Reliable Viscosity Calculation from Equilibrium Molecular Dynamics Simulations: A Time Decomposition Method.

Area of Science:

• Computational physics
• Fluid dynamics
• Chemical engineering

Background:

• Equilibrium molecular dynamics with Green-Kubo integrals is standard for fluid shear viscosity.
• This method is computationally intensive and prone to variability due to difficulties in identifying integral plateau regions.

Purpose of the Study:

• To develop a more robust and automatable method for computing shear viscosity.
• To reduce uncertainty and improve objectivity in Green-Kubo viscosity calculations.

Main Methods:

• Proposed a time decomposition approach using multiple independent trajectories.
• Applied Green-Kubo relation to each trajectory and averaged running integrals.
• Fitted averaged integrals to a double-exponential function with a weighting function derived from standard deviation.

Main Results:

• The developed weighting function minimizes uncertainty in shear viscosity estimation.
• An objective method for determining integration cutoff time (tcut) was established.
• The approach was successfully applied to ethanol and [BMIM][Tf2N] ionic liquid, yielding reproducible and reliable viscosity values.

Conclusions:

• The time decomposition approach offers an objective, automatable, and robust method for shear viscosity computation.
• This technique is suitable for computational screening studies and reduces reliance on human intervention.
• The method provides reliable viscosity data for diverse liquids across temperatures.