The equilibrium and non-equilibrium properties of matter reflect the interatomic and intermolecular forces. Elementary considerations of the mere existence of liquids lead us to conclude that molecules attract one another when they are far apart. Also the fact that liquids and solids have finite densities of the magnitude observed under normal conditions means that molecules repel at short range. There is therefore a balance point of zero force or minimum energy. These basic truths are represented in Figure 1.1 which shows the interaction energy u(R) of two spherical atoms as a function of their separation R. The separation Re at the minimum is a measure of the diameter of an atom. In the caseof two interacting argon atoms the well depth e is 0.196 × 10−20J and Re = 3.76 × 10−10m.