[gmx-developers] Possible bug in grompp?

[Berk Hess]

Walter Ash wrote:

> I've been working on a new molecule topology and I've noticed what I 
> believe is a bug in grompp...  I am using the ffopls forcefield with 
> genpairs=yes, fudgeQQ and fudgeLJ = 0.5,  and combination rule 3.  
> This is with the 3.2.1 distribution version.
>
> In the molecular topology, under [ pairs ], it appears I have to 
> specify LJ14 parameters in C6 and C12 format despite the use of sigma 
> and epsilon everywhere else in this forcefield (comb rule 3).  If I 
> use sigma and epsilon in a [ pairs ] definition, my LJ14 energies are 
> orders of magnitude too high as grompp intereprets these parameters as 
> C6 and C12 values instead.  Putting C6 and C12 parameters here gives 
> the expected behavior.
>
> This is contrary to what is stated in the manual and what I would 
> expect the behavior to be.

A bug indeed.
It has already come up several times on the gmx-users list the past weeks.
I have fixed it in CVS and for the next release.

Berk.