When is H2O not water?

We have combined a computational search strategy with first-principles density-functional-theory calculations to identify metastable phases of H2O under pressure. The most stable structures consist of water molecules, while the most energetic metastable phases consist of oxygen and hydrogen molecules. In between lie many other metastable phases, consisting of various combinations of a few small molecules. It may be possible to synthesize some of these metastable phases, and we use our results to understand the nature of the crystalline metastable phase of H2O recently synthesized by Mao [Science 314, 636 (2006)]. (C) 2007 American Institute of Physics.