State Variables for the Glassy State: Numerical Studies of Amorphous Ice

The potential energy landscape (PEL) formalism is a valuable approach within statistical mechanics for describing supercooled liquids and glasses. We use the PEL formalism and computer simulations to study the transformation between low-density (LDA) and high-density amorphous ice (HDA). We employ the ST2 water model that exhibits a LDA-HDA first-order phase transition, as observed in experiments. We prepare samples of LDA and HDA following completely uncorrelated thermodynamic paths and study the evolution of these LDA and HDA samples during compression and decompression at temperatures sufficiently low that annealing is absent. It is found that the evolution of these LDA/HDA glasses, during the compression and decompression processes, is uniquely determined by a few quantities associated of the starting glass: (i) the thermodynamic variables (N,V,T) and (b) a few properties of the PEL. In other words, for isothermal compression/decompression processes, these few quantities define the ‘glass state’ of amorphous ice. We interpret our results in terms of the PEL formalism and suggests that the identified “state variables” may be applicable to amorphous solids in general.