Linear response and correlation of a self-propelled particle in the presence of external fields

TL;DR

This paper investigates the non-equilibrium linear response and correlation functions of active particles under external fields, comparing different computational methods and analyzing the effects of variable transformations on response predictions.

Contribution

It introduces a detailed analysis of the response functions of AOUPs, highlighting how variable transformations influence the response and comparing the AOUP model with the UCNA approximation.

Findings

Response depends on perturbation placement and variable choice.

UCNA approximates stationary properties well but not non-equilibrium responses.

Large persistence times lead to significant deviations in non-equilibrium properties.

Abstract

We study the non-equilibrium properties of non interacting active Ornstein-Uhlenbeck particles (AOUP) subject to an external nonuniform field using a Fokker-Planck approach with a focus on the linear response and time-correlation functions. In particular, we compare different methods to compute these functions including the unified colored noise approximation (UCNA). The AOUP model, described by the position of the particle and the active force acting on it, is usually mapped into a Markovian process, describing the motion of a fictitious passive particle in terms of its position and velocity, where the effect of the activity is transferred into a position-dependent friction. We show that the form of the response function of the AOUP depends on whether we put the perturbation on the position and keep unperturbed the active force in the original variables or perturb the position and maintain unperturbed the velocity in the transformed variables. Indeed, as a result of the change of variables the perturbation on the position becomes a perturbation both on the position and on the fictitious velocity. We test these predictions by considering the response for three types of convex potentials: quadratic, quartic and double-well potential. Moreover, by comparing the response of the AOUP model with the corresponding response of the UCNA model we conclude that although the stationary properties are fairly well approximated by the UCNA, the non equilibrium properties are not, an effect which is not negligible when the persistence time is large.