Here γ γ j represents the surface (Gibbs free) energy per unit area of the j th crystal face and O j is the area of said face. Δ Δ G i represents the difference in energy between a real crystal composed of i molecules with a surface and a similar configuration of i molecules located inside an infinitely large crystal. This quantity is therefore the energy associated with the surface. The equilibrium shape of the crystal will then be that which minimizes the value of Δ Δ G i .