From: Peter Freddolino (
Date: Wed May 23 2018 - 22:07:13 CDT

Rather than change the level of theory, which really breaks the whole
parameterization procedure, you are probably better off in your case to
instead restrict the range of angles that you're sampling. For highly
constrained ring systems like the one you're working with, it is probably
better to restrict your scan to a region around the equilibrium geometry
that could be reasonably achieved in the context of your molecule (and
then you'll end up fitting the dihedral potential to this - you should also
consider carefully what multiplicity for the dihedrals are appropriate in
your case; if the molecule is constrained such that only one region of
dihedral space can reasonably be sampled, as would be the case for some
atoms involved in the amantadine ring system, a multiplicity of 1 may be
appropriate even if the chemistry would typically suggest 3).


On Wed, May 23, 2018 at 2:28 PM, McGuire, Kelly <> wrote:

> I am on the Scan Torsions step of the FFTK plugin, and my molecule has a
> carbon cage (amantadine). Some of the dihedral scans struggle
> to finish. I set the scan to do +/- 180 for all of the dihedrals. I'm
> wondering what the best procedure for a cage type molecule would be
> for a torsion scan. Most of them finish with the default MP2/6-31(d), but
> about 5 of them either end with the internal coordinates error, constraints
> errors, or just stops without errors. However, if I change to HF/6-31(d),
> they finish with normal termination. I don't fully understand the
> difference between HF
> and MP2 other than MP2 is a hartree-fock approximation but with an
> electron correlation energy correction. How much would it change my
> results on this
> step if I used HF instead of MP2, or is there a better way to get the scan
> torsion to work for a cage molecule? Thanks!!
> *Kelly L. McGuire*
> *PhD Scholar*
> *Department of Physiology and Developmental Biology*
> *Brigham Young University*
> *LSB 3050*
> *Provo, UT 84602*