The main isomerisation reaction is initiated in the S 1 excited state, for which a conical instersection (CX) between S 0 and S 1 occurs at approximately 90 • twisting. In Figure , the potential energy surface interpolating the 11-cis, the CX and the all-trans rPSB computed at the SF-TDDFTB is shown. A comparison with all-electron SF-TDDFT and MRSF-TDDFT is added as reference on the same interpolated geometries. Overall, the topology of the surfaces at the SF/MRSF-TDDFT and SF-TDDFTB level are very similar. Interestingly, SF-TDDFTB compares better to MRSF-TDDFT grounds tate, indicating a smaller spin contamination than the all-electron SF-TDDFT. The ground state average difference between MRSF-TDDFT and SF-TDDFT is 2.04 kcal/mol, with a standard deviation of 1.90 kcal/mol. For the excited state it is 5.23 kcal/mol with a standard deviation of 1.53 kcal/mol. The main changes come at the the CX. The CX is computed at the SF-TDDFTB level and features a 99 • twist between the two moieties. At this geometry, MRSF-TDDFT has an energy gap of 6.03 kcal/mol. This is indicating that the MRSF-TDDFT CX is not exactly at the same geometry as SF-TDDFTB. The 11-cis conformer is 4.17 kcal/mol less stable than the all-trans conformer in SF-TDDFTB, in good agreement with the 5.42 kcal/mol of MRSF-TDDFT. In the excited state from all-trans to CX, SF-TDDFTB features a barrier of 9.03 kcal/mol, in comparison with the 10.39 kcal/mol in MRSF-TDDFT. For the 11-cis to CX the barrier is slightly larger, with 14.36 kcal/mol in SF-TDDFTB compared to 10.39 kcal/mol in MRSF-TDDFT.