Enabling parallel computing in CRASH
  We present the new parallel version (pCRASH2) of the cosmological radiative
transfer code CRASH2 for distributed memory supercomputing facilities. The code
is based on a static domain decomposition strategy inspired by geometric
dilution of photons in the optical thin case that ensures a favourable
performance speed-up with increasing number of computational cores. Linear
speed-up is ensured as long as the number of radiation sources is equal to the
number of computational cores or larger. The propagation of rays is segmented
and rays are only propagated through one sub-domain per time step to guarantee
an optimal balance between communication and computation. We have extensively
checked pCRASH2 with a standardised set of test cases to validate the
parallelisation scheme. The parallel version of CRASH2 can easily handle the
propagation of radiation from a large number of sources and is ready for the
extension of the ionisation network to species other than hydrogen and helium.
