| 1. |
- Nguyen-Cong, K., et al.
(author)
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Billion atom molecular dynamics simulations of carbon at extreme conditions and experimental time and length scales
- 2021
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In: SC '21: Proceedings of the International Conference for High Performance Computing, Networking, Storage and Analysis. - New York, NY, USA : Association for Computing Machinery (ACM).
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Conference paper (peer-reviewed)abstract
- Billion atom molecular dynamics (MD) using quantum-Accurate machine-learning Spectral Neighbor Analysis Potential (SNAP) observed long-sought high pressure BC8 phase of carbon at extreme pressure (12 Mbar) and temperature (5,000 K). 24-hour, 4650 node production simulation on OLCF Summit demonstrated an unprecedented scaling and unmatched real-world performance of SNAP MD while sampling 1 nanosecond of physical time. Efficient implementation of SNAP force kernel in LAMMPS using the Kokkos CUDA backend on NVIDIA GPUs combined with excellent strong scaling (better than 97% parallel efficiency) enabled a peak computing rate of 50.0 PFLOPs (24.9% of theoretical peak) for a 20 billion atom MD simulation on the full Summit machine (27,900 GPUs). The peak MD performance of 6.21 Matom-steps/node-s is 22.9 times greater than a previous record for quantum-Accurate MD. Near perfect weak scaling of SNAP MD highlights its excellent potential to advance the frontier of quantum-Accurate MD to trillion atom simulations on upcoming exascale platforms.
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| 2. |
- Nikitin, Sergey V., et al.
(author)
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Activity of phenoxy-imine titanium catalysts in ethylene polymerization : A quantum chemical approach
- 2016
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In: Journal of Molecular Catalysis A: Chemical. - : Elsevier BV. - 1381-1169. ; 423, s. 285-292
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Journal article (peer-reviewed)abstract
- The mechanism of ethylene polymerization on phenoxy-imine (FI) titanium catalysts was studied theoretically to identify the major factors affecting the catalytic activity. Geometry optimizations of FI ligands, octahedral titanium dichloride complexes, active cationic species, and their π‐complexes with ethylene as well as calculations of the energy profile of chain propagation were performed at the BP86-D3 level. We found that the calculated energy gaps between frontier orbitals (HOMO and LUMO) in the active cations of the catalysts correlate with the experimental activity values. High activities of FI catalysts with α‐Cumyl groups were attributed to smaller HOMO-LUMO gaps due to hyperconjugation between π-systems of α‐Cumyl and (N‐aryl)salicylaldimine moieties in the active cations. The correlation provides a qualitative estimate of the catalytic activity for further design of new FI titanium complexes.
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| 3. |
- Nguyen-Cong, Kien, et al.
(author)
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Extreme Metastability of Diamond and its Transformation to the BC8 Post-Diamond Phase of Carbon
- 2024
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In: The Journal of Physical Chemistry Letters. - : American Chemical Society (ACS). - 1948-7185. ; 15:4, s. 1152-1160
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Journal article (peer-reviewed)abstract
- Diamond possesses exceptional physical properties due to its remarkably strong carbon-carbon bonding, leading to significant resilience to structural transformations at very high pressures and temperatures. Despite several experimental attempts, synthesis and recovery of the theoretically predicted post-diamond BC8 phase remains elusive. Through quantum-accurate multimillion atom molecular dynamics (MD) simulations, we have uncovered the extreme metastability of diamond at very high pressures, significantly exceeding its range of thermodynamic stability. We predict the post-diamond BC8 phase to be experimentally accessible only within a narrow high pressure-temperature region of the carbon phase diagram. The diamond to BC8 transformation proceeds through premelting followed by BC8 nucleation and growth in the metastable carbon liquid. We propose a double-shock compression pathway for BC8 synthesis, which is currently being explored in experiments at the National Ignition Facility.
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| 4. |
- Nguyen-Cong, Kien, et al.
(author)
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First Principles Molecular Dynamics Simulations of High-Pressure Melting of Diamond
- 2020
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In: Shock compression of condensed matter - 2019. - : AIP Publishing.
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Conference paper (peer-reviewed)abstract
- Although the high-pressure phase diagram of carbon at extreme temperatures and pressures is in focus of theoretical and experimental dynamic compression studies, there still exist outstanding problems including disagreement between theoretical predictions and experiments. Using first-principles molecular dynamics simulations at high temperatures and pressures and employing large unit cells, we construct an accurate phase diagram of carbon using two-phase and Z-methods. In accord with previous simulations, a large positive slope of the melting line is observed for pressures from 0 to 200 GPa, whereas at pressures above 500 GPa a very small negative slope exists, which is in contrast to most of previous simulations and experiment. Our accurate results demonstrate the necessity for future dynamic compression experiments to clarify behavior of carbon at extreme conditions including its melting line.
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| 5. |
- Willman, Jonathan T., et al.
(author)
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Machine learning interatomic potential for simulations of carbon at extreme conditions
- 2022
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In: Physical Review B. - : American Physical Society (APS). - 2469-9950 .- 2469-9969. ; 106:18
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Journal article (peer-reviewed)abstract
- A spectral neighbor analysis (SNAP) machine learning interatomic potential (MLIP) has been developed for simulations of carbon at extreme pressures (up to 5 TPa) and temperatures (up to 20 000 K). This was achieved using a large database of experimentally relevant quantum molecular dynamics (QMD) data, training the SNAP potential using a robust machine learning methodology, and performing extensive validation against QMD and experimental data. The resultant carbon MLIP demonstrates unprecedented accuracy and transferability in predicting the carbon phase diagram, melting curves of crystalline phases, and the shock Hugoniot, all within 3% of QMD. By achieving quantum accuracy and efficient implementation on leadership-class high-performance computing systems, SNAP advances frontiers of classical MD simulations by enabling atomic-scale insights at experimental time and length scales.
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| 6. |
- Willman, Jonathan T., et al.
(author)
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Quantum Accurate SNAP Carbon Potential for MD Shock Simulations
- 2020
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In: Shock compression of condensed matter - 2019. - : AIP Publishing.
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Conference paper (peer-reviewed)abstract
- We present a new quantum accurate Spectral Neighbor Analysis Potential (SNAP) machine-learning potential for simulating carbon under extreme conditions of dynamic compression (pressures up to 1 TPa and temperatures up to 10,000 K). The development of SNAP potential involves (1) the generation of the training database comprised of the consistent and meaningful set of first-principles DFT (Density Functional Theory) data for carbon materials at high pressure and temperature; (2) the robust and physically guided training of the SNAP parameters on first-principles data involving statistical data analysis; and (3) the validation of the SNAP potential in MD simulations of carbon at high PT conditions. The excellent performance of quadratic SNAP potential is demonstrated by simulating the radial distribution functions at high pressure-temperature conditions and melt curve of diamond, which were found in good agreement with DFT.
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