  We study rare events in systems of diffusive fields driven out of equilibrium
by the boundaries. We present a numerical technique and use it to calculate the
probabilities of rare events in one and two dimensions. Using this technique,
we show that the probability density of a slowly varying configuration can be
captured with a small number of long wave-length modes. For a configuration
which varies rapidly in space this description can be complemented by a local
equilibrium assumption.
