From a pure physical viewpoint, molecules are just a collection of nuclei and electrons in a geometry that minimize their energy. However, we cannot be oblivious to the existence of a structural pattern that shows something connecting the nuclei, mostly in a pairwise atom to atom fashion. This is implicit when drawing molecules with ball-and-stick or skeletal models, but it is also observed from computational and experimental images (see Fig. ). This is the structural (also called topological ) approach to the chemical bond. If we believe that electrons make the bond, the interatomic glue has to be the electron density. This idea is rooted in the Lewis electron pair, and was thoroughly developed through many models, possibly the most famous being the Quantum Theory of Atoms in Molecules (QTAIM ), which we will briefly discuss below.