In Figure , the potential energy surfaces interpolating the S 0 /S 1 CX and the cis and trans conformers for the imidazolinone (I) twisting and the phenolate (P) twisting in the gas phase. These correspond to the deactivation pathways, which are blocked by the protein scaffold that enforces the fluorescence. A comparison with all-electron SF-TDDFT and MRSF-TDDFT is added as reference on the same interpolated geometries for both the I and P pathways. Overall, the topology of the surfaces at the SF-TDDFT, MRSF-TDDFT and SF-TDDFTB level are in good accordance. The ground state average difference between MRSF-TDDFT and SF-TDDFT is 0.06 kcal/mol, with a standard deviation of 0.81 kcal/mol for the I pathway and 2.32 kcal/mol, with a standard deviation of 3.12 kcal/mol for the P pathway. For the excited state, the differences with respect to MRSF-TDDFT are 1.21 kcal/mol with a standard deviation of 1.95 kcal/mol for the I pathway, and 8.43 kcal/mol with a standard deviation of 4.46 kcal/mol for the P pathway. The excitation energy at the S 0 mininum for is 89.55 kcal/mol at the SF-TDDFTB level, in good agreement to the 92.25 kcal/mol of MRSF-TDDFT. Previous CASPT2 calculations of a similar chromophore reported an excitation energy of 88.78 kcal/mol. For the P pathway, the main changes come nearby the CX, in which the MRSF-TDDFT gap is 24.46 kcal/mol. On the other hand, for the I pathway, the CX gap is 10.86 kcal/mol. On the one hand, the energetic difference between the 0 • and 180 • isomers of the I twisting is 1.91 kcal/mol (SF-TDDFT), in close agreement to the 3.21 kcal/mol value given by MRSF-TDDFT. On the other hand, the energetic difference between the 0 • and 180 • isomers of the I twisting is 0.19 kcal/mol (SF-TDDFT), in close agreement to the 0.35 kcal/mol value given by MRSF-TDDFT. As indicated in the previous literature for anionic HBDI, the I pathway is more favourable than the P pathway. However, the I twisting CX was predicted for a similar chromophore to be 12 kcal/mol lower at the CASPT2 level than the FC S 1 energy, whereas in the interpolated SF-TDDFTB it is 5.54 kcal/mol higher in energy. In the case of the P twisting, the CX is 28.50 kcal/mol above the FC S 1 excitation.