Rajiv K. Kalia

Condensed matter theories

Topological Phase Transitions in Strongly Interacting Fermi Systems (J W Clark) Semifluxon Dynamics in Extended Josephson Junctions (H Farhan) On the Quantum Hall Effect in Graphene (S Fujita) Ergodicity and Chaos in a System of Harmonic Oscillators (M H Lee) Fluid Helium-4 in Thermal Equilibrium (M Ristig) On the Generalised Slater Approximation (E Suraud) An Informative Method for the Diagnostics of Superconductors (K Rostami) Numerical Study of Pi-Junction Using Spin Filtering Barriers (S Kawabata) RPA Approach to Nonlinear Transport in Quantum Dots (B Tanatar) and other papers.

Hybrid finite-element/molecular-dynamics/electronic-density-functional approach to materials simulations on parallel computers

A hybrid simulation approach is developed to study chemical reactions coupled with long-range mechanical phenomena in materials. The finite-element method for continuum mechanics is coupled with the molecular dynamics method for an atomic system that embeds a cluster of atoms described quantum-mechanically with the electronic density-functional method based on real-space multigrids. The hybrid simulation approach is implemented on parallel computers using both task and spatial decompositions. Additive hybridization and unified finite-element/molecular-dynamics schemes allow scalable parallel implementation and rapid code development, respectively. A hybrid simulation of oxidation of Si(111) surface demonstrates seamless coupling of the continuum region with the classical and the quantum atomic regions.

Interaction potential for silicon carbide: A molecular dynamics study of elastic constants and vibrational density of states for crystalline and amorphous silicon carbide

An effective interatomic interaction potential for SiC is proposed. The potential consists of two-body and three-body covalent interactions. The two-body potential includes steric repulsions due to atomic sizes, Coulomb interactions resulting from charge transfer between atoms, charge-induced dipole-interactions due to the electronic polarizability of ions, and induced dipole-dipole (van der Waals) interactions. The covalent characters of the Si–C–Si and C–Si–C bonds are described by the three-body potential. The proposed three-body interaction potential is a modification of the Stillinger-Weber form proposed to describe Si. Using the molecular dynamics method, the interaction potential is used to study structural, elastic, and dynamical properties of crystalline (3C), amorphous, and liquid states of SiC for several densities and temperatures. The structural energy for cubic (3C) structure has the lowest energy, followed by the wurtzite (2H) and rock-salt (RS) structures. The pressure for the structural t...

Incipient phase separation in Ag/Ge/Se glasses : clustering of Ag atoms

Abstract Structural and dynamical properties of three-component chalcogenide glasses, Ag/Ge/Se, are studied using the molecular dynamics (MD) method. Effective potentials for GeSe2 and Ag2Se, which have already been well characterized, are combined to describe the interatomic interactions in the ternary system. The MD results for bond lengths, the static structure factor, and the vibrational density of states are in good agreement with neutron-scattering results. Our results elucidate the role played by silver ions as network modifiers in glass-forming GeSe2 with the medium-range correlations. Upon cooling we observe clustering of Ag atoms which indicates a tendency toward phase separation.

Multiscale simulation of nanosystems

The authors describe simulation approaches that seamlessly combine continuum mechanics with atomistic simulations and quantum mechanics. They also discuss computational and visualization issues associated with these simulations on massively parallel computers. Scientists are combining continuum mechanics and atomistic simulations through integrated multidisciplinary efforts so that a single simulation couples diverse length scales. However, the complexity of these hybrid schemes poses an unprecedented challenge, and developments in scalable parallel algorithms as well as interactive and immersive visualization are crucial for their success. This article describes such multiscale simulation approaches and associated computational issues using recent work as an example.

Linear-scaling density-functional-theory calculations of electronic structure based on real-space grids: Design, analysis, and scalability test of parallel algorithms

We have implemented parallel algorithms for density-functional-theory (DFT) based electronic-structure calculations. These include a plane-wave based algorithm, a real-space-grid algorithm based on a high-order finite difference method, and a linear-scaling real-space algorithm using localized orbitals. Parallelization schemes are described for these algorithms, and the computational complexity and the communications involved in the resulting parallel algorithms are analyzed. Scalability tests of these algorithms on massively parallel computers show that the linear-scaling DFT algorithm is highly scalable. For a 110,592-atom gallium arsenide system on 1024 IBM SP3 processors, the parallel efficiency is as high as 93%.

Interaction potentials for alumina and molecular dynamics simulations of amorphous and liquid alumina

Structural and dynamical properties of crystalline alumina α-Al2O3 and amorphous and molten alumina are investigated with molecular dynamics simulation based on an effective interatomic potentials consisting of two- and three-body terms. Structural correlations are examined through pair distribution functions, coordination numbers, static structure factors, bond angle distributions, and shortest-path ring analyses. The calculated results for neutron and x-ray static structure factors are in good agreement with experimental results. Dynamical correlations, such as velocity autocorrelation function, vibrational density of states, current-current correlation function, and frequency-dependent conductivity, are also discussed.

Phonons in graphitic tubules: A tight‐binding molecular dynamics study

Using the tight binding molecular dynamics method (TBMD), the structural and dynamical properties of graphitic tubules are studied. The phonon dispersion and density of states of graphitic tubules with various helicities and diameters are calculated. Phonon modes in tubules are softened by the curvature when compared with graphite. Unique features of the graphitic tubule, with special emphasis on low‐frequency modes, are discussed. The symmetry of phonon modes is analyzed, and infrared and Raman active modes are identified. Sound velocities in graphitic tubules are also calculated as functions of tubule helicity and diameter.

Multiresolution molecular dynamics algorithm for realistic materials modeling on parallel computers

Abstract For realistic modeling of materials, a molecular-dynamics (MD) algorithm is developed based on multiresolutions in both space and time. Materials of interest are characterized by the long-range Coulomb, steric and charge-dipole interactions as well as three-body covalent potentials. The long-range Coulomb interaction is computed with the fast multipole method. For bulk systems with periodic boundary conditions, infinite summation over repeated image charges is carried out with the reduced cell multipole method. Short- and medium-range non-Coulombic interactions are computed with the multiple time-step approach. A separable tensor decomposition scheme is used to compute three-body potentials. For a 4.2 million-particle SiO 2 system, one MD step takes only 4.8 seconds on the 512-node Intel Touchstone Delta machine and 10.3 seconds on 64 nodes of an IBM SP1 system. The constant-grain parallel efficiency of the program is η ′ = 0.92 and the communication overhead is 8% on the Delta machine. On the SP1 system, η ′ = 0.91 and communication overhead is 7%.

Brittle dynamic fracture of crystalline cubic silicon carbide "3C-SiC… via molecular dynamics simulation

Brittle fracture dynamics for three low-index crack surfaces, i.e., (110), (111), and (100), in crystalline cubic silicon carbide (3C-SiC) is studied using molecular dynamics simulation. The results exhibit significant orientation dependence: (110) fracture propagates in a cleavage manner; (111) fracture involves slip in the {111¯} planes; and crack branching is observed in (001) fracture. Calculated critical energy release rates, which characterize fracture toughness, are compared with available experimental and ab initio calculation data.