Using the fluctuation-dissipation theorem for non-conservative forces

Kiryl Asheichyk, Matthias Krüger

Published 2019-08-29Version 1

An equilibrium system which is perturbed by an external potential relaxes to a new equilibrium state, a process obeying the fluctuation-dissipation theorem. In contrast, perturbing by non-conservative forces yields a nonequilibrium steady state, and the fluctuation-dissipation theorem can in general not be applied. Here we exploit a freedom inherent to linear response theory: Force fields which perform work that does not couple statistically to the considered observable can be added without changing the response. Using this freedom, we demonstrate that the fluctuation-dissipation theorem can be applied for certain non-conservative forces. We discuss the case of a non-conservative force field linear in particle coordinates, where the mentioned freedom can be formulated in terms of symmetries. In particular, for the case of shear, this yields a new response formula, which we find advantageous over the known Green-Kubo relation in terms of statistical accuracy.