  There are now more Particle-in-Cell (PIC) codes than ever before that
researchers use to simulate intense laser-plasma interactions. To date, there
have been relatively few direct comparisons of these codes in the literature,
especially for relativistic intensity lasers interacting with thin overdense
targets. To address this we perform a code comparison of three PIC codes:
EPOCH, LSP, and WarpX for the problem of laser-driven ion acceleration in a
2D(3v) geometry for a $10^{20}$ W cm$^{-2}$ intensity laser. We examine the
plasma density, ion energy spectra, and laser-plasma coupling of the three
codes and find strong agreement. We also run the same simulation 20 times with
different random seeds to explore statistical fluctuations of the outputs. We
then compare the execution times and memory usage of the codes (without
"tuning" to improve performance) using between 1 and 48 processors on one node.
We provide input files to encourage larger and more frequent code comparisons
in this field.
