THE DOMINANT ONE-PHOTON-EXCITED AND 2-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 descri ption of the nonlinear optical response of the octatetraene molecule, have been characterized by the ab initio complete active space self-co nsistent-field/multiconfiguration second-order perturbation theory (CA SSCF/CASPT2) technique. We compare the nature of the main electronic c ouplings among excited states, as obtained at the ab initio level, to that provided by the semiempirical Pariser-Parr-Pople (PPP) method. Bo th approaches indicate the presence of a dominant A(g) excited state a t about 1.6 times the optical gap. However, in contrast to the PPP res ults, other two-photon states are found in the ab initio calculations to contribute significantly to the cubic polarizability. (C) 1997 Else vier Science B.V.