$CH5+:$ 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 $Cs(1)$ structure of $CH5+$ is lowest, followed by $Cs(2)$ and $C2v.$ 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 $C2v$ -structure is discussed. Accurate predictions of the proton affinity of CH₄ and the binding energy between $CH3+$ and H₂ are also given.