Support of virtual sites for creating an Interchange from_openmm

[hannahbaumann]

Description
I'm trying to create an Interchange object from an OpenMM system that contains TIP4P water and got this Error: UnsupportedImportError: A particle is a virtual site, which is not yet supported.
Are there any plans to support virtual sites in the from_openmm call or would you recommend a different procedure in a case like this?

[mattwthompson]

No plans right now (but also not planning on never doing this!). I've added it to the board to see how others in the org wish to assign its its priority.

At a technical level I think it's doable, and something narrowly-scoped to 4-site water models is certainly easier than general support. I think it's doable. Compared to a three-site water model, it's just an extra key in the ElectrostaticsCollection and constructing a VirtualSiteCollection from scratch, but only needing to support one style.

[hannahbaumann]

Awesome, thank you for letting me know!