Accurate computation of shear viscosity from equilibrium molecular dynamics simulations

The accuracy of the Green–Kubo formulation for computing shear viscosity from equilibrium molecular dynamics simulations depends on the quality of the potential and on how the viscosity computation is carried out. We examine the role of the duration of the simulation, the number of particles used, and how the correlations are accumulated on the accuracy of the computed viscosity. We propose as a measure of the accuracy the standard deviation of five independently computed shear viscosity values based on independent components of the stress tensor. Using this measure, we examine the shear viscosity calculation for molten NaCl to determine the values of the run length, window width, and spacing between windows and obtain a good compromise between calculation time and viscosity quality. Significantly we note that even though viscosity can be calculated using relatively few particles, reducing state point uncertainty requires more, rather than less, particles.

Keywords

• Viscosity,
• Molecular dynamics,
• NaCl,
• Green–Kubo