One of the earliest derivations of perturbation theories of molecular interactions that included the effects of molecular overlap was given by Murrell et al. as well as Salem and Musher in the mid-1960s. Expressions presented by them were up to the secondorder in terms of the intermolecular interaction operator. However, those formulations only included the exchange effects at the level of the single electron pair approximation. As it was shown (vide infra), the single-pair approximation might be very poor for systems with strongly diffused monomers, such as monomers with low-lying excited states or anionic states .