Ring angles

The angle terms applied to atoms in or around a ring have not received a lot of attention. Concerns have been raised that they may be overly stiff or broad. For example, as briefly mentioned by Bill Swope and Alberto Gobbi: 2022-10-13 Force Field Release Meeting notes

Lorenzo D’Amore compared OpenFF 2.0 performance to GAFF 2.11 with a specific focus on ring moieties: https://openforcefieldgroup.slack.com/archives/CKSHCE7SB/p1670282381642029 (powerpoint also attached here for posterity)

Open

As summarised by David Mobley :

One key thing from Lorenzo’s analysis seems to be that performance of the same angle parameters is different across the several categories of angles he notes:

1. All atoms are in a ring

2. Two atoms are in a ring

3. One atom is in a ring

4. No atoms are in a ring

This probably suggests we should try some fitting experiments — at least for angle parameters with larger errors — where we separate out these categories (either individually or in groups) and fit separately.

In general, Lorenzo concludes that Sage performs better than GAFF when all atoms are in a ring (case 1), but underperforms in all other cases (2, 3, 4).

Sage 2.1 separated some ring terms out, so Lorenzo’s analysis may not be totally applicable. However, these terms still don’t fully specify how many atoms are in the ring, so we should still attempt to split out parameters based on this and see how they perform.