As expected, the differences for the T 0 state are smaller than for the S 0 state where larger deviations are observed for the intermediate points. These results support the use of the traditional approach to compute vertical excitations at minimia or adiabatic energies between optimized structures of different spin states. In this case, the XMS-CASPT2 adiabatic energy difference, ∆E S 0 -T 0 , on the TPSSh geometries is only +0.1 kcal/mol from the value on the XMS-CASPT2 structure (Tables and). On the other hand, if one wishes to analyze energy differences on the order of 2 to 3 kcal/mol at intermediate geometries, structure choice would likely impact the energy splittings reported. These differences can be important if one wishes to understand reactivity based on transition state structures or photophysical processes resulting from intersystem crossing.