We first assume that the photoisomerization process deposits the system into each of the four metastable substates in the same proportions as observed in the QM surface-hopping MD calculations (Section 3.1), i.e., MS1:MS2:MS3:MS4=7:2:4:0. Thus, with an overall photoisomerization rate constant of k photo , the rate constant for arriving in MS1 is 7 13 k photo , and so on. The steady state probabilities of the system's states are plotted as a function of k photo in Fig 9A . At low k photo , the stable form has a probability near unity and the metastable substates have probabilities near zero. As k photo rises above ∼ 10 5 s -1 , the probability begins to shift from the stable form to the various metastable substates; and The motor's rotation rate (Figure , orange curve) has a similarly sigmoid dependence on k photo . The asymptotic maximal rotation rate, 1.5MHz, is approached as the stable form spends a negligible amount of time waiting for photoisomerization, so that THI is rate-limiting. The k photo at which the rate is 50% of its asymptotic maximum, is k photo,50 = 1 × 10 6 s -1 .