PNNL

Abstract

A sensitivity analysis of bulk water thermodynamics is presented in an effort to understand the relation between details of molecular potentials and the properties that they predict. The analysis is incorporated in molecular dynamics simulation and investigates the sensitivity of the Helmholtz free energy, internal energy, entropy, heat capacity, pressure, thermal pressure coefficient, and static dielectric constant to details of the potential. The sensitivities of the properties are calculated with respect to the van der Waals repulsive and the attractive parts, plus short and long range Coulomb parts of three four site empirical water potentials: TIP4P, Dang-Chang and TTM2R. The polarization sensitivity is calculated for the polarizable Dang-Chang and TTM2R potentials. The analysis indicates that all investigated properties are most sensitive to the van der Waals repulsive, the short range Coulomb and the polarization components of the potentials. The sensitivity of the Helmoltz free energy, internal energy, and entropy due to polarizaion is almost 30% of total electrostatic sensitivity. In addition the similarities in the trends of the observed sensitivities for nonpolarizable and polarizable potentials lead to the conclusion that the complexity of the model is not of critical importance for the calculation of these properties for bulk water. The van der Waals attractive and the long range Coulomb sensitivities are relatively small for the entropy, heat capacity, thermal pressure coefficient and the static dielectric constant, while small changes in any of the potential contributions will significantly effect the pressure. The analysis suggests a procedure for modification of the potentials and their improved prediction of thermodynamic properties. Based on the proposed procedure the water cluster potential TTM2R was adapted for simulation of bulk water properties.