Dear Prateek, The error message is: " File "/home/sina/anaconda3/lib/python3.6/site-packages/yt/analysis_modules/halo_analysis/halo_finding_methods.py", line 117, in _parse_old_halo_list halo_properties['particle_mass'][0][i] = halo.virial_mass().in_cgs() AttributeError: 'int' object has no attribute 'in_cgs' " when I deleted the "in_cgs()" part, there is no error but the particle positions are all zero and the virial mass and virial radius are all -1. Here is the full error message: """ Number of initial groups:51218 Number of groups:643 FOF CPU TIME: 0.049968 secs yt : [INFO ] 2019-08-06 15:33:28,674 Parsing outputs yt : [WARNING ] 2019-08-06 15:33:28,728 Field ('all', 'particle_mass') was added without specifying units, assuming units are g ERROR:root:Internal Python error in the inspect module. Below is the traceback from this internal error. Traceback (most recent call last): File "/home/sina/anaconda3/lib/python3.6/site-packages/IPython/core/interactiveshell.py", line 2963, in run_code exec(code_obj, self.user_global_ns, self.user_ns) File "<ipython-input-5-6b9262d90adc>", line 1, in <module> hc.create() File "/home/sina/anaconda3/lib/python3.6/site-packages/yt/analysis_modules/halo_analysis/halo_catalog.py", line 337, in create self._run(save_halos, save_catalog, njobs=njobs, dynamic=dynamic) File "/home/sina/anaconda3/lib/python3.6/site-packages/yt/utilities/parallel_tools/parallel_analysis_interface.py", line 301, in barrierize return func(*args, **kwargs) File "/home/sina/anaconda3/lib/python3.6/site-packages/yt/analysis_modules/halo_analysis/halo_catalog.py", line 406, in _run self.halos_ds = self.finder_method(self.data_ds) File "/home/sina/anaconda3/lib/python3.6/site-packages/yt/analysis_modules/halo_analysis/halo_finding_methods.py", line 44, in __call__ return self.function(ds, *self.args, **self.kwargs) File "/home/sina/anaconda3/lib/python3.6/site-packages/yt/analysis_modules/halo_analysis/halo_finding_methods.py", line 62, in _fof_method halos_ds = _parse_old_halo_list(ds, halo_list) File "/home/sina/anaconda3/lib/python3.6/site-packages/yt/analysis_modules/halo_analysis/halo_finding_methods.py", line 117, in _parse_old_halo_list halo_properties['particle_mass'][0][i] = halo.virial_mass().in_cgs() AttributeError: 'int' object has no attribute 'in_cgs' During handling of the above exception, another exception occurred: Traceback (most recent call last): File "/home/sina/anaconda3/lib/python3.6/site-packages/IPython/core/interactiveshell.py", line 1863, in showtraceback stb = value._render_traceback_() AttributeError: 'AttributeError' object has no attribute '_render_traceback_' During handling of the above exception, another exception occurred: Traceback (most recent call last): File "/home/sina/anaconda3/lib/python3.6/site-packages/IPython/core/ultratb.py", line 1095, in get_records return _fixed_getinnerframes(etb, number_of_lines_of_context, tb_offset) File "/home/sina/anaconda3/lib/python3.6/site-packages/IPython/core/ultratb.py", line 311, in wrapped return f(*args, **kwargs) File "/home/sina/anaconda3/lib/python3.6/site-packages/IPython/core/ultratb.py", line 345, in _fixed_getinnerframes records = fix_frame_records_filenames(inspect.getinnerframes(etb, context)) File "/home/sina/anaconda3/lib/python3.6/inspect.py", line 1490, in getinnerframes frameinfo = (tb.tb_frame,) + getframeinfo(tb, context) File "/home/sina/anaconda3/lib/python3.6/inspect.py", line 1448, in getframeinfo filename = getsourcefile(frame) or getfile(frame) File "/home/sina/anaconda3/lib/python3.6/inspect.py", line 696, in getsourcefile if getattr(getmodule(object, filename), '__loader__', None) is not None: File "/home/sina/anaconda3/lib/python3.6/inspect.py", line 742, in getmodule os.path.realpath(f)] = module.__name__ AttributeError: module has no attribute '__name__' """ Best, Sina