MixOccupations / NoMixOccupations

The subkeys MixOccupations / NoMixOccupations (MIXOCC / NOMIXOCC). Our GIAO formulation of the shielding does not allow for broken (non-integer) occupation numbers that are used in ADF for partially filled, degenerate orbitals. The reason is that we use the "S-Matrix" for the occupied-occupied contributions to the paramagnetic shielding which in turn avoids degenerate perturbation theory. Hence, the program aborts when these broken occupation numbers occur (key NoMixOcc, the default). This default can be overwritten, key MixOcc, in which case we use the occupied-occupied U-matrix instead of the S-matrix, but only for combinations of occupied MOs that do not have equal occupation numbers. This is mostly an experimental feature.