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 low est stationary structures of CH5+ for CH4, and for CH3+ + H-2. The bas is set limit of the total energy at all levels of the treatment of cor relation is reached within similar to 1 kcal/mol. At the CCSD(T)-R12 l evel the same accuracy is achieved for CH4 with respect to its ''exper imental'' non-relativistic energy. Accordingly, energy differences sho uld 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 stabilizat ion of the C-2v -structure is discussed. Accurate predictions of the p roton affinity of CH4 and the binding energy between CH3+ and H-2 are also given. (C) 1997 American institute of Physics.