Polyamorphism of ice at low temperatures from constant-pressure   simulations
  We report results of MD simulations of amorphous ice in the pressure range 0 - 22.5 kbar. The high-density amorphous ice (HDA) prepared by compression of Ih ice at T = 80 K is annealed to T = 170 K at intermediate pressures in order to generate relaxed states. We confirm the existence of recently observed phenomena, the very high-density amorphous ice and a continuum of HDA forms. We suggest that both phenomena have their origin in the evolution of the network topology of the annealed HDA phase with decreasing volume, resulting at low temperatures in the metastability of a range of densities. 
R. Martonak, D. Donadio and M. Parrinello (Computational Science,   Department of Chemistry and Applied Biosciences, ETH Zur...