Helmholtz Parameterizations of Fluid EOS Suitable for High Pressure Work

Many common forms for the parameterization of fluid equations of state are unreliable, sometimes highly divergent, not only in regimes of P and T where data do not yet exist but also for the computation of thermodynamic variables other than those of the primary data. It has been shown however that, by following some simple rules, EOS based on expansions of the Helmholtz free energy can be expected to give reasonable results in both cases. We present such EOS for water and for ammonia. These EOS are in accord with the lower pressure data, shock data, and speed of sound measurements made in the diamond-anvil cell.