, where φ is the torsion angle of the dihedral being described. The OPLS FF also employs an improper dihedral term through the same potential form, using only a V 2 parameter. The final term accounts for all non-bonded interactions between pairs of atoms seperated by a distance r ij . The standard Coulomb potential is used to calculate the interaction between two charges q i and q j . Finally, the short-range repulsion and longerrange attractive van der Waals interactions are described using the L-J 12-6 potential. Here