Theoretical Studies of the Structure of a Model Bulk Ice Iₕ near 300 K Using a Central Force Potential Model

We report preliminary results of a Metropolis Monte Carlo study of the stability of a model periodic ice I _h structure near 300 K which suggests that the model bulk ice sample melts between 200 and 300 K. The 192 water molecules in the periodic unit cell interact via the revised central force potentials of Stillinger and Rahman (J. Chem. Phys., 68, 666 (1978)) and the initial configuration of the unit cell has small total dipole and quadrupole moments. Cumulative Metropolis Monte Carlo averages of binding energy per molecule, (two dimensional) ice Ih structure factors, unit cell dipole moment, and mean square oxygen displacement are examined at 300 K, and compared with averages previously determined for the (equilibrated) ice model at 20 and 200 K. The 300 K system is run for 4.6 million Monte Carlo steps. The results indicate that the binding energy per molecule decreases linearly with temperature from 20 to 300 K. However, a sharp discontinuity in the natural logarithm of the average ice I _h structure factors is observed between 200 and 300 K. This is accompanied by a large (1.2 A and increasing with Monte Carlo step number) average root mean square oxygen displacement at 300 K. These preliminary results at 300 K suggest that the model periodic ice I _h sample undergoes a phase change between 200 and 300 K. A complete equilibration at 300 K and a study of the periodic bulk ice sample at temperatures between 200 and 300 K are in progress.