Bulletin of the American Physical Society
APS March Meeting 2023
Volume 68, Number 3
Las Vegas, Nevada (March 5-10)
Virtual (March 20-22); Time Zone: Pacific Time
Session AAA08: V: Matter under Extreme Conditions |
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Sponsoring Units: DCOMP Chair: Pravinkumar Ghodake, Indian Institute of Technology Bombay Room: Virtual Room 8 |
Wednesday, March 22, 2023 12:30PM - 12:42PM |
AAA08.00001: Planck Gravity and Its Relation to Relativity Matt Dray 2.5549..x10^{59} in m^{3}/JS^{4} and m/KgS^{2} is a value derived from 2πc^{3}/h = c^{3}/h-bar = G/l_{p}^{2} and it is a gravitational constant which should be known as Planck Gravity. This constant not only shows that gravity can be described as a rate of flow, but it can be used to normalize the derived Planck units in a way which expands out relativity. |
Wednesday, March 22, 2023 12:42PM - 12:54PM |
AAA08.00002: Building a database of thermoelastic properties using ab initio workflows Qi Zhang, Renata M Wentzcovitch Materials computations of thermoelastic properties, especially of the ab initio kind, are intrinsically complex. These difficulties have inspired us to develop a workflow framework, express, to automate long and extensive sequences of ab initio calculations. Various materials properties can be computed in express, e.g., static equation of state, phonon density of states, thermal equation of state, and other thermodynamic properties. It helps users in the preparation of inputs, execution of simulations, and analysis of data. It also tracks the operations and steps that users perform and thus can restore interrupted or failed jobs. |
Wednesday, March 22, 2023 12:54PM - 1:06PM |
AAA08.00003: pgm: A Python package for free energy calculation Hongjin Wang, Jingyi Zhuang, Zhen Zhang, Renata M Wentzcovitch The quasiharmonic approximation (QHA) is a powerful method for computing the free energy and thermodynamic properties of materials at high pressures (P) and temperatures (T). However, anharmonicity, electronic excitations in metals, or both, introduce an intrinsic T-dependence on the phonon frequencies, making the QHA inadequate. Here we present a Python package, pgm, for free energy and thermodynamic property calculations. It is based on the concept of phonon quasiparticles and the phonon gas model (PGM). The free energy is obtained by integrating the entropy, which can be readily calculated for a system of phonon quasiparticles. This method is useful for computing the free energy in anharmonic insulators and harmonic or anharmonic metals. The current implementation offers properties in a continuum P,T range. The necessary inputs are ab initio T_{el}-dependent static energies and T-dependent phonon quasiparticle frequencies at several discrete volumes and the user-specified P,T range. We employ techniques like just-in-time (JIT) compiling and parallel computing to accelerate the numerical computation. We demonstrate successful applications of pgm to hcp-iron (ε-Fe) at extreme conditions [1] and cubic CaSiO3-perovskite [2], a strongly anharmonic system. Anharmonic free energies computed using thermodynamic integration (TI) agree with the results produced by this approach using an equivalent q-point mesh [3]. Therefore, a dense q-point mesh should produce anharmonic free energies equivalent to those obtained on TI simulations on 10^{4} atoms and beyond. |
Wednesday, March 22, 2023 1:06PM - 1:18PM |
AAA08.00004: Plane-wave-based stochastic-deterministic density functional theory for extended systems Qianrui Liu, Mohan Chen Traditional finite-temperature Kohn-Sham density functional theory (KSDFT) is one of the most popular quantum-mechanics-based methods in modeling materials since it balances the accuracy and efficiency well. However, traditional KSDFT based on the diagonalization method (DG) needs to solve all occupied bands, which increases fast as the temperature rises according to the Fermi-Dirac function. Thus, it is inefficient and also takes up a lot of memory at high temperatures, which limits its further applications. The evaluation of the ground-state density in KSDFT can be replaced by the Chebyshev trace (CT) method. Recently, stochastic density functional theory [Phys. Rev. Lett. 111, 106402 (2013)] (SDFT) and its improved theory, mixed stochastic-deterministic density functional theory [Phys. Rev. Lett. 125, 055002 (2020)] (MDFT) are developed based on the CT method and stochastic orbitals, which makes it possible to simulate high-temperature systems more efficiently. We have implemented the four methods based on the plane-wave basis set within the first-principles package ABACUS. In addition, all methods are adapted to the norm-conserving pseudopotentials and periodic boundary conditions with the use of k-point sampling in the Brillouin zone. By testing the Si and C systems from the DG method as benchmarks, we systematically evaluate the accuracy and efficiency of the SDFT, and MDFT methods by examining a series of physical properties, which include the electron density, free energy, atomic forces, stress, and density of states. We conclude that they not only reproduce the DG results with a sufficient accuracy but also exhibit several advantages over the DG method. We expect these methods can be of great help in studying high-temperature systems such as warm dense matter and dense plasmas. |
