Melting is a familiar phase transition, but the atomic-scale details of that process have been a mystery. A computational study now suggests that the traditional description, in which random thermal fluctuations lead to a small liquid nucleus, is incorrect. Instead, melting of crystalline solids originates in defects such as a hole in the crystal lattice or an extra atom in an interstitial space, according to a study by Amit Samanta and Weinan E of Princeton University and Tang-Qing Yu and Mark E. Tuckerman of New York University (Science 2014, DOI: 10.1126/science.1253810). By studying melting of copper and aluminum, the researchers found that when defects migrating through a material meet, they can join together into lines, called dislocations, or clusters. The liquid then nucleates and grows out of those dislocations or clusters. If a solid is heated above its melting temperature, however, then melting is driven by instability created through atomic vibrations. The high energy barriers associated with melting make the process inaccessible to standard computer simulations, a challenge the scientists addressed using a computational method known as sampling. Now they are working to adapt the method to more complicated molecular solids, such as ices, E and Tuckerman say.