SCF Calculations of the Electronic States of Magnesium Monoxide

Abstract

Ab initio calculations based on Nesbet's symmetry and equivalence restriction version of the LCAO—MO—SCF method, using an extended double‐zeta basis set, have been performed at a number of internuclear separations for six configurations, in order to account for the features of the spectrum of MgO and to ascertain the nature of the ground state. Most arguments coming from both experimental data and theoretical calculations corrected by semiempirical estimates of correlation energy differences between the configurations, are in favor of a closed‐shell ¹Σ⁺ ground state, although the lowest Hartree—Fock state is a less correlated ³II open‐shell state. Higher‐lying observed states (D¹Δ and C¹Σ⁻) as well as unobserved ones up to the first ionization limit are accounted for by variational or reliable virtual orbital calculations, and they are presented in an energy‐level diagram. In this connection, the properties of the variational ²II and ²Σ⁺ low‐lying levels of MgO⁺ are discussed. It is then suggested that, in view of this, the unclassified ultraviolet bands should most likely be partly assigned to triplet—triplet transitions involving the very low‐lying ³II state. Finally, reasonable predictions about other as yet unobserved band systems are made.