Error Creating ForceField from GAFF Foyer XML

[umesh-timalsina]

I created a gaff gmso XML file using the gaff foyer XML using the following code:

>>> from pathlib import Path
>>> import pkg_resources
>>> from gmso.external.convert_foyer_xml import from_foyer_xml
>>> from_foyer_xml(Path(pkg_resources.resource_filename('antefoyer', 'xml/gaff.xml')).resolve(), overwrite=True)

The file can be accessed here.
And when I try to create a ForceField from the file. I get the following error:

>>> from gmso import ForceField
>>> ff = ForceField('gaff_gmso.xml', strict=False)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/umesh/work/mosdef-hub/gmso/gmso/core/forcefield.py", line 82, in __init__
    ff = ForceField.from_xml(xml_loc, strict, greedy)
  File "/home/umesh/work/mosdef-hub/gmso/gmso/core/forcefield.py", line 509, in from_xml
    child_tag='ImproperType'
  File "/home/umesh/work/mosdef-hub/gmso/gmso/utils/ff_utils.py", line 329, in parse_ff_connection_types
    ctor_kwargs['expression'])
  File "/home/umesh/work/mosdef-hub/gmso/gmso/utils/ff_utils.py", line 63, in _parse_params_values
    'Parameter {} is in units but cannot be found in parameters list'.format(param))
gmso.exceptions.ForceFieldParseError: Parameter delta0 is in units but cannot be found in parameters list
>>> 

[zijiewu3]

Had similar issues. See #522

[daico007]

Should have been addressed by #765