Structure of Ice-VII and Ice-VIII:  A Quantum Mechanical Study

Ab initio quantum mechanical methods have been used to investigate the structures of Ice-VII and Ice-VIII. In the case of proton-ordered Ice-VIII, accurate structural data enable us to benchmark the methodology. The disorder in H-bond orientations of Ice-VII causes difficulties in understanding its structure from either an experimental or a theoretical viewpoint. In this work, we explore the consequence of randomness of H-bond arrangements in Ice-VII by examining all ice-rule-allowed configurations in a unit cell of 16 water molecules. Noticeable relative displacement (ε) between the two sublattices, previously thought to exist only in Ice-VIII, was found in many of these configurations. We find a correlation between the energy of a particular configuration of Ice-VII and ε, an energy gap of 1.5 mhartree that separates a unique ground state from all the others. Our results have proven that ab initio calculations can be used to resolve ambiguities in interpreting neutron diffraction data of Ice-VII.