CH5+: The story goes on. An explicitly correlated coupled-cluster study

CCSD(T)-R12 calculations (as well as R12 calculations at lower CC and MPn level) with large basis sets have been performed for the three lowest stationary structures of CH5+, for CH4, and for CH3+ + H2. The basis set limit of the total energy at all levels of the treatment of correlation is reached within ∼1 kcal/mol. At the CCSD(T)-R12 level the same accuracy is achieved for CH4 with respect to its "experimental'' non-relativistic energy. Accordingly, energy differences should be correct to within a small fraction of a kcal/mol. In agreement with the most accurate previous calculations, the C_s(1) structure of CH5+ is lowest, followed by C_s(2) and C_2v. For the relative energies of the two latter structures we predict energy differences of 0.1 and 0.8 kcal/mol respectively. The possibility of a vibrational stabilization of the C_2v -structure is discussed. Accurate predictions of the proton affinity of CH4 and the binding energy between CH3+ and H2 are also given.