where âµ,σ (â † µ,σ ) annihilates (creates) an electron with spin σ on site µ, and nµ = σ â † µσ âµσ counts the total number of electrons on the µ atom. The parameters entering the first line of the above Hamiltonian are defined as follows: ε µ is the on-site orbital energy, while t µν the hopping amplitude between atomic sites µ and ν. In particular, in the PPP model, hopping occurs only between 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 µ 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 = 0 for boron atoms, Z µ = 1 for carbon and aza nitrogen atoms, Z µ = 2 for pyrrole nitrogens). We follow the V µν expression given by Ohno: 58,61,66