Fortunately, there are many indications that the "nonuniqueness" of the potential is harmless for Kohn-Sham applications because the wave function remains unique even when the potential is not. However, I do feel that all multivariable DFTs have another hidden problem -namely, to what extent can the intensive variables of the theory be varied arbitrarily and independently of each other? This problem may be particularly severe when there are global constraints enforced by symmetry -for example, the total angular momentum of a rotationally symmetric system. Or there may be inequalities, whereby the maximum value of a density (say, the spin density) can never exceed some maximum value that is controlled by another density (say, the particle density). Such constraints affect the intensive variables but not their conjugate fields, which can always be varied independently. This is one of the main reasons (if not the only one) why the map from densities to potentials is generally not invertible. ) formulation of spin DFT for a noncollinear spin density seems to be nearing a "phase transition" with the appearance of new DFAs. There exist several interesting ideas for the construction of noncollinear spin functionals using gauge-invariant blocks 655 and orbital functionals: 656 they should be pursued. The expectation value of any observable quantity is a functional of the ground-state density. As far as I am aware, there is little progress in developing exact or approximate functionals for general observables, except for the total energy and the density itself. The definition of the density functional of any observable is known -for example, if Q is the operator for the observable Q, then the density functional Q[ρ] is given by