Re: [lammps-users] About the use of OPLS improper parameters

Hi,
OPLS-AA uses the same functional form for proper and improper torsions (which are used to increase the in-plane stiffness of atoms in planar conformations, not to conserve chirality). Only the order of the atoms changes: in proper torsions the 4 atoms 1-2-3-4 are bonded as shown, whereas in improper torsions the 3rd atom is the central one with bonds to the other 3.
If you are considering OPLS for ionic liquids I’d suggest the parameter set developed in my group https://github.com/agiliopadua/clandp or the one by Orlando Acevedo.
I also provide a tool to build the input stack and data file for LAMMPS https://github.com/agiliopadua/fftool that handles the improper torsions.
Finally, you should think about scaling charges in fixed-charge force fields if you are interested in representing dynamics reasonably well (integer ion charge models tend to give sluggish dynamics). You’ll know this from your diffusion coefficients.
Agilio

Dear Agilio,

Thank you so much for sharing this information. It is very helpful. Now, I have figured out how to use the OPLS improper torsion in LAMMPS.

Best,