Three parallel algorithms for classical molecular dynamics are presented. The first assigns each processor a fixed subset of atoms; the second assigns each a fixed subset of inter-atomic forces to compute; the third assigns each a fixed spatial region. The algorithms are suitable for molecular dynamics models which can be difficult to parallelize efficiently—those with short-range forces where the neighbors of each atom change rapidly. They can be implemented on any distributed-memory parallel machine which allows for message-passing of data between independently executing processors. The algorithms are tested on a standard Lennard-Jones benchmark problem for system sizes ranging from 500 to 100,000,000 atoms on several parallel supercomputers--the nCUBE 2, Intel iPSC/860 and Paragon, and Cray T3D. Comparing the results to the fastest reported vectorized Cray Y-MP and C90 algorithm shows that the current generation of parallel machines is competitive with conventional vector supercomputers even for small problems. For large problems, the spatial algorithm achieves parallel efficiencies of 90% and a 1840-node Intel Paragon performs up to 165 faster than a single Cray C9O processor. Trade-offs between the three algorithms and guidelines for adapting them to more complex molecular dynamics simulations are also discussed.

Fast parallel algorithms for short-range molecular dynamics

CALYPSO: A method for crystal structure prediction

The thermodynamic properties of 254 end-members, including 210 mineral end-members, 18 silicate liquid end-members and 26 aqueous fluid species are presented in a revised and updated internally consistent thermodynamic data set. The PVT properties of the data set phases are now based on a modified Tait equation of state (EOS) for the solids and the Pitzer & Sterner (1995) equation for gaseous components. Thermal expansion and compressibility are linked within the modified Tait EOS (TEOS) by a thermal pressure formulation using an Einstein temperature to model the temperature dependence of both the thermal expansion and bulk modulus in a consistent way. The new EOS has led to improved fitting of the phase equilibrium experiments. Many new end-members have been added, including several deep mantle phases and, for the first time, sulphur-bearing minerals. Silicate liquid end-members are in good agreement with both phase equilibrium experiments and measured heat of melting. The new dataset considerably enhances the capabilities for thermodynamic calculation on rocks, melts and aqueous fluids under crustal to deep mantle conditions. Implementations are already available in thermocalc to take advantage of the new data set and its methodologies, as illustrated by example calculations on sapphirine-bearing equilibria, sulphur-bearing equilibria and calculations to 300 kbar and 2000 °C to extend to lower mantle conditions.

An improved and extended internally consistent thermodynamic dataset for phases of petrological interest, involving a new equation of state for solids

Modeling hardness of polycrystalline materials and bulk metallic glasses

In comparing a material's resistance to distort under mechanical load rather than to alter in volume, Poisson's ratio offers the fundamental metric by which to compare the performance of any material when strained elastically. The numerical limits are set by ½ and -1, between which all stable isotropic materials are found. With new experiments, computational methods and routes to materials synthesis, we assess what Poisson's ratio means in the contemporary understanding of the mechanical characteristics of modern materials. Central to these recent advances, we emphasize the significance of relationships outside the elastic limit between Poisson's ratio and densification, connectivity, ductility and the toughness of solids; and their association with the dynamic properties of the liquids from which they were condensed and into which they melt.

/pdf/poisson-s-ratio-and-modern-materials-96egici4f6.pdf

Poisson's ratio and modern materials

The superconducting CeFe1−xCoxAsO (Co=0.1) oxyarsenide with a transition temperature (Tc) 11.4 K has been investigated by in situ high pressure synchrotron x-ray diffraction, magnetization, and resistivity measurements. The experiments performed at 10 K up to 6 GPa and at room temperature (RT) up to 55 GPa indicate large anisotropic lattice compression. A pressure induced structural change to a collapsed tetragonal structure is observed above 10 GPa at RT. We report here the enhancement of Tc from 11.4 to 12.3 K with a small increase in pressure up to 0.4 GPa and is first observed in an electron doped Ce-1111 system.

Pressure induced structural transition and enhancement of superconductivity in Co doped CeFeAsO

The single-crystal elastic moduli of the MgSiO_3 orthoenstatite end-member have been measured by Brillouin spectroscopy at ambient conditions. The aggregate elastic moduli at 1 atm are K_s = 107.6(15) GPa and µ = 76.8(7) GPa, for the adiabatic bulk modulus and shear modulus, respectively. These values are in excellent agreement with the results of previous acoustic studies of synthetic MgSiO_3 . Comparison of our results with pressure-volume (P-V) measurements do not support the presence of a change in the P-V trajectory of orthoenstatite at approximately 4 GPa, as suggested previously on the basis of X-ray measurements to 8.5 GPa. The effects of chemical composition on the elasticity of orthopyroxenes are well represented by the variation of compressional and shear velocities with density, which exhibit well defined trends for both synthetic and natural samples.

Elasticity of MgSiO_3 orthoenstatite

[1] The adiabatic bulk (KS) and shear (G) moduli of a polycrystalline majorite-garnet solid solution with composition (Na0.92Mg2.08)(Mg0.02Al1.06Si0.92)Si3O12 were measured at ambient conditions by Brillouin spectroscopy. KS and G were found to be 171.6 (3.5) GPa and 103.5 (2.0) GPa, respectively. Assuming a linear dependence of the elastic moduli on composition for the Mg2Si2O6 (enstatite-majorite, En-Mj) – NaAlSi2O6 (jadeite-majorite, Jd-Mj) join, the calculated KS and G of the fictive Jd-Mj end-member are 178 and 125 GPa, respectively. At ambient conditions, the shear velocity change associated with the pyroxene to majorite transition increases dramatically with increasing Na content in the system. In contrast, there is virtually no shear velocity increase associated with the pyroxene to majorite transition in MgSiO3. If this relationship holds at high pressures and temperatures, it suggests that for mineral assemblages containing an abundance of large radius cations such as Ca and Na, the pyroxene→garnet reaction will produce higher shear velocity gradients than assemblages that are relatively deficient in these elements. Such chemical effects could play a significant role in explaining high seismic velocity gradients in the transition zone.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2002GL015106

Elastic Moduli of Jadeite‐Enstatite Majorite

http://absimage.aps.org/image/MAR14/MWS_MAR14-2013-004417.pdf

X-ray induced mobility of molecular oxygen at extreme conditions

The single-crystal elastic properties of Ca 0.07 Mg 1.93 Si 2 O 6 orthopyroxene (space group Pbca ) have been investigated by Brillouin spectroscopy at ambient conditions. The aggregate bulk and shear moduli, K 0 ,S = 102.5 GPa (1.5) and μ = 74.2 GPa (1.1), respectively, are ~5% and ~3% lower than commonly accepted values for MgSiO 3 end-member ( K 0 ,S = 107.6, μ = 76.8 GPa). These results indicate that the incorporation of small amount of Ca in the orthoenstatite structure does not greatly affect its elastic properties. As a consequence, the increase in bulk modulus reported in natural orthopyroxenes relative to the Mg-end-member is not related to the substitution of Ca in the M2 octahedral sites, but more probably to the substitution of Al in tetrahedral sites.

Stanislav V. Sinogeikin

Papers

Pressure induced structural transition and enhancement of superconductivity in Co doped CeFeAsO

Elasticity of MgSiO_3 orthoenstatite

Elastic Moduli of Jadeite‐Enstatite Majorite

X-ray induced mobility of molecular oxygen at extreme conditions

Single-crystal elastic properties of Ca0.07Mg1.93Si2O6 orthopyroxene