v ext is the external potential acting on the interacting system (at minimum, for a molecular system, the electron–nuclei interaction), E H is the Hartree (or Coulomb) energy E H [ ρ ρ ] = e 2 2 ∫ ∫ d r ∫ ∫ d r ′ ρ ρ ( r ) ρ ρ ( r ′ ) | r − − r ′ | ,