Molecular dynamics simulations are performed to investigate structure, mechanical properties, and thermal transport in amorphous silicon nitride under uniform dilation. As the density is lowered, we observe the format...
Molecular dynamics simulations are performed to investigate structure, mechanical properties, and thermal transport in amorphous silicon nitride under uniform dilation. As the density is lowered, we observe the formation of pores below ρ = 2.6 g/cc and at 2.0 g/cc the largest pore percolates through the entire system. Effects of porosity on elastic constants, phonons and thermal conductivity are investigated. Thermal conductivity and Young’s modulus are found to scale as ρ1–5 and ρ3–6, respectively.
We investigate the onset and growth of the neck between amorphous Si3N4 nanoclusters at zero and finite pressures. Local stress fluctuations and atomic self-diffusion in the interface region are found to be responsibl...
We investigate the onset and growth of the neck between amorphous Si3N4 nanoclusters at zero and finite pressures. Local stress fluctuations and atomic self-diffusion in the interface region are found to be responsible for neck formation. External pressure has a dramatic influence on the rate of sintering.
Using a reactive empirical bond-order potential (REBOP) model for hydrocarbons1, large scale molecular dynamics simulations of carbon systems are carried out on parallel machines. Structural and dynamical correlations...
Using a reactive empirical bond-order potential (REBOP) model for hydrocarbons1, large scale molecular dynamics simulations of carbon systems are carried out on parallel machines. Structural and dynamical correlations of amorphous carbon at various densities are studied. The calculated structure factor agrees well with neutron scattering experiments and the results of tight- binding molecular dynamics simulations. The dynamic behavior of crack propagation through graphite sheet is also investigated with the molecular-dynamics method. Effects of external stress and initial notch shape on crack propagation in graphite are studied. It is found that graphite sheet fractures in a cleavage-like or branching manners depending on the orientations of the graphite sheet with respect to the external stress. The roughness of crack surfaces is analyzed. Two roughness exponents are observed in two different regions.
A quantum molecular dynamics (QMD) simulation scheme has been developed which is suitable for the study of highly nonlinear electron dynamics far from equilibrium in semiconducting devices of nanometer size. The core ...
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A quantum molecular dynamics (QMD) simulation scheme has been developed which is suitable for the study of highly nonlinear electron dynamics far from equilibrium in semiconducting devices of nanometer size. The core components of the QMD algorithm are: (i) solution of the time-dependent Kohn-Sham or Schrodinger equations;and (ii) solution of the Poisson equation for the direct electron-electron interaction. Efficient parallel algorithms for these components are the space-splitting Schrodinger solver and the dynamical-simulated-annealing Poisson solver. Both algorithms are scalable and require only nearest-neighbor communications. These algorithms are implemented on an 8,192-node MasPar computer. Timing tests are carried out to compare these algorithms with other parallel Schrodinger and Poisson solvers. The timing results on the MasPar are also compared with those on other parallel and vector architectures.
For realistic modeling of materials, a molecular-dynamics (MD) algorithm is developed based on multiresolutions in bath space and time. materials of interest are characterized by the long-range Coulomb, steric and cha...
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For realistic modeling of materials, a molecular-dynamics (MD) algorithm is developed based on multiresolutions in bath 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 nonCoulombic interactions are computed with the multiple time-step approach. A separable tenser decomposition scheme is used to compute three-body potentials. For a 4.2 million-particle SiO2 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 eta' = 0.92 and the communication overhead is 8% on the Delta machine. On the SP1 system, eta' = 0.91 and communication overhead is 7%.
We present a computational scheme to propagate wave packets describing S states of two-electron systems. Body-fixed polar coordinates (r1,r2,θ) are used to represent the time-dependent wave functions on a grid in thr...
We present a computational scheme to propagate wave packets describing S states of two-electron systems. Body-fixed polar coordinates (r1,r2,θ) are used to represent the time-dependent wave functions on a grid in three dimensions. We handle the Coulomb singularities by transformation of the wave function and a modified finite-difference formula to evaluate the kinetic energy. We use a finite-difference split-operator scheme for the time propagation and, as a first application, we compute the low-energy spectrum of helium and H−.
Pore interface growth and the roughness of fracture surfaces in silica glasses have been investigated with large-scale molecular-dynamics simulations. During uniform dilation, the average pore radius R and the width W...
