CH(5)(+) - THE NEVER-ENDING STORY OR THE FINAL WORD

The closely related C(s)(1), C(s)(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-zeta 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-zeta 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 angstrom. 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.