  Typical density functional theory (DFT) and approximations thereto solve the
many-electron ground state problem by working from a non-interacting Kohn-Sham
reference system. In practice, DFT benefits from useful minimization conditions
that let approximate solutions iteratively approach the optimal energy and
density. The present work reveals that stationary conditions also apply to all
excited states of general electronic systems; and may be defined
straightforwardly from ensemble extensions of DFT. However, it also reveals
that the stationary condition for the non-interacting Kohn-Sham reference
systems that make density functional theory numerically efficient is more
complex than the ground state problem. The extra complexity reflects a
contribution from density-driven correlations. Some implications for
approximations and self-consistency are discussed.
