Valence bond energy curves for He²⁺₂

Ab initio valence bond self-consistent field calculations have been carried out on He²⁺₂ in a large Gaussian basis set for several sized structure expansions of the wavefunction. The results are used to validate the method and to test the essential elements of the avoided crossing semi-empirical VB model of Gill and Radom. The energy dissociation curve of the 19 structure expansion is close to the complete configuration interaction curve in the same basis set. In the neighborhood of the ¹∑⁺_g ground state energy minimum the VB coupling matrix elements between the lowest energy covalent (He⁺ He⁺) and ionic (He²⁺ He) structures is found to be large and of similar magnitude to the ionic structure diagonal energy. The covalent structure energy curve is not purely Coulombic repulsive, as assumed in the semi-empirical model, but shows an attractive component due to a polarization contribution. The proportionality constant in the semi-empirical formula for the coupling matrix elements is found to be lower than the assumed value due to the omission of this attractive component.