[DNM]: Add experimental "auto" idivf

[mattwthompson]

Description

Requires installing a branch of the same name from the toolkit. Here's a script that demonstrates it:

import openmm
from openff.toolkit import ForceField, Molecule, Quantity, unit
from openff.toolkit.typing.engines.smirnoff.parameters import (
    ParameterList,
    ProperTorsionType,
)


def get_torsion_force_constants(system: openmm.System) -> set[float]:
    k = list()
    for force in system.getForces():
        if isinstance(force, openmm.PeriodicTorsionForce):
            for torison_index in range(force.getNumTorsions()):
                torsion = force.getTorsionParameters(torison_index)
                k.append(torsion[6]._value)

    return set(k)


force_field = ForceField("openff-2.1.0.offxml")

force_field.deregister_parameter_handler("ImproperTorsions")

# This is super illegal
force_field["ProperTorsions"]._parameters = ParameterList(
    [
        ProperTorsionType(
            smirks="[#1:1]-[*:2]~[*:3]-[#1:4]",
            periodicity=[1],
            phase=[0],
            k=[Quantity(10.0, unit.kilojoule_per_mole)],
            idivf=["experimental-auto"],
            id="expt1",
        )
    ]
)

for smiles in ["CC", "CO", "C=C", "C#C"]:
    print(
        smiles,
        get_torsion_force_constants(
            force_field.create_openmm_system(Molecule.from_smiles(smiles).to_topology())
        ),
    )

# CC {1.1111111111111112}
# CO {3.3333333333333335}
# C=C {2.5}
# C#C {10.0}

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 60.00000% with 10 lines in your changes missing coverage. Please review.

Project coverage is 91.80%. Comparing base (1c4e383) to head (cd88a77). Report is 440 commits behind head on main.

Additional details and impacted files