The Dynamics of Non-Crystalline Silica: Insight from Molecular Dynamics   Computer Simulations
  Using a large scale molecular dynamics computer simulation we investigate the dynamics of a supercooled melt of SiO_2. We find that with increasing temperature the temperature dependence of the diffusion constants crosses over from an Arrhenius-law, with activation energies close to the experimental values, to a power-law dependence. We show that this crossover is related to the fact that at low temperatures the dynamics of the ions is dominated by hopping processes, whereas at high temperatures it shows the continuous flow-like motion proposed by the ideal version of mode-coupling theory (MCT). Finally we show that at low temperatures the dynamics of the system in the beta-relaxation regime obeys the factorization property, in agreement with MCT. 
Walter Kob, Jurgen Horbach, and Kurt Binder (Institute of Physics,   Mainz, Germany)