Structures at high pressure from random searching

An approach to searching for the structures of crystals is described. We choose an ensemble of "random" starting structures and relax them, often constraining the starting structures and/or relaxation to bias the search in some manner. The methodology is illustrated by applications to high-pressure phases of group IV hydrides, especially silane (SiH4), and to solid hydrogen, using first-principles density-functional-theory methods. We discuss the issue of possible decomposition of group IV hydrides into their constituent elements under pressure. A new molecular-hydrogen structure is described whose enthalpy is calculated to be slightly lower than all other known structures in the pressure range 75-108 GPa, and we mention its possible relevance to phase II of hydrogen. (C) 2009 WILEY-VCH Verlag GmbH & Co. KGaA. Weinheim