Rational function approximation of the configuration interaction optimized 1Σ +g ground state of H2 and He 2+2

Abstract

A rational approximation of the 1s basis function: χ=(ζr^n′+ a)^–1 exp (–ζrⁿ) is used with (non-) integer values of n and n′ to approximate the (1 σ_g)² ground state of H₂(at R= 1.4 a.u.) and He²⁺₂(at R= 1.3 a.u.) within a CI framework and using numerical integration over the molecular orbitals. Results are compared with the usual 1s STO, 1s GTO and the non-integer-n 1s STO. The most significant improvement in energy is reached by using orbitals with non-integer values of n′.