The phase diagram of water at negative pressures: Virtual ices

摘要

The phase diagram of water at negative pressures as obtained from computer simulations for twomodels of water, TIP4P/2005 and TIP5P is presented. Several solid structures with lower densitiesthan ice Ih, so-called virtual ices, were considered as possible candidates to occupy the negativepressure region of the phase diagram of water. In particular the empty hydrate structures sI, sII, andsH and another, recently proposed, low-density ice structure. The relative stabilities of thesestructures at 0 K was determined using empirical water potentials and density functional theorycalculations. By performing free energy calculations and Gibbs–Duhem integration the phasediagram of TIP4P/2005 was determined at negative pressures. The empty hydrates sII and sH appearto be the stable solid phases of water at negative pressures. The phase boundary between ice Ih andsII clathrate occurs at moderate negative pressures, while at large negative pressures sH becomes themost stable phase. This behavior is in reasonable agreement with what is observed in densityfunctional theory calculations.