I am interested to use LAMMPS to find the phase transition of amino acid. Which potential is applicable for this system especially this have C, H, O and N?
There are a lot of force fields available for the study of biomolecules such as amino acids. You can do a quick literature search (with Web of Science, Scopus, or even the Google) to find several appropriate references.
Specifically CHARMM and AMBER ff, both of which
can be specified via LAMMPS commands. See the howto
section of the manual - sec 4.3