The ionic 1 1 B + u state comprises a one-electron HOMO → LUMO transition. On the other hand, the multi-configurational 2 1 A - g state 87-89,95 is dominated by the HOMO-1 → LUMO and HOMO → LUMO+1 one-electron and HOMO 2 → LUMO 2 two-electron transitions. The radically different nature of the two states makes its correct description difficult. The corresponding internal conversion (IC) between the 1 1 B + u and the 2 1 A - g states 87-89 has long been argued to occur through a true crossing between their potential energy surfaces (PESs). After the π → π * transition to 1 1 B + u , the crossing is reached along the bond length alternation (BLA) displacement mode. In a recent study on s-trans-butadiene and s-trans-hexatriene with various quantum chemical theories, 12 the 1 1 B + u < 2 1 A - g state ordering at the FC geometry and the existence of the planar