A first approach to study the electronic structure of these macrocycles must include these four π orbitals. In addition to the orbitals from the corrole itself, relevant metal orbitals should be included in the active space. For Cu, the d x 2 -y 2 orbital is the highest in energy in a square-planar geometry and has the correct symmetry to form bonding and anti-bonding σ-type interactions with the corrole. The remaining d-orbitals are doubly occupied in all of the copper oxidation states of interest, making the occupation of the d x 2 -y 2 orbital central to oxidation state assignment.