The two lowest excited singlet states of all-trans-1,3,5,7-octatetraene, 2 1A−g and 1 1B+u, are studied by means of high level ab initio methods computing the vertical and adiabatic excitation energies for both states and the vertical emission energy for the 11Ag−←2 1A−g transition. The results confirm the known assignment of two energies, the 2 1A−g adiabatic excitation energy and the 2 1A−g vertical emission energy, for which well defined experimental values are available, with an excellent agreement between theory and experiment. In the experimental absorption spectrum, the maximum of the band describing the 1 1B+u←11Ag− excitation is the first peak and it has been assigned to the (0-0) vibrational transition, but in literature it is normally compared with the theoretical vertical excitation energy. This comparison has been questioned in the past, but a conclusive demonstration of its lack of foundation has not been given. The analysis reported here, while confirming the assignment of the highest peak in the experimental spectrum to the (0-0) adiabatic transition, indicates that it cannot be used as a reference for the vertical excitation energy. The theoretical vertical excitation energies for the 2 1A−g and 1 1B+u states are found to be almost degenerate, with a value, ≃ 4.8 eV, higher than that normally accepted in the literature, 4.4 eV. The motivations which have induced in the past other authors to consider this a correct value are discussed and the origin of their feebleness are analyzed
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