The cell theory of liquids. III. Fusion

Correlation effects in liquids at high densities and solids are investigated. Convergence of the ‘correlation expansion’ of the phase integral is tested for a ‘harmonic force’ model, and found to be very satisfactory. The binary correlation correction for the 12–6 potential at the temperature kT/∊ = 0∙75 is evaluated approximately, and shown to give rise to a concave region in the curve of free energy against volume, indicating the existence of two condensed phases. For this temperature the pressure at which the two phases are in equilibrium is negative, i. e. the solid phase is more stable at zero pressure. An approximate extrapolation indicates that the zero pressure freezing-point is about kTF/∊ = 0∙90, somewhat higher than the value 0∙70 observed for argon. The most likely of several possible explanations of the discrepancy is the effect of non-additive interactions in reducing the cohesion of solid argon. The calculated properties of the liquid are in better agreement with observed values for argon than those calculated neglecting correlations.