  We investigate the jump motion among potential energy minima of a
Lennard-Jones model glass former by extensive computer simulation. From the
time series of minima energies, it becomes clear that the energy landscape is
organized in superstructures, called metabasins. We show that diffusion can be
pictured as a random walk among metabasins, and that the whole temperature
dependence resides in the distribution of waiting times. The waiting time
distribution exhibits algebraic decays: $\tau^{-1/2}$ for very short times and
$\tau^{-\alpha}$ for longer times, where $\alpha\approx2$ near $T_c$. We
demonstrate that solely the waiting times in the very stable basins account for
the temperature dependence of the diffusion constant.
