Microscopic mechanisms of equilibrium melting of a solid.

The melting of a solid, like other first-order phase transitions, exhibits an intrinsic time-scale disparity: The time spent by the system in metastable states is orders of magnitude longer than the transition times between the states. Using rare-event sampling techniques, we find that melting of representative solids-here, copper and aluminum-occurs via multiple, competing pathways involving the formation and migration of point defects or dislocations. Each path is characterized by multiple barrier-crossing events arising from multiple metastable states within the solid basin. At temperatures approaching superheating, melting becomes a single barrier-crossing process, and at the limit of superheating, the melting mechanism is driven by a vibrational instability. Our findings reveal the importance of nonlocal behavior, suggesting a revision of the perspective of classical nucleation theory.