One reason for the efficiency of the AB2 approach compared to FNO comes from focusing on the bonding orbitals one at a time rather than the whole occupied space at once. It is then important to start by localizing the occupied space, which is known to be a cubic scaling iterative procedure for e.g. the Boys and Pipek-Mezey localization measures. Then, one must also distinguish between localized orbitals with different character: specifically core, bonding, and non-bonding, e.g. lone pairs. Our implementation uses an automatic bonding detection option that runs before AB2. The detection process is simply determined by Pipek's delocalization measure on Mulliken charges, where measures amounting to 1 indicate an orbital localized on an atom (core or non-bonding) and measures around 2 correspond to orbitals split between two atoms.