, where α is a HF mixing parameter) may be defined using ensemble generalized Kohn-Sham theory. An additional challenge in ensemble DFT is that the correlation energy is more complicated than its standard (pure-state) DFT counterpart. Firstly, because it must address multiple states at once. Secondly, because it contains density-driven (DD) correlations, 429 which are a consequence of the fact that Kohn-Sham states reproduce the correct total density, but not the correct densities of the individual interacting states included in the ensemble. Gould and Pittalis defined DD correlations in special types of ensembles. Fromager then provided a rigorous general scheme for understanding DD correlations. 430 (3.7.3) Fromager : Just for the sake of clarity, it is probably good to explain why a distinction has to be made between thermal ensembles and other types of ensembles like, for example, the Gross-Oliveira-Kohn (GOK) ensemble 422 or the more recent N-centred ensembles 423 , which are (somehow artificially) constructed in order to compute neutral or charged excitation energies in a completely time-independent formalism.