  Active matter systems are driven out of equilibrium at the level of
individual constituents. One widely studied class are systems of athermal
particles that move under the combined influence of interparticle interactions
and self-propulsions, with the latter evolving according to the
Ornstein-Uhlenbeck stochastic process. Intuitively, these so-called active
Ornstein-Uhlenbeck particles (AOUPs) systems are farther from equilibrium for
longer self-propulsion persistence times. Quantitatively, this is confirmed by
the increasing equal-time velocity correlations (which are trivial in
equilibrium) and by the increasing violation of the Einstein relation between
the self-diffusion and mobility coefficients. In contrast, the entropy
production rate, calculated from the ratio of the probabilities of the position
space trajectory and its time-reversed counterpart, has a non-monotonic
dependence on the persistence time. Thus, it does not properly quantify the
departure of AOUPs systems from equilibrium.
