Tuning Simulation Parameters for a Series of Alcohols to Match Experiments

Molecular Dynamics (MD) simulations are a useful tool to model liquids and predicts their properties. To make these calculations, MD simulations use force fields that have been adjusted to reproduce key properties. One such forcefield is the Optimized Potentials for Liquid Simulations (OPLS) forcefield. However, OPLS simulations sometimes give results at variance with some experimental properties for polar liquids, which suggests that OPLS does not accurately model the electrostatic interactions between such molecules. One such property is the dielectric constant, which is a measure of electric field responsiveness and dipole fluctuations. Atomic partial charges affect the electrostatic interactions and dipole fluctuations, and hence the dielectric constant, which suggests that OPLS partial charges may require adjustment to better reproduce liquid properties. To explore this possibility, we investigate a series of alkyl alcohols, methanol, ethanol, and propanol, which present a series of polar liquids with a range of dielectric constants, to see how scaling the partial charges affects the dielectric constant. Of course, scaling the partial charges may affect other liquid properties such as the free energy of self-solvation, which determines vapor pressure. Ideally, a consistent adjustment to the partial charges will give improved predictions for such properties. We find that kinetic and structured property in these polar fluids are also sensitive indications of electrostatic attractions. Adjusting affects the prevalence of string-like aggregation of molecular dipoles, which in in turn influences self-diffusion. Overall, we find a modest increase in partial charges results in improved values for all three properties- dielectric constant, self-solvation, and self-diffusion.