(112b) Evaluating the Grand-Canonical Partition Function of Atomic and Molecular Fluids Using Expanded Wang-Landau Simulations

The Wang-Landau (WL) sampling is a powerful method that allows for a direct determination of the density of states. However, applications to the calculation of the thermodynamic properties of realistic fluids have been limited so far. By combining the Wang-Landau method with expanded grand-canonical simulations, we are able to obtain a high-accuracy estimate for the grand-canonical partition function for atomic and molecular fluids. Then, using the formalism of statistical thermodynamics, we are able to calculate the thermodynamic properties of these systems, for a wide range of conditions spanning the single-phase regions as well as the vapor-liquid phase boundary (including the interfacial free energies). Excellent agreement with prior simulation work and with the available experimental data is obtained for Argon and carbon dioxide, thereby establishing the accuracy of the method for the calculation of thermodynamic properties such as the Gibbs free energies, the Helmholtz free energies as well as the entropies.