The dominant one- and two-photon excited states in the nonlinear optical response of octatetraene: Ab initio versus semiempirical theoretical descriptions

The one- and two-photon singlet excited states, relevant for the description of the nonlinear optical response of the octatetraene molecule, have been characterized by the ab initio complete active space self-consistent-field/multiconfiguration second-order perturbation theory (CASSCF/CASPT2) technique. We compare the nature of the main electronic couplings among excited states, as obtained at the ab initio level, to that provided by the semiempirical Pariser-Parr-Pople (PPP) method. Both approaches indicate the presence of a dominant A_g excited state at about 1.6 times the optical gap. However, in contrast to the PPP results, other two-photon states are found in the ab initio calculations to contribute significantly to the cubic polarizability.