Symmetry-adapted perturbation theory calculation of the intra-atomic correlation contribution to the short-range repulsion of helium atoms

The first-order exchange energy in the interaction of two helium atoms is expanded as a double-perturbation series with respect to the Moller–Plesset-type fluctuation potentials for the isolated atoms. The resulting perturbation corrections have been calculated using orbital basis sets of varying size. Results in our largest [7s4p2d1f] basis are compared with an accurate value of the first-order energy computed using Gaussian-type geminals. It is shown that the Moller–Plesset-type expansion for the exchange repulsion energy converges moderately fast and that the major part of the intra-atomic correlation correction to the first-order exchange energy is due to one-particle correlation effects included in the Brueckner orbitals.