Dissociation of methane into hydrocarbons at extreme (planetary) pressure and temperature
ABSTRACT Constant-pressure, first-principles molecular dynamic simulations were used to investigate the behavior of methane at high pressure and temperature. Contrary to the current interpretation of shock-wave experiments, the simulations suggest that, below 100 gigapascals, methane dissociates into a mixture of hydrocarbons, and it separates into hydrogen and carbon only above 300 gigapascals. The simulation conditions (100 to 300 gigapascals; 4000 to 5000 kelvin) were chosen to follow the isentrope in the middle ice layers of Neptune and Uranus. Implications on the physics of these planets are discussed.
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ABSTRACT: We describe here in detail the recently introduced methodology for simulation of structural transitions in crystals. The applications of the new scheme are illustrated on various kinds of crystals and the advantages with respect to previous schemes are emphasized. The relevance of the new method for the problem of crystal structure prediction is also discussed.Zeitschrift für Kristallographie 12/2004; 220(5-6):489-498. DOI:10.1524/zkri.220.5.489.65078 · 1.26 Impact Factor
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ABSTRACT: Prediction of the stable crystal structure on the basis of only the chemical composition is one of the central problems of condensed matter physics, which for a long time remained unsolved. The recently developed evolutionary algorithm USPEX (Universal Structure Predictor: Evolutionary Xtallography) made an important progress in this field, enabling efficient and reliable prediction of structures with up to 30-40 atoms in the unit cell using ab initio methods (or up to 100-200 atoms/cell with classical forcefields). Here we review this methodology, illustrating its variation operators and visualising evolutionary runs using specifically formulated similarity matrices. We also show several recent applications – (1) prediction of new high-pressure phases of CaCO 3 , (2) search for the structure of the polymeric phase of CO 2 ("phase V"), (3) search for new phases of transition metals, (4) high-pressure phases of oxygen, (4) new high-pressure phases of FeS and FeO, (5) exploration of possible stable compounds in the Xe-C system at high pressures, (6) investigation of previously proposed cluster-based insulators Al 13 K and Al 12 C.