  Using the minima hopping global geometry optimization method on the density
functional potential energy surface we study medium size and large boron
clusters. Even though for isolated medium size clusters the ground state is a
cage like structure they are unstable against external perturbations such as
contact with other clusters. The energy landscape of larger boron clusters is
glass like and has a large number of structures which are lower in energy than
the cages. This is in contrast to carbon and boron nitride systems which can be
clearly identified as structure seekers in our minima hopping runs. The
differences in the potential energy landscape explain why carbon and boron
nitride systems are found in nature whereas pure boron fullerenes have not been
found.
