[gmx-users] question of improper dihedral of opls aa

[Tom]

Dear Gromacs Users,

I created a new residue on aminoacids.rtp file
and assigned improper dihedral after [ bonds ], for example

[ impropers ]
      C2    H      C3     S1

Then I want to add the parameters of this dihedral angle potential
onto ffbonded.itp

I know how to assign dihedral. Dihedral can be assigned with
these 4 atom types with the format as the follow
  Br     C      CB     CT      3      0.00000   0.00000   0.00000   0.00000
  0.00000   0.00000 ; acyl halide

*But the format of improper dihedral potential is not clear to me.*
*Can anyone help with the format? How to type these parameters
on ffbonded.itp ?*

I am using oplsaa.ff

Thanks a lot for the help!

Tom