Inhomogeneous Electron Gas

This paper deals with the ground state of an interacting electron gas in an external potential v(r). It is proved that there exists a universal functional of the density, F[n(r)], independent of v(r), such that the expression E = integral v(r)n(r)dr + F[n(r)] has as its minimum value the correct ground state energy associated with v(r). The functional F[n(r)] alone is sufficient to determine the exact ground state energy and density. Approximations to the functional are discussed within the framework of the slowly varying density.