showing an equivalent decay obtained with the two electronic structure software. When propagating on the support of energies, gradients, and couplings calculated on-the-fly with any flavor of CASPT2, the ultrafast decays within the first 40fs, using the same ICs, are clearly distinct with respect to the CASSCF one. Three main differences can be noticed: i) the initial decay occurs slightly later in time, after 10 fs with respect to the 8 of CASSCF, ii) the only RMS-CASPT2 shows some degree of reflection of population, while the other dynamics show a constant population of S 1 after the initial decay. For completeness, the population profile along the first 100fs at the CASPT2 levels is shown in the Supporting Information (Figure ). We limited to reporting XMS-CASPT2 and RMS-CASPT2 profiles, as they are the only two dynamics that ensure excellent energy conservation for all the TBFs along the 100fs, while, in some of the TBFs propagated along MS-CASPT2 and XDW-CASPT2 surfaces, jumps in total energy can be observed after passing the CoIns. It can be observed that the main deactivation of the population with all CASPT2 flavors occurs step-wise over longer times. The stronger population transfer to the ground state happens after around 75 fs. At this point, both XMS-CASPT2 and RMS-CASPT2 show a reflection of around 10% of the population comparable to the population trace of CASSCF. After 100 fs, both CASPT2 flavors predict more than 70% of S 1 population having decayed to the ground state.