A rigid sphere model for the melting of argon

A model is proposed for liquid and solid argon, in which the repulsive forces are represented by rigid cores and the attractive forces by a uniform background potential which depends on density but not on temperature. Thermodynamic consistency implies the equation of state:[image omitted]where p0 is the pressure exerted by rigid spheres at the same number density n and temperature T, and a is independent of n and T. When p0 is taken from the calculations of Alder and Wainwright it becomes possible to make an absolute calculation of the values of several dimensionless thermodynamic quantities characteristic of the triple point. All of these except certain second derivatives of the free energy agree very well with experiment, suggesting that the structure of liquids and solids near the triple point is mainly determined by the repulsive forces, the attractive forces serving merely to keep the molecules together.