  The oxygen and silicon dynamics in silica is compared via computer
simulations. In agreement with experimental data and previous simulations a
decoupling of oxygen and silicon dynamics is observed upon cooling. The origin
of this decoupling is studied in the framework of the potential energy
landscape. From analysis of the transition features between neighboring
superstructures of minima, denoted metabasins, the differences between the
oxygen and the silicon dynamics can be quantified. The decoupling can be
explicitly related to the presence of generalized rotational processes, giving
rise to oxygen but not to silicon displacement. Closer analysis of these
processes yields important insight into the nature of the potential energy
landscape of silica. The physical picture of relaxation processes in silica,
obtained in previous work for the oxygen dynamics, is consistent with the
decoupling effects, elucidated here.
