At the level of theory for which the XMS-CASPT2 geometries were optimized, the separation between the S 0 and T 0 states (∆E S 0 -T 0 ) is only 2.1 kcal/mol for the unfunctionalized corrole (Table ). This increases to 4.9 kcal/mol in the triphenyl copper corrole (Table ). For both systems, the vibrational contributions to the difference in enthalpies (∆H S 0 -T 0 ) are small. When the free energy difference between the states (∆G S 0 -T 0 ) is computed, a smaller splitting is obtained at 1.3 and 3.1 kcal/mol for the unfunctionalized and triphenyl corroles, respectively. Since the S 1 state was also optimized for the unfunctionalized corrole, the energy differences between the two singlet states can also be reported at the same level of theory. Specifically, ∆E S 0 -S 1 is 7.3 kcal/mol, while ∆H S 0 -S 1 and ∆G S 0 -S 1 are 7.5 and 6.6 kcal/mol, respectively. Single-point calculations with a larger restricted active space were used to compute the electronic energy splitting, but keeping the thermal corrections for those obtained with the optimized level of theory, to obtain best estimates of 2.8 and 6.8 kcal/mol for ∆H S 0 -T 0 for the two corroles, respectively (Table and). Note that with RASPT2, ∆E S 0 -T 0 is 7.3 kcal/mol for the triphenyl copper corrole. Pierloot et al.