑ states of BeO

Ab initio calculations have been carried out on the lowest ³∑⁻ and ³∑⁺ states of beryllium oxide. A “double zeta plus polarization” set of Slater functions was used. The self-consistent-field wavefunction for the ³∑⁻ state dissociates properly to ground state Be and O atoms and is repulsive. Electron correlation was explicitly considered for the ³∑⁺ state using “first-order” wavefunctions, which have yielded reliable dissociation energies for other diatomic molecules. The ³∑⁺ state, which has not been observed experimentally, is predicted to lie 1.91 eV above the lowest ¹∑⁺ state. Predicted spectroscopic constants for the ³∑⁺ state are r_e = 1.384 Å, ω_e = 1234 cm⁻¹, and B_e = 1.527 cm⁻¹.