Density functional theory relies on the theorems of Hohenberg and Kohn [144].
These two theorems could be summarized in the following: (I) That the ground state of a many-electron system (under the effects of an external potential) is uniquely defined by the ground-state electron density, and (II) The existence of an energy functional of the density that is minimal for the ground state density
giving the ground state energy.

Rocio Yanes