cannot optimize scale14 with DoubleExponential

[pavankum]

Attached a minimal FB input for reproducing the case. I can optimize scale14 in regular vdW non-bonded FF (Sage) but not with the DoubleExponential functional form. Edit: The gradient is always zero with DoubleExponential form for scale14.

The following additions to the file handlers/nonbonded.py fixed it, I don't know if both the changes are needed or just adding the parameter attribute is enough

at line397: scale14 = ParameterAttribute(default=0.5)
at line453: global_parameters = ["alpha", "beta", "scale14"]

dexp-opt-scale14.tar.gz

cc: @jthorton @j-wags

[mattwthompson]

I would be curious if you only needed to add it to global_parameters; it seems like it should already be a ParameterAttribute by nature of importing from CustomNonbondedHandler?

smirnoff-plugins/smirnoff_plugins/handlers/nonbonded.py

Lines 20 to 26 in b7d8856

	class CustomNonbondedHandler(ParameterHandler, abc.ABC):
	"""The base class for custom parameter handlers which apply nonbonded parameters."""
	_OPENMMTYPE = openmm.CustomNonbondedForce
	_DEPENDENCIES = [vdWHandler, VirtualSiteHandler]
	scale14 = ParameterAttribute(default=0.5, converter=float)

[pavankum]

Thank you @mattwthompson. Yeap, that works by just adding to global parameters list.

[mattwthompson]

Could you open a PR that includes this change? I don't know how to the best behavior of it, though.

[pavankum]

Yeah, just made the PR, will close this issue then.