As detailed in Results and Discussion, we tested various choices of what parameters to optimize, and we found that optimizing dihedral (proper and improper) amplitudes alone sufficed for Motor S and N, whereas Motor O needed more changes. Our procedures ensure that these parameters remain non-negative during optimization. No changes were considered to the functional form of the force field, the multiplicities and phases of torsional terms, and the partial atomic charges. Initial force field parameters were taken from general Amber force field (GAFF) version 2.1 implemented in AmberTools18 , with AM1/BCC partial charges . A subtlety of atom type selection for the motors is discussed in the Appendix C. For the dichloromethane solvent at room temperature and pressure, the GAFF2 parameters gave a density of 1.25 g cm -3 , in good agreement with the experimental value of 1.33 g cm -3 , so the GAFF2 parameters were used as is for the solvent.