LigParGen is a web-based service that provides force field (FF) parameters for organic molecules or ligands, offered by the Jorgensen group.

It is available here. http://zarbi.chem.yale.edu/ligpargen/

LigParGen provides bond, angle, dihedral, and Lennard-Jones OPLS-AA parameters with 1.14CM1A or 1.14CM1A-LBCC partial atomic charges. Server provides parameter and topology files for commonly used molecular dynamics and Monte Carlo packages OpenMM, Gromacs, NAMD, CHARMM, LAMMPS, CNS/X-PLOR, Q, DESMOND, BOSS and MCPRO. Also, the PQR file is generated. Supported input formats: SMILES, MOL and PDB.Maximum ligand size allowed is 200 atoms.

It is also possible to install LigParGen locally on Mac or Linux machines using Anaconda as described here

More details here

LigParGen web server: an automatic OPLS-AA parameter generator for organic ligands, DOI