Atomic and molecular exchange-correlation charges in Kohn-Sham theory

A new definition of the exchange-correlation charge has recently been proposed (S. Liu, P. W. Ayers and R. G. Parr, J. Chem. Phys., 1999, 111, 6197). This charge, which is related to the exchange-correlation potential by the Poisson equation, is attractive due to its dependence on a single electronic coordinate. Using the method of Zhao, Morrison and Parr (Phys. Rev. A., 1994, 50, 2138), accurate charges have previously been calculated for the spherical systems He, Ne and Ar. By using a gaussian basis set implementation of this method, we demonstrate that the charge can be routinely determined for both atoms and small molecules; we consider He, Ne, HF, CO and N-2. We highlight the deficiencies of finite gaussian basis sets, and graphically examine the charges. The molecular charges show the same characteristics as those observed in the atomic charges, with structure that is closely related to that of the associated exchange-correlation potentials. The determination of accurate exchange-correlation charges may aid the development of improved exchange-correlation functionals and model potentials.