Prediction of the Gibbs energies and an improved equation of state for water at extreme conditions from ab initio energies with classical simulations

The hydration free energies of water at two state points (2344 K and 1.0 g/cm³, and 2000 K and 1.8 g/cm³) were calculated using the ABC-FEP method (Wood et al., 1999) that combines interaction energies calculated from ab initio quantum mechanics with configurations sampled from classical simulations. We start with a calculation of the Gibbs energy of an approximate classical water model by a standard simulation method. Next, the difference in Gibbs energy of the approximate model potential and more accurate ab initio interaction potential was obtained using the free energy perturbation method. At 2000 K and 1.8 g/cm³, it was necessary to derive a better classical model from the ab initio results due to the inaccuracy of the classical model at short distances. The estimated accuracy of the predicted free energies is ± 15 kJ/mol. The calculated results are used to examine the predictive capability of previously proposed classical simulations and equations of state. The equation of Pitzer and Sterner (1994) was reparameterized using the available experimental data and the newly calculated data points.