Comparative study of unconventional 1 s basis functions for the 1 Σ g + ground state of H 2 and He 2 ++

We investigated various nonstandard 1 s basis functions (generalized Slater‐Gaussian, ellipsoidal Gaussian, floating spherical and ellipsoidal Gaussian, rational function, Hulthén approximation, two‐Slater‐type orbital, generalized Guillemin–Zener function, and various noninteger‐ n elliptical orbitals) for approximating the 1 Σ   g +ground state of H 2 and He 2 ++ . A CI trial wave‐function including Σ g ‐type MO's is adopted and molecular integrals are evaluated numerically. The energy improvement on the 1 s STO is small except for noninteger‐ n orbitals which closely approach the “ SCF limit”.