Global nine-dimensional potential energy surface for the H-5 system. I. Abinitio multiple reference single and double excitation configuration interaction computations

The ground-state adiabatic potential energy surface was computed for 2836 c onformations of H-5, using a multiple reference single and double excitatio n configuration interaction program and an extended basis set. The transiti on state obtained is that of a pentagon with D-5h symmetry. We find the ene rgy barrier to lie at 81 kcal/mol within +/-1 kcal/mol if a multireference Davidson's correction is employed, while the D-e (H-2) at this level was 10 9 kcal/mol. The calculations presented here clearly show the energy of H-5 in D-5h symmetry to be 28 kcal/mol below the dissociation energy of H-2, an d therefore a bimolecular mechanism, in the presence of hydrogen atoms, cou ld indeed be considered for the exchange process: H-2+D-2+H-->2HD+H. (C) 19 99 American Institute of Physics. [S0021-9606(99)30716-9].