Abstract
Multireference Møller–Plesset perturbation theory with complete active space configuration interaction (CASCI-MRMP) is applied to the study of the valence π→π* excited states of all-trans linear polyenes C_2nH_2n+2 (n = 3–14). Our focus was to determine the nature of the four lowest-lying singlet excited states, 2¹A_g⁻, 1¹B_u⁺, 1¹B_u⁻ and 3¹A_g⁻ and their ordering. The ionic 1¹B_u⁺ state is the lowest optically allowed excited state, while the covalent 2¹A_g⁻, 1¹B_u⁻ and 3¹A_g⁻ states are the optically forbidden states. Theory predicts that the 1¹B_u⁻ state becomes lower than the 1¹B_u⁺ state at n ≥ 7 and that the 3¹A_g⁻ state also becomes lower than the 1¹B_u⁺ state at n ≥ 11.