  We develop an exact Green-Kubo formula relating nonequilibrium averages in
systems of interacting active Brownian particles to equilibrium
time-correlation functions. The method is applied to calculate the
density-dependent average swim speed, which is a key quantity entering coarse
grained theories of active matter. The average swim speed is determined by
integrating the equilibrium autocorrelation function of the interaction force
acting on a tagged particle. Analytical results are validated using Brownian
dynamics simulations.
