Lammps interatomic potentials

I’m running lammps for pvc polymer and i would like to know where i will get interatomic potential for H-C-Cl

In classical models the situation is different from quantum chemistry. You not the parameters (or basis sets) for a specific element, but the parameters vary for the chemical environment. I suggest you spend some time with your adviser/tutor discussion force fields and study text books on force fields and how “atom typing” works in classical MD. The specific parameters then still depend on your choice of force field. Most force fields have databases for parameter sets and rules to tools that help to assign them. It is discouraged to mix-and-match parameters from different force fields, as they have inconsistent balances for the different components.

If parameters for specific moieties are not available, you need to search the published literature, if somebody has created additional parameters for them and make sure they are compatible with your force field of choice.