Viscosity is a crucial property of liquid lubricants, and it is theoretically a well-definedquantity in molecular dynamics (MD) simulations. However, no standardized protocol has beendefined for calculating this property from equilibrium MD simulations. While best practices doexist, the actual calculation depends on several ad hoc decisions during the post-processing of theraw MD data. A common protocol for calculating the viscosity with equilibrium MD simulationsis called the time decomposition method (TDM). Although the TDM attempts to standardize theviscosity calculation using the Green–Kubo method, it still relies on certain empirical rules andsubjective user observations, e.g., the plateau region of the Green–Kubo integral or the integrationcut-off time. It is known that the TDM works reasonably well for low-viscosity fluids, e.g., athigh temperatures. However, modified heuristics have been proposed at high pressures, indicatingthat no single set of rules works well for all circumstances. This study examines the effect ofheuristics and ad hoc decisions on the predicted viscosity of a short, branched lubricant molecule,2,2,4-trimethylhexane. Equilibrium molecular dynamics simulations were performed at variousoperating conditions (high pressures and temperatures), followed by post-processing with threelevels of uncertainty quantification. A new approach, “Enhanced Bootstrapping”, is introduced toassess the effects of individual ad hoc parameters on the viscosity. The results show a strong linearcorrelation (with a Pearson correlation coefficient of up to 36%) between the calculated viscosity andan ad hoc TDM parameter, which determines the integration cut-off time, under realistic lubricationconditions, particularly at high pressures. This study reveals that ad hoc decisions can lead topotentially misleading conclusions when the post-processing is performed ambiguously.
Abstract