Extended Hückel method calculation of polarization energies: the case of a benzene dimer.

We adapted Hoffmann's extended Hückel method to an interacting molecular system and use this approach to compute the electron affinity and ionization potential of benzene dimers. We restrict the added charge to one of the molecules and argue that the dimer energy computed in this manner is the relevant energy in any meaningful thermally activated hopping rate expression. The dimer electron affinity and ionization potential differs from the isolated molecule corresponding quantity by what is called polarization energy. The polarization energy normally stabilizes the anion and this is particularly relevant for benzene, given that its isolated anion is unstable with respect to charge detachment. We found that the anionic benzene dimer is only stabilized in certain conformations, suggesting that the stabilization of a benzene anion in an amorphous environment is very unlikely. The modest computational cost of the method makes it a viable alternative to compute the energy of charged molecules in amorphous molecular films, a central issue in the problem of charge transport in organic electronics.