Comment on "Optical conversion of conical intersection to avoided crossing" by Y. Arasaki and K. Takatsuka, Phys. Chem. Chem. Phys., 2010, 12, 1239.

A recent paper in this journal proposed the conversion of conical intersections to avoided crossings by lowering the symmetry with an optical field. The article also claimed that the characters of nonadiabatic transitions caused by avoided crossings and conical intersections are qualitatively different. The present comment shows that this proposal and this claim result from an incorrect appreciation of the nature of conical intersections and avoided crossings. Conical intersections are moved, not removed, by almost all perturbations. Furthermore, there is no dichotomy between avoided crossing mechanisms and conical intersection mechanisms; as the parameters of the problem change and the typical locally avoided crossing involved in nonadiabatic dynamics becomes farther from the conical intersection, there is a gradual shift in the nature of the nonadiabatic transitions, with a continuum of possible behaviors, not just two.