Hello everyone. I am currently using YT to run rockstar on my simulation. I have been following the instructions which are around and about and have managed to write a script to run rockstar via MPI readers = int(ncpu/4.) #Reserve one cpu for the server writers = ncpu - readers - 1 print 'Running rockstar with %i writers and %i readers'%(writers, readers) rh = RockstarHaloFinder(es, num_readers=readers, num_writers=writers, particle_type="dark_matter", dm_only=True) rh.run() using a particle filter on the datasets @yt.particle_filter("dark_matter", requires=[('particle_mass')]) def dark_matter(pfilter, data): if ('all', 'particle_age') in data.ds.field_list: return data[("all", "particle_age")] == 0.0 else: arr = np.zeros(len(data['particle_mass'])) return arr == 0.0 def setup_ds(ds): #Return only dark matter particles, and assert that the filter holds print 'Here' assert(ds.add_particle_filter("dark_matter")) It seems to run fine.... e.g