[gmx-users] OPLS united atom

[Krzysztof Murzyn]

Hi,

> >
> > My question is how I could properly transfer these parameters in the
> > Gromacs world. An obvious solution would be to list all terms for each
> > torsion with func eq 1 but this implies that one Gromacs dihedral
> > definition would consist of several entries/lines. Would it be OK?
> >
>
> No, rewrite it as a Ryckaert-Bellemans dihedrals instead; this is
> described in the manual, and I've already done it for OPLS-AA.
>
> The reason why we don't use several dihedrals for one bond is that you
> would end up calculating cos(phi) two or three times, and this is a
> somewhat costly operation. With a series we can calculate it once and
> then just multiply it to get higher powers.
>
Thanks for your answer, Erik. Now, having OPLS dihedral parameters
converted into the Ryckaert-Bellemans form I wonder what happens with 1-4
interactions. According to the gromacs manual, the use of RB dihedral
potential implies that all 1-4 interactions are excluded while in the
united-atom OPLS forcefield two scaling factors  (generate pairs = yes and
FudgeLJ = 8.0, FudgeQQ = 2.0) are used to include them in the total
potential.

Regards,
Krzysztof