Benchmark ab initio potential curves for the light diatomic hydrides. Unusually large nonadiabatic effects in BeH and BH

In order to assess the relative importance of the intrinsic error in high-level conventional ab initio methods and errors inherent in the Born-Oppenheimer approximation, benchmark quality potential curves and spectroscopic constants have been computed for the light diatomic hydrides H-2, LiH, BeH, and BH. The intrinsic error is found to be comparable to or smaller than the nonadiabatic effects. Revised spectroscopic constants are proposed for BeH. BH is predicted to have an unusually large nonadiabatic contribution of about 0.0025 Angstrom to its bond length. Previous arguments favoring the use of atomic rather than nuclear masses are further corroborated. (C) 1998 Elsevier Science B.V.