Non-equilibrium molecular dynamics simulation of the shear viscosity of liquid methanol: adaptation of the Ewald sum to Lees-Edwards boundary conditions

Abstract

The Ewald sum method is commonly used in equilibrium simulations of polar fluids to enhance convergence of long-range Coulombic forces within modest-sized cubic simulation cells. In this work, we derive a form of the standard Ewald sum technique for use with non-equilibrium molecular dynamics (NEMD) simulations of viscosity that make use of the Lees–Edwards boundary conditions. This generalized Ewald sum can be used for any parallelepiped simulation cell. The method was tested by performing NEMD simulations at various temperatures and densities for simulated liquid methanol. The results were in excellent agreement with experimental data for methanol and Green–Kubo simulations of the viscosity using the standard cubic-cell Ewald sum. A simple truncation of the polar interactions at 10 Å was found to produce errors of over 200% in the simulated viscosities. Values obtained with the polar interactions turned off (i.e. using only dispersion forces) were generally 40–60% below the experimental values. These results show that long-range Coulombic interactions may be significant in simulated liquid viscosities and that they can be accurately handled in NEMD simulations with the proposed extension of the Ewald sum.