Benchmark Free Energies and Entropies for Saturated and Compressed Water

Abstract

We use molecular simulation to compute the thermodynamic properties of 7 rigid models for water (SPC/E, TIP3P, TIP4P, TIP4P/2005, TIP4P/Ew, TIP5P, OPC) over a wide range of temperature and pressure. Carrying out Expanded Wang-Landau simulations, we obtain a high accuracy estimate for the grand-canonical partition function which, in turn, provides access to all properties, including the free energy and entropy, both at the vapor-liquid coexistence and for compressed water. The results at coexistence highlight the close connection between the behavior of the statistical functions and the thermodynamic properties. They show that the subgroup {SPC/E,TIP4P/2005,TIP4P/Ew} provides the most accurate account of the vaporliquid equilibrium properties. For compressed water, the comparison of the simulation results to the experimental data establishes that the TIP4P/Ew model performs best among the 7 models considered here, and captures the experimental trends for the dependence of entropy and molar Gibbs free energy on pressure.

Stabilization of Nanobubbles under Hydrophobic Confinement

“…In all cases, the free energy profile connects a first minimum, reached for a low value of the reaction coodinate S * around 200, to a second minimum, for a value of S * around 13500. The order parameter S * is extensive 49 , since it is calculated as the total entropy of nanoconfined water, and takes into account the increase in N , the number of water molecules confined in the nanotube (note that the total entropy is not strictly proportional to N , since it is function of the number of molecules, as well as of the amount of order within the confined fluid). As a result, the first minimum for a low total S * is associated with configurations with low N values, corresponding to a vapor phase adsorbed in the CNT.…”

“…We use the SP C/E force field 31 for water, which models well the thermodynamics of the vapor-liquid equilibrium for water [43][44][45][46][47][48][49] . In particular, prior simulations, performed using the Expanded Wang Landau simulation method 50 , have allowed us to identify the chemical potential for the SP C/E model at the vapor-liquid coexistence 49 . The thermodynamic conditions used in µV T − S simulations, with µ ranging from -4120 kJ/kg to -4160 kJ/kg for T = 500 K, are chosen close to the coexistence for bulk water.…”

“…In all cases, the free energy profile connects a first minimum, reached for a low value of the reaction coodinate S * around 200, to a second minimum, for a value of S * around 13500. The order parameter S * is extensive49 , since it is calculated as the total entropy of nanoconfined water, and takes Water in(26,26) CNT: Free energy profiles for µ ranging from -4120 kJ/kg to -4160 kJ/kg as a function of the entropic order parameter…”

Interactions of Water and Alkanes: Modifying Additive Force Fields to Account for Polarization Effects

“…Therefore, one of the water/hexadecane slabs was also simulated using TIP4P-Ew, a model that yields more realistic condensed-phase properties over a wide range of temperatures and pressures. 14,68…”

Entropy in Molecular Fluids: Interplay between Interaction Complexity and Criticality

“…74 This force field provides a very good account of the vapor−liquid equilibria properties of water. 15,75,76 Simulation Framework. To compute entropy, we calculate the canonical and grand-canonical partition functions.…”