Making ice from liquid water is simple enough. Figuring out the molecular processes underlying the transformation, however, has proved far more difficult. But in a report published today in the journal Nature scientists describe having used a supercomputer to pinpoint the specific conditions required to start the crystallization of water into solid ice.

Previous simulations of the freezing of water had imposed artificial conditions on the system or resulted in a crystallized state different than the one that forms at atmospheric pressure. A detailed understanding of water's freezing processes is complicated by the substance's inclination to achieve a supercooled state, in which it remains a liquid despite being cooled well below its freezing point. Supercooled water turns to ice only after the formation of a so-called critical nucleus that grows until the entire sample crystallizes. In the new work, Masakazu Matsumoto and colleagues at Nagoya University in Japan set out to uncover the molecular-level details of how such a critical nucleus forms in liquid water. They computed changes in the relative orientations of 512 water molecules using calculations of the forces acting on and between them. The spontaneous appearance of several unusually long-lasting hydrogen bonds between adjacent water molecules, they report, is the first step of ice formation. "The initial nucleus then slowly changes shape and size until it reaches a stage that allows rapid expansion," the authors write, "resulting in crystallization of the entire system."

Though the newfound understanding of the processes at work in a freezer won't change the ice that comes out of it, the findings do offer more general insight into crystallization. According to Srikanth Sastry of the Jawaharlal Nehru Center for Advanced Scientific Research in India, "understanding the behavior of water could help us to tackle other questions, such as when a liquid readily forms a disordered glass state¿an important issue in designing non-crystalline materials."

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