Analysis of intra-atomic correlation effects on the first order interaction energy between He atoms

The first order interaction energy between two He atoms is calculated with a large CI function (reproducing 97.2 per cent of the correlation energy of the He atom) for interatomic distances R within the range 3.0-5.6 a0. It is shown that the intra-atomic correlation effect gives a repulsive contribution which increases with R up to 7 per cent of the uncorrelated result in the region of the van der Waals minimum (R=5.6 a0). Basis set effects and approximate methods of estimation of the intramolecular correlation effects are also discussed. (author)