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

Differential scanning calorimetry (DSC) is a powerful tool to address some of the most challenging issues in glass science and technology, such as the nonequilibrium nature of the glassy state and ...

Understanding Glass through Differential Scanning Calorimetry

In several series of lithium, sodium, and potassium borosilicate glasses whose compositions traverse known regions of liquid–liquid phase separation, we have applied triple-quantum magic-angle spinning (3QMAS) 11B and 17O NMR to obtain high-resolution information about short-range structure and connections among various network structural units, and their variation with composition and thermal history. Oxygen-17 3QMAS spectra reveal changes in connectivities between silicate and BO3 ([3]B) and BO4 ([4]B) units, by quantifying populations of bridging oxygens such as B–O–B, Si–O–B and Si–O–Si, and of non-bridging oxygens. [3]B–O–Si and [4]B–O–Si as well as [3]B–O–[3]B and [4]B–O–[3]B linkages can be distinguished. 11B MAS and 3QMAS at a magnetic field of 14.1 T allow proportions of several borate units to be determined, including [3]B in boroxol ring and non-ring sites and [4]B with 3 versus 4 Si neighbors. By combining the 11B and 17O NMR results, detailed information on Si/B mixing in sodium borosilicates can be derived, showing, for example, that [4]B and non-ring [3]B tend to mix with silicate units, while ring [3]B is mainly connected to borate groups. In a preliminary study of the effects of varying alkali cation, potassium-containing glasses are similar to those in the sodium borosilicate system, but a lithium borosilicate seems to exhibit considerably greater chemical heterogeneity. In annealing experiments that converted an optically clear to obviously phase-separated glasses, the ratio of [3]B to [4]B does not change significantly, but part of the non-ring [3]B converts to ring [3]B as the degree of unmixing increases.

Solid-State NMR Study of Metastable Immiscibility in Alkali Borosilicate Glasses

Molecular dynamics simulation and luminescence properties of Eu3+ doped molybdenum gadolinium borate glasses for red emission

In this research, the radiation shielding parameters for two glass systems with composition of 20MgO-(20-x)Al2O3-57SiO2-3B2O3-xCr2O3 (x = 0, 0.1, 0.25, 0.5 and 1 mol%) and Na2O-Al2O3 SiO2 glasses were studied. For this purpose, a scintillation detector namely 3 × 3 inch NaI(Tl) has been designed for detection of photons through MCNPX simulation code. Thus, the mass attenuation coefficients (μ/ρ) and some other gamma shielding quantities were calculated. The estimated MCNPX data of all the glasses were compared and approved with those of WinXCOM program. The gamma ray buildup factors for the two glass systems were also determined in a wide energy range of 0.02–15 MeV for penetration depths up to 15 mfp. The results revealed that the mass attenuation coefficients for the glass samples in the first glass system are higher than those in the glasses in the second system and thus the glasses coded as MASBC have more ability in reducing radiation than NAS glasses. Among the selected glasses, C4 has the largest values of μρ and lowest half value layer, and therefore this glass sample has better shielding features than other samples. Besides, 3—1 glass sample has the lowest values; therefore this sample has the least ability to block and reduce the incoming gamma radiation. The results also showed that the effective atomic number (Zeff) of the glasses in the first composition increased with increasing the content of Cr2O3, and Zeff in the second glass system (i.e. NAS) increased with increasing the Al2O3 content. The values of mass removal cross section for neutron (ΣR) vary between 0.09112 and 0.09163 and 0.08415–0.08710 for the first and second glass system respectively. The obtained results from the present investigation can be useful to understanding of influence of Cr2O3 and Al2O3 additive on nuclear radiation shielding properties of tellurite MgO-Al2O3-SiO2-B2O3 and Na2O-Al2O3-SiO2 glasses.

