Numerical evidence of two aligned conical intersections induced by the Renner effect in bent C 2 H cation

Characterizing and localizing conical intersections 1 are important for the control of molecular processes. In an earlier work (see, e.g., Halász et al., J Chem Phys 2007 126, 154309), applying topological phase consideration, we showed, that the Renner effect in polyatomic molecules with more than three nuclei is necessarily accompanied by a so called symmetry‐allowed conical intersection. By using a perturbational approach Vértesi and Englman (J Phys B: At Mol Opt Phys 2008 41, 025102) pointed out that not only one as it was revealed by us, but also two aligned conical intersections will be created in the slightly bent Renner–Teller tetra‐atomic radical cation C 2 H . The aim of this present work is to support this finding both by symmetry arguments and by ab initio calculations.