Perturbation calculations on the variation of hydrogen---bond energies with intermolecular distance

In previous perturbation calculations on the hydrogen bond [6] the short-range repulsion was seriously underestimated. It is shown that this can be remedied by choosing a more realistic model system and using exact 3-centre integrals

Double-exchange contributions to the first-order interaction energy between closed-shell molecules

Explicit formulas are presented for the double-exchange first-order interaction energy of two closed-shell molecules. Calculations on He2 show that the double-exchange terms contribute significantly for R < 3.5 au