Publications

Thompson, Aidan P.

Abstract not provided.

Physical Review B

Wood, M.A.; Cusentino, Mary A.; Wirth, B.D.; Thompson, Aidan P.

The central approximation made in classical molecular dynamics simulation of materials is the interatomic potential used to calculate the forces on the atoms. Great effort and ingenuity is required to construct viable functional forms and find accurate parametrizations for potentials using traditional approaches. Machine learning has emerged as an effective alternative approach to develop accurate and robust interatomic potentials. Starting with a very general model form, the potential is learned directly from a database of electronic structure calculations and therefore can be viewed as a multiscale link between quantum and classical atomistic simulations. Risk of inaccurate extrapolation exists outside the narrow range of time and length scales where the two methods can be directly compared. In this work, we use the spectral neighbor analysis potential (SNAP) and show how a fit can be produced with minimal interpolation errors which is also robust in extrapolating beyond training. To demonstrate the method, we have developed a tungsten-beryllium potential suitable for the full range of binary compositions. Subsequently, large-scale molecular dynamics simulations were performed of high energy Be atom implantation onto the (001) surface of solid tungsten. The machine learned W-Be potential generates a population of implantation structures consistent with quantum calculations of defect formation energies. A very shallow (<2nm) average Be implantation depth is predicted which may explain ITER diverter degradation in the presence of beryllium.

Brown, Judith A.; Kittell, David E.; Wood, Mitchell A.; Thompson, Aidan P.; Bolintineanu, Dan S.

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Brown, Judith A.; Kittell, David E.; Wood, Mitchell A.; Thompson, Aidan P.; Bolintineanu, Dan S.

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Wood, Mitchell A.; Cusentino, Mary A.; Thompson, Aidan P.

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Koski, Jason K.; Leung, Kevin L.; Thompson, Aidan P.; Lane, James M.

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Thompson, Aidan P.; Wood, Mitchell A.; Cusentino, Mary A.

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Thompson, Aidan P.

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Tranchida, Julien G.; Thompson, Aidan P.; Plimpton, Steven J.; Thibaudeau, Pascal T.

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Wood, Mitchell A.; Cusentino, Mary A.; Thompson, Aidan P.

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Kittell, David E.; Yarrington, Cole Y.; Bolintineanu, Dan S.; Lechman, Jeremy B.; Wood, Mitchell A.; Knepper, Robert; Olles, Joseph D.; Thompson, Aidan P.; Wixom, Ryan R.

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Molecular Physics

Mattox, Timothy I.; Larentzos, James P.; Moore, Stan G.; Stone, Christopher P.; Ibanez, Daniel A.; Thompson, Aidan P.; Lísal, Martin; Brennan, John K.; Plimpton, Steven J.

Simulating energetic materials with complex microstructure is a grand challenge, where until recently, an inherent gap in computational capabilities had existed in modelling grain-scale effects at the microscale. We have enabled a critical capability in modelling the multiscale nature of the energy release and propagation mechanisms in advanced energetic materials by implementing, in the widely used LAMMPS molecular dynamics (MD) package, several novel coarse-graining techniques that also treat chemical reactivity. Our innovative algorithmic developments rooted within the dissipative particle dynamics framework, along with performance optimisations and application of acceleration technologies, have enabled extensions in both the length and time scales far beyond those ever realised by atomistic reactive MD simulations. In this paper, we demonstrate these advances by modelling a shockwave propagating through a microstructured material and comparing performance with the state-of-the-art in atomistic reactive MD techniques. As a result of this work, unparalleled explorations in energetic materials research are now possible.

Wood, Mitchell A.; Cusentino, Mary A.; Thompson, Aidan P.

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Tranchida, Julien G.; Wood, Mitchell A.; Moore, Stan G.; Plimpton, Steven J.; Thompson, Aidan P.; Thibaudeau, Pascal T.

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Cochrane, Kyle C.; Desjarlais, Michael P.; Lane, James M.; Leung, Kevin L.; Thompson, Aidan P.; Wright, Alan F.

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Yarrington, Cole Y.; Kittell, David E.; Bolintineanu, Dan S.; Lechman, Jeremy B.; Wood, Mitchell A.; Knepper, Robert; Olles, Joseph D.; Thompson, Aidan P.; Wixom, Ryan R.

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Yarrington, Cole Y.; Kittell, David E.; Bolintineanu, Dan S.; Lechman, Jeremy B.; Wood, Mitchell A.; Knepper, Robert; Olles, Joseph D.; Thompson, Aidan P.; Wixom, Ryan R.

