[gmx-users] Dihedral angle in OPLS-AA force field

David van der Spoel spoel at xray.bmc.uu.se
Mon Sep 11 15:14:20 CEST 2006

alokjain at iitk.ac.in wrote:
> Dear All,
> I have a basic question on Dihedral angle in OPLS-AA force field.
> I read some of the papers related to development of OPLS force field where
> they have different formula for calculating the torsion angle
> I think that is Ryckaert-Bellemans function, but there is no separate term
> for Improper dihedral term.
> when I tried to run a simulation using OPLS-AA force field gromacs 3.2.1
> version (pdb2gmx option 3) I got following energy terms in my log file.
> Energies (kJ/mol)
> Bond          Angle           Proper Dih.   Ryckaert-Bell.   LJ-14
> 1.15231e+03   2.92374e+03    1.73760e+02    1.32334e+03    1.86599e+03
> Coulomb-14      LJ (SR)      LJ (LR)      Coulomb (SR)    Coulomb (LR)
> 5.03326e+03  1.39004e+05   -1.66722e+03    -9.54632e+05    -5.37046e+04
> Potential       Kinetic En.   Total Energy   Temperature    Pressure (bar)
> -8.58528e+05   1.57277e+05   -7.01250e+05    3.04174e+02   -4.80269e+01
> so I am getting two proper dihedral term in output(Proper Dih. and
> Ryckaert-Bell.).But when I checked the ffoplsaabon.itp file I got
> following comments
>  Improper OPLS dihedrals to keep groups planar.
> ; (OPLS doesnt use improper for chiral atoms).
> ; Since these functions are periodic of the form 1-cos(2*x), the are
> ; actually implemented as proper dihedrals [1+cos(2*x+180)] for the moment,
> ; to keep things compatible.
> So its mean Proper Dih. which I am getting, In reality it  is Improper
> Dihedral? and Ryckaert-Bell for proper dihedral, (please correct me if
> I am wrong).But in original OPLS force field Improper Dihedral was not
> defined so how we can justify uses of Improper Dihedral.

yes that's correct. AFAIK improper dihedrals are used in OPLS for 
instance for keeping rings flat.

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://folding.bmc.uu.se

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