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 molecul ar-dynamics simulations of a periodic model of the hexagonal ice (0001) sur face were carried out. Partial disorder in the uppermost bilayer was observ ed at a simulation temperature of 190 K, which is similar to 30 K below the estimated bulk melting point, qualitatively validating earlier classical m olecular-dynamics studies of this phenomenon. Over 0.5 ps, the time scale o f a simulation, there were three particularly useful (and complementary) me asures of disorder: The pair distribution function g(r), the distance of th e oxygen atoms from the bottommost bilayer, and the distribution of angles phi and theta 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. (C) 2000 Ame rican Institute of Physics. [S0021-9606(00)71247-5].