  We report results of molecular dynamics simulations of amorphous ice for
pressures up to 22.5 kbar. The high-density amorphous ice (HDA) as prepared by
pressure-induced amorphization of Ih ice at T=80 K is annealed to T=170 K at
various pressures to allow for relaxation. Upon increase of pressure, relaxed
amorphous ice undergoes a pronounced change of structure, ranging from the
low-density amorphous ice (LDA) at p=0, through a continuum of HDA states to
the limiting very high-density amorphous ice (VHDA) regime above 10 kbar. The
main part of the overall structural change takes place within the HDA
megabasin, which includes a variety of structures with quite different local
and medium-range order as well as network topology and spans a broad range of
densities. The VHDA represents the limit to densification by adapting the
hydrogen-bonded network topology, without creating interpenetrating networks.
The connection between structure and metastability of various forms upon
decompression and heating is studied and discussed. We also discuss the analogy
with amorphous and crystalline silica. Finally, some conclusions concerning the
relation between amorphous ice and supercooled water are drawn.
