  We use classical molecular dynamics combined with the recently developed
metadynamics method [A. Laio and M. Parrinello, Procs. Natl. Acad. Sci. USA 99,
20 (2002)] to study the process of bulk melting in hexagonal ice. Our
simulations show that bulk melting is mediated by the formation of topological
defects which preserve the coordination of the tetrahedral network. Such
defects cluster to form a defective region involving about 50 molecules with a
surprisingly long life-time. The subsequent formation of coordination defects
triggers the transition to the liquid state.
