Interop: Apply separate force fields to different portions of a topology about openff-interchange HOT 3 CLOSED

I think this is a holdover from the days when we thought that we could have a "pre-parameterization interchange" - Like, we could add two unparameterized molecules loaded from different places, and later apply a FF to them. We decided at some point that Interchanges would strictly be "post-parameterization", and in that context I think this ticket is totally closed. If there's a use case I'm not imagining here, folks can reopen this/open a new issue.

from openff-interchange.

Something to consider is that, when mixing two force fields from a line (i.e. Parsley), each SMIRKS in a potential handler is not guaranteed to be unique if not tracking the force field each is associated with. The SMIRKS patterns between versions of Parsley are mostly the same, and there is plenty of overlap between those and smirnoff99Frosst.

Maybe the best way to handle this is also track the source force field in the PotentialKey objects.

from openff-interchange.

The __add__ functionality added in #148 may or may not do this clearly. I have not tested it yet. The Parsley + GAFF/ff19sb case should be mostly covered by testing if Parsley + Foyer-style force fields can co-exist.

from openff-interchange.