Ensemble density-functional theory of charged and neutral electronic excitations: Exact construction of energy levels and beyond

It has been shown recently that charged and neutral electronic excitation processes can be described in principle exactly within a unified (so-called N-centered) ensemble density-functional theory (DFT) where the number of electrons is artificially held constant and equal to an integer. The practical advantage is that, unlike in conventional formulations of DFT, the derivative discontinuities of the density-functional exchange-correlation (xc) energy that occur when electrons are excited can essentially be removed from the theory. Their contribution to the physical excitation energies is then taken care of by the universal ensemble xc functional, through its ensemble weight dependence. In this talk, I will review the theory and discuss further extensions to spin-DFT and DFT for electrons and nuclei. If time permits, the extraction of linear response properties such as the oscillator strengths will also be discussed.