Quantum-Chemical and Force-Field Investigations of Ice Ih: Computation of Proton-Ordered Structures and Prediction of Their Lattice Energies

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 H2O, 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.