Disorder in ice polymorphs:: A Monte Carlo simulation study

Two water molecules connected by hydrogen bond in hexagonal ice can have four possible configurations. These configurations are distinguished by the relative orientation of the two molecules and termed for obvious reasons as c-cis, h-cis, c-trans, and h-trans. The occurrence of symmetry permitted dimer orientations is a characteristic feature of each ice polymorph. In the proton-ordered structures the occurrence of orientations is strictly determined, while in the proton-disordered structures it can vary within certain limits. We performed Monte Carlo simulations using the so-called TIP5P-EW, TIP4P-EW and TIP4P-2005 interaction models to study this isomerism for the polymorphs of ice. We found that the variation of energy with the frequency of different dimer orientations in the proton-disordered phases is large enough to influence the results of phase stability studies. Knowing the distributions of dimer orientations of the ice IX-ice III ordered-disordered polymorph pairs, we could estimate the internal energy of ice IX using dimer energies assigned to certain orientations in the disordered phase of ice III. In agreement with experimental evidences at low temperatures the TIP4P-EW and TIP4P-2005 potentials predicted lower energy for ice VIII than for ice VII. (c) 2007 Elsevier B.V. All rights reserved.