Interpretation of thermodynamic processes in the lower crust, mantle and core are limited by the lack of data for the high temperatures and pressures characteristic of these regions. To contribute to the solution of this problem, we have developed corresponding states models and methods of simulation for closed electronic shell, non-polar systems. These methods can be used to extrapolate behaviour from lower temperatures and pressures to much higher T and P with accuracy close to that of

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... ents. For sufficiently high temperatures, important components of natural fluids, such as CH4, CO2, H20, N2, satisfy the non-polar and closed shell criteria. Their interparticle interactions can be well approximated by a potential of the form: where (~ is an arbitrary function and ~ and ~ are two temperature-and pressure-independent parameters. Under these conditions, the coresponding states theory shows that the thermodynamic functions of the system scale with the values of o and g. If an accurate free energy model is available for one species, with potentials well approximated by Eq. (1), all the thermodynamic properties of another species, also described by Eq. (1) but with different values of and o, can be found by scaling the temperature and pressure. The value of o and r for a particular system can be calculated from limited data at low P and T or can be found in the literature. We have developed a corresponding states model based on a highly accurate free energy equation for the CH 4 system. This equation of state was parameterized using the extensive experimental data available for this system as well as from simulation PVT data for high temperatures and pressures. Using this equation as a basis and values of ~ and g obtained from low P,T data sets, we have predicted PVT properties for species such as H20, CO 2 and N2 that are better than those calculated by published models which were parameterized directly from data taken for these species. To treat mixtures, the corresponding states model for pure systems may be generalized by assuming composition dependent ~ and g parameters calculated from the following mixing rules: For sufficiently high pressures, fluid/fluid phase coexistence has been observed for the I-IzO-N2 system. Since the corresponding states model is based on a free energy description, phase equilibria and phase coexistence in the H20-N2 system can be calculated using our free energy model of CH4 and the mixing rules: Eqs. (2) and (3) . Results are presented in Fig. 1 . As in the case of PVTX data, the predicted compositions, temperatures and pressures are close to measured values. The only justification of the mixing rules given by Eqs. (2) and (3) is the TABLE 1. compares predictions with data not used in the model parameterization