If accessible from the Franck-Condon region, they can provide pathways for nonadiabatic (i.e., "curve-crossing") transitions to other electronic states including the ground state with up to 100 % quantum efficiency. Thus, internal conversion through CIs is responsible for the high efficiency and selectivity of many photochemical reactions. In the case of photochemical electrocyclic ringopening reactions, the ground state minima of the photoproduct are connected with the Franck-Condon region of the ππ * state by a CI in the vicinity of the pericyclic minimum of the π * π * excited state . Indeed, in some cases, the "pericyclic minimum" may not be a proper minimum at all, but rather a conical intersection. are shown as a blue line for the disrotatory path, and a orange line for the conrotatory path. The optimized geometries for the reactant CHD, the product (cZc-HT), the minimum energy conical intersection on the conrotatory pathway (MECIcon), along with the transition state (TS) on both the conrotatory (TScon) and disrotatory (TSdis) path, are shown on the left of the PES. (b) Solid lines: The ground-state (S0) MEP of the CHD electrocyclic ring-opening through disrotatory (left) and conrotatory (right) reactions. Dashed lines: The corresponding first excited state (S1) energies of both MEPs. Values in parentheses represent the root mean square deviation (RMSD) between a given critical geometry and the reactant CHD structure.