"1.4 Types of binding ... The most important types of force are as follows: (a) Electrostatic forces. In an ionic crystal the attraction is mainly due to the Coulomb interaction between point charges. This is particularly amenable to calculation, and a great deal of work has been done on it. The force is a 'two-body' force, i.e. the interaction between two given ions is independent of the positions of any other ions that may be present. ... (b) Van der Waals forces. This name describes the effect that a neutral and isotropic atom can acquire a polarization under the influence of an electric field, and even two neutral isotropic atoms will induce small dipole moments in each other, due to the fluctuating moments which they possess because of the existence of virtual excited states. ... (c) Homopolar binding. These are forces like those effective in homopolar molecules, and we know they are due to the exchange of electrons between the atoms. In molecular crystals (H2, Cl2, etc.) these bonds can easily be localized and we can start from a description of the molecular by the methods of quantum chemistry and then add the relatively weak forces between different molecules. In other cases, however, such as diamond or graphite, each atom shares some valence electrons with each of its neighbors, and it is therefore not possible to single out any given groups of atoms that may be regarded as chemically saturated. The quantitative discussion of such forces is not easy. ... (d) Overlap. If two atoms approach so closely that their electron shells overlap, then there is a strong repulsive force between them. ... (e) Metallic bond. ... it is worth noting that in the case of a metal the presence and motion of the conduction electrons is an important factor in holding the crystal together and in determining its structure."