We report the development of an ab initio constant pressure extended molecular dynamics method with variable cell shape. This is a symmetry conserving method which allows for efficient structural searches and optimizations in spaces with preselected symmetry groups. We have used it to investigate MgSiO3, a perovskite, the marjor Earth-forming mineral phase which exists particularly in the lower mantle. We predict its structural behavior up to pressures which exceed the highest values reached in this region.
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