Statistics – Computation
Scientific paper
Dec 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006agufm.p42b..05f&link_type=abstract
American Geophysical Union, Fall Meeting 2006, abstract #P42B-05
Statistics
Computation
5422 Ices, 5460 Physical Properties Of Materials, 5470 Surface Materials And Properties, 5494 Instruments And Techniques
Scientific paper
Planetary bodies are essentially crystalline aggregates. The structure and evolution of all such bodies are thus fundamentally due to the microscopic behaviour of the component crystals - equilibrium structures, elasticity, and transport properties for example. Such properties can be calculated with high precision from quantum mechanical first principles. Any property we wish to determine may be found from derivatives of the total energy of the crystal as it is perturbed from its equilibrium state. For example, the elastic constants are 2nd derivatives of the energy as a function of some applied strain. Whilst we can sample the total energy hypersurface any way we desire, many planetary ices have complex crystal structures (low symmetry and large unit cells) so ab initio calculations become too expensive. Experimentally, we can readily sample parts of the energy hypersurface - for example, by obtaining equilibrium crystal structures, measuring thermal expansion or incompressibility - and employ this data to fit simple two- and three-body interatomic potentials. These potentials, applied with care, may then be used to investigate other parts of the energy hypersurface, that is to calculate other physical properties at low computational cost. We have used powder neutron diffraction to study the crystal structures, thermal expansion tensors, and incompressibilities of a range of likely planetary (icy moon) materials, including ice polymorphs, ammonia hydrates, MgSO4.7H2O, MgSO4.11H2O, Na2SO4.10H2O, H2SO4.6½H2O, H2SO4.8H2O, and most recently CH3OH.H2O. Some of our latest results are presented, and the status of calculations and their relation to modelling of planetary interiors are discussed.
Fortes Dominic A.
Vocadlo Lidunka
Wood Ian Geoffrey
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