How many water molecules does it take to make a crystal? About 275 ± 25, according to Christoph Pradzynski of the University of Göttingen and colleagues. The researchers developed a technique that allowed them to measure for the first time the infrared spectra of precisely selected molecular clusters in a size range that was previously inaccessible. Studying size-selected clusters with the number of water molecules ranging from 85 to 475, they found that the absorption band at around 3200 cm–1, which is indicative of crystalline ice, first appears in clusters of about 275 molecules.

The results should help to refine the interaction potentials of water that are used in predicting water’s macroscopic properties and the techniques could also be applied to other substances.