The torque available from THI, averaged over the 180 • rotation cycle of the motor, can be estimated as the free energy change, 7.8 kcal mol -1 , divided by π radians, which gives a torque of about 18 pN nm. This may be compared with 40 pN nm for the highly evolved F1-ATPase motor . Note that this analysis omits any contribution from the photoisomerization step, which also runs in the forward direction and could in principle contribute more to the average torque. We tested the numerical reliability of the free energy evaluations by rerunning all the simulations with different random number seeds, and obtained free energy differences within 0.3 kcal mol -1 of the original results. In addition, rerunning with umbrella sampling spring constant of 100 instead of 200 kj mol -1 affected the result by less than 0.3 kcal mol -1 , whereas varying WHAM parameters and the integral bounds within reason gave even smaller changes. Thus, the numerical uncertainties in these results appear to be well below 1 kcal mol -1 , and we expect that the force field is a larger source of error than the numerical uncertainty. Furthermore, these force field-based free energy differences are close to those previously obtained by purely QM methods for a similar motor .