where µ and ν run on atomic sites, â( †) µ,σ is the fermionic annihilation (creation) operator for an electron with spin σ in the atomic orbital (AO) on site µ, and nµ = Hamiltonian describe e-e interaction: U µ measures the repulsion between two electrons on the same site, V µν is the repulsion between electrons on different sites and Z µ measures the charge on atom µ upon removal of the π-electrons (Z µ = 1 for carbon and aza nitrogen atoms, Z µ = 2 for pyrrole nitrogens). We adopt the Ohno expression for V µν : 42,45,52