# Attachment energy and layer energy

Crystallization energy is the total of the two terms:

Indeed, let us “peal” a crystal face by removing a layer of molecules of *minimum* thickness d_{hkl}, so that the layer obtained allows the crystal to be reproduced by simple orthogonal translation of itself. In order to detach the unit of surface of such a layer, we would need to expend the energy equivalent to the attachment energy Efix, which is calculated by considering the molecules situated in a hemisphere centered on the molecule in question, with a radius of 3-4 mm.

On the other hand, each molecule of the layer is bonded to the other molecules of the same layer. The corresponding energy calculation E_{cou} is done by considering the molecules situated in a radius of 3 or 4 mm.

Within a crystal, each layer is in contact with the rest of the layers in the crystal on both faces, hence the coefficient 2 in equation [1.1].

Equation [1.1] shows that when considering the faces directed in different ways, energies E_{cou} and E_{fix} *vary in opposite ways* due to their total remaining constant and equal to Ecris.

Energies E_{fix} are negative and in the order of 100 MJ.kmol^{-1}.