concepts such as molecular similarity and transferability between molecules.[@ParrRGAyersPWNalewajskiRF:2005JPCA] Since the concept of AIM is basically about how one should divide the electrons, more specifically the charge density distribution (CDD) $\rho(\mathbf{r})$ in the molecule between the different “atoms”, this leads to two obvious categories of approaches in which most of the methods used for defining AIM can be divided. The first category of approaches is based on the wave-function/states of the system, and most of the work is performed in the Hilbert space of the basis functions used. One of the most famous examples here is (probably) the Mulliken approach.[@MullikenRS:1935JChemPhys; @MullikenRS:1955aJChemPhys] The second category of approaches is based on the division of the CDD as it exists in real-space space. In these real-space approaches the molecule is split into atomic basins, that can overlap such as in the Hirshfeld[@HirshfeldFL:1977TCA] and derived methods,[@BultinckP:2007FaradDisc_HirRef; @BultinckP:2007JCP_HirRef; @LillestolenTCWheatleyRJ:2008ChemComm; @LillestolenTCWheatleyRJ:2009JChemPhys; @ManzSholl:JChemTheoryComp2010] or that are non-overlapping such as in Bader’s approach.[@BaderRFW:1991ChemRev]\
The concept of AIM is strongly linked to the concept of transferability. Because both are central in chemistry, and chemists mainly focus on molecules, they are mostly used for molecules.[@FranciscoEPendasAM:2011CTC; @FranciscoEPendasAM:2011TCA] There is, however, no reason why these concepts should not be applicable for periodic 