We have extended a form of double perturbation theory for molecules by including a screening constant in the calculation. We have computed the ground state energies for the diatomic systems H2+, HHe2+, H2 and HHe+ with inclusion of screening. A comparison of the results with those in the unscreened approximation is interesting. Thus for the one-electron systems the energies up to second order are better if we do not include screening. However, for the two-electron systems the energies up to second order are improved by screening except at small internuclear separation.
Bibliographical noteFunding Information:
One of us (K. B. L.) would like to acknowledge the support of the Science Research Council (U.K.). We would also like to acknowledge the support of the U.S. Army through its European Research Office.