@leeping @MSchauperl @jchodera @davidlmobley @dgasmith

I'd like to restart the discussion of semiempirical charge methods, and how we can encode them in the SMIRNOFF spec.

Currently, users have two options to charge their molecules:

1. adding the <ToolkitAM1BCC/> tag in an OFFXML file, which will go run AM1-BCC with either Quacpac or Antechamber/sqm on the backend.
2. importing charges from the reference molecules loaded into the openforcefield toolkit

We do not currently have a complete implementation of the <ChargeIncrementModel> behavior from the SMIRNOFF spec, as it didn't seem like the format laid out there was general enough.

In this Issue, I'd like for us (and anyone else interested in working with our charge model) to start collecting input and moving toward an API for our charge-generating QM methods. This API should be general enough to support our method development for a year or two.

Even if you don't have a complete solution in mind, it would be good to collect requirements or ideas.

As an added dimension, there's a good amount of interest in a graph-based charge model (lacking geometry), so we can think of how that might tie in as well.