Prediction of fluid–fluid and fluid–solid equilibria in the molecular system helium–hydrogen up to 1 Mbar

By using variational theory together with an extended van der Waals one-fluid model, as has been developed by Ree, fluid–fluid coexistence surfaces of He–H2 mixtures are calculated up to 1 Mbar. The exponential-six potential parameters of the system are chosen such that good agreement is obtained with experimental data up to 75 kbar and 300 K. Moreover, using these parameters, fluid–solid equilibria and the three-phase line are calculated assuming that the hard-sphere packing fraction of the fluid along the freezing surface is a linear function of composition. The calculated triple point composition along the three-phase line agrees with experiment up to the highest pressure.