MOCASSIN's principal limitation is imposed by the computer power available. The great volume of data which has to be handled in a three-dimensional simulation, implies the need for a system with multi-processing capabilities in order to accelerate the computational time. However, the fast development of Beowulf Linux clusters is making parallel computing more affordable, and this is also a reason why the MPI formalism was chosen (as opposed to openMP), as this allows information to be passed from one processor to another and, therefore, it does not necessarily require a system with shared memory facilities. Such systems, which include the Silicon Graphics Origin 2000 machine used for this work, are generally much more expensive than Beowulf clusters.
Pure-dust models are less computationally expensive than gas or dust and gas ones and reasonably sizes grids can be feasibly run on single processor machines.
MOCASSIN was designed for the modelling of the photoionised region of planetary nebulae and H II regions and it does not, at present, include the high energy physical processes which are needed, for example, for the modelling of AGNs. However the inclusion of processes such as inner shell photoionisation and Compton heating is straightforward and this is intended to be one of the developments of the near future. Moreover, current efforts are geared toward the inclusion of a self-consistent treatment for PDR processes, this requires the incorporation of a chemical network.
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