Reliable Viscosity Calculation from High-Pressure Equilibrium Molecular Dynamics: Case Study of 2,2,4-Trimethylhexane

Abstract

Viscosity is a fundamental property of liquid lubricants, yet it is challenging to determine accurately, especially at high pressures. Although equilibrium molecular dynamics (EMD) simulations are a promising alternative to resource-intensive experiments, practical challenges remain in assessing the sufficiency of simulation time and in controlling uncertainties in the Green-Kubo formalism due to the finite amount of trajectory data. In this work, we extend the STable AutoCorrelation Integral Estimator (STACIE), a recently developed algorithm for estimating transport properties. First, we introduce the Lorentz model to estimate the viscosity and the exponential correlation time from the low-frequency power spectrum of deviatoric pressure fluctuations. Second, we show how to supplement the three conventional off-diagonal elements of the pressure tensor ($P_{xy}$, $P_{yz}$ and $P_{zx}$) with two additional uncorrelated deviatoric pressure components for shear viscosity calculations. Using these improvements, we apply STACIE to calculate the shear viscosity of 2,2,4-trimethylhexane from EMD simulations. We demonstrate STACIE's capability to reliably calculate viscosity under high-pressure conditions, offering a robust and automated solution with validated uncertainty quantification. Our results, when compared to the outcomes of the 10th International Fluid Properties Simulation Challenge, underscore the need for long EMD simulations. Large deviations from experimental viscosities in previous works were primarily due to insufficient simulation times and ad hoc post-processing choices, rather than the limitations of the force fields used. Unlike previous studies, our viscosity estimates agree well with experimental results (relative error < 6%) up to the highest pressure of 1 GPa, highlighting the improved reliability and accuracy of STACIE's systematic approach to viscosity predictions.

Author accepted manuscript

References in corpus (16)

• The first principles calculation of transport coefficients
• Transport properties for liquid silicon-oxygen-iron mixtures at Earth's core conditions
• Breakdown of the Stokes-Einstein relation in two, three and four dimensions
• Origin of hyperdiffusion in generalized Brownian motion
• Calculation of shear viscosity using Green-Kubo relations within a parton cascade
• Accurate thermal conductivities from optimally short molecular dynamics simulations
• Nature of Intrinsic Uncertainties in Equilibrium Molecular Dynamics Estimation of Shear Viscosity for Simple and Complex Fluids
• Long Time Tail of the Velocity Autocorrelation Function in a Two-Dimensional Moderately Dense Hard Disk Fluid
• Determination of pressure-viscosity relation of 2,2,4-trimethylhexane by all-atom molecular dynamics simulations
• Shear viscosity of nucleonic matter
• Shear viscosity in magnetized neutron star crust
• Fast Vibrational Modes and Slow Heterogeneous Dynamics in Polymers and Viscous Liquids
• Assessment of elastic models in supercooled water: A molecular dynamics study with the TIP4P/2005f force field
• Dependence of the Atomic Level Green-Kubo Stress Correlation Function on Wavevector and Frequency. Molecular Dynamics Results from a Model Liquid
• STable AutoCorrelation Integral Estimator (STACIE): Robust and accurate transport properties from molecular dynamics simulations
• A hybrid Green-Kubo (hGK) framework for calculating viscosity from short MD simulations