The nature of the S₁/S₀ conical intersection of fulvene

An S₁/S₀ conical intersection of fulvene is located with the help of the Longuet-Higgins sign change theorem. Its nature is related to the fact that this conical intersection is surrounded by two minima and two transition states between them. There are two distinct isomerization reactions around the exocyclic double bond, each characterized by a different transition state. The coordinates leading to the conical intersection are constructed by combinations of these two reaction coordinates. The possible existence of two separate conical intersections, the second occurring upon scission of the exocyclic CC double bond, is discussed.