Hello! I have two questions: the first is related to saving new field specs, and the second is related to reading in the file itself. I was following along with the documentation that describes saving new field specs, and I defined a new field spec as (it should be noted that my extra columns are for both gas and star particles, and I was unsure as to how to tell yt that it applied for both types, so I made a best guess): my_field_def = ("Coordinates", "Velocities", "ParticleIDs", "Mass", ("InternalEnergy", "Gas"), ("Density", "Gas"), ("SmoothingLength", "Gas"), ("RII", "Stars"), ("RIa", "Stars"), ("StellarAge", "Stars"), ("C", "Gas", "Stars"), ("O", "Gas", "Stars"), ("Ca", "Gas", "Stars"), ("Cr", "Gas", "Stars"), ("Mn", "Gas", "Stars"), ("Fe", "Gas", "Stars"), ) And a unit base: unit_base = {'UnitLength_in_cm' : 3.085678e+21, 'UnitMass_in_g' : 1.989e+43, 'UnitVelocity_in_cm_per_s' : 1.0e+5} Firstly, when I tried to add my custom field definition to the gadget_field_specs by doing: from yt.frontends.sph.definitions import gadget_field_specs I get the error: Traceback (most recent call last): File "<stdin>", line 1, in <module> ImportError: No module named definitions Just wondering what I was doing wrong? Also, when I try to load in the snapshot, I am doing: fname = 'snapshot_000.0' ds = yt.GadgetDataset(fname, unit_base=unit_base, field_spec=my_field_def) ds.index ad = ds.all_data() pz = yt.ProjectionPlot(ds, 'z', ('gas', 'density')) pz.show() The output is: yt : [INFO ] 2015-08-18 17:24:45,103 Calculating time from 2.500e-01 to be 6.950e+16 seconds yt : [INFO ] 2015-08-18 17:24:45,144 Parameters: current_time = 6.95032608578e+16 s yt : [INFO ] 2015-08-18 17:24:45,144 Parameters: domain_dimensions = [2 2 2] yt : [INFO ] 2015-08-18 17:24:45,144 Parameters: domain_left_edge = [ 0. 0. 0.] yt : [INFO ] 2015-08-18 17:24:45,145 Parameters: domain_right_edge = [ 50000. 50000. 50000.] yt : [INFO ] 2015-08-18 17:24:45,145 Parameters: cosmological_simulation = 1 yt : [INFO ] 2015-08-18 17:24:45,145 Parameters: current_redshift = 3.00000005201 yt : [INFO ] 2015-08-18 17:24:45,145 Parameters: omega_lambda = 0.726 yt : [INFO ] 2015-08-18 17:24:45,145 Parameters: omega_matter = 0.274 yt : [INFO ] 2015-08-18 17:24:45,145 Parameters: hubble_constant = 0.702 yt : [INFO ] 2015-08-18 17:24:45,146 Allocating for 4.194e+06 particles yt : [INFO ] 2015-08-18 17:24:45,664 Identified 3.077e+05 octs Traceback (most recent call last): File "yt_test_script.py", line 72, in <module> ds.index File "/home/user/Research/Tools/yt/yt-x86_64/src/yt-hg/yt/data_objects/static_output.py", line 321, in index self, dataset_type=self.dataset_type) File "/home/user/Research/Tools/yt/yt-x86_64/src/yt-hg/yt/geometry/particle_geometry_handler.py", line 53, in __init__ super(ParticleIndex, self).__init__(ds, dataset_type) File "/home/user/Research/Tools/yt/yt-x86_64/src/yt-hg/yt/geometry/geometry_handler.py", line 65, in __init__ self._detect_output_fields() File "/home/user/Research/Tools/yt/yt-x86_64/src/yt-hg/yt/geometry/particle_geometry_handler.py", line 125, in _detect_output_fields fl, _units = self.io._identify_fields(dom) File "/home/user/Research/Tools/yt/yt-x86_64/src/yt-hg/yt/frontends/gadget/io.py", line 208, in _identify_fields field, req = field ValueError: too many values to unpack The snapshot I'm trying to load is a 128^3 50Mpc snapshot distributed onto 4 files and the error occurs on a call to ds.index, if that helps. Thank you! -Jared