Characterization of a broad range gamma-ray and neutron shielding properties of MgO-Al2O3-SiO2-B2O3 and Na2O-Al2O3-SiO2 glass systems

A set of 12 glass compositions with distinct structural features have been designed over a broad composition space in the per-alkaline region of the Na2O – Al2O3 – SiO2 ternary system. As expected from a per-alkaline system, aluminum has been found to be tetrahedrally coordinated in all the glasses using 27Al magic angle spinning – nuclear magnetic resonance (MAS-NMR) spectroscopy and from structure models generated using molecular dynamic (MD) simulations. The physical properties of glasses, for example, density, coefficient of thermal expansion (CTE), glass transition, elastic moduli and Vickers hardness and brittleness have been measured experimentally and their trends have been explained based on the atomic structure of glasses, from both simulations and experiments. A reasonable agreement has been observed between the composition – structure – property relationship trends obtained experimentally when compared with those predicted by MD simulations. This demonstrates that MD simulation is a promising technique for predictive modeling and designing novel glass compositions for functional applications.

Composition – structure – property relationships in alkali aluminosilicate glasses: A combined experimental – computational approach towards designing functional glasses

A series of calcium-aluminoborosilicate glasses with different ratios of B2O3/SiO2 have been isostatically compressed at 1 and 2 GPa at temperatures above their glass transition temperatures. Boron and aluminum coordination numbers were quantified for recovered samples by 11B magic angle spinning (MAS) nuclear magnetic resonance (NMR) and 27Al MAS NMR spectroscopy at ambient pressure and temperature. The average coordination numbers increase as the glass was compressed to higher pressure and the glass with higher concentration of boron shows higher recoverable densification. The atomic structure changes can be represented by oxygen packing density, Young's modulus and Vickers hardness number show good correlations with oxygen packing density as well. In general Poisson's ratio decreases as the average coordination number of network formers increases, however the interpretation of the changes in Poisson's ratio with pressure for each glass composition presents a more complicated case and no straightforward trend with compaction was found.

Composition and pressure effects on the structure, elastic properties and hardness of aluminoborosilicate glass

In-situ Brillouin light scattering (BLS) experiments were carried out to measure both longitudinal and shear sound velocities of air-cooled and annealed sodium borate glasses xNa2O–(100−x)B2O3 (x = 10, 15, 20, 25, 30, and 35 mol%) from room temperature to temperatures beyond the glass transition temperature (Tg) for each composition. This allows us to access the complete set of elastic moduli at high temperatures and to study the effect of thermal history on physical properties of this system. On heating air-cooled glasses of lower Na2O content, elastic moduli increase anomalously with increasing temperature just below their Tg, whereas this behavior is absent in corresponding annealed glasses. This anomalous increase in elastic moduli with temperature was not observed in glasses of higher Na2O content. These differences were explained by different structural relaxation mechanisms in the glass transition range in sodium borate glasses of different compositions based on Raman spectroscopy studies in this work and in literature.

Understanding Sodium Borate Glasses and Melts from Their Elastic Response to Temperature

Understanding the structural origin of intermediate glasses

For most glass-forming liquids, the temperature dependence of viscosity is non-Arrhenius. Despite the technological and geological importance, the origin of this non-Arrhenius temperature dependence of viscosity remains elusive to date and constitutes an important but unsolved problem in condensed matter physics. It has become increasingly clear in recent years that high-temperature elasticity and viscosity of glass-forming liquids are strongly correlated. This work proposes a modified elastic model to predict equilibrium viscosity of glass-forming liquids. The new elastic model considers the configurational entropy as a factor controlling the activation energy for viscous flow in addition to the high-frequency shear modulus as in the Dyre shoving model. It works much better than the shoving model in fitting equilibrium viscosity for both strong and fragile systems. The new model also has the capability to estimate the nonequilibrium isostructural viscosity of glass from the equilibrium viscosity and the temperaturedependent elasticity of the glassy state.

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Siva Priya Jaccani

Papers

Composition – structure – property relationships in alkali aluminosilicate glasses: A combined experimental – computational approach towards designing functional glasses

Composition and pressure effects on the structure, elastic properties and hardness of aluminoborosilicate glass

Understanding Sodium Borate Glasses and Melts from Their Elastic Response to Temperature

Understanding the structural origin of intermediate glasses

Modified elastic model for viscosity in glass-forming systems