The Hohenberg-Kohn theorem [3] states that the ground state
electron density 
minimises the energy functional
where F is a universal functional and 
is the potential due
to the static ions. Furthermore the minimum value of E is 
, the
ground state electronic energy.
This introduces the concept of electron density as a fundamental
variable. If the functional 
can be minimised with respect
to the electron density, this will yield the ground state electronic
energy and density of the system. It is important to note that this
means that calculations based on DFT are restricted to systems in
their ground state.
