When pyrene is excited at 4 eV, the vibrational temperature is 582 K, and the Boltzmann populations of S1 and S2 are 99.96 and 0.04%, respectively. The strong S2→S0 oscillator strength cannot compensate for this near-zero S2 population, and the S1 emission dominates the spectrum [Figure ]. Nevertheless, the differential emission rate is a factor of ten smaller than in the high-energy excitation cases. Assuming a factor 15 increase of the S1→S0 oscillator strength due to vibronic coupling enhances the S1 dominance, and the S2 shoulder disappears [Figure (d)], as also observed experimentally. These VCA simulations show two main relevant variables affecting pyrene's fluorescence: the relative population and the vibronic coupling between S1 and S2. The relative population controls the relative amount of emission from S2, whereas the vibronic coupling controls the emission intensity from S1. Nevertheless, the vibronic coupling was included on a crude ad hoc basis. Section IV.2 discusses this feature on more solid grounds based on vibronic couplings estimates within the nuclear ensemble approach.