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

[alokjain at iitk.ac.in]

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.


Best regards,
Alok