 Finally, the “Atoms in Molecules” program (Bader 1991; see Gillespie and Popelier 2001, Chs. 6 &amp; 7 for an exposition) suggests that we can hold on to the structural conception of the bond understood functionally, but reject Lewis’ ideas about how electrons realize this relationship. Bader, for example, argues that we can define ‘bond paths’ in terms of topological features of the molecule-wide electron density. Such bond paths have physical locations, and generally correspond closely to classical covalent bonds. Moreover they partially vindicate the idea that bonding involves an increase in electron density between atoms: a bond path is an axis of maximal electron density (leaving a bond path in a direction perpendicular to it involves a decrease in electron density). There are also many technical advantages to this approach. Molecule-wide electron density exists within the ontology of quantum mechanics, so no quantum-mechanical model could exclude it. Further, electron density is considerably easier to calculate than other quantum mechanical properties, and it can be measured empirically using X-ray diffraction techniques.