ON THE RELATIONSHIP BETWEEN 1ST-ORDER EXCHANGE AND COULOMB INTERACTION ENERGIES FOR CLOSED-SHELL ATOMS AND MOLECULES

First-order Heitler-London interaction energies E(HL)(1), and their fi rst-order Coulomb and exchange components E(C)(1) and E(X)(1), have be en computed for the Rg-Rg', H-2-Rg and N2-Rg interactions, where the r are gas Rg or Rg' can be He, Ne, Ar or Kr. High quality self-consisten t field wavefunctions are used to represent the isolated species and t he results are presented as a function of the interspecies distance an d relative orientation (where appropriate). The results are used to in vestigate the relationship between the first-order exchange and Coulom b energies. and it is found that the often used linear relationship, E (X) = -gamma(1 + aR)E(C)(1), requires modification for interactions in volving molecules for physically significant (small) values of R. The modification proposed is E(X)(1) = -gamma(1 + aR)(E(C)(1) - E(S)(1), w here E(S)(1) is a correction term for the singularity occurring in E(C )(1), and two versions of E(S)(1) are developed. The importance of the singularity, and the validity of the relationships between E(X)(1) an d E(C)(1), are discussed as a function of R, orientation, and intrinsi c versus basis set incompleteness errors in the models. The analysis r eported here augments earlier studies, related to the original model, based on interactions involving H atoms and the He-He interaction usin g a product of two screened 1s orbitals to represent He. The general u sefulness of the ab initio results for E(HL)(1), E(X)(1) and E(C)(1) i s also discussed briefly.