  Available estimates for the energy resolution of DUNE vary by as much as a
factor of four. To address this controversy, and to connect the resolution to
the underlying physical processes, we build an independent simulation pipeline
for neutrino events in liquid argon, combining the public tools GENIE and
FLUKA. Using this pipeline, we first characterize the channels of
non-hermeticity of DUNE, including subthreshold particles, charge
recombination, and nuclear breakup. Particular attention is paid to the role of
neutrons, which are responsible for a large fraction of missing energy in all
channels. Next, we determine energy resolution, by quantifying event-to-event
stochastic fluctuations in missing energy. This is done for several sets of
assumptions about the reconstruction performance, including those available in
the literature. The resulting migration matrices, connecting true and
reconstructed neutrino energies, are presented. Finally, we quantify the impact
of different improvements on the experimental performance. For example, we show
that dropping particle identification information degrades the resolution by a
factor of two, while omitting charge deposits from de-excitation gammas worsens
it by about 25%. In the future, this framework can be used to assess the impact
of cross section uncertainties on the oscillation sensitivity.
