CEPA calculations on open-shell molecules. III. Potential curves for the six lowest excited states of He2 in the vicinity of their equilibrium distances

Abstract

CEPA-PNO and PNO-CI calculations have been performed for the potential energy curves of the He2+ ground state and the six lowest excited states of He2 in the range of 1.4 a0 ≤R ≤ 3.5 a0. The calculated equilibrium distances as well as the spectroscopic constants are in very good agreement with molecular constants as derived experimentally from the rotation-vibration spectrum of He2 by Ginter, except for thec3∑g+ state. This latter discrepancy is probably due to an “obligatory” hump in thec3∑g+ state occurring at 3.5 a0 which cannot be properly treated in our calculation. The relative energetic positions of the six lowest states and their ionization energies are reproduced by our calculations with an accuracy of 0–400 cm−1. Extrapolation of our results to infinite basis sets leads to estimates of the dissociation energies of He2 excited states which cannot be measured spectroscopically because of the humps in all these states.