Priya Vashishta

Structural and dynamical correlations in Ag2Se: A molecular dynamics study of superionic and molten phases

We study the structural properties, single‐particle dynamics, and the charge transport in superionic and molten Ag2Se using the method of molecular dynamics. The calculations are based on a model of interionic potentials in which the ions interact through Coulomb interaction, steric repulsion, and charge–dipole interaction due to the large electronic polarizability of the selenium ions. In the superionic phase the Ag ions diffuse through a stable bcc lattice of Se atoms. Structural and dynamical correlations are studied at five temperatures in the superionic phase and three temperatures in the molten phase. Among the structural correlations the results are presented for partial pair distribution functions, coordination numbers, partial structure factors, bond angle distributions, and the wave vector and temperature dependence of the Bragg intensities. Detailed comparison with the neutron and x‐ray single crystal diffraction results are made whenever possible. Diffuse neutron and x‐ray scattering is calcul...

Hybrid quantum mechanical/molecular dynamics simulation on parallel computers: density functional theory on real-space multigrids

A hybrid quantum mechanical/molecular dynamics simulation scheme is developed, in which a quantum mechanical system described by the density functional theory on real-space multigrids is embedded in a classical system of atoms interacting via an empirical interatomic potential. Handshake atoms coupling the quantum and the classical systems are treated by a novel scaled position method. The scheme is implemented on parallel computers using both task and spatial decompositions. An application to oxidation of Si (100) surface demonstrates seamless coupling of the quantum and the classical systems.

A Multilevel Parallelization Framework for High-Order Stencil Computations

Stencil based computation on structured grids is a common kernel to broad scientific applications. The order of stencils increases with the required precision, and it is a challenge to optimize such high-order stencils on multicore architectures. Here, we propose a multilevel parallelization framework that combines: (1) inter-node parallelism by spatial decomposition; (2) intra-chip parallelism through multithreading; and (3) data-level parallelism via single-instruction multiple-data (SIMD) techniques. The framework is applied to a 6 th order stencil based seismic wave propagation code on a suite of multicore architectures. Strong-scaling scalability tests exhibit superlinear speedup due to increasing cache capacity on Intel Harpertown and AMD Barcelona based clusters, whereas weak-scaling parallel efficiency is 0.92 on 65,536 BlueGene/P processors. Multithreading+SIMD optimizations achieve 7.85-fold speedup on a dual quad-core Intel Clovertown, and the data-level parallel efficiency is found to depend on the stencil order.

Interfacial colloidal crystals and melting transition

Polystyrene spheres (diameter approximately 3000 AA) trapped at the air-water interface and interacting via dipole-dipole repulsion have been seen to form a two-dimensional triangular lattice. The authors report the first molecular dynamics study of such an interfacial colloidal crystal. It is shown that the system undergoes a first-order melting transition in the region Gamma =59-65, where Gamma is the ratio of the average potential to kinetic energy. The transition entropy per particle is 0.3 kB.

Parallel Lattice Boltzmann Flow Simulation on Emerging Multi-core Platforms

A parallel Lattice Boltzmann Method (pLBM), which is based on hierarchical spatial decomposition, is designed to perform large-scale flow simulations. The algorithm uses critical section-free, dual representation in order to expose maximal concurrency and data locality. Performances of emerging multi-core platforms--PlayStation3 (Cell Broadband Engine) and Compute Unified Device Architecture (CUDA)--are tested using the pLBM, which is implemented with multi-thread and message-passing programming. The results show that pLBM achieves good performance improvement, 11.02 for Cell over a traditional Xeon cluster and 8.76 for CUDA graphics processing unit (GPU) over a Sempron central processing unit (CPU). The results provide some insights into application design on future many-core platforms.

Ionic motion in superionic Ag2S

A model of the superionic conductor Ag2S is proposed and studies by the molecular dynamics technique using effective pair potentials. The self-diffusion for silver, DAg(T), is in good agreement with experiment. The Ag density map and its temperature dependence is calculated. The authors also calculate the X-ray and neutron structure factors which explain the anomalous intensity at Q=(1.6, 1, 0) observed by Cava and McWhan (1980).

Low temperature phase transformation in superionic conductors: A molecular dynamics study of silver sulfide

Upon lowering the temperature below 450 K the superionic solid Ag2 S undergoes a structural phase transformation from a bcc sulfur lattice with the space group Im3m to a monoclinic lattice with the space group P21 /c. We have successfully modeled this transformation using the variable size–shape form of molecular dynamics and a pair potential constructed earlier to model structural and transport properties in the superionic phase. Very good agreement is found between the partial pair distribution functions for the transformed crystal and the distribution functions calculated using the atomic positions obtained in x‐ray studies. We also find good agreement between the three‐body angular distributions calculated for the transformed system and distributions calculated for the structure based on the x‐ray positions.

Ground‐state and finite‐temperature energetics and topologies of germanium microclusters

We have investigated the ground‐state and finite‐temperature properties of Ge microclusters (N=2 to 14) using molecular dynamics (MD) simulation along with the method of steepest‐descent quench (SDQ). The interaction potential adopted is the three‐body Stillinger–Weber potential as modified by Ding and Andersen for amorphous Ge. Our results indicate that the experimentally observed greater stability of certain cluster sizes can be explained by the topology and energetics of the clusters at finite temperature rather than by the binding energies of the ground‐state structures.

Inelastic neutron scattering and dynamics of ions in the super-ionic conductor Ag2S

The dynamics of ions in the super-ionic conductor Ag2S and its manifestations in the inelastic neutron scattering function are studied using the molecular dynamics technique. The frequency spectrum of density fluctuations is investigated in the vicinity of Q=(1.6,1,0). The results for the intensities, linewidths and the temperature dependence of the quasi-elastic peak agree with those from the experiments of Grier, Shapiro and Cava (1984). The low-energy excitation ( approximately=2 meV) observed below the super-ionic transition is also explained.

Use of computer simulation technique to study atomic migration in solids

Abstract Nature of ionic motions in superionic conductors AgI and Ag 2 S are studied using molecular-dynamics (MD) technique. Our model of these superionic conductors is based on the use of effective pair-potentials. To determine the constants in these potentials, cohesive energy and bulk modulus are used as input; in addition one uses notions of ionic size based on the known crystal structures. The α⇄β structural phase transition in AgI is studied with the use of the new molecular dynamics (NMD) technique which allows for a dynamical variation of the shape and size of the cell. In the present model, upon heating the βAgI, the iodine ions undergo a hcp→bcc transformation and silver ions became mobile, whereas the reverse transformation is observed on cooling of αAgI. The calculated α⇄β transition temperature, and structural and dynamical properties are in good agreement with experiments. It has also become possible to observe crystal growth of α and β phases when a system of molten AgI is cooled to different temperatures. Transition temperatures determined from the studies of α⇄β structural transitions and from crystal growth studies are in good agreement. The calculated transition temperatures are also in good accord with the experimental results. A model of superionic conductor Ag 2 S is also studied. The silver density map and its temperature dependence is calculated. We have also calculated the x-ray and neutron structure factors which explain the anomalous intensity at and near Q =(1.6, 1, 0) observed in diffuse scattering experiments. A study of low-energy excitations in Ag 2 S is in progress.