Quantum-Chemical and Force-Field Investigations of Ice Ih: Computation of Proton-Ordered Structures and Prediction of Their Lattice Energies
Abstract
The different possible proton-ordered structures of ice Ih for an orthorombic unit cell with 8 water molecules were derived. The number of unique structures was found to be 16. The crystallographic coordinates of these are reported. The energetics of the different polymorphs were investigated by quantum-mechanical density-functional theory calculations and for comparison by molecular-mechanics analytical potential models. The polymorphs were found to be close in energy, i.e., within approximately 0.25 kcal/mol H 2 O, on the basis of the quantum-chemical DFT methods. At 277 K, the different energy levels are about evenly populated, but at a lower temperature, a transition to an ordered form is expected. This form was found to agree with the ice phase XI. The difference in lattice energies among the polymorphs was rationalized in terms of structural characteristics. The most important parameters to determine the lattice energies were found to be the distributions of water dimer H-bonded pair conformations, in an intricate manner.
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