Transient subtraction: A control variate method for computing transport coefficients

TL;DR

This paper introduces the transient subtraction method, a variance reduction technique for computing transport coefficients in molecular dynamics, by subtracting coupled equilibrium trajectories to improve estimator accuracy.

Contribution

It proposes a novel variance reduction approach for transport coefficient estimation using transient dynamics and coupled equilibrium trajectories.

Findings

Reduced variance in transport coefficient estimators

Mathematical analysis of bias and variance

Numerical validation on various systems

Abstract

In molecular dynamics, transport coefficients measure the sensitivity of the invariant probability measure of the stochastic dynamics at hand with respect to some perturbation. They are typically computed using either the linear response of nonequilibrium dynamics, or the Green--Kubo formula. The estimators for both approaches have large variances, which motivates the study of variance reduction techniques for computing transport coefficients. We present an alternative approach, called the \emph{transient subtraction technique} (inspired by early work by Ciccotti and Jaccucci in 1975), which amounts to simulating a transient dynamics started off equilibrium and relaxing towards the equilibrium state, from which we subtract a sensibly coupled equilibrium trajectory, resulting in an estimator with smaller variance. We present the mathematical formulation of the transient subtraction technique, give error estimates on the bias and variance of the associated estimator, and demonstrate the relevance of the method through numerical illustrations for various systems.