The Kohn–Sham orbital energies ε i, in general, have little physical meaning (see Koopmans' theorem). The sum of the orbital energies is related to the total energy as E = ∑ ∑ i N ε ε i − − E H [ ρ ρ ] + E xc [ ρ ρ ] − − ∫ ∫ δ δ E xc [ ρ ρ ] δ δ ρ ρ ( r ) ρ ρ ( r ) d r .