Following optimization, the force field energy surfaces (Fig 4C ) correctly represent all local minima and yield energy barriers similar to the QM results. It is notable that the optimization reduces the amplitudes of torsional energy terms associated with the central double bond and thus addresses the issue that the initial force field overestimated the barrier heights. In addition, optimization causes the various improper dihedral terms to have more varied amplitudes, ranging from essentially zero to 10.0 kcal mol -1 , whereas all the GAFF2 amplitudes are 1.1 kcal mol -1 . These changes presumably enable the optimized parameters to provide correct ring conformations. Note that only proper and improper dihedral terms were adjusted; other force field terms, such as bond-stretches and bond-angles, were left unchanged. For Motor N, the initial GAFF2 parameters again yield potential energy surfaces significantly different from the QM surfaces, and optimization of dihedral terms again yielded potential energy surfaces much closer to QM (Figure in the ESI). We therefore tried optimizing not only dihedral terms for Motor O, but also the σ and ϵ values of each Lennard-Jones (LJ) term, as well as the force constants and equilibrium lengths of bond-stretch and angle-bend terms. These terms strongly affect the steric interactions between the stator and rotor during THI. To avoid nonphysically large changes in LJ terms, flat bottomed restraints were applied, to allowed maximal changes of 0.1 Å in σ and 0.015 kJ mol -1 in ϵ. The resulting force field is softened, with σ and ϵ smaller in most cases, and also reduced bond-stretch and angle-bend force constants. As expected, using the softened LJ terms in classical MD simulations allowed a greater THI rate that matches experiments. However, in this case the optimization algorithm made bond and angle force constants unphysically small, causing the molecule to be overly flexible during simulation. For example, the force constant of the central double bond, which is conjugated and thus should have a bond order between one and two, fell from the GAFF2 value of 855 to 312 kJ mol -1 Å -2 . Note that even the GAFF2 value is probably on the low side, because the force constant for a benzene C-C bond, which also is of order between one and two, has been reported as 1124 kJ mol -1 Å -2 .