The presence of the pseudo-Jahn-Teller (PJT) effect has been investigated in the heavier analogues of graphene, namely silicene, germanene and stanene, by applying the orbital vibronic coupling density theory. In order to do so, we have made a visa -vis analogy with their respective planar, honeycomb molecular cluster models, namely hexasilabenzene (Si 6 H 6), hexagermabenzene (Ge 6 H 6) and hexastannabenzene (Sn 6 H 6). One-to-one mapping of the occupied crystal orbitals and unoccupied crystal orbitals in two-dimensional (2D) Si, Ge and Sn systems to the occupied molecular orbitals and unoccupied molecular orbitals of the corresponding molecular units are used to identify PJT-active bands and compute the crystal orbital vibronic coupling density (c-OVCD) and crystal orbital vibronic coupling constants (c-OVCCs). c-OVCD and c-OVCC show the local picture of the PJT coupling in these 2D systems. This article exemplifies the fruitfulness of deciphering the structural aspects in materials based on orbitals of their corresponding simple molecular units-a reductionist quantum chemical approach to materials.

The physical chemistry of iron at the inner-core conditions is key to understanding the evolution and habitability of Earth and super-Earth planets. Based on full first-principles simulations, we report cooperative diffusion along the longitudinally fast ⟨111⟩ directions of body-centered cubic (bcc) iron in temperature ranges of up to 2000-4000 K below melting and pressures of ∼300-4000 GPa. The diffusion is due to the low energy barrier in the corresponding direction and is accompanied by mechanical and dynamical stability, as well as strong elastic anisotropy of bcc iron. These findings provide a possible explanation for seismological signatures of the Earth’s inner core, particularly the positive correlation between P wave velocity and attenuation. The diffusion can also change the detailed mechanism of core convection by increasing the diffusivity and electrical conductivity and lowering the viscosity. The findings need to be considered in future geophysical and planetary models and should motivate future studies of materials under extreme conditions.

A wide-range (0 to 1044.0 g/cm 3 and 0 to 10 9 K) equation-of-state (EOS) table for a CH 1.72 O 0.37 N 0.086 quaternary compound has been constructed based on density-functional theory (DFT) molecular-dynamics (MD) calculations using a combination of Kohn-Sham DFT MD, orbital-free DFT MD, and numerical extrapolation. The rst-principles EOS data are compared with predictions of simple models, including the fully ionized ideal gas and the Fermi-degenerate electron gas models, to chart their temperature-density conditions of applicability. The shock Hugoniot, thermodynamic properties, and bulk sound velocities are predicted based on the EOS table and compared to those of C-H compounds. The Hugoniot results show the maximum compression ratio of the C-H-O-N resin is larger than that of CH polystyrene due to the existence of oxygen and nitrogen; while the other properties are similar between CHON and CH. Radiation hydrodynamic simulations have been performed using the table for inertial connement fusion targets with a CHON ablator and compared with a similar design with CH. The simulations show CHON outperforms CH as the ablator for laser-direct-drive target designs.

The physical chemistry of iron at inner-core conditions is key to understanding the evolution and habitability of Earth-like planets. Based on full first-principles simulations, we report cooperative diffusion along the longitudinally fast〈111〉directions of body-centred cubic (bcc) iron in temperature ranges of 2000-4000 K below the melting curve and pressures up to 3 TPa. The diffusion is due to low energy barriers in the corresponding direction and accompanied by mechanical and dynamic stability of bcc iron. With the assistance of the diffusion, bcc iron demonstrates strong elastic anisotropy and variations with temperature and pressure, