It was recalled that pair potentials provided a quantitative starting-point for studying the vacancy formation energy in hot close-packed crystals. The relationship of the vacancy formation energy to the melting point was considered with respect to the role played by 3-body forces. It was asserted that a close-packed crystal would melt when the internal energy which was required in order to create a localized hole (an unrelaxed vacancy in a hot crystal) was equal to the change in internal energy at the melting point which was required in order to expand the liquid by one atomic volume. When N-body forces became important, and were treated by means of so-called glue models, the important role which was played by the shear modulus in determining the vacancy formation energy (at absolute zero) and the divacancy binding energy became apparent. It was argued that the vacancy formation energy in Al, for example, was closely related to the surface energy because of the similarity of conduction-electron density profiles around a vacant site and through a planar surface.

Force Fields in Close-Packed Crystals and their Melts in Relation to Defects, Surface Energies and Mechanical Properties. N.H.March: Philosophical Magazine A, 2000, 80[6], 1335-48