Hi again Nathan, I have modified my script to run on the output_00080 cosmological RAMSES run from http://yt-project.org/data/. The script is uploaded with link: http://use.yt/upload/9d91ea29. When I run this example I get a similar traceback: Traceback (most recent call last): File "/home/lhw28/.local/lib/python3.6/site-packages/yt/data_objects/data_containers.py", line 308, in _generate_fluid_field finfo.check_available(gen_obj) File "/home/lhw28/.local/lib/python3.6/site-packages/yt/fields/derived_field.py", line 204, in check_available validator(data) File "/home/lhw28/.local/lib/python3.6/site-packages/yt/fields/derived_field.py", line 470, in __call__ raise NeedsGridType(self.ghost_zones,self.fields) yt.fields.field_exceptions.NeedsGridType: (0, None) During handling of the above exception, another exception occurred: Traceback (most recent call last): File "yt_issue.py", line 58, in <module> center=centre, data_source=box) File "/home/lhw28/.local/lib/python3.6/site-packages/yt/data_objects/construction_data_containers.py", line 270, in __init__ self.get_data(field) File "/home/lhw28/.local/lib/python3.6/site-packages/yt/data_objects/construction_data_containers.py", line 344, in get_data self._handle_chunk(chunk, fields, tree) File "/home/lhw28/.local/lib/python3.6/site-packages/yt/data_objects/construction_data_containers.py", line 450, in _handle_chunk d = chunk[field] * dl File "/home/lhw28/.local/lib/python3.6/site-packages/yt/data_objects/data_containers.py", line 255, in __getitem__ self.get_data(f) File "/home/lhw28/.local/lib/python3.6/site-packages/yt/data_objects/data_containers.py", line 1382, in get_data self._generate_fields(fields_to_generate) File "/home/lhw28/.local/lib/python3.6/site-packages/yt/data_objects/data_containers.py", line 1402, in _generate_fields fd = self._generate_field(field) File "/home/lhw28/.local/lib/python3.6/site-packages/yt/data_objects/data_containers.py", line 292, in _generate_field tr = self._generate_fluid_field(field) File "/home/lhw28/.local/lib/python3.6/site-packages/yt/data_objects/data_containers.py", line 310, in _generate_fluid_field rv = self._generate_spatial_fluid(field, ngt_exception.ghost_zones) File "/home/lhw28/.local/lib/python3.6/site-packages/yt/data_objects/data_containers.py", line 330, in _generate_spatial_fluid ind += o.select(self.selector, self[field], rv, ind) File "/home/lhw28/.local/lib/python3.6/site-packages/yt/data_objects/data_containers.py", line 255, in __getitem__ self.get_data(f) File "/home/lhw28/.local/lib/python3.6/site-packages/yt/data_objects/data_containers.py", line 1382, in get_data self._generate_fields(fields_to_generate) File "/home/lhw28/.local/lib/python3.6/site-packages/yt/data_objects/data_containers.py", line 1402, in _generate_fields fd = self._generate_field(field) File "/home/lhw28/.local/lib/python3.6/site-packages/yt/data_objects/data_containers.py", line 292, in _generate_field tr = self._generate_fluid_field(field) File "/home/lhw28/.local/lib/python3.6/site-packages/yt/data_objects/data_containers.py", line 312, in _generate_fluid_field rv = finfo(gen_obj) File "/home/lhw28/.local/lib/python3.6/site-packages/yt/fields/derived_field.py", line 256, in __call__ dd = self._function(self, data) File "/home/lhw28/.local/lib/python3.6/site-packages/yt/fields/particle_fields.py", line 133, in particle_density pos = data[ptype, coord_name].convert_to_units("code_length") File "/home/lhw28/.local/lib/python3.6/site-packages/yt/data_objects/octree_subset.py", line 73, in __getitem__ tr = super(OctreeSubset, self).__getitem__(key) File "/home/lhw28/.local/lib/python3.6/site-packages/yt/data_objects/data_containers.py", line 255, in __getitem__ self.get_data(f) File "/home/lhw28/.local/lib/python3.6/site-packages/yt/data_objects/data_containers.py", line 1325, in get_data self.get_data(apply_fields[filter_type]) File "/opt/ioa/software/anaconda/anaconda36/envs/20181213_2_py36/lib/python3.6/contextlib.py", line 88, in __exit__ next(self.gen) File "/home/lhw28/.local/lib/python3.6/site-packages/yt/data_objects/particle_filters.py", line 53, in apply raise YTIllDefinedFilter(self, tr.shape, filter.shape) yt.utilities.exceptions.YTIllDefinedFilter: Filter '<yt.data_objects.particle_filters.ParticleFilter object at 0x7f25c23609e8>' ill-defined. Applied to shape (1175, 3) but is shape (31990,). I realised I never mentioned what version of YT I am using. In case it's important, it's 3.5.0. (And I'm using Python 3.6.3). Thanks for your help, Lewis