The BH5 molecule contains a weak two-electron-three-center bond and it requires extremely high level of theories to calculate the energy and structure correctly. The potential energy of the hydrogen scrambling in BH5 has been generated by the multiconfiguration molecular mechanics algorithm with 15 high-level Shepard interpolation points, which would be practically impossible to obtain otherwise. The high-level interpolation points were obtained from the multicoefficient correlated quantum mechanical methods. The more high-level points are used, the better the shape of the potential energy surface. The rate constants are calculated using the variational transition state theory including multidimensional tunneling approximation. The potential energy curve for the BH5 dissociation has also been calculated, and the variational transition state was located to obtain the dissociation rate constants. Tunneling is very important in the scrambling, and there is large variational effect on the dissociation. The rate constants for the scrambling and the dissociation are 2.1x10(9) and 2.3x10(12) s(-1) at 300 K, respectively, which suggests that the dissociation is three orders of magnitude faster than the scrambling. (C) 2004 American Institute of Physics.