Using the computer to understand the chemistry of conical intersections.

The application of computational chemistry in resolving photochemical and photobiological problems depends on theoretical models and calculation strategies that allow an understanding of how and when the energy contained in a photon can be used or dissipated by a molecule. Progress in this arena has been the result of a scientific journey that can be traced from the 1960s and 1970s with the development of the concept of the "photochemical funnel", and again from the 1980s and 1990s with the development and application of ab initio multiconfigurational quantum chemistry. Mainly following the viewpoint and contributions of the corresponding author (together with others), here we show that both the early and the ongoing research points to a central role of conical intersections in the molecular-level control of the selectivity and efficiency of photochemical reactions and internal conversion processes.