(3.4.2) Rebolini : Although Kohn-Sham theory is in principle valid for all systems, in practice existing DFAs mostly fail at describing multi-reference systems. However, one may want to distinguish between properties that depend on the total density of the system, which may still be properly described, and properties which depend specifically on the strongly-correlated electrons where DFAs are almost "expected" to fail -for instance, DFAs can be used to study the equilibrium structure, phonon spectrum, and polarization of strongly-correlated materials but fail to describe most magnetic properties.