  The electronic structure calculations based upon energy density functionals
are highly successful and widely used both in solid state physics and quantum
chemistry. Moreover, the Hohenberg-Kohn theorems and the Kohn-Sham method
provide them with a firm basis. However, several basic issues are not solved,
and hamper the progress to achieve high accuracy. In this paper we focus on the
conceptual problem of size consistency, basing our analysis on the
non-intensive character of the (spin) electronic density in the presence of
degeneracy. We also briefly discuss some of the issues concerning fractional
electron numbers from the same point of view, analyzing the behavior of the
exact functionals for the He and the Hooke's atom series when the number of
electrons fluctuates between one and two.
