A quantum mechanical study has shed more light on the hexamer structures that make up the smallest drops of water—that is, the clusters with the fewest number of water molecules capable of holding a three-dimensional shape via hydrogen bonding (J. Am. Chem. Soc., DOI: 10.1021/ja304528m). The hexamer is the prototypical system scientists use to understand the structure and dynamics of bulk water. Yimin Wang and Joel M. Bowman of Emory University and Volodymyr Babin and Francesco Paesani of the University of California, San Diego, studied the clusters through simulations. They found that the “cage” and “prism” structures exist in almost equal amounts at temperatures near absolute zero. As the temperature increases to 60 K, more of the cage structure forms. A third structure—the “book”—appears at warmer temperatures. Interplay between energetic and entropic effects drives the differences in cluster species population, with entropy favoring the floppier cage and book clusters, Paesani says. The minute energy differences between the structures make them particularly sensitive to their environment, the researchers say, which may explain different amounts of the structures observed for different carrier gases in supersonic expansion experiments on water clusters reported earlier this year (C&EN, May 21, page 33).