We modeled the kinetics of ISC by applying NA-TST 23 using the LZ formula, Eq. ( ). We carried out these calculations using the open-source master equation code MESMER, which has been described in detail previously. For the sake of simplicity, we focused on the canonical formalism. Rovibrational densities of states were computed using the rigid-rotor harmonicoscillator approximation. MESMER input files with full details of the calculations are given in SI.