Multi-state (MS)-CASPT2 computations were also performed showing significant coupling between the ground and first excited singlet states (S 0 and S 1 ) at intermediate geometries along the saddling angle. These geometries were computed via constrained optimizations at the DFT level and the large coupling was reported to arise from known failures in the MS-CASPT2 approach. In 2016, Vlaisavljevich and Shiozaki performed geometry optimizations of the bare copper corrole using the implementation of fully internally contracted (FIC) analytical gradients with the extended multistate (XMS)-CASPT2 method. They employed a small active space including four electrons in five orbitals (4e,5o) (Figure ) and used a double-ζ quality basis set (cc-pVDZ) on all atoms. Although the structural features from this optimization are in reasonable agreement with experiment, differences in the electronic Additionally, weak coupling is reported between the S 0 and S 1 . Nevertheless, the goal of the study was to demonstrate the FIC-CASPT2 gradient implementation and further discussion of the copper corrole electronic structure was not included.