NONEQUILIBRIUM MOLECULAR-DYNAMICS SIMULATION OF THE SHEAR VISCOSITY OF LIQUID METHANOL - ADAPTATION OF THE EWALD SUM TO LEES-EDWARDS BOUNDARY-CONDITIONS

The Ewald sum method is commonly used in equilibrium simulations of po lar fluids to enhance convergence of long-range Coulombic forces withi n modest-sized cubic simulation cells. In this work, we derive a form of the standard Ewald sum technique for use with nonequilibrium molecu lar dynamics (NEMD) simulations of viscosity that make use of the Lees -Edwards boundary conditions. This generalized Ewald sum can be used f or any parallelepiped simulation cell. The method was tested by perfor ming NEMD simulations at various temperatures and densities for simula ted liquid methanol. The results were in excellent agreement with expe rimental data for methanol and Green-Kubo simulations of the viscosity using the standard cubic-cell Ewald sum. A simple truncation of the p olar interactions at 10 Angstrom was found to produce errors of over 2 00% in the simulated viscosities. Values obtained with the polar inter actions turned off (i.e. using only dispersion forces) were generally 40-60% below the experimental values. These results show that long-ran ge Coulombic interactions may be significant in simulated liquid visco sities and that they can be accurately handled in NEMD simulations wit h the proposed extension of the Ewald sum.