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Journal of Chemical Physics

Wood, Mitchell A.; Thompson, Aidan P.

The Spectral Neighbor Analysis Potential (SNAP) is a classical interatomic potential that expresses the energy of each atom as a linear function of selected bispectrum components of the neighbor atoms. An extension of the SNAP form is proposed that includes quadratic terms in the bispectrum components. The extension is shown to provide a large increase in accuracy relative to the linear form, while incurring only a modest increase in computational cost. The mathematical structure of the quadratic SNAP form is similar to the embedded atom method (EAM), with the SNAP bispectrum components serving as counterparts to the two-body density functions in EAM. The effectiveness of the new form is demonstrated using an extensive set of training data for tantalum structures. Similar to artificial neural network potentials, the quadratic SNAP form requires substantially more training data in order to prevent overfitting. The quality of this new potential form is measured through a robust cross-validation analysis.

Tranchida, Julien G.; Thompson, Aidan P.; Plimpton, Steven J.; Thibaudeau, Pascal T.

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Zimmerman, Jonathan A.; Song, Bo S.; Thompson, Aidan P.; Medlin, Douglas L.; San Marchi, Christopher W.

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Wood, Mitchell A.; Kittell, David E.; Yarrington, Cole Y.; Thompson, Aidan P.

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Tranchida, Julien G.; Thompson, Aidan P.; Plimpton, Steven J.; Pillars, Jamin R.; Monson, Todd M.; Langlois, Eric L.; Cangi, Attila C.; Thibaudeau, Pascal T.

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Thompson, Aidan P.

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Wood, Mitchell A.; Thompson, Aidan P.

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Physical Review B

Wood, M.A.; Kittell, D.E.; Yarrington, Cole Y.; Thompson, Aidan P.

Shock wave interactions with defects, such as pores, are known to play a key role in the chemical initiation of energetic materials. The shock response of hexanitrostilbene is studied through a combination of large-scale reactive molecular dynamics and mesoscale hydrodynamic simulations. In order to extend our simulation capability at the mesoscale to include weak shock conditions (<6 GPa), atomistic simulations of pore collapse are used to define a strain-rate-dependent strength model. Comparing these simulation methods allows us to impose physically reasonable constraints on the mesoscale model parameters. In doing so, we have been able to study shock waves interacting with pores as a function of this viscoplastic material response. We find that the pore collapse behavior of weak shocks is characteristically different than that of strong shocks.

Thompson, Aidan P.; Trott, Christian R.

Within the EXAALT project, the SNAP [1] approach is being used to develop high accuracy potentials for use in large-scale long-time molecular dynamics simulations of materials behavior. In particular, we have developed a new SNAP potential that is suitable for describing the interplay between helium atoms and vacancies in high-temperature tungsten[2]. This model is now being used to study plasma-surface interactions in nuclear fusion reactors for energy production. The high-accuracy of SNAP potentials comes at the price of increased computational cost per atom and increased computational complexity. The increased cost is mitigated by improvements in strong scaling that can be achieved using advanced algorithms [3].

Trott, Christian R.; Plimpton, Steven J.; Thompson, Aidan P.

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Moore, Stan G.; Thompson, Aidan P.; Wood, Mitchell W.

LAMMPS is a classical molecular dynamics code (lammps.sandia.gov) used to model materials science problems at Sandia National Laboratories and around the world. LAMMPS was one of three Sandia codes selected to participate in the Trinity KNL (TR2) Open Science period. During this period, three different problems of interest were investigated using LAMMPS. The first was benchmarking KNL performance using different force field models. The second was simulating void collapse in shocked HNS energetic material using an all-atom model. The third was simulating shock propagation through poly-crystalline RDX energetic material using a coarse-grain model, the results of which were used in an ACM Gordon Bell Prize submission. This report describes the results of these simulations, lessons learned, and some hardware issues found on Trinity KNL as part of this work.

Wood, Mitchell A.; Kittell, David E.; Yarrington, Cole Y.; Thompson, Aidan P.

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Tranchida, Julien G.; Thompson, Aidan P.; Plimpton, Steven J.

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Tranchida, Julien G.; Thompson, Aidan P.; Plimpton, Steven J.

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Wood, Mitchell A.; Kittell, David E.; Yarrington, Cole Y.; Thompson, Aidan P.

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Thompson, Aidan P.

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Kittell, David E.; Yarrington, Cole Y.; Wood, Mitchell A.; Thompson, Aidan P.

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Wood, Mitchell A.; Thompson, Aidan P.

The purpose of this short contribution is to report on the development of a Spectral Neighbor Analysis Potential (SNAP) for tungsten. We have focused on the characterization of elastic and defect properties of the pure material in order to support molecular dynamics simulations of plasma-facing materials in fusion reactors. A parallel genetic algorithm approach was used to efficiently search for fitting parameters optimized against a large number of objective functions. In addition, we have shown that this many-body tungsten potential can be used in conjunction with a simple helium pair potential1 to produce accurate defect formation energies for the W-He binary system.

Wood, Mitchell A.; Kittell, David E.; Yarrington, Cole Y.; Thompson, Aidan P.

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Thompson, Aidan P.; Wood, Mitchell A.

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Yarrington, Cole Y.; Thompson, Aidan P.; Kittell, David E.; Wood, Mitchell A.

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AIP Conference Proceedings

Matthew, J.; Lane, D.; Thompson, Aidan P.; Vogler, Tracy V.

We have recently shown that the final density of silicon under shock compression is anomalously enhanced by introducing voids in the initial uncompressed material. Using molecular simulation, we also demonstrated a molecular mechanism for the effect, which is seen in a growing class of other similar materials. We have shown that this mechanism involves a premature local phase transition nucleated by local shear strain. At higher shock loads we show here that this transition becomes frustrated producing amorphous silicon.We also observe local melting below the equilibrium melt line for bulk silicon. Large-scale non-equilibrium molecular dynamics (NEMD) and Hugoniostat simulations of shock compressed porous silicon are used to study the mechanism. Final stress states and strength were characterized versus initial porosity and for various porosity microstructures.

Wills, Ann E.; Thompson, Aidan P.; Raman, Sumathy R.

We establish an atomistic view of the high- and low-temperature phases of iron/steel as well as some elements of the phase transition between these phases on cooling. In particular we examine the 4 most common orientation relationships between the high temperature austen- ite and low-temperature ferrite phases seen in experiment. With a thorough understanding of these relationships we are prepared to set up various atomistic simulations, using tech- niques such as Density Functional Theory and Molecular Dynamics, to further study the phase transition, in particular, quantities needed for Phase Field Modeling, such as the free energies of bulk phases and the phase transition front propagation velocity.

Decolvenaere, Elizabeth D.; Thompson, Aidan P.; Stephens, John A.

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Thompson, Aidan P.; Plimpton, Steven J.

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Thompson, Aidan P.

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Physical Review B

Shan, Tzu R.; Wixom, Ryan R.; Thompson, Aidan P.

In both continuum hydrodynamics simulations and also multimillion atom reactive molecular dynamics simulations of shockwave propagation in single crystal pentaerythritol tetranitrate (PETN) containing a cylindrical void, we observed the formation of an initial radially symmetric hot spot. By extending the simulation time to the nanosecond scale, however, we observed the transformation of the small symmetric hot spot into a longitudinally asymmetric hot region extending over a much larger volume. Performing reactive molecular dynamics shock simulations using the reactive force field (ReaxFF) as implemented in the LAMMPS molecular dynamics package, we showed that the longitudinally asymmetric hot region was formed by coalescence of the primary radially symmetric hot spot with a secondary triangular hot zone. We showed that the triangular hot zone coincided with a double-shocked region where the primary planar shockwave was overtaken by a secondary cylindrical shockwave. The secondary cylindrical shockwave originated in void collapse after the primary planar shockwave had passed over the void. A similar phenomenon was observed in continuum hydrodynamics shock simulations using the CTH hydrodynamics package. The formation and growth of extended asymmetric hot regions on nanosecond timescales has important implications for shock initiation thresholds in energetic materials.

Thompson, Aidan P.; Shan, Tzu-Ray S.; Wixom, Ryan R.; Yarrington, Cole Y.

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Littlewood, David J.; Mitchell, John A.; Thompson, Aidan P.

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Trott, Christian R.; Shan, Tzu-Ray S.; Moore, Stan G.; Thompson, Aidan P.; Plimpton, Steven J.

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Trott, Christian R.; Thompson, Aidan P.; Plimpton, Steven J.; Moore, Stan G.; Shan, Tzu-Ray S.

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Shan, Tzu-Ray S.; Thompson, Aidan P.

Scientific impact: The project supports the investigation of energetic materials. This work is providing fundamental insight into initiation mechanisms in energetic materials.

Shan, Tzu-Ray S.; Thompson, Aidan P.

End of year summary including report on project milestones, project productivity, and next steps.