Charged Excitations Made Neutral: N-Centered Ensemble Density Functional Theory of Fukui Functions

An in-principle exact working equation to compute electronic affinity and ionization Fukui functions is derived within the N-centered (Nc) ensemble extension of density functional theory (DFT). It circumvents the kernel derivative discontinuity problem of DFT for fractional electron numbers, whose contribution is recovered through weight derivatives of the ensemble density functional potential. Thus, it allows for the design of alternative and effective approximations, such as the weight-dependent scaling of regular functionals or the interpolation between known limits of Nc ensembles.