An exchange functional for accurate virtual orbital energies

We show that a simple Gaussian model for exchange yields Kohn-Sham virtual orbital energies that are in significantly better agreement with Hartree-Fock theory than those arising from functionals based on the uniform electron gas. Also we show that normalization of the Gaussian model significantly improves the accuracy of total exchange energies, and that reparametrizing a Becke-type asymptotic correction leads to total exchange energies which are only slightly less accurate than B88. Errors in HOMO-LUMO gaps for this new functional are typically less than a third of the corresponding B88 errors. Many-body perturbation theory using B88 exchange in the zeroth-order Kohn-Sham problem is shown to be divergent or very slowly convergent for some typically well-behaved closed shell systems; using the functional presented here, though, convergence is in each case at a rate comparable with normal Moller-Plesset perturbation theory. (C) 2000 American Institute of Physics. [S0021- 9606(00)30216-1].