Metals or other highly crystalline materials exhibit Hookean elastic behaviour at strains typically less than 0.2 %. Their elasticity is dominantly energy – driven. The displacement of the atoms in the lattice accompanying the stress causes an increase internal energy. The entropy remains approximately constant. If entropic effects are neglected, the elastic force causing the displacement of the atoms from their equilibrium states is equal to the slope in the plot of potential against displacement. The potential exhibits the following approximate dependence near the equilibrium point