(3.4.3) Neese : The first thing that comes to my mind in this context is the multiplet problem -that is, the simple fact that a single electron configuration (meaning a distribution of electrons among orbitals with occupation 0, 1 or 2) gives, in general, a number of different many-particle states with different spin couplings among the unpaired electrons. This is not an esoteric formal remark. In open-shell transitionmetal complexes, for example, the multiplet problem is extremely prevalent and affects all of their physical (spectroscopic) properties in a profound way. Take a simple L-edge (2p → 3d) excitation. In a d 5 system, this excitation leads to as many as 1512 different final states that all contribute to the L-edge absorption spectrum. Yet, in a particle-hole theory such as TDDFT, one only has 15 particle-hole pairs to work with. How to describe 1512 states with only 15 particle-hole pairs is not clear. In practice, the failures are dramatic. Another example are the d-d spectra of these ions, which frequently show low-lying double excitations that are completely absent from the DFT-computed spectra. Similar remarks hold for many multi-determinantal (as opposed to multi-configurational) problems, like spin-coupled openshell ions. Surely, broken-symmetry DFT is a highly useful tool, yet it is a bit of a crutch and I am not aware of a satisfactory formal solution that would also be practical.