VAN-DER-WAALS THEORY OF ORDER-DISORDER TRANSITIONS

In analogy with the well-known Van der Waals theory for disordered flu id phases we propose a simple analytic expression for the Helmholtz fr ee energy of an ordered crystalline phase. It is based on a free-volum e approximation to the entropic contribution due to the hardcore repul sions and a static lattice energy approximation to the contribution of the attractions. In this way we are able to describe in a simple way phase transitions between two disordered phases, between two ordered p hases and between an ordered and a disordered phase in systems of any spatial dimensionality.