R. K. Nesbet,
IBM Almaden Research Center, San José, U.S.A.

For polyatomic systems, electronic structure calculations are greatly facilitated if electronic correlation can be modeled by a mean potential field acting on otherwise independent electrons. For ground states and thermal ensembles, total energy can be expressed as a functional of electronic orbital functions. This implies a formally exact orbital functional theory (OFT), a variational mean-field theory in which generally nonlocal potential functions are defined by orbital functional derivatives. Reduction to the local potentials postulated in density functional theory (DFT) is not possible in general, but can be a very useful approximation in particular circumstances. Practical implementation of OFT will be discussed. The methodology, not restricted to local potentials, retains the the computational efficiency of DFT.