An analytical second-order description of the S0/S1 intersection seam:: fulvene revisited

A comprehensive picture of the extended S (0) /S (1) intersection seam of fulvene is presented, using a complete second-order description. We are now able to discuss the connectivity of all of the seam critical points using the eigenvectors of the intersection-space Hessian. The conjecture (Deeb et al. in Chem Phys 325:251, 2006) that there are two disjoint S (0) /S (1) conical intersections seams in fulvene is found to be incorrect. Previously reported high symmetry planar and perpendicular structures are shown to be second-order saddle points on the intersection seam. In addition, the pyramidalized planar structure is shown to be a first-order saddle point, a previously located low symmetry structure is also shown to be a saddle point, and a new global intersection minimum has been found. The local topology (sloped vs. peaked) of these five seam critical points is analysed and interpreted using the second-order analysis.