(2.1.30) Galli : In the Hohenberg-Kohn formulation, DFT is an exact theory of ground and excited states, entirely based on the electron density. That is, the density determines uniquely the potential, hence both ground and excited state properties of the system may in principle be derived. However there is no practical recipe on how to derive such potential and hence on how to derive neither ground or excited state properties. The Kohn-Sham formulation, in contrast, is applicable only to ground-state properties, although in practice it is applied also to excited states.