cannot add a vdW parameter to the default vdW handler in 0.8.1

[trevorgokey]

Describe the bug

Previously, before commit 907423a#diff-4055de40d998a929690d778b556442506df263af162f482a3f47ad9dd98f1883

I was able to add a vdW parameter as follows using the following dictionary:

                param_dict = dict(
                    epsilon="0.5 * kilocalorie/mole",
                    rmin_half="1.2 * angstrom",
                    smirks="[*:1]",
                    id="n1",
                )

via the standard ph.add_parameter(param_dict). Now with the most recent version, it no longer works.

I couldn't find anywhere in the release notes that describe the intended break/change in behavior, so it seems something is off here.

To Reproduce

from openforcefield.typing.engines.smirnoff import ForceField
ff = ForceField('openff-1.3.0.offxml')
param = dict(epsilon="0.5 * kilocalorie/mole", rmin_half="1.2 * angstrom", smirks="[*:1]", id="n99")
ff.get_parameter_handler("vdW").add_parameter(param)

Output

with 0.7.2, no output (it works)

with 0.8.1:

/home/tgokey/.local/miniconda3/envs/oFF-test/lib/python3.7/site-packages/openforcefield/typing/engines/smirnoff/parameters.py in add_parameter(self, parameter_kwargs, parameter, after, before)
   2090         # If a dict was passed, construct it; if a ParameterType was passed, do nothing
   2091         if parameter_kwargs:
-> 2092             new_parameter = self._INFOTYPE(**parameter_kwargs)
   2093         elif parameter:
   2094             new_parameter = parameter

/home/tgokey/.local/miniconda3/envs/oFF-test/lib/python3.7/site-packages/openforcefield/typing/engines/smirnoff/parameters.py in __init__(self, **kwargs)
   3502                 )
   3503 
-> 3504             super().__init__(**kwargs)
   3505 
   3506             if sigma:

/home/tgokey/.local/miniconda3/envs/oFF-test/lib/python3.7/site-packages/openforcefield/typing/engines/smirnoff/parameters.py in __init__(self, smirks, allow_cosmetic_attributes, **kwargs)
   1828         # This is just to make smirks a required positional argument.
   1829         kwargs["smirks"] = smirks
-> 1830         super().__init__(allow_cosmetic_attributes=allow_cosmetic_attributes, **kwargs)
   1831 
   1832     def __repr__(self):

/home/tgokey/.local/miniconda3/envs/oFF-test/lib/python3.7/site-packages/openforcefield/typing/engines/smirnoff/parameters.py in __init__(self, allow_cosmetic_attributes, **kwargs)
    914         for key, val in smirnoff_data.items():
    915             if key in allowed_attributes:
--> 916                 setattr(self, key, val)
    917             # Handle all unknown kwargs as cosmetic so we can write them back out
    918             elif allow_cosmetic_attributes:

/home/tgokey/.local/miniconda3/envs/oFF-test/lib/python3.7/site-packages/openforcefield/typing/engines/smirnoff/parameters.py in __setattr__(self, name, value)
   3512             super().__setattr__(key=name, value=value)
   3513             if name == "rmin_half":
-> 3514                 super().__setattr__("sigma", value / 2 ** (1 / 6))
   3515                 self._extra_nb_var = "sigma"
   3516 

TypeError: unsupported operand type(s) for /: 'str' and 'float'

[mattwthompson]

Thanks for the report - it does like a regression to me. In that setter, I overlooked that the values may be strings, and only tested unpacked quantities. I think the solution would be to check the type and, if it is a string, unwrap it with object_to_quantity?

[j-wags]

Pretty much. Specifically I think we want to use convert_and_validate instead of object_to_quantity, though I need to look closely at the pattern of what-is-calling-what here.

[j-wags]

Ah, you're right, @mattwthompson. It should be object_to_quantity.

I had just started a fix branch when you opened the PR. Doc Thompson, fastest PR in the west. Ain't nobody can best him.

[trevorgokey]

Awesome! This is really nice to see get tackled and sorted out so quickly.