Search for conical intersection points (CI) by Newton trajectories

Recently, valley–ridge inflection points on the potential energy surface for the ring opening of the cyclopropyl radical have been determined using Newton trajectories (NTs) [W. Quapp, J.M. Bofill, A. Aguilar-Mogas, Theor. Chem. Acc. 129 (2011) 803]. This Letter is the report about the utilization of NTs for the search for conical intersection (CI) points. These points play a main role in the understanding of intersections of different electronic surfaces which open the door for photochemical reactions. We explain the reason why Newton trajectories can find CI points, and report a CI seam on the CASSCF (3,3) surface of the allyl radical ring closure.