• Proper torsions: Each of the 154 torsion types is associated with an N-term Fourier series of potential energy contributions, where ≤ 6, and each term, , is of the form = cos + . We optimized all of the amplitudes that were defined in SMIRNOFF99Frosst, comprising 154, 62, 26, 5, 4 and 3 values of 1 , 2 , 3 , 4 , 5 , and 6 respectively, for a total of 254 parameters. Parameters t156, t157, t158 represent torsion angles containing a linear angle, and their values of 1 were kept at 0.0 during fitting. The phase parameters, and the selection of Fourier terms used for each torsion were not optimized in this release.