  Preheating can convert superhorizon fluctuations of light scalar fields
present at the end of inflation into observable density perturbations. We show
in detail how lattice field theory simulations and the separate universes
approximation can be used to calculate these perturbations and make predictions
for the nonlinearity parameter fNL . We also present a simple approximation
scheme that can reproduce these results analytically. Applying these methods to
the massless preheating model, we determine the parameter values that are ruled
out by too high levels of non-Gaussianity.
