A good example for determining the difference between complete and incomplete premelting is ice. From vacuum ultraviolet (VUV) frequencies upwards the polarizability of ice is greater than that of water, at lower frequencies this is reversed. Assuming there is already a film of thickness formula_7 on the solid it is easy for any components of electromagnetic waves to travel through the film in the direction perpendicular to the solid surface as long formula_7 is small. Hence as long as the film is thin compared to the frequency interaction from the solid to the whole film is possible. But when formula_7 gets large compared to typical VUV frequencies the electronic structure of the film will be too slow to pass the high frequencies to the other end of the liquid phase. Thus this end of the liquid phase feels only a retarded van der Waals interaction with the solid phase. Hence the attraction between the liquid molecules themselves will predominate and they will start forming droplets instead of thickening the film further. So the speed of light limits complete premelting. This makes it a question of solid and surface free energies whether complete premelting occurs. Complete surface melting will occur when formula_31 is monotonically decreasing. If formula_31 instead shows a global minimum at finite formula_7 than the premelting will be incomplete. This implies: When the long range interactions in the system are attractive than there will be incomplete premelting — assuming the film thickness is larger than any repulsive interactions. Is the film thickness small compared to the range of the repulsive interactions present and the repulsive interactions are stronger than the attractive ones than complete premelting can occur.