Summary of exchange perturbation calculations for H2 at large separations

This paper is a summary and appraisal of the recent work of the authors together with Kolos and Wolniewicz which applied four types of perturbation procedures to the calculation of the energy of the ground state and first excited state of the hydrogen molecule at large separations. All of the perturbation techniques gave good values for the exchange energy but less accurate values for the Coulombic energy. The reasons why the second-order Eisenschitz-London (or van der Avoird or Hirschfelder HAV) energy approaches one-half of the correct limit at large separations are discussed. There seems to be no uniqueness to the orders in an exchange perturbation problem.