Dynamical aspects of isomerization and melting transitions in [H2O](8)

We present a transition path sampling study of the dynamics of isomerizatio n between the S-4 and the D-2d cubic structures of the water octamer. The r eaction mechanism involves a transition state characterized by a distorted face exhibiting a diagonal hydrogen bond. Analysis of an ensemble of trajec tories shows that the isomerization requires concerted flips of double prot on donor molecules and the interchange between dangling and bonding hydroge ns in single proton donor molecules. At a total energy E = -60.5 kcal/mol, we calculated that the characteristic time for the interconversion is of th e order of milliseconds. We have also investigated pathways for the melting transition at temperatures T approximate to 200 K. We find that the barrie r for solid-liquid interconversions never exceeds 2k(B)T measured from the liquid side. Such transitions between liquid and solid do not involve the p assage over an energetic barrier; instead, the stabilization of the liquid phase is the result of a cancellation between energetic and entropic contri butions.