Exchange perturbation theory. II. Eisenschitz-London type

An exchange perturbation theory is developed which is identical through first order in the primitive function G with the Eisenschitz-London (EL) theory. It is shown that in higher orders, G is different from the EL primitive function and from the primitive functions of related theories. The function G is least distorted from the zeroth-order function F⁰, a product of functions for the subsystems when the interactions have been set equal to zero. The potential which distorts F⁰ into G is more thoroughly screened than in any other theory we have examined. We argue that this EL-type theory should be used when the unscreened interactions are strong.