EXCHANGE ENERGY FUNCTIONALS BASED ON THE DENSITY-MATRIX EXPANSION OF THE HARTREE-FOCK EXCHANGE TERM

In a recent paper (Koehl, R. M., Odom, G. K., and Scuseria, G. E., 199 6, Molec. Phys. 87, 835), we proposed a generalized version of the den sity matrix expansion of Negele and Vautherin. In the present paper, w e have further modified this expansion by augmenting the second order term to explicitly include gradient corrections and correcting the Fer mi momentum to account for the non-uniformity of the electron density. The resulting functional gives better results than the original excha nge functional according to some preliminary hybrid calculations using Hartree-Fock densities for the G2 set of Pople and co-workers. While most of our investigations only concern closed shell molecules, we sho w that the results are easily generalized to the open shell case. The functionals thus obtained offer the possibility of improving the accur acy of existing exchange-correlation functionals.