Third Order Hyperpolarizability of Polythiophene and Thiophene Oligomers

We have performed a CNDO calculation on polythiophene. A new representation of the virtual excited states in terms of electron-hole distance is proposed. A careful size consistent calculation of γ is presented and leads to a finite value per unit volume for an infinite chain (χ3 = 1.0 10-11 esu at ω = 0). The same procedure is used to check the influence of the polymer configuration.