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 CH

_{5}^{+},

for CH

_{4}

, and for CH

_{3}^{+}

+ H

_{2}

. 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 CH

_{4}

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 CH

_{5}^{+}

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 CH

_{4}

and the binding energy between CH

_{3}^{+}

and H

_{2}

are also given.

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