Exchange and Correlation Energy in Density Functional Theory

Several different versions of density functional theory (DFT) that satisfy Hohenberg-Kohn theorems are characterized by different definitions of a reference or model state determined by an N-electron ground state. A common formalism is developed in which exact Kohn-Sham equations are derived for standard Kohn-Sham theory, for reference-state density functional theory, and for unrestricted Hartree-Fock (UHF) theory considered as an exactly soluble model Hohenberg-Kohn theory. A natural definition of exchange and correlation energy functionals is shown to be valid for all such theories. An easily computed necessary condition for the locality of exchange and correlation potentials is derived. While it is shown that in the UHF model of DFT the optimized effective potential (OEP) exchange satisfies this condition by construction, the derivation shows that this condition is not, in general, sufficient to define an exact local exchange potential. It serves as a test to eliminate proposed local potentials that are not exact for ground states. © 2000 John Wiley & Sons, Inc.