and E xc is the exchange–correlation energy. The Kohn–Sham equations are found by varying the total energy expression with respect to a set of orbitals, subject to constraints on those orbitals, to yield the Kohn–Sham potential as v eff ( r ) = v ext ( r ) + e 2 ∫ ∫ ρ ρ ( r ′ ) | r − − r ′ | d r ′ + δ δ E xc [ ρ ρ ] δ δ ρ ρ ( r ) , where the last term v xc ( r ) ≡ ≡ δ δ E xc [ ρ ρ ] δ δ ρ ρ ( r ) is the exchange–correlation potential. This term, and the corresponding energy expression, are the only unknowns in the Kohn–Sham approach to density functional theory. An approximation that does not vary the orbitals is Harris functional theory.