Chemical theory of dislocation mobility

The differences just outlined cannot be explained by means of a classical mechanical model. However, they can be explained by considering the chemical bonding in differing solid crystals. In particular, hardness depends on the degree of localization of the valence (bonding) electrons. Qualitatively, this is a very old idea. What is new is that it is possible to provide quantitative theories by using the results of quantum chemistry, and relatively simple analysis. Selected sets of crystal types must be treated, of course, just as selected sets of molecules are treated in theoretical chemistry. Otherwise the rationalization becomes unmanageable.