On the importance of numerical integration details for homogeneous flow simulation

TL;DR

This paper presents a reversible, energy-conserving numerical integration scheme for the Sllod equations, improving the accuracy of homogeneous flow simulations at the atomic scale, especially at high flow rates.

Contribution

The authors develop and implement a reversible, energy-conserving integration scheme for the Sllod equations in LAMMPS, addressing subtle issues in existing methods.

Findings

Enhanced accuracy in transient response simulations

More reliable viscosity calculations at high flow rates

Energy conservation reduces systematic pressure tensor errors

Abstract

The Sllod equations of motion enable modeling of homogeneous flow at the atomic scale, and are commonly used to predict fluid properties such as viscosity. However, few publicly available codes support such simulations, and those that do often do not implement a reversible numerical integration scheme or have other subtle problems. Here, we demonstrate a reversible and energy-conserving integration scheme for the Sllod equations of motion with error on the order of $\delta t^3$, in line with typical operator splitting integrators used in standard molecular dynamics simulations. We discuss various implementation details, and implement the scheme in LAMMPS where we find that our changes enable more accurate simulation of transient responses, mixed flows, and steady states, especially at high rates of flow. Importantly, we show that a lack of energy conservation can manifest as a systematic error in the direct ensemble average of the pressure tensor, leading to an error in the calculated viscosity which becomes significant at high flow rates.