conical intersection
Point of crossing between two electronic states of the same spin @[email protected] (most commonly @[email protected] or @[email protected]).
  1. In a polyatomic molecule two @[email protected] are allowed to cross along a (3N − 8)-dimensional subspace of the (3N − 6)-dimensional nuclear coordinate space (the @[email protected]) even if they have the same spatial/spin symmetry (N is the number of nuclei). Each point of the @[email protected] corresponds to a conical intersection. If the energy is plotted against two special internal geometrical coordinates, x1 and x2, which define the so-called @[email protected], the potential energy surface would have the form of a double cone in the region surrounding the @[email protected] In the remaining (3N − 8) directions, the energies of the @[email protected] and @[email protected] remain degenerate; movement in the @[email protected] lifts the @[email protected]
  2. From a mechanistic point of view, conical intersections often provide the @[email protected] mediating @[email protected] and @[email protected]
PAC, 2007, 79, 293. (Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006)) on page 317 [Terms] [Paper]