Generalized Fluctuation-Dissipation relations holding in non-equilibrium dynamics

TL;DR

This paper derives simplified generalized Fluctuation-Dissipation Relations applicable to both equilibrium and non-equilibrium stochastic systems, enabling analysis without full steady-state distributions.

Contribution

It introduces a new form of FDR that depends only on microscopic dynamics, applicable to a wide range of passive and active particle models.

Findings

Validated in equilibrium colloids with Langevin dynamics

Extended to active particle models like ABP and AOUP

Revealed generalized virial and equipartition relations far from equilibrium

Abstract

We derive generalized Fluctuation-Dissipation Relations (FDR) holding for a general stochastic dynamics that includes as subcases both equilibrium models for passive colloids and non-equilibrium models used to describe active particles. The relations reported here differ from previous formulations of the FDR because of their simplicity: they require only the microscopic knowledge of the dynamics instead of the whole expression of the steady-state probability distribution function that, except for linear interactions, is unknown for systems displaying non-vanishing currents. From the response function, we can extrapolate generalized versions of the Mesoscopic Virial equation and the equipartition theorem, which still holds far from equilibrium. Our results are tested in the case of equilibrium colloids described by underdamped or overdamped Langevin equations and for models describing the non-equilibrium behavior of active particles. Both the Active Brownian Particle and the Active Ornstein-Uhlenbeck particle models are compared in the case of a single particle confined in an external potential.