I find it important to distinguish between multi-determinantal and multi-configuration problems. In the former case, there is a single electronic configuration but spin coupling of the unpaired electrons leads to a multi-determinantal wave function. In the latter case, there is an actual mixing of configurations with different orbital occupations. The former case is far easier and far more frequent. Recognizing this distinction may help (and, in fact, has already helped) to design more tailored approximations -for example, spin-flip methods.