  We characterise the steady states of a suspension of two-dimensional active
brownian particles (ABPs). We calculate the steady-state probability
distribution to lowest order in Peclet number. We show that macroscopic
quantities can be calculated in analogous way to equilibrium systems using this
probability distribution. We then derive expressions for the macroscopic
pressure and position-orientation correlation functions. We check our results
by direct comparison with extensive numerical simulations. A key finding is the
importance of many-body effective interactions even at very low densities.
