[gmx-users] 1-4 interactions in CHARMM force field

[Justin Lemkul]

On 5/15/17 2:02 PM, Dawid das wrote:
> Dear Gromacs Users,
>
> I would like to hear your advice on scaling 1,4-interactions for both
> classical and
> QM/MM molecular dynamics simulations. I am using CHARMM27 force field.
>
> Firstly,  for classical MD, according to this post
> https://www.charmm.org/ubbthreads/ubbthreads.php?ubb=showflat&Number=1048
>
> I should not rescale 1,4 iternactions, so if I use MDP options you suggest
> on
> Gromacs website I do not rescale them, right?
>

Right.  If you mess with 1-4 scaling, you invalidate the force field.  The 
dihedrals are parametrized under the assumption that 1-4 interactions are at 
full strength.  Dihedrals are correction factors for inaccuracies in the 
additive nonbonded functional form and therefore are tied to the strength of the 
1-4 interactions.  Some force fields "fudge" the 1-4 terms to compensate (in 
part) for such errors.  CHARMM does not.

> Secondly, do you have any experience with QM/MM MD? Should I rescale the
> 1,4 interactions? I searched for that but found nothing. Perhaps I've
> missed a
> paper.
>

1-4 interactions are purely a construct of the empirical force field.  They are 
not relevant to the QM subsystem because no such empirical model exists there, 
it is QM.

-Justin

-- 
==================================================

Justin A. Lemkul, Ph.D.
Ruth L. Kirschstein NRSA Postdoctoral Fellow

Department of Pharmaceutical Sciences
School of Pharmacy
Health Sciences Facility II, Room 629
University of Maryland, Baltimore
20 Penn St.
Baltimore, MD 21201

jalemkul at outerbanks.umaryland.edu | (410) 706-7441
http://mackerell.umaryland.edu/~jalemkul

==================================================