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.