  The origin of the fragile-to-strong crossover in liquid silica is
characterized in terms of properties of the potential energy landscape (PEL).
Using the standard BKS model of silica we observe a low-energy cutoff of the
PEL. It is shown that this feature of the PEL is responsible for the occurrence
of the fragile-to-strong crossover and may also explain the avoidance of the
Kauzmann paradox. The number of defects, i.e. deviations from the ideal
tetrahedral structure, vanishes for configurations with energies close to this
cutoff. This suggests a structural reason for this cutoff.
