Density-Functional Theory for Fractional Particle Number: Derivative Discontinuities of the Energy

Abstract
The Hohenberg-Kohn theorem is extended to fractional electron number N, for an isolated open system described by a statistical mixture. The curve of lowest average energy EN versus N is found to be a series of straight line segments with slope discontinuities at integral N. As N increases through an integer M, the chemical potential and the highest occupied Kohn-Sham orbital energy both jump from EMEM1 to EM+1EM. The exchange-correlation potential δExcδn(r) jumps by the same constant, and limrδExcδn(r)>~0.