Quantum cluster equilibrium theory of liquids: Freezing of QCE/3-21G waterto tetrakaidecahedral "Bucky-ice"

We extend the Quantum Cluster Equilibrium (QCE/3-21G-level) theory of liqui d water to include larger icelike clusters, including tetrahedral and fulle renelike clusters up to 26-mers. A low-energy tetrakaidecahedral 24-mer (la beled w24_0, with two hexagonal apical faces bridged by 12 pentagons in a c ooperatively charge-balanced H-bond pattern) is found to lead to a new low- T phase that bounds both liquid and vapor regions in first-order transition lines, giving rise to a true QCE triple point. We characterize the microst ructural composition and macroscopic properties of this "Bucky-ice" phase a nd examine its dependence on calculated energetics and vibrational properti es of constituent clusters. Although the Bucky-ice phase differs in signifi cant respects from physical ice-I (e.g., melting point similar to 20 K too high, molar volume similar to 5% too low), it manifests qualitatively corre ct thermodynamic features of true ice polymorphs, suggesting an important r ole of clathratelike clusters in the liquid/solid transition region. (C) 19 99 American Institute of Physics. [S0021-9606(98)50640-X].