Equilibrium and non-equilibrium molecular dynamics approaches for the linear viscoelasticity of polymer melts

TL;DR

This study compares equilibrium and non-equilibrium molecular dynamics methods for calculating the viscoelastic properties of polymer melts, demonstrating their equivalence and efficiency under proper data processing.

Contribution

It provides a comprehensive comparison of EMD and NEMD approaches, introducing noise reduction techniques and optimized analysis methods for accurate viscoelastic property computation.

Findings

EMD with Green-Kubo yields smooth stress relaxation profiles

NEMD with stress pre-averaging and Fourier analysis is computationally efficient

Both methods produce quantitatively equivalent results when properly applied

Abstract

Viscoelastic properties of polymer melts are particularly challenging to compute due to the intrinsic stress fluctuations in molecular dynamics (MD). We compared equilibrium and non-equilibrium MD approaches for extracting the storage (G') and loss moduli (G") over a wide frequency range from a bead-spring chain model, in both unentangled and entangled regimes. We found that, with properly chosen data processing and noise reduction procedures, different methods render quantitatively equivalent results. In equilibrium MD (EMD), applying the Green-Kubo relation with a multi-tau correlator method for noise filtering generates smooth stress relaxation modulus profiles, from which accurate G' and G" can be obtained. For unentangled chains, combining the Rouse model with a short-time correction provides a convenient option that circumvents the stress fluctuation challenge altogether. For non-equilibrium MD (NEMD), we found that combining a stress pre-averaging treatment with discrete Fourier transform analysis reliably computes G' and G" with much shorter simulation length than previously reported. Comparing the efficiency and statistical accuracy of these methods, we concluded that EMD is both reliable and efficient, and is suitable when the whole spectrum of linear viscoelastic properties is desired, whereas NEMD offers flexibility when only some frequency ranges are of interest.