[gmx-users] opls LJ combination rules

David van der Spoel spoel at xray.bmc.uu.se
Fri Dec 3 11:11:13 CET 2004

On Fri, 2004-12-03 at 09:53 +0200, Markus O Kaukonen wrote:
> Dear All,
> When reading OPLS author (jorgensen-JACS-v118-pp11225-11236-y1996,
> below eq.1 ) the combination rules for LJ parameters is
> sigma_ij=sqrt(sigma_i*sigma_j). This is not what Gromacs manual says
> (eq. 4.7 in Gromacs-3.2 manual).
The manual is wrong, the files are correct. This is controlled in the
ffoplsaa.itp file, the combination rule should be 3.
> Questions:
> Is the formula for OPLS potential in gromacs as in
> jorgensen-JACS-v118-pp11225-11236-y1996?
> Are the phase angles f1, f2, f3 in torsion potential always zero in
> Gromacs?

The OPLS torsional potentials are implemented as RYckaert-Bellemans
potentials, meaning the higher order terms are zero.
> I find these questions of some importance when using OPLSAA-gromacs
> and hope someone who has done the gromacs coding could comment.
> Or is the only way to 'look at the source code'.
> Terveisin Markus
David van der Spoel, PhD, Assoc. Prof., Molecular Biophysics group,
Dept. of Cell and Molecular Biology, Uppsala University.
Husargatan 3, Box 596,          75124 Uppsala, Sweden
phone:  46 18 471 4205          fax: 46 18 511 755
spoel at xray.bmc.uu.se    spoel at gromacs.org   http://xray.bmc.uu.se/~spoel

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