Dihydroazulene (DHA)/vinylheptafulvene (VHF) photochromism has been investigated by studying the isomerization of 1,2,3,8a,9-pentahydrocyclopent[a)azulene-9,9-dicarbonitrile through complete active space-self consistent field calculations on the ground (So) and first excited (SI) states of smaller model compounds. In each case, the S, reaction coordinate is characterized by a transition structure for adiabatic ring opening, connecting a DHA-like intermediate to a much more stable VHF-like structure. This VHF-like structure is not a real S, minimum but a crossing (i.e., a conical intersection) between the excited- and ground-state potential energy surfaces. The existence of such a crossing is consistent with the lifetime of similar to600 fs recently measured for the DHA-like intermediate on S, (Em, J.; Petermann, M.; Mrozek, T.; Daub, J.; Kuldova, K.; Kryschi, C. Chem. Phys. 2000, 259, 331-337). The shape of the crossing is also crucial; it not only explains the fact that the quantum yield approaches 1.0 for the forward DHA --> VHF reaction, but also the lack of any fluorescence or photochemical back-reaction from VHF. These findings are supported by ab initio direct dynamics calculations. This work suggests that calculating and understanding the topology of excited-state potential energy surfaces will be useful in designing photochromic molecules.