Nonadiabatic Quantum Chemistry-Past, Present, and Future

The current state of the art of theoretical descriptions, and computational treatments, of nonadiabatic processes, is discussed. Mead and Truhlar studied the failure of the curl condition to derivative couplings outside the space and concluded that a rigorous diabatic basis only exists in the trivial case. Robb and co-workers characterized the topography of a point of conical intersection within the seam or intersection space using a Hessian matrix projected into the intersection space. Tully and co-workers observed consequence of the large number of closely spaced states in the vicinity of the Fermi level, provided by the metal. Robb and co-workers emphasized on the the relevant points in the intersection space are those where the reaction path meets the seam, and this need not be at the minimum of the seam and the radiation-less decay takes place in the branching plane as one passes through the conical intersection.