FRACTIONAL OCCUPATION NUMBERS IN DENSITY-FUNCTIONAL THEORY

Density-functional theory for ensembles defined by fixed weights and f ractional occupation numbers is derived using a Lagrange-multiplier ve rsion of the constrained-search procedure. This is related to an adiab atic connection between a generalized Kohn-Sham ensemble for nonintera cting electrons and a minimum-energy ensemble including the electronic Coulomb interaction. Using this formalism, it is shown that discontin uous changes of the chemical potential of a finite electronic system r esult from constraints on density variations imposed by Fermi statisti cs. Such discontinuities do not exclude the possibility of defining an exchange-correlation energy functional that is free of derivative dis continuities for unconstrained density variations.