Electronic States of Diatomic Molecules: The Oxygen Molecule

Abstract

A scheme is proposed for the computation of electronic energy levels of diatomic molecules with fair accuracy and minimum labor. The theoretical LCAO MO method including configuration interaction is employed. However, the number of configurations considered is kept small; the energies of the asymptotic dissociation products for the various states are taken from atomic data in the manner of Pariser; the calculation of the interaction energies is simplified by the neglect of overlap in all terms and by the neglect of differential overlap in the electronic repulsion terms; and certain core parameters are fitted empirically. The first application is made to the oxygen molecule, using one empirical parameter. The vertical excitation energies from the ground state to the ¹Δ_g, ¹Σ_g⁺, ³Σ_u⁺, and ³Σ_u⁻ states are computed as functions of distance between 1.16A and 1.63A, and they agree with the observed values within 0.2 ev; excitation energies to the unobserved ¹Σ_u⁻, ³Δ_u, ¹Δ_u, and ¹Σ_u⁺ states are also computed. The results are in substantial accord with a previous more involved calculation by Moffitt. A justification of the proposed scheme is presented which makes use of the orthonormalized atomic orbitals of Löwdin.