Calculation of Cauchy stress tensor in molecular dynamics system with a generalized Irving-Kirkwood formulism

TL;DR

This paper generalizes the Irving-Kirkwood formulism to include both spatial and temporal averaging, deriving a new expression for the Cauchy stress tensor in molecular dynamics, and compares it with traditional methods through numerical experiments.

Contribution

It introduces a systematic generalization of the IK formulism incorporating spatial and temporal averages for stress calculation in molecular dynamics.

Findings

The generalized IK formulism provides a more accurate stress measure.

The relationship between Cauchy and Piola-Kirchhoff stress is clarified.

Numerical results validate the theoretical analysis.

Abstract

Irving and Kirkwood formulism (IK formulism) provides a way to compute continuum mechanics quantities at certain location in terms of molecular variables. To make the approach more practical in computer simulation, Hardy proposed to use a spacial kernel function that couples continuum quantities with atomistic information. To reduce irrational fluctuations, Murdoch proposed to use a temporal kernel function to smooth the physical quantities obtained in Hardy's approach. In this paper, we generalize the original IK formulism to systematically incorporate both spacial and temporal average. The Cauchy stress tensor is derived in this generalized IK formulism (g-IK formulism). Analysis is given to illuminate the connection and difference between g-IK formulism and traditional temporal post-process approach. The relationship between Cauchy stress and first Piola-Kirchhoff stress is restudied in the framework of g-IK formulism. Numerical experiments using molecular dynamics are conducted to examine the analysis results.