Some methods that accept the compound keyword fail or don't return the expected set of values

[mnmelo]

For at least moment_of_inertia and asphericity the use of compounds seems to be broken:

Expected behavior

from MDAnalysis.tests.datafiles import TRZ_psf, TRZ
u = mda.Universe(TRZ_psf, TRZ) # universe with 24 residues

u.atoms.asphericity(compound='residues')
# expected a 1D array with 24 values of asphericity, one per residue
# equivalent to
np.array([res.atoms.asphericity() for res in u.residues])

u.atoms.moment_of_inertia(compound='residues')
# expected a 3D array with 24 MoI 3x3 tensors, one per residue, with shape (24,3,3)
# equivalent to
np.array([res.atoms.moment_of_inertia() for res in u.residues])

Actual behavior

u.atoms.asphericity(compound='residues')
~/src/MDAnalysis/mdanalysis/package/MDAnalysis/lib/util.py in wrapper(group, *args, **kwargs)
   1841         # has already been thrown:
   1842         if group.isunique or warn_if_not_unique.warned:
-> 1843             return groupmethod(group, *args, **kwargs)
   1844         # Otherwise, throw a DuplicateWarning and execute the method.
   1845         method_name = ".".join(

~/src/MDAnalysis/mdanalysis/package/MDAnalysis/core/groups.py in wrapped(group, *args, **kwargs)
    386                 raise ValueError(
    387                     "both 'pbc' and 'unwrap' can not be set to true")
--> 388         return function(group, *args, **kwargs)
    389     return wrapped
    390

~/src/MDAnalysis/mdanalysis/package/MDAnalysis/core/topologyattrs.py in asphericity(group, pbc, unwrap, compound)
   1418             pbc=pbc, unwrap=unwrap, compound=compound)
   1419         if compound != 'group':
-> 1420             com = (com * group.masses[:, None]
   1421                    ).sum(axis=0) / group.masses.sum()
   1422

ValueError: operands could not be broadcast together with shapes (24,3) (8184,1)

u.atoms.moment_of_inertia(compound='residues')
~/src/MDAnalysis/mdanalysis/package/MDAnalysis/lib/util.py in wrapper(group, *args, **kwargs)
   1841         # has already been thrown:
   1842         if group.isunique or warn_if_not_unique.warned:
-> 1843             return groupmethod(group, *args, **kwargs)
   1844         # Otherwise, throw a DuplicateWarning and execute the method.
   1845         method_name = ".".join(

~/src/MDAnalysis/mdanalysis/package/MDAnalysis/core/groups.py in wrapped(group, *args, **kwargs)
    386                 raise ValueError(
    387                     "both 'pbc' and 'unwrap' can not be set to true")
--> 388         return function(group, *args, **kwargs)
    389     return wrapped
    390

~/src/MDAnalysis/mdanalysis/package/MDAnalysis/core/topologyattrs.py in moment_of_inertia(group, pbc, **kwargs)
   1260             pbc=pbc, unwrap=unwrap, compound=compound)
   1261         if compound != 'group':
-> 1262             com = (com * group.masses[:, None]
   1263                    ).sum(axis=0) / group.masses.sum()
   1264

ValueError: operands could not be broadcast together with shapes (24,3) (8184,1)

Calling the method on a ResidueGroup seems to default to using compound='group' and yield a single value. More puzzingly, calling it on a ResidueGroup and explicitly passing compound='residues' works (no ValueError) but yields the same single value (with some precision difference betraying a different logic route):

u.residues.asphericity()
>> 0.5655677527570107

u.residues.asphericity(compound='residues')
>> 0.5655677527570102

u.residues.moment_of_inertia()
>> array([[1332923.85838838,  480989.6752955 , -227811.7564208 ],
          [ 480989.6752955 ,  612813.7537505 ,  231931.75813526],
          [-227811.7564208 ,  231931.75813526, 1509650.41986839]])

u.residues.moment_of_inertia(compound='residues')
>> array([[1332923.85838838,  480989.6752955 , -227811.7564208 ],
          [ 480989.6752955 ,  612813.7537505 ,  231931.75813526],
          [-227811.7564208 ,  231931.75813526, 1509650.41986839]])```

A consolidated compound-splitting strategy (#3000) might help fix these issues and uniformize the API.

Current version of MDAnalysis

• Which version are you using? 2.0.0-dev0 (at commit beb5232)
• Which version of Python? 3.6.9
• Which operating system? Ubuntu 18.04