From the density functional to the single-particle Green function
  An analysis shows that the ground state of the inhomogeneous system of interacting electrons in the static external field, which satisfies the thermodynamic limit, can be consistently described only using the Green function theory based on the quantum field theory methods (perturbation theory diagram technique). In this case, the ground state energy and inhomogeneous electron density in such a system can be determined only after calculating the single-particle Green function. 
V.B. Bobrov
cond-mat.stat-mech cond-mat.str-el