Identities for density functionals linking functionals of different   densities
  In electron density functional theory formal properties of density functionals play an important role in constructing and testing approximate functionals. In this paper it is shown that a set of density functionals satisfy an equation that links functionals of densities of N and N-1 systems of the same Hamiltonian. In particular the identity is satisfied by the sum of the Hartree and exchange-correlation energies. 
Daniel P. Joubert
cond-mat.mtrl-sci