Labelling fails if v-site handler added after library charge handler

[SimonBoothroyd]

Describe the bug

Labelling a molecule with a FF that contains a library charge handler that was added after a v-site handler fails

This flag should likely be inside the next loop.

To Reproduce

from openff.toolkit.typing.engines.smirnoff import ForceField
from openff.toolkit.typing.engines.smirnoff.parameters import (
    LibraryChargeHandler,
    VirtualSiteHandler
)
from simtk import unit as simtk_unit

force_field = ForceField()
force_field.get_parameter_handler("Electrostatics")

vsite_handler: VirtualSiteHandler = force_field.get_parameter_handler(
    "VirtualSites"
)
vsite_handler.add_parameter(
    {
        "smirks": "[#6:1][#9:2]",
        "name": "EP",
        "type": "BondCharge",
        "distance": 1.0 * simtk_unit.angstrom,
        "match": "once",
        "charge_increment1": 0.2 * simtk_unit.elementary_charge,
        "charge_increment2": 0.1 * simtk_unit.elementary_charge,
        "sigma": 1.0 * simtk_unit.angstrom,
        "epsilon": 0.0 * simtk_unit.kilocalorie_per_mole,
    }
)

library_handler: LibraryChargeHandler = force_field.get_parameter_handler(
    "LibraryCharges"
)
library_handler.add_parameter(
    {
        "smirks": "[F:2][C:1]([H:3])([H:4])([H:5])",
        "charge": [
            0.3 * simtk_unit.elementary_charge,
            -0.15 * simtk_unit.elementary_charge,
            -0.05 * simtk_unit.elementary_charge,
            -0.05 * simtk_unit.elementary_charge,
            -0.05 * simtk_unit.elementary_charge
        ]
    }
)

print(force_field.label_molecules(Molecule.from_smiles("CF").to_topology()))

Output

                if param_is_list:
                    for match in matches:
>                       parameter_matches[match] = [
                            m.parameter_type for m in matches[match]
                        ]
E                       TypeError: '_Match' object is not iterable

/xxx/openff/toolkit/typing/engines/smirnoff/forcefield.py:1435: TypeError

Computing environment (please complete the following information):

• Operating system
• Output of running conda list

openff-toolkit-base       0.10.3             pyhd8ed1ab_0    conda-forge

Additional context

[jthorton]

Duplicate of #1047, I could have a quick go at this one.

[mattwthompson]

Is this still a bug after #1252 (version 0.10.5+)?

[jthorton]

Yeah, can confirm this is still broken with a fresh 0.10.6 install.

[j-wags]

Oh, I didn't see that Simon had posted a proposed fix here - This may be quick so I'd be happy to give this a shot this morning @jthorton.

[jthorton]

@j-wags there also seems to be a print statement that was left in here that causes the toolkit to fill the Bepokefit gateway logs, could this also be removed I can make a separate issue for it if you prefer.

[j-wags]

Yes, I'd seen that in my testing as well and removed it. I'll include that fix in the same PR.

Yesterday I started working on this. It's a bit more complex that just Simon's recommendation because VirtualSiteHandler's fine_matches function returns a list, due to the fact that ValenceDict isn't exactly appropriate for vsites. So I'm experimenting with some fixes, and will open a PR once I have one ready to go.