On the generalized Langevin equation for a Rouse bead in a nonequilibrium bath

TL;DR

This paper derives the generalized Langevin equation for a bead in a Rouse chain within a nonequilibrium bath, analyzing effects of constant and active forces on the system's dynamics and fluctuation-dissipation relations.

Contribution

It extends the generalized Langevin equation framework to nonequilibrium baths with active forces, revealing new dynamical regimes and modifications to fluctuation-dissipation relations.

Findings

Active forces induce frenetic contributions to fluctuation-dissipation relations.

The middle bead exhibits regimes of normal diffusion, subdiffusion, and superdiffusion.

System approaches a new equilibrium after constant force perturbation.

Abstract

We present the reduced dynamics of a bead in a Rouse chain which is submerged in a bath containing a driving agent that renders it out-of-equilibrium. We first review the generalized Langevin equation of the middle bead in an equilibrated bath. Thereafter, we introduce two driving forces. Firstly, we add a constant force that is applied to the first bead of the chain. We investigate how the generalized Langevin equation changes due to this perturbation for which the system evolves towards a new equilibrium state after some time. Secondly, we consider the case of stochastic active forces which will drive the system to a nonequilibrium state. Including these active forces results in a frenetic contribution to the second fluctuation-dissipation relation, in accord with a recent extension of the fluctuation-dissipation relation to nonequilibrium. The form of the frenetic term is analysed for the specific case of Gaussian, exponentially correlated active forces. We also discuss the resulting rich dynamics of the middle bead in which various regimes of normal diffusion, subdiffusion and superdiffusion can be present.