Iteration The ground and first excited state energies almost converge by the second iteration. These energies are obtained by performing a CASPT2 calculation with the ASC determined at the SCF level, updating the ASC using the relaxed density, and performing another CASPT2 calculation with the updated ASC. Unlike single-reference methods, a significant part of the density is determined by the zeroth-order wavefunction for both the initial and final states, while the first-order correction from the PT2 calculation is perturbative. Consequently, relaxation has a minimal impact. The first cycle yields an energy decrease of 0.02 eV and 0.03 eV for the S 0 and S 1 states, respectively, indicating that full relaxation is not critical. Furthermore, the difference in equilibrium energies between the PTE (3.034 eV) and PTED (3.036 eV) approaches is only 0.002 eV (≈ 0.5 kcal mol -1 ), suggesting that, in practice, the relaxation correction for transition energies is below the expected accuracy of the MRPT method. For this specific case, the PTE approach accurately reproduces the PTED energy difference, and thus, PTE energies are used hereafter. It is important to note that these energies represent equilibrium energies and are not directly applicable to absorption energy calculations, which require a non-equilibrium solvation model.