(3.7.1) Savin : When discussing ensembles, we should be careful to distinguish between the different cases. Are we interested in describing ensembles associated with degenerate states? For example, do we want to construct universal density functionals that have the same value for all the densities of the ensemble? Do we mean errors that show up due to the locality in our approximations 419 -for example, at dissociation? Do we mean ensembles that show up in (even accurate) Kohn-Sham calculations (cf. the pure-state v-representability discussion in ref. )? Do these ensembles survive at weak interactions? If a multireference treatment is needed in wave-function theory, can we use ensembles with (semi)local DFAs in DFT? We should keep in mind that the same classical ensemble may correspond to different wave functions, by ignoring the effect of the interference term produced by the sign (phase) of the coefficients. Ensembles are introduced for very different reasons for ground states, excited states and high temperature.