The relationship between the density functional   Hartree-plus-exchange-correlation potential for an integer N-electron and   (N-1)-electron system
  It is shown that the Hartree-plus-exchange-correlation density functional potential for an integer N-electron system differs by a constant form the corresponding potential for an (N-1)-electron system if the densities are determined from the same external potential. 
Daniel P. Joubert
cond-mat.mtrl-sci