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 an 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 standardized set of test cases to validate the parallelization 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 ionization network to species other than hydrogen and helium.