Molecular Dynamics Simulations on the Glass-to-liquid Transition in High Density Amorphous Ice

It is an open question whether high density amorphous (HDA) ice is a glassy material structurally related to an ultraviscous high density liquid (HDL) or a nanocrystalline material unrelated to a liquid. In order to shed light on this question we have performed molecular dynamics simulations on a HDA model system at a pressure of p approximate to 0.3 GPa using the COMPASS force field. After removing the irreversible structural relaxation effect by initial isobaric heating cooling cycles, we observe a deviation from linearity in the density vs. temperature plot in the range 170 +/- 15 K in subsequent cycles, which we attribute to the glass-transition temperature T-g. This assignment of T-g is corroborated by two independent methods, namely from a rapid increase in the diffusion coefficient at approximate to 169 K and a deviation from linearity at approximate to 174 K in an enthalpy versus temperature plot. The structure of the model system is in good agreement with the experimentally determined structure of HDA. We, thus, suggest that HDA may indeed be a low temperature structural proxy of an ultraviscous liquid HDL.