  Interaction of charges in CCDs with the already accumulated charge
distribution causes both a flux dependence of the point-spread function (an
increase of observed size with flux, also known as the brighter/fatter effect)
and pixel-to-pixel correlations of the Poissonian noise in flat fields. We
describe these effects in the Dark Energy Camera (DECam) with charge dependent
shifts of effective pixel borders, i.e. the Antilogus et al. (2014) model,
which we fit to measurements of flat-field Poissonian noise correlations. The
latter fall off approximately as a power-law r^-2.5 with pixel separation r,
are isotropic except for an asymmetry in the direct neighbors along rows and
columns, are stable in time, and are weakly dependent on wavelength. They show
variations from chip to chip at the 20% level that correlate with the silicon
resistivity. The charge shifts predicted by the model cause biased shape
measurements, primarily due to their effect on bright stars, at levels
exceeding weak lensing science requirements. We measure the flux dependence of
star images and show that the effect can be mitigated by applying the reverse
charge shifts at the pixel level during image processing. Differences in
stellar size, however, remain significant due to residuals at larger distance
from the centroid.
