UNIT CELLS FOR THE SIMULATION OF HEXAGONAL ICE

A number of periodic lattices have historically been used to represent ice-lh in computer simulations. These vary in size, shape, and method of generation, and while they have served their intended purposes, th eir properties have rarely been documented in detail and their interco mpatibility is unknown. We develop a method for generating sets of int ernally consistent lattices and apply it to determine eight unit cells containing from 96 to 768 water molecules in both near-cubic and slab arrangements. It can easily be applied to generate additional (larger ) cells or representations of specific crystal faces. Each unit cell i n this set has zero net dipole moment and minimal net quadrupole momen t and is optimized using four different criteria to measure the random ness of the hydrogen bonding; if required, these criteria can easily b e modified to suit the intended application and alternate sets thus ge nerated. We find that Cota and Hoover's much used constraint for selec ting unit cells with zero dipole moment is too restrictive, not permit ting a fully random hydrogen-bonding network; also, unit-cell generati on methods based on potential-energy minimization are found to prefer unrepresentative, highly ordered structures. (C) 1997 American Institu te of Physics.