In the general many-electron case, the ensemble Hxc functional needs to know if it has to compute the Hxc energy of a pure ground state or of an ensemble consisting of ground and excited states, hence the ξ ξ ξ dependence in E ξ ξ ξ Hxc [ρ]. The extraction of excitation energies from an ensemble DFT calculation reveals the importance of this weight dependence. In particular, it has been shown that ensemble density-functional weight derivatives ∂ E ξ ξ ξ Hxc [ρ]/∂ ξ I are directly connected to the derivative discontinuities that the Hxc potential exhibits when a given excited state is incorporated into the ensemble. In DFT for (canonical, for simplicity) thermal ensembles, the ensemble weights are controlled by a single parameter -the temperature or, equivalently, the inverse temperature β . In thermal DFT, the variational principle is extended to the total (Helmholtz) energy, which contains an entropic contribution: