Bulletin of the American Physical Society
APS March Meeting 2021
Volume 66, Number 1
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session F23: Materials in Extremes: Advanced SimulationsFocus Live

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Sponsoring Units: GSCCM Chair: Aidan Thompson, Sandia National Laboratories 
Tuesday, March 16, 2021 11:30AM  11:42AM Live 
F23.00001: Melting line of dense hydrogen from hierarchical machine learning using diffusion Monte Carlo data Hongwei Niu, Yubo Yang, Scott Jensen, Markus Holzmann, CARLO PIERLEONI, David M Ceperley The phase transitions of dense hydrogen are of interest in astrophysics and in condensed matter physics. Ab initio simulations suffer from errors due to small system sizes, while empirical potentials lack the accuracy needed to describe these transitions. To minimize these errors, we use diffusion Monte Carlo (DMC) forces to train a hierarchical machinelearning potential and perform largescale twophase classical molecular dynamics and quantum path integral molecular dynamics simulations to estimate the melting line for dense hydrogen in the pressure range 50200 GPa. We estimate the effect on the melting temperature coming from the assumed density functional, from the machinelearning procedures, and from the nuclear zeropoint motion. 
Tuesday, March 16, 2021 11:42AM  11:54AM Live 
F23.00002: Quantum Monte Carlo determination of the principal Hugoniot of deuterium Michele Ruggeri, Markus Holzmann, David M Ceperley, CARLO PIERLEONI In this talk I will present recent Quantum Monte Carlo results on the determination of the principal Hugoniot of deuterium, obtained using a combination of a finite temperature, path integral method, Coupled Electron Ion Monte Carlo, with ground state techniques [1]. The importance of a careful determination of the properties of the reference state and the relevance of electronic thermal effects and nuclear quantum effects will also be discussed. Our Quantum Monte Carlo results are in agreement with experimental data from shock wave experiments [2] for temperatures up to 4000 K and pressures up to 20 GPa. At higher temperatures and pressures our simulations predict a more compressible Hugoniot than experimental measurements; the reason of this discrepancy is likely linked to the presence of strong electronic correlation as deuterium molecules dissociate. 
Tuesday, March 16, 2021 11:54AM  12:06PM Live 
F23.00003: Isotope effects in the phase diagram of hydrogen and deuterium Graeme Ackland, Hongxiang Zong, Heather Wiebe, Sebastiaan van der Bund

Tuesday, March 16, 2021 12:06PM  12:42PM Live 
F23.00004: Elucidating Long Timescale Chemical Events in Reactive Materials Invited Speaker: Nir Goldman Knowledge of the equation of state and chemical kinetics of materials under reactive conditions is needed for a wide number of research areas, including studies of planetary interiors, astrobiology, and highpressure detonations of energetic materials. In this regard, we have developed a family of atomistic simulation models which yield similar accuracy to higher order quantum approaches (KohnSham DFT) while yielding orders of magnitude increase in computational efficiency. This talk will focus on three different types of models in development in our research group: (1) semiempirical quantum simulation approaches, (2) reactive force fields for molecular dynamics simulations, and (3) spin lattice models for solid phase reactivity. These efforts will be discussed in the context of corrosion on actinide and other metal surfaces, shock compression of organics and energetic materials, and prebiotic synthesis in impacting astrophysical ices. Our methods provide a straightforward way to conduct computationally efficient and highly accurate simulations over a broad range of conditions, where physical and chemical properties can be difficult to interrogate directly and there is historically a significant reliance on theoretical approaches for interpretation and validation of experimental results. 
Tuesday, March 16, 2021 12:42PM  12:54PM Live 
F23.00005: Quantumaccurate SNAP Carbon Potential for MD Simulations of Carbon at Extreme Conditions Jonathan Willman, Ashley Williams, Kien Nguyen Cong, Anatoly B Belonoshko, Mitchell Wood, Aidan Thompson, Ivan Oleynik Highlyaccurate interatomic potentials are urgently sought for realistic MD simulations of high strainrate experiments of carbon materials response to high temperatures and pressures. With that goal in mind, we have developed a novel Spectral Neighbor Analysis Potential (SNAP) machinelearning potential to describe properties of carbon at extreme pressures (up to 5 TPa) and temperatures (up to 10,000 K). SNAP is formulated in terms of the bispectrum components, which play a role of descriptors that characterize the local neighborhood of each atom. Machine learning approaches are used to train the SNAP potential on a large dataset of firstprinciples training data. SNAP development includes (1) generation of the training database comprising the consistent and meaningful set of firstprinciples DFT calculations; (2) the robust and physically guided fit of the SNAP parameters; and (3) the validation of the SNAP potential in MD simulations of carbon at extreme conditions. SNAP potential is applied to investigate shock response of diamond at extreme conditions. 
Tuesday, March 16, 2021 12:54PM  1:06PM Live 
F23.00006: Time dependent boundary conditions for large scale atomistic simulations of shocked surface instabilities James Edward Hammerberg, Ramon Ravelo, Timothy C Germann Shock induced surface instabilities such as RichtmyerMeshkov instabilities at perturbed metal vacuum/gas interfaces result in metal material ejecta. For strong shock waves the material ejected is initially in the form of fluid sheets when the surface perturbation is twodimensional. These sheets ultimately break up to form a distribution of droplets. Largescale nonequilibrium molecular dynamics simulations of such instabilities allow the investigation of the dynamics of the breakup process but are limited in length and time scales by rarefaction wave reflections at the boundaries ultimately leading to spall that may affect the instability growth. A timedependent boundary condition based on the selfsimilar character of the release wave is presented that mitigates boundary reflections and reduces unwanted tensile waves behind the perturbed interface zone. This boundary condition can be used to increase the wavelength and time scale for breakup simulations. We discuss he details of this method and results of NEMD simulations of a shocked Cu interface with a single mode perturbation characterized by a wavelength of λ= 13 nm and a wavenumber amplitude product kh_{0} = 1 
Tuesday, March 16, 2021 1:06PM  1:18PM Live 
F23.00007: Manufacturing Machine Learned Interatomic Potentials for Shock Physics Ben Nebgen, Justin Steven Smith, Kipton Barros, Gowri Srinivasan The ability to reliably generate efficient interatomic potentials capable of accurately simulating novel materials under nonequilibrium conditions would be a major advancement for materials science. Machine learning has facilitated the generation of such potentials in a variety of individual studies with excellent agreement to both experiment and other theoretical methods. To generate a neural network potential, quantum calculations determine energies and forces on a training set of atomic configurations generated through active learning. This dataset is then used to train a Neural Network (NN) to predict an energy as a function of atomic coordinates for a given material. These NNs are useful for performing MD simulations and computing physical properties. This presentation will focus on the consistency and reproducibility of this methodology by generating potentials for a variety of materials utilizing the same protocol. Generated potentials will be validated through the computation of bulk properties and simulated phase diagrams. Finally, shock simulations were performed with these potentials to demonstrate their ability to perform under nonequilibrium conditions. 
Tuesday, March 16, 2021 1:18PM  1:30PM Live 
F23.00008: Reaction Profiles and Energy Surfaces of Compressed Species Under Extreme Conditions Jacob Spooner, Noham Weinberg Both experiment and first principles calculations unequivocally indicate that properties of elements and their compounds undergo a tremendous transformation at ultrahigh pressures exceeding 1 Mbar due to the fact that the difference between intra and intermolecular interactions disappears under such conditions. Yet, even at much milder pressures of 50300 kbar, when molecules still retain their individual identity, their chemical properties and reactivity change dramatically. Since kinetics and mechanisms of condensedphase reactions are described in terms of their potential energy (PES) or Gibbs energy (GES) surfaces, chemical effects of high pressure can be assessed through analysis of pressureinduced deformations of GES of solvated reaction systems. We use quantum mechanical and molecular dynamics simulations to construct GES and reaction profiles of compressed species, and analyze how topography of GES changes in response to compression. We also discuss the important role of volume profiles in assessing pressureinduced deformations and show that the highpressure GES are well described in terms of these volume profiles and the reference zeropressure GES. 
