<i>N</i>‐centered ensemble density‐functional theory for open systems
Bruno Senjean, Emmanuel Fromager
Abstract
Abstract Two (so‐called left and right) variants of N ‐centered ensemble density‐functional theory (DFT) are presented. Unlike the original formulation of the theory, these variants allow for the description of systems with a fractional electron number. While conventional DFT for open systems uses only the true electron density as basic variable, left/right N ‐centered ensemble DFT relies instead on (a) a fictitious ensemble density that integrates to a central (integral) number N of electrons, and (b) a grand canonical ensemble weight α which is equal to the deviation of the true electron number from N . Within such a formalism, the infamous derivative discontinuity that appears when crossing an integral number of electrons is described exactly through the dependence in α of the left and right N ‐centered ensemble Hartree‐exchange‐correlation density functionals. Incorporating N ‐centered ensembles into existing density‐functional embedding theories is expected to pave the way toward the in‐principle‐exact description of an open fragment by means of a pure‐state N ‐electron many‐body wavefunction. Work is currently in progress in this direction.