Pore interface growth and the roughness of fracture surfaces in silica glasses have been investigated with large-scale molecular-dynamics simulations. During uniform dilation, the average pore radius R and the width W scale with the pore size s as R∼sη and η=0.48±0.03 with η=0.48±0.03 and μ=0.50±0.03. When the mass density is reduced to 1.4 g/cm3, the pores grow catastrophically to cause fracture. The roughness exponent for fracture surfaces, α=0.87±0.02 supports experimental claims about the universality of α.
Lattice dynamics of solid C60 is studied using a unified interaction model which consists of a tight-binding potential for the intramolecular interaction and a Lennard-Jones and bond-charge model for the intermolecula...
Lattice dynamics of solid C60 is studied using a unified interaction model which consists of a tight-binding potential for the intramolecular interaction and a Lennard-Jones and bond-charge model for the intermolecular interaction. Phonon dispersion and density of states of solid C60 are calculated in the energy range from 0 to 210 meV. The intermolecular-phonon density of states shows peaks around 2.3 and 3.7 meV, and extends to 7.6 meV. Intermolecular phonons and intramolecular vibrational modes are well separated by a gap of nearly 22 meV. The intermolecular-phonon modes show strong dispersion, so do the intramolecular modes, especially those with energy below 70 meV. The calculated phonon spectrum agrees very well with recent inelastic neutron-scattering experiments. The effects of orientational disorder and pressure on the intermolecular and intramolecular phonons of solid C60 are investigated. Intermolecular-phonon modes are sensitive to orientational disordering, and orientational disordering softens libron modes. The pressure dependence of the phonon dispersion and density of states is studied for solid C60 in the range of pressures from 0 to 56 kbar. The intermolecular-phonon spectrum shows strong broadening when pressure is applied. The libron modes shift to higher frequencies with a rate of about 0.4 cm−1/kbar. Intramolecular modes broaden or split, and shift toward higher frequencies at a rate of up to 0.88 cm−1/kbar. Most Raman and infrared active modes show strong pressure dependence. Intermolecular- and intramolecular-phonon spectra at various pressures are presented and results are compared with the available experiments.
For particles interacting via two- and three-body potentials, a domain-decomposition algorithm is used to implement molecular dynamics (MD) on distributed memory MIMD (multiple-instruction multiple-data) computers. Th...
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For particles interacting via two- and three-body potentials, a domain-decomposition algorithm is used to implement molecular dynamics (MD) on distributed memory MIMD (multiple-instruction multiple-data) computers. The algorithm employs the linked-cell-list method and separable three-body force calculation. The force calculation is accelerated by the multiple-time-step (MTS) method. For a 1.54 million particle SiO2 system, the MD program runs at a speed of 660 time steps per hour (1100 steps/h without the three-body interaction) on a 64-node Intel iPSC/860. The parallel algorithm is highly efficient (parallel efficiency = 0.973), as it involves only 3% communication overhead. Utilizing the second derivatives of the potential energy, the conjugate-gradient search for a local minimum underlying an MD configuration is accelerated by a factor of 13.
The effect of pressure on inter‐ and intramolecular phonons in solid C60 is studied using a unified model which consists of a tight‐binding potential for the intramolecular interaction and a Lennard‐Jones and bond ...
The effect of pressure on inter‐ and intramolecular phonons in solid C60 is studied using a unified model which consists of a tight‐binding potential for the intramolecular interaction and a Lennard‐Jones and bond charge model for the intermolecular interaction. At various pressures (up to 56 kbar), the phonon dispersion and density of states of solid C60 are calculated in the energy range from 0 to 210 meV. At zero pressure, the intermolecular phonon density of states shows peaks around 2.3 and 3.7 meV, and extends to 7.6 meV. Not only the intermolecular phonon modes but also the intramolecular modes show significant dispersions, especially those modes with energy below 70 meV. Under pressure, the intermolecular phonon spectrum shows strong broadening. The libron modes shift to higher frequencies at a rate of 0.40 cm−1/kbar. The effect of pressure on intramolecular modes manifests in two ways: (i) the lower‐frequency modes broaden and shift toward higher energies, and (ii) the higher‐frequency modes split and shift upward in energy. Intramolecular modes shift to higher energy at a rate up to 0.88 cm−1/kbar. Most Raman and infrared active modes show strong pressure dependence. Inter‐ and intramolecular phonon spectra at various pressures are presented and results are compared with experiments.
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