CH+5: The never-ending story or the final word?

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Abstract
The closely related Cs(1), Cs(2), and C2v(3) structures of CH5+ have been reinvestigated with high level ab initio theory through the coupled cluster with single and double substitutions (CCSD), and CCSD with perturbatively included connected triple excitations [CCSD(T)] levels, employing a triple‐ζ plus double polarization functions basis set, with f‐functions on carbon as well as d‐functions on the hydrogens [TZ2P(f,d)]. Vibrational frequencies have been computed up to TZ2P+f CCSD; the inclusion of f‐functions on carbon is critical for the configuration interaction with single and double excitations (CISD) and coupled cluster methods using the triple‐ζ basis sets. The changes in geometries between the CISD and CCSD levels are very small, e.g., the C–H bond lengths vary by at most 0.005 Å. Thus, the optimizations are essentially converged within theoretical limits. The differences in energies of 1, 2, and 3 decrease and essentially vanish at the most sophisticated levels when the zero point vibrational energy corrections are applied. Hence, there is essentially no barrier to complete hydrogen scrambling.