The active spaces of the complexes are summarized in Table , similar to our previous work on nitrosyl complexes. The active spaces consist of all five Fe(3d) orbitals, all (possible) five Fe(4d) orbitals to account for the double-shell effect, 26 all (possible) Fe-ligand σ orbitals, and a set of ten NO-based orbitals. The latter set includes two NO(π) and the correlating two NO(π*) orbitals; the NO(σ) orbital and the correlating NO(σ*) orbital, two NO(π′) orbitals to account for the radial correlation of the NO(π*) orbitals, one nitrogen 2s orbital and the correlating orbital. Four Gouterman's π orbitals were also included, as porphyrin could be noninnocent in some states (See Table ). The natural active orbitals are shown in Figures .