Force Fields

Good afternoon
With respect to the Molecular Dynamics Simulation Studies of Benzene,
and p-Xylene , i have the potential parameters from an article in which
benzene ring is modeled as a rigid-body, but three C-H bonds in the methyl
groups of toluene (C6H5CH3) and p-xylene (CH3C6H4CH3) are not assumed to
be rigid and they are rotating according to a torsional potential.In my
lammps input file i didn't consider benzene as rigid body so i need the
parameters for the bonds,angles and dihedrals coming from the benzene ring
also, is it?
or how can i consider such benzene rigid-body in our lammps md simulation ?
Grateful to u
Best Regards

Depending on the system and properties you are trying to simulate you could consider using the OPLS all atom FF or the TraPPE united atom one. Both of the above have shown good transferability for molecules of the type you are referring to.