Wednesday, March 22, 2023 1:18PM - 1:30PM |
AAA08.00005: Ab initio melting temperatures of bcc and hcp iron under the Earth's inner core condition Yang Sun, Mikhail I Mendelev, Feng Zhang, Cai-Zhuang Wang, Renata M Wentzcovitch, Kai-Ming Ho There has been a long debate on the stable phase of iron under the Earth's inner core conditions. Due to the solid-liquid coexistence at the inner core boundary, the thermodynamic stability of solid phases directly relates to their melting temperatures, which remains considerable uncertainty. In the present study, we utilized a semi-empirical potential fitted to high-temperature ab initio data to perform a thermodynamic integration from classical systems described by this potential to ab initio systems. This method provides a smooth path for the thermodynamic integration and significantly reduces the uncertainty of determining the melting temperatures caused by the finite size effect down to 15 K. Our results suggest the hcp phase is the ground state of pure iron under the inner core conditions, while the free energy difference between the hcp and bcc phases is tiny, on the order of 10s meV/atom. It suggests the effect of nickel and light elements must be taken into account to get a complete picture of the crystalline structure of the solid inner core phase. |
Wednesday, March 22, 2023 1:30PM - 1:42PM |
AAA08.00006: The epsilon phase of solid oxygen: The importance of the magnetic order Le The Anh, Hiroshi Fukui, Toshiaki Iitaka, Osamu Sugino The epsilon phase of solid oxygen is a unique molecular crystal with C2/m space group symmetry being composed of the (O_{2})_{4} clusters. It has been a challenge for decades to elucidate the anomalous pressure dependence of the lattice parameters at 10-20 GPa. In previous calculations [1, 2], the antiferromagnetic structure with 8-atom primitive unit cell of the C2/m has been assumed as the most stable structure (called AFM2). We show that the problems are remained due to the possible mismatch between the C2/m space group and the magnetic orders. To treat this problem, we use an extended conventional unit cell (16 atom/cell) instead of 8-atom primitive cell with spin-polarized SCAN-rVV10 functionals. We found that the anti-ferromagnetic structure with 16-atom unit cell (called AFM1), is more stable than the AFM2 in pressure range from 10 GPa to 20 GPa. In the AFM1 order, there are four oxygen molecules in the computational cell where the constituent O atoms have the same value for the magnetic moment, while there four oxygen molecules having non-equivalent magnetic moments. The former ones are arranged making the (anti-)ferromagnetic order in the b- (a-)direction, while the latter ones are arranged making the (anti-)ferromagnetic order in the a- (b-)direction. This AFM1 arrangement is inconsistent with the magnetic space group corresponding to the C2/m space group. This result also suggests that there is breaking of the time-reversal symmetry, prompting future experimental investigation. The observed lattice parameters [3] and neutron powder diffraction patterns [4, 5] are compared with the calculations considering of the AFM1 structure. |
Wednesday, March 22, 2023 1:42PM - 1:54PM |
AAA08.00007: Deep machine-learning potential for atomistic simulation of δ-AlOOH at high pressures and temperatures Chenxing Luo, Yang Sun, Renata M Wentzcovitch δ-AlOOH (δ) is a critical high-pressure hydrous phase for understanding the Earth’s geological water cycle. Our earlier ab initio study [1] offered a practical multi-configuration model for understanding the pressure-dependent behavior of the H-bond, including H-bond disorder, tunneling, and symmetrization. Since H-bond behavior in δ is likely typical of H-bonds in other hydrous or nominally anhydrous phases, it would be helpful to develop a more complete understanding of its behavior beyond the symmetrization transition pressures. However, further simulations on δ require large-scale simulations that are prohibitive to ab initio calculations. |
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