  A dedicated simulator, Poisson_CCD, has been constructed which models
astronomical CCDs by solving Poisson's equation numerically and simulating
charge transport within the CCD. The potentials and free carrier densities
within the CCD are self-consistently solved for, giving realistic results for
the charge distribution within the CCD storage wells. The simulator has been
used to model the CCDs which are being used to construct the LSST digital
camera. The simulator output has been validated by comparing its predictions
with several different types of CCD measurements, including astrometric shifts,
brighter-fatter induced pixel-pixel covariances, saturation effects, and
diffusion spreading. The code is open source and freely available.
