Taking into account the full chemical and molecular structure, we calculate the two-photon absorption spectrum (TPA) and the frequency-dependent third-harmonic generation (THG) of a long thiophene oligomer (T7, the alpha-heptathiophene) by means of a multireference-determinant-configuration interaction method based on the intermediate-neglect-of-differential-overlap Hamiltonian. The TPA spectrum is dominated by a very intense peak associated with an mA excited state (m = 6). Three excited states (1B, mA, and m'A) contribute primarily to the static third-order nonlinear optical response; however, a more complex model including both the 2A state and higher-lying B states is needed for description of all the features observed m the frequency-dependent THG spectrum. Extrapolation of the results from the oligomer to the polymer is a nontrivial task because of the presence of double resonances that are characteristic of finite chains.
