Relative Permittivity of Stockmayer-Type Model Fluids from MD Simulations and COFFEE

The relative permittivity is a property that depends on the orientation structure of a fluid. Thus, using state-of-the-art equations of state it can be modeled only when using high level techniques, such as nonprimitive models. This is a drawback, especially when electrolytes are present: when using a primitive model, an auxiliary model for the relative permittivity must be introduced. Here, we pursue a different approach to predict the relative permittivity. The recently developed 'Co-Oriented Fluid Functional Equation for Electrostatic Interactions' (COFFEE) framework considers the mutual orientation of molecular dipole moments. It thereby enables a prediction of the relative permittivity when combined with Kirkwood's theory. To assess these predictions obtained from COFFEE, we present a comprehensive set of molecular simulation data for the relative permittivity of Stockmayer-type model fluids at wide ranges of temperature and density. Continuing a previous work, we extend the range of states considered to the homogeneous vapor, liquid, and supercritical regions. We find that when plotting the relative permittivity as a function of the ratio of density and temperature, all data points for a certain fluid collapse onto a single master curve. To a good approximation, the same holds for experimental data of the relative permittivity of water. By comparing the molecular simulation results with COFFEE, a good overall agreement is observed. Both data sets follow the same qualitative trends. Also reasonable quantitative agreement is found for many state points, the only exception being dense states at low temperatures.