Conical intersections in molecular photochemistry - the role of phase change

Conical intersections are now known to be involved in a large variety of ph otochemical transformations. In this paper, we present a simple method for determining the existence of a conical intersection for a given chemical re action. The method, based on the phase change theorem, is easy to apply, do es not require quantum chemical calculations, and uses ground state surface properties only. The method can be used to analyze all kinds of photochemi cal reactions, including isomerizations, rearrangements and dissociations. It is shown that if the system passes through a conical intersection on its return to the ground state surface, at least two different products are ne cessarily formed (in addition, the system may return to the reactant by int ernal conversion). (C) 2000 Elsevier Science B.V. All rights reserved.