The results presented above focus on the mechanisms and kinetics of the motor rotation. Here, we present results for the free energy difference between the metastable form and the stable form, and discuss the implications of this quantity as a performance metric. We used umbrella sampling to obtain the PMF for rotation around the central double bond, defined here by torsion 43-15-14-29. As shown in Fig for Motor S, the metastable form is separated from the stable form by a free energy barrier. Integration along the PMF (Section 2.2) gives a favorable free energy release of 7.8 kcal mol -1 on going from the metastable to the stable form (right to left in the figure). Note that the height and width of the barrier makes this free energy difference insensitive to the precise choice of boundary between the stable form's free energy well on the left and the metastable form's free energy well on the right. The corresponding PMF for Motor N is similar in shape (Fig , and the free energy difference is slightly greater, at 8.8 kcal mol -1 . Thus, the available free energy from THI is similar to the standard free energy released by hydrolysis of ATP, the cell's energy currency, ∼8 kcal mol -1 .