Hi, Takahiro
The CLAYFF is a kind of force field combining coulombic, van der Waals, bonded stretch and angle bend potentials. The four potentials can be found in pair_style of LAMMPS.
For detailed information about this 4 potentials, you can also read paper: J.Phys.Chem.B 2004, 108,1255-1266. ( Randall T.Cygan, Jian-Jie Liang, and Andrey G.Kalinichev. )