Conical Intersections:  The New Conventional Wisdom

During the past decade the perception of conical intersections has changed. It is now appreciated that what was once viewed largely as a theoretical curiosity is an essential aspect of electronically nonadiabatic processes. Concomitantly, our understanding of this singular consequence of the Born−Oppenheimer separation of nuclear and electronic motion has grown enormously. In this work the theory of conical intersections is reviewed.