Kohn-Sham approximation scheme for an interacting Bose-condensed gas

The grand canonical density functional theory for inhomogeneous systems of interacting bosons is developed in the effective action approach. The Legendre transform of the generating functional for Green's functions is used to define the effective action as a functional of both the particle density and the order parameter. Expanding the thermal effective action in powers of the Planck constant we obtain a systematic approximation scheme, which practically implements the Kohn-Sham idea: the problem of interacting bosons is reduced to a single-particle system in a fictitious external potential. The Kohn-Sham potential, the density and the order parameter have to be determined self-consistently in a given order approximation.