bond_style mm3 command
bond_style mm3 bond_coeff 1 100.0 107.0
The mm3 bond style uses the potential that is anharmonic in the bond as defined in (Allinger)
where r0 is the equilibrium value of the bond, and K is a prefactor. The anharmonic prefactors have units angstrom^(-n): -2.55 angstrom^(-1) and (7/12)2.55^2 angstrom^(-2). The code takes care of the necessary unit conversion for these factors internally. Note that the MM3 papers contains an error in Eq (1): (7/12)2.55 should be replaced with (7/12)2.55^2
This bond style can only be used if LAMMPS was built with the USER_YAFF package. See the Build package doc page for more info.