where âµ,σ and â † µ,σ annihilates and creates, respectively, an electron with spin σ on atom µ, and nµ = σ â † µσ âµσ counts the total number of electrons on site µ. The parameters in the first line of the Hamiltonian above are defined as follows: ε µ is the on-site orbital energy, while t µν the hopping amplitude between atomic sites µ and ν. Specifically, in the PPP model, hopping is restricted to atoms connected by a σ-bond. The second line of the Hamiltonian accounts for electron-electron interactions: U µ represents the repulsion between two electrons on the same atom, V µν denotes the repulsion between electrons on different atoms, while Z µ is the charge on site µ when π-electrons are removed (Z µ = 1 for both carbon and aza nitrogen atoms). We follow the Ohno expression for V µν : 68,71,101