Tuesday, March 16, 2021 1:30PM  1:42PM Live 
F23.00009: Temperature Effects on Compaction and Strength During Shock Compression of Porous Silica Jason Koski, Keith Jones, Tracy John Vogler, J Matthew Lane We use molecular dynamics simulations to investigate the effect of initial temperature on the Hugoniot response of shock compressed porous amorphous silica. We find that initial temperatures ranging from 77 to 1000 K can have an unusually significant effect on the final Hugoniot states for systems just above the elastic limit at pressures between 1.5 and 6 GPa. Outside this narrow range, temperature plays a much smaller role in the compression behavior. We use the constantstress Hugoniostat methodology with the BKS interatomic potential for silicon dioxide. We will present data characterizing the effect on compaction and strength for a range of initial porosities. Multiple pore structures are also compared to investigate the role of microstructure. 
Tuesday, March 16, 2021 1:42PM  1:54PM Live 
F23.00010: Electronphonon interactions and reactivity in solids under high pressures and temperatures Anguang Hu The accurate nonperturbation simulations of electronphonon interactions from finite differences have been developed using allelectron quantum solidstate chemistry. The simulations demonstrated that all electronic degeneracies couple specific vibrational motion, establishing the correlation of reactivity between crystal and electronic structures in solids under high pressures and temperatures. The electronic degeneracies around the Fermi level of the band structure are electronic reaction centers, strongly coupled to the atom motion associated with the specific vibrational mode. When the relevant vibrational motion removes these degeneracies in the band structure, the specific reaction mode is to be selected and then becomes extremely reactive. It means that a reaction can take place when the specific vibrational mode becomes reactive due to vibrationally mediated electronelectron interactions, resulting in immediately coupling of mechanical work to thermal heat. Therefore, such a reaction is a nonequilibrium process initiated by the specific reaction mode. The simulations have been validated in good agreement with several highpressure experimental observations on chemical transformations in terms of both transition pressures and temperatures. 
Tuesday, March 16, 2021 1:54PM  2:06PM Live 
F23.00011: Investigating DislocationObstacle Interactions in Tungsten using a novel Parallel Replica Dynamics Method Nithin Mathew, Enrique Martinez Saez, Danny Perez Plasma Facing Materials (PFMs) in fusion reactors are subjected to extreme temperatures and high particle flux of H isotopes and He. Tungsten (W) is the main candidate for PFM in the International Thermonuclear Experimental Reactor but He irradiation of W results in creation of He_{n}V_{m} complexes and modification of surface microstructure. This leads to increased retention of H isotopes and degradation of thermomechanical stability. In this work, we study the interaction of these He_{n}V_{m} complexes with edge dislocations using accelerated molecular dynamics. We use a novel Parallel Replica Dynamics method where states and transitions are identified onthefly using a diffusion distance metric calculated from an approximation of the Koopman operator of the dynamics. Using up to 600 replicas, we are able to investigate the interactions between edge dislocations and He_{n}V_{m} complexes at temperatures ranging from 3001200 K and at rates that span ~4 orders of magnitude, reaching microsecond timescales. Effect of bubble pressure on the interaction with dislocations and bridging to higher lengthscale models will be discussed. 
Tuesday, March 16, 2021 2:06PM  2:18PM On Demand 
F23.00012: Phonon Anharmonicity In The Vibrational Entropy Of Transition Metals BIMAL K C, Celia Garcia Amparan, Ramon Ravelo

Tuesday, March 16, 2021 2:18PM  2:30PM On Demand 
F23.00013: Development of RFMEAM interatomic potentials for high temperature diborides Bikash Timalsina, Alin Niraula, Andrew Ian Duff, Gregory E Hilmas, William G. Fahrenholtz, Ridwan Sakidja Reference free (RF) MEAM interatomic potentials have been developed to investigate the thermodynamic stability and thermomechanical properties of ultrahigh temperature binary including ZrB2 and entropystabilized diborides by fitting the energy, forces, stress obtained from the DFT datasets including lattice deformations and hightemperature NVT abinitio ensembles. The fitting technique implements a conjugate gradient minimizer along with Genetic Algorithm (GA) using the MEAMfit code. A few critical materials properties of the diborides including the bulk modulus, elastic constants, cohesive and point defect energy were calculated and compared with the experimental and DFT results to verify the interatomic potentials. In addition, the vibrational characteristics of these compounds were calculated to elucidate their experimentally observed thermal expansion and thermal conductivity at ultrahigh temperatures. The support from CMMI Division of NSF (Award No. 1902069) is gratefully acknowledged. 
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