What Governs the Proton Ordering in Ice XV?

Powder neutron diffraction and Raman
spectroscopy experiments for
ice XV, the recently discovered proton-ordered polymorph of ice VI,
suggest that the protons arrange in an antiferroelectric structure
with P1̅ symmetry, contrary to several density
functional theory predictions of a ferroelectric Cc structure. Here, we find that higher-level fragment-based second-order
perturbation theory (MP2) and coupled cluster theory (CCSD­(T)) electronic
structure calculations predict that the experimentally proposed proton
ordering is indeed slightly more stable than the other possible structures.
These calculations reveal a close competition between the structure
with the strongest local hydrogen bonding (Cc) and
the one with the most favorable “delocalized” hydrogen
bond cooperativity effects (P1̅), with the
latter being preferred by ∼0.4 kJ/mol per molecule. The results
reiterate the importance of viewing ice networks as a whole instead
of focusing on pairwise hydrogen-bonding interactions.