THE NONADDITIVE EXCHANGE ENERGIES OF H-3 AND HE-3

Non-additive SCF energies are calculated for H3 (quartet spin state) a nd He3 using Hartree-Fock quality basis sets containing only s functio ns. The results are analysed using a modification of an effective Gaus sian model, replacing the explicit dependence on Gaussians by related dimer and/or trimer properties. A good fit to the SCF results is obtai ned using a model based on dimer and trimer charge density overlap int egrals, and two adjustable parameters.