Melting curves of molecular solids

Theoretical melting curves of various solidified gases using the model of Lennard-Jones and Devonshire's equation of state, including the order-disorder transformation, are calculated. The input parameters are ε and σ of the Lennard-Jones potential function and their values are taken from the literature. The theoretical results are compared with the experimental values listed in the article by Pollack and measured by Kennedy. Agreement between theory and experiment is very good for the rare gases, and fair for ammonia, nitrogen, carbon dioxide, and methane. Selected melting curves of halogens, halogen compounds, and a few polar compounds are also calculated but no comparison can be made since no experimental data are available at this time. The effect of raising the temperature above the melting point on the shape of the isotherms is also considered for neon and xenon.