Nevertheless, there is one main concern we must deal with: fluorescence is a long-term process. In pyrene, for instance, it should take place within 100 ns, which is much longer than the state-of-the-art simulation capabilities. We can overcome this problem by noticing that nonadiabatic relaxation is ultrafast, occurring on the sub-picosecond scale. Then, supposing that the molecular motion is ergodic after this initial relaxation, we can propagate many independent, relatively short trajectories, discard the initial relaxation in each of them, and sample points from the remaining dynamics to compose the nuclear ensemble. The technical details of this procedure are discussed in Section III.2.