[gmx-users] Transference of dihedral potentials between united atom and all-atom force fields

[Justin Lemkul]

On 6/29/19 12:39 PM, Peter Stern wrote:
> No, of course not.  United atom potentials are optimized taking into account that the hydrogens are not included.  Think about it.  If you use the United atom dihedral potential in an all-atom force field you will also be adding to that all the dihedral potentials which include the hydrogen atoms.  Clearly this is not going to be the same for a particular C-C bond.
>
> And in general, you cannot transfer parameters from one force field to another.  I think that has been emphasized repeatedly in this forum.

100% agree with everything Peter has said above and I will add this (I 
have said it before but will say it again): dihedrals do not correspond 
to any real physical force. They are mathematical Band-Aids that account 
for inaccuracies in our treatment of nonbonded terms in 1-4 
interactions. Some force fields apply scaling factors to these 
interactions, others do not. In any case, a dihedral term is absolutely 
connected to a force field parameter set (like other force field terms, 
but perhaps even more obviously so) because it is inextricably linked to 
the (1) atomic charges, (2) LJ terms, and (3) 1-4 scaling factor, if 
applicable.

-Justin

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Justin A. Lemkul, Ph.D.
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