We used a Newton-Raphson algorithm implemented in the open-source package ForceBalance to systematically optimize force field parameters, so that the force field replicates as much as possible the QM energy surfaces (Section 2.3.1). During optimization, force field energies and gradients of the unsolvated motor molecule were compared with the corresponding QM energies and gradients. The objective function is the sum of squared energy and gradient differences between the force field and QM across the grid of conformations. For Motor O, it proved necessary to employ additional regularization terms, i.e. penalty terms to discourage unphysically large changes of certain parameters, as described in Results and Discussion.