Papers by Timothy Germann
We present the results of 138 million and 1.8 billion atom Non-Equilibrium Molecular Dynamics (NE... more We present the results of 138 million and 1.8 billion atom Non-Equilibrium Molecular Dynamics (NEMD) simulations for Al-Al sliding friction at pressures of 15 GPa. Three-dimensional samples comprised of 4 nm, 20 nm and 50 nm grains were studied to times of 117 ns for the largest systems. We discuss the evolution of the initial grain size distribution to a steady state distribution that is statistically similar for all initial grain sizes. We compare the results for the frictional force to a rate dependent model that incorporates plasticity and discuss the relationships among grain size, grain morphology, dislocations and other defect structures, and plasticity.
Large-order dimensional perturbation theory has proven to be a useful technique for the study of ... more Large-order dimensional perturbation theory has proven to be a useful technique for the study of ground and excited states of two-electron atoms. The authors use the 1/D expansion for L = O wave functions to elucidate the evolution between the harmonic (large-D) and physical wave functions, including the transition of the large dimension molecular structure to that of a singly
We demonstrate the excellent performance and scalability of a classical molecular dynamics code, ... more We demonstrate the excellent performance and scalability of a classical molecular dynamics code, SPaSM, on the IBM BlueGene/L supercomputer at LLNL. Simulations involving up to 160 billion atoms (micron-size cubic samples) on 65,536 processors are reported, consistently achieving 24.4ñ25.5 Top/s for the commonly used Lennard-Jones 6-12 pairwise interaction potential. Two extended production simulations (one lasting 8 hours and the other
AIP Conference Proceedings, 2004
ABSTRACT
International Journal of Impact Engineering, Dec 1, 2006
Using multimillion-atom classical molecular dynamics simulations, we have studied the impact dyna... more Using multimillion-atom classical molecular dynamics simulations, we have studied the impact dynamics of solid and liquid spherical copper clusters (10-30 nm radius) with a solid surface, at velocities ranging from 100 m/s to 2 km/s. The resulting shock, jetting, and fragmentation processes are analyzed, demonstrating three distinct mechanisms for fragmentation. At early times, shock-induced ejection and hydrodynamic jetting produce fragments in the normal and tangential directions, respectively, while sublimation (evaporation) from the shock-heated solid (liquid) surface produces an isotropic fragment flux at both early and late times.
Equilibrium molecular dynamics (MD) simulation of high explosives can provide important informati... more Equilibrium molecular dynamics (MD) simulation of high explosives can provide important information on their thermal decomposition by helping to characterize processes with timescales that are much longer than those attainable with non-equilibrium MD shock studies. A reactive force field is used with MD to probe the chemisty induced by intense heating (`cook-off') of 1,3,5-triamino-2,4,6-trinitrobenzene (TATB). The force field (ReaxFF) was
Equilibrium molecular dynamics (MD) simulation of high explosives can provide important informati... more Equilibrium molecular dynamics (MD) simulation of high explosives can provide important information on their thermal decomposition by helping to characterize processes with timescales that are much longer than those attainable with non-equilibrium MD shock studies. A reactive force field is used with MD to probe the chemisty induced by intense heating (`cook-off') of 1,3,5-triamino-2,4,6-trinitrobenzene (TATB). The force field (ReaxFF) was
International Journal of Modern Physics C, 2004
We have performed parallel large-scale molecular-dynamics simulations on the QSC-machine at Los A... more We have performed parallel large-scale molecular-dynamics simulations on the QSC-machine at Los Alamos. The good scalability of the SPaSM code is demonstrated together with its capability of efficient data analysis for enormous system sizes up to 19 000 416 964 particles. Furthermore, we introduce a newly-developed graphics package that renders in a very efficient parallel way a huge number of spheres necessary for the visualization of atomistic simulations. These abilities pave the way for future atomistic large-scale simulations of physical problems with system sizes on the μ-scale.
Materials Science and Engineering: A, 2008
It is surprising that no full molecular dynamics calculations with correct crystallography exist ... more It is surprising that no full molecular dynamics calculations with correct crystallography exist for the two most commonly studied martensitic phase transitions, iron and NiTi. This is due to the lack of interatomic potentials for these systems, which in turn can be traced to a lack of understanding of the underlying physics. We discuss the discrepancies between theory and experiment
Equilibrium molecular dynamics (MD) simulation of high explosives can provide important informati... more Equilibrium molecular dynamics (MD) simulation of high explosives can provide important information on their thermal decomposition by helping to characterize processes with timescales that are much longer than those attainable with non-equilibrium MD shock studies. A reactive force field is used with MD to probe the chemisty induced by intense heating (`cook-off') of 1,3,5-triamino-2,4,6-trinitrobenzene (TATB). The force field (ReaxFF) was
Equilibrium molecular dynamics (MD) simulation of high explosives can provide important informati... more Equilibrium molecular dynamics (MD) simulation of high explosives can provide important information on their thermal decomposition by helping to characterize processes with timescales that are much longer than those attainable with non-equilibrium MD shock studies. A reactive force field is used with MD to probe the chemisty induced by intense heating (`cook-off') of 1,3,5-triamino-2,4,6-trinitrobenzene (TATB). The force field (ReaxFF) was developed by van Duin, Goddard and coworkers [1] at CalTech and has already shown promise in predicting the chemistry in small samples of RDX under either shock compression or intense heat. Large-system simulations are desired for TATB because of the high degree of carbon clustering expected in this material. We will show results of 800-particle simulations at several temperatures, and detail current capabilities for large-scale (10^4 -- 10^5 atoms) systems carried out with the massively parallel GRASP MD software developed at Sandia National...
