The QM energy surfaces of Motor S provided by TorsionDrive are smooth and well-converged, as evident in Fig 4A . The two deep minima in each energy surface correspond to the stable forms in Fig , and the shallower minima correspond to the metastable form. This clean separation of stable form and metastable form was not achieved by other plausible choices of reaction coordinate, such as when the central torsion angle was defined by atoms 16-15-14-13 or when using the stator pucker as a scan coordinate. The lack of perfect 180 • periodicity along the central torsion results from the geometric properties of the torsion and the nonplanarity of the rotor and stator. The motors rotate from right to left in these graphs (negative direction of the central torsion) because the metastable to stable transition along negative x direction has a moderate barrier height, whereas the metastable to stable transition along the positive x direction has an extremely large barrier. The GAFF2 force field yields energy surfaces (Fig ) that deviate significantly from QM (Fig 4A ). The energy barriers between minima are higher and wider than QM, and the metastable local minimum at around (40, 0) in QM (Fig ) is missed by GAFF2. Furthermore, we found that the unoptimized force field gave fundamentally incorrect conformations in MD simulations. For example, the stator heterocycle adopted a chair conformation, whereas experiments and QM suggest a boat conformation. GAFF1 yielded similar problems, and we expect other general force fields, if not tuned for alkene motors, to be inadequate in quantitative studies, if not qualitatively wrong.