Since the definition of the density functional of a general observable is known, the development of an approximate or exact density functional Q[ρ] means trying to find an exact or approximate expression that does not depend on the unknown interacting state Ψ ρ , but which depends only on the density and also on quantities that can be obtained from a Kohn-Sham calculation (e.g., the Kohn-Sham state, orbitals, and eigenvalues) and which is more accurate than the obvious approximation in eqn (72). Recently, we managed to write down such a density functional for the magnetization density in DFT (not spin DFT) for open-shell systems in the absence of an external magnetic field 657 .