eV. As for nitrogen atoms, a unique set of PPP parameters is not available. We set the hopping integral for C-N bonds to the same value as for C-C bonds. Typically, the on-site e-e repulsion is slightly larger for N atoms than that for C atoms. Best agreement with experimental data is obtained here setting U N ∼ 15 eV and 15.5 eV for pyrrole and aza nitrogens, respectively. These choices marginally affect the results (see ESI, Section S2). As for the molecular geometry, for the sake of simplicity, we set all angles to 120°and all bond lengths to 1.4 Å (results for different geometries are marginally different, see ESI Fig. ).