Phys Rev E, 2007
The dependence of macroscopic detonation properties of a two-dimensional diatomic (AB) molecular ... more The dependence of macroscopic detonation properties of a two-dimensional diatomic (AB) molecular system on the fundamental properties of the molecule were investigated. This includes examining the detonation velocity, reaction zone thickness, and critical width as a function of the exothermicity of the gas-phase reaction and the gas-phase dissociation energy for. Following previous work, molecular dynamics (MD) simulations with a reactive empirical bond-order potential were used to characterize the shock-induced response of a diatomic AB molecular solid, which exothermically reacts to produce gaseous products. MD simulations reveal that there is a linear dependence between the square of the detonation velocity and each of these molecular parameters. The detonation velocities were shown to be consistent with the Chapman-Jouguet model, demonstrating that these dependencies arise from how the Equation of State of the products and reactants are affected.
Interfacial sliding under high pressure loading at high velocities (0 < v < 1 km/s) results in a ... more Interfacial sliding under high pressure loading at high velocities (0 < v < 1 km/s) results in a variety of mesosc ale phenomena at extreme strain rates. For ductile metal interfacial pairs, these include nano- and mesoscale dynami c strucutral transitions, local melting and amorphization, material mixing, and localization of plastic deformation. We illustrate these phenomena with large scale NonEquilibrium Molecular Dynamics (NEMD) simulations for Cu/Ag, Ta/Al, and Al/Al interfaces. These suggest universal behavior in sliding velocity for the frictional force and a scaling form for the frictional force vs. velocity at high velocities which will be discussed.
A linear algebraic method for exact computation of the coefficients of the 1/D expansion of the S... more A linear algebraic method for exact computation of the coefficients of the 1/D expansion of the Schrödinger equation. [The Journal of Chemical Physics 101, 5987 (1994)]. Martin Dunn, Timothy C. Germann, David Z. Goodson ...
We report on two-dimensional large-scale Molecular Dynamics simulations for Lennard-Jones solids,... more We report on two-dimensional large-scale Molecular Dynamics simulations for Lennard-Jones solids, with initially flat commensurate and incommensurate interfaces, as a function of sliding velocity for various compressions. The high velocity behavior is similar for both commensurations, exhibiting an inverse power law for the frictional force when the velocity exceeds approximately one tenth the sound speed. At lower velocities, the frictional behavior differs, with the incommensurate case exhibiting a frictional force linear in velocity at low velocities followed by a maximum related to a phase transformation with a phase front propagating normally to the original interface. The frictional force for the commensurate interface approaches a finite value at low velocities.
Physical Review a, Dec 1, 1998
The energy levels of diamagnetic hydrogen as a function of two independent parameters, magnetic f... more The energy levels of diamagnetic hydrogen as a function of two independent parameters, magnetic field strength B, and angular momentum m, are examined. Avoided crossings appear between these energy levels as either parameter is varied while the other is held fixed. These avoided crossings are directly related to degeneracies (Fermi resonances) occurring at zeroth order in perturbation theory. The mathematical basis of these degeneracies are the square-root branch points that connect the energy levels. It is found that the locations of avoided crossings in either constant-B or constant-m spectra can be predicted by visually scanning the locations of these branch points in the complex-δ plane, where δ=1/(2+2\|m\|) is the perturbation parameter used in this research.
We have developed a large-scale stochastic simulation model to investigate the spread of a pandem... more We have developed a large-scale stochastic simulation model to investigate the spread of a pandemic strain of influenza virus through the U.S. population of 281 million people, to assess the likely effectiveness of various potential intervention strategies including antiviral agents, vaccines, and modified social mobility (including school closure and travel restrictions) [1]. The heterogeneous population structure and mobility is based on available Census and Department of Transportation data where available. Our simulations demonstrate that, in a highly mobile population, restricting travel after an outbreak is detected is likely to delay slightly the time course of the outbreak without impacting the eventual number ill. For large basic reproductive numbers R0, we predict that multiple strategies in combination (involving both social and medical interventions) will be required to achieve a substantial reduction in illness rates. [1] T. C. Germann, K. Kadau, I. M. Longini, and C. A. Macken, Proc. Natl. Acad. Sci. (USA) 103, 5935-5940 (2006).
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Papers by Timothy Germann