Equation of state of the energy landscape of SPC/E water

The contributions to the thermodynamics of liquid SPC/E water from its inherent structures have been determined over a broad range of temperature and density (220-500 K, 0.8-1.25 g/cm³). Molecular dynamics simulations of shifted-force SPC/E water give a retracing locus of density maxima in the equilibrium liquid with a retracing point at ca. 275 K and -200 bar, showing that over a large range of temperature and pressure the inherent structures correspond to a liquid with negative thermal expansion. Both the pressure and potential energy in the inherent structures show a strong dependence on the temperature of the liquid from which the mechanically stable packings are generated. The potential energy of the inherent structures also shows a striking behavior at low temperatures, attaining lower energies than the ground states of pure crystalline forms. This contrasts with simple molecular liquids having no orientation-dependent interactions and suggests novel microscopic interpretations for a number of water's anomalous properties, such as the polyamorphic transition between its glassy phases, and a fragile-to-strong transition as the liquid is supercooled to its vitreous form.