""" Changes the LJ coefficient terms only for a specific type-pair """

def ChLJPair(parm, atom_1, atom_2, rmin, eps, one_4=False):
   
   if one_4:
      key = 'LENNARD_JONES_14'
   else:
      key = 'LENNARD_JONES'

   # Make sure that atom type 1 comes first
   atom_1, atom_2 = sorted([atom_1, atom_2])
   
   # Find the atom1 - atom2 interaction (adjusting for indexing from 0)
   term_idx = parm.parm_data['NONBONDED_PARM_INDEX'][
                   parm.pointers['NTYPES']*(atom_1 - 1) + atom_2 - 1] - 1
   
   # Now change the ACOEF and BCOEF arrays, assuming pre-combined values
   parm.parm_data['%s_ACOEF' % key][term_idx] = eps * rmin ** 12
   parm.parm_data['%s_BCOEF' % key][term_idx] = 2 * eps * rmin ** 6