Proton-ordered ice structures at zero pressure. A quantum-mechanical investigation

The results of quantum-mechanical computations are presented concerning the properties of two periodic structures of ice: ice XI (space group Cmc2(1)), the only experimentally known proton-ordered structure at zero pressure, and the hypothetical structure here denoted as P-ice (space group Pna2(1)). The latter is selected as a simple model of ordinary disordered ice. The equilibrium geometry and relative stability of the two phases are studied by performing periodic Hartree-Fock calculations. The difference in stability of the two structures is within the estimated computational accuracy, and they may be considered to be essentially iso-energetic. We also present the results of an embedded-cluster Hartree-Fock study of the substitutional KOH impurity in the two structures: the interest in this problem comes from the fact that the disorder-order transition in ice XI is catalyzed by the presence of non-negligible quantities of dissolved potassium hydroxide. The KOH solution in ice XI is favored with respect to that in P-ice by a few kcal/mol. This difference could support the hypothesis that microcrystals of ice XI are formed around individual KOH molecules.