Fe(II)-porphyrin. The next system is a Fe(II)-porphyrin model complex whose properties were intensely discussed in our previous publications. We use the converged stochastic CASSCF natural orbitals from our previous work 9 with a (32,34) active space that consists of nine doubly-occupied π-orbitals, seven empty π * -orbitals, five metal centered 3d-orbitals, four doubly-occupied σ N -orbitals, four empty orbitals of the (4s4p) shell, and five empty d -orbitals (double-shell orbitals). The natural MOs for this system are delocalized, but not over the whole molecule. They are still confined to certain regions and can be identified as metal-centered, aromatic, or σ-donating nitrogen orbitals. So there is spatial separation of MOs, but not as strongly as for the stack of five or ten benzene molecules.