Migration of conical intersections induced by the renner effect in bent C2H2+ cation

► Conical intersections (CIs) have been shown to play an important role in the nonadiabatic molecular processes involved in photochemistry and photophysics. In an earlier work [see, e.g., Halász et al., J. Chem. Phys. 126 (2007) 154309], applying topological line integral technique, we pointed out, 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 T. Vértesi, R. Englman [J. Phys. B: Atom. Mol. Opt. Phys. 41 (2008) 025102] showed that not only one as it was revealed by us, but two aligned conical intersections will be created in the slightly bent Renner-Teller tetra-atomic radical cation C2H2+. This finding was supported by us both by symmetry arguments and by ab initio calculations [see, Halász et al., Int. J. Quant. Chem. (2010), doi:10.1002/qua.22598.]. ► Continuing the line of this latter work additional ab initio calculations have been performed to map the seams of the conical intersections in a significant region of the configuration space (CS). Among others we could show that slightly increasing the displacement value of the H1 atom from the CC axis the seam of the second CI begins to migrate and finally disappears while the first CI moves only a little bit away.