[AMBER] optimizing GAFF2 torsional parameters

From: Christos Deligkaris <deligkaris.gmail.com>
Date: Wed, 11 Sep 2019 13:22:38 -0500

dear all,

I am trying to improve the GAFF2 dihedral parameters for a torsion on a
small molecule. My small molecule has 5 torsions, only one is problematic,
I have been able to improve the dihedral parameters for the other 4
torsions by adjusting the force constants of the GAFF2 dihedral terms.

In the figure shown here (PDF file):

red is QM, green is GAFF2, blue is one of my attempts to improve the
results. The somewhat improved energy maximum at 0 degrees comes with an
artificial minimum at around -30 degrees. This minimum seems to also be
present at the default GAFF2 torsional profile but it is more shallow.

I can increase the weights of the points around the artificial minimum to
make them more consistent with QM data, but of course that comes at the
expense of less agreement with QM data at around -180- -120 degrees. I have
also tried optimizing the phase angles, without restricting them to 0 or
180, but that introduces an additional artificial energy minimum at around
-150 degrees. I have also tried all possible multiplicities, 1,2,3 without
any luck. I am using Mathematica to do the optimization.

What strategies have people used in the past to improve agreement of
dihedral parameters with QM data? Are there reasons to prefer no artificial
energy minima versus better agreement on energy barriers?

Best wishes,

Christos Deligkaris, PhD
Assistant Professor of Physics, University of Southern Indiana
SC2220, 8600 University Blvd, Evansville IN, 47712
Office Phone: (812) 228-5056
www.deligkaris.org .DeligkarisGroup
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Received on Wed Sep 11 2019 - 11:30:02 PDT
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