Computing shear viscosities from molecular dynamics simulations: Comparing the OrthoBoXY approach with the Green-Kubo...

We calculated shear viscosities of 15 molecular liquids from equilibrium molecular dynamics simulations using the OrthoBoXY approach and compared them to viscosities calculated via the Green-Kubo method. Data from both methods agree reasonably well. We discuss how to avoid pitfalls while computing the OrthoBoXY data to obtain optimal results. From simulations of multiple system sizes, we verify that the viscosity of molecular liquids is not influenced by finite size effects down to systems as small as 250 molecules. Moreover, we also demonstrate that the standard error of the viscosity is nearly independent of the system size. This is a consequence of a compensation effect of an increasing accuracy of the self-diffusion coefficients with increasing system size and the system-size dependent weighting according to the OrthoBoXY equation. Consequently, we suggest that it is preferable to run simulations of smaller systems with longer simulation times rather than larger systems with shorter simulation runs. Moreover, we discuss a refinement of the recently introduced "recipe" for OrthoBoXY simulations block-lengths: for highly viscous systems, the value of τblock might safely be scaled by a factor of 1/8, significantly reducing the computational resources. For less viscous systems, the value of τblock might safely be scaled by a factor of 1/4. For systems with high fluidity, the value of τblock should not be scaled down in order to achieve reliable results. When using a smaller system size of 250 molecules, these refinements are leading up to a 24-fold reduction in computational cost compared to the previous recommended setup without sacrificing numerical accuracy.