Changing multiplicity for the other dihedral produced strange results. I started getting peaks inside of the wells.
Yes. We're trying to reproduce the QM. The QM has peaks inside the wells. One thing you could do is visualize the QM scan and see whether any of the C(sp²) centers became slightly nonplanar around the energy minimum. That is a possible source of the peaks, and in that case, we don't necessarily want to reproduce them.
At any rate, I didn't say changing
multiplicity - I said adding
a multiplicity, so that you have a 2-fold and
a 4-fold term for the same dihedral. There are tons of resources (most importantly, the tutorial) that show how this works. But check the planarity first.
Yesterday, I forgot to look at the scale of the graph. I now discovered it's from 0 to 1 kcal/mol; in other words, the discrepancies around the minimum are are small in terms of energy. As far as I am concerned, the fit is acceptable. But I still think you can get the OCCC even better by adding a 4-fold (and/or possibly a 6-fold, but let's not go there for now).
By the way, I need to set up explicit solvation and periodic boundary conditions. Offhand can you recommend any good resources?
The script archive comes to mind...