To compute the energy barrier of the thermal back reaction a concerted adiabatic scan is performed for the two breaking CC single bonds of QC-to-NBD, which are constrained to 20 values from 1.5 Åout to 2.2 Å, starting from the QC structure with the lowest energy. The highest energy structure and energy is used as an estimate for the transition state. The absorption spectra of NBD are computed by the sTDA-xTB method 27 using the lowest energy GFN2-xTB structure.