From: Mayne, Christopher G (
Date: Tue May 17 2016 - 15:45:02 CDT


The problem that you are encountering is that the global minima are offset. Since the QME and MME profiles are not on the same absolute scale, we rescale the global minimum. Here, the global QME minimum is much lower than the MME minimum, causing the whole MME profile to be offset too low. There are a few things to look into.

1) Use the torsion explorer to inspect the conformation of the QME global minimum to check for anything strange going on with the structure.
2) Check the bond and angle parameters for something that is raising the MME energy
3) Split the optimization into two phases, one of which isolates the parameters associated with the QM scan that is generating the global minimum.

Christopher Mayne

On May 17, 2016, at 1:23 PM, Seth Axen <<>> wrote:

   I am using ffTK to fit several dihedral parameters. For several of my molecules, through iterative sampling, I am able to fit the MM potential energy surface curve to the shape of the QM curve. Generally, the global minima also match in height. However, additional simulated annealing or downhill, regardless of the tolerance, is unable to fit the heights of the curves. I was hoping someone could shed some light on what might be the cause of this. Attached is an example.