One of the first wavepacket dynamics studies focusing on a photochemically allowed electrocyclic reaction was a study of cyclobutene . It confirmed the Woodward-Hoffman picture of a strictly disrotatory pathway. The response of the excited state wavepacket in the Franck-Condon region of the ππ * excited state is a simultaneous bond lengthening in combination with a disrotatory rehybridization of the two methylene groups of cyclobutene. The disrotatory motion leads to the dissociation of the bond between the methylene units. The ππ * state potential energy surface is repulsive with respect to conrotatory motion in the Franck-Condon region and, thus, sets the course of the reaction already at photoexcitation. Wavepacket dynamics studies of the CHD system were pioneered by the de Vivie-Riedle group using reduced dimensionality quantum dynamics along the reactive coordinates on the excited S1 state . Later investigations employed nonadiabatic dynamics simulations in the full dimensionality of the potential energy surfaces . Our own simulations of the nonadiabatic dynamics of CHD , αTp , and αPh (78) using the ab initio multiple spawning (AIMS) method ( ) demonstrate that the reaction can be entirely described within the framework of the electronic ground state (S0) and the first excited state (S1). The S1 state has ππ * diabatic character in the Franck-Condon region. The CI between ππ * state and π * π * state is not directly involved in the pathway of the excited state wavepacket. The wavepacket evolves instead around the lower cone of the CI, leading to a gradual change of the diabatic character of S1. Thus, despite the indirect interaction, the CI plays a crucial role in shaping the profile of the excited state potential energy surface and in providing access to the second CI with the ground state, which in turn provides access to the minima of the open chain photoproduct.