  We use the non-equilibrium statistical field theory for classical particles
recently developed by Mazenko and Das and Mazenko, together with the free
generating functional for particles initially correlated in phase space derived
in Bartelmann et al. to study the impact of initial correlations on the
equation of state of real gases. We first show that we can reproduce the well
known van der Waals equation of state for uncorrelated initial conditions using
this approach. We then impose correlated initial conditions and study their
qualitative and quantitative effect on the equation of state of a van der Waals
gas. The correlations impose a significant correction to the pressure of an
ideal gas which is an order of magnitude larger than the correction due to
particle interactions.
