First-principles molecular-dynamics study of surface disordering of the (0001) face of hexagonal ice

In order to study surface disordering of ice at temperatures below the bulk melting point as a function of depth into the bulk, Car–Parrinello molecular-dynamics simulations of a periodic model of the hexagonal ice (0001) surface were carried out. Partial disorder in the uppermost bilayer was observed at a simulation temperature of 190 K, which is ∼30 K below the estimated bulk melting point, qualitatively validating earlier classical molecular-dynamics studies of this phenomenon. Over 0.5 ps, the time scale of a simulation, there were three particularly useful (and complementary) measures of disorder: The pair distribution function g(r), the distance of the oxygen atoms from the bottommost bilayer, and the distribution of angles φ and θ formed by the molecular dipole vector and the Cartesian axes. Our results set the stage for future studies addressing the effect of the disordered ice surface on heterogeneous atmospheric chemistry.