The contributions from two important mechanisms to the hyperfine pressure shift (HPS) that are not included in restricted Hartree-Fock (RHF) calculations on interacting atoms are analyzed using the Li-He system as a specific example. First, it is shown that the influence of exchange core polarization, which is neglected in the RHF approximation, leads to about 40% of the direct contribution from distortions of the valence $2s$ orbital. Secondly, the long-range van der Waals polarization effect is missing in the RHF approximation and it is shown that this also can lead to a substantial contribution (about 20% of the direct contribution and of opposite sign) to the HPS. All of the three contributions to the HPS considered here—namely, from the direct polarization, exchange core polarization, and van der Waals mechanisms—are found, using two choices of intermolecular potentials, to be quite sensitive to the intermolecular potential, while their ratios are less sensitive. Thus, although the results for the total HPS from the present work are not to be taken as quantitative, nevertheless, they indicate that the exchange core polarization and van der Waals contributions, together with correlation effects, have to be included for a complete theory of HPS.

• Received 8 November 1971

DOI:https://doi.org/10.1103/PhysRevA.8.1673