Coupled electron-ion Monte Carlo simulation of hydrogen molecular crystals
[3];
[4]
- Sapienza Univ. of Rome (Italy). Dept. of Physics
- Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States). Physics Division
- Univ. of Illinois, Urbana-Champaign, IL (United States). Dept. of Physics
- Univ. of L’Aquila, L’Aquila (Italy). Dept. of Physical and Chemical Sciences; Commissariat a l'Energie Atomique et aux Energies Alternatives (CEA-Saclay), Gif-sur-Yvette (France); Univ. Paris-Sud, Gif-sur-Yvette (France); Univ. Paris-Saclay, Gif-sur-Yvette (France). Maison de la Simulation
We performed simulations for solid molecular hydrogen at high pressures (250 GPa ≤ P ≤ 500 GPa) along two isotherms at T = 200 K (phase III) and at T = 414 K (phase IV). At T = 200 K, we considered likely candidates for phase III, the C2c and Cmca12 structures, while at T = 414 K in phase IV, we studied the Pc48 structure. We employed both Coupled Electron-Ion Monte Carlo (CEIMC) and Path Integral Molecular Dynamics (PIMD). The latter is based on Density Functional Theory (DFT) with the van der Waals approximation (vdW-DF). The comparison between the two methods allows us to address the question of the accuracy of the exchange-correlation approximation of DFT for thermal and quantum protons without recurring to perturbation theories. In general, we find that atomic and molecular fluctuations in PIMD are larger than in CEIMC which suggests that the potential energy surface from vdW-DF is less structured than the one from quantum Monte Carlo. We find qualitatively different behaviors for systems prepared in the C2c structure for increasing pressure. Within PIMD, the C2c structure is dynamically partially stable for P ≤ 250 GPa only: it retains the symmetry of the molecular centers but not the molecular orientation; at intermediate pressures, it develops layered structures like Pbcn or Ibam and transforms to the metallic Cmca-4 structure at P ≥ 450 GPa. Instead, within CEIMC, the C2c structure is found to be dynamically stable at least up to 450 GPa; at increasing pressure, the molecular bond length increases and the nuclear correlation decreases. For the other two structures, the two methods are in qualitative agreement although quantitative differences remain. Finally, we discuss various structural properties and the electrical conductivity. Here, we find that these structures become conducting around 350 GPa but the metallic Drude-like behavior is reached only at around 500 GPa, consistent with recent experimental claims.
- Research Organization:
- Lawrence Livermore National Laboratory (LLNL), Livermore, CA (United States)
- Sponsoring Organization:
- USDOE National Nuclear Security Administration (NNSA)
- Grant/Contract Number:
- AC52-07NA27344; NA0001789
- OSTI ID:
- 1512594
- Alternate ID(s):
- OSTI ID: 1405552
- Report Number(s):
- LLNL-JRNL-773208; 965437
- Journal Information:
- Journal of Chemical Physics, Vol. 148, Issue 10; ISSN 0021-9606
- Publisher:
- American Institute of Physics (AIP)Copyright Statement
- Country of Publication:
- United States
- Language:
- English
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