Application of the overlap model to calculating correlated exchange energies

We develop methods for approximating correlated exchange energies which require only Hartree-Fock self-consistent field (SCF) exchange energies and SCF and correlated charge density overlaps. We benchmark the methods using results calculated for the water dimer at the second-order Moller-Plesset (MP2) level using symmetry-adapted perturbation theory. Assuming that the exchange/overlap ratio is transferable between SCF and MP2 calculations gives a weighted RMS error of 3.2% with no fitted parameters. Including a single overall scaling parameter gives an error of 2.3%. (C) 2000 Published by Elsevier Science B.V.