I want to do some molecular dynamics simulations for silicene with hydrogen and halogen atoms deposited at the surface. Where can I find Stillinger-Weber potentials to use for this? There doesn't seem to be any available in the list of potentials in the LAMMPS installation.
Generally, if you want to perform simulations with some force field, you will have to search the literature for published FFs tailored for your problem. The Stillinger-Weber (SW) model is very popular to describe bulk (diamond) silicon, for which it was designed in the first place. But several reparametrizations have also been published. If you happen to find the publication that matches your interest, you'll have to see if the authors provide a suitable parameter file themselves, or if you can make one yourself based on the published parameters. If you closely follow the descriptions in the LAMMPS manual, this is not too hard for SW. But you will have to run some benchmark calculations before you use it for production simulations.
You may also take a look at the NIST potentials database, which collects many FFs for various systems, most of which are provided in LAMMPS-readable format. If you click on Si, you will find many FFs, some of which are in fact parametrized for silicene in particular (references are also provided for all FFs). I do not see multi-component systems, though, which are generally much harder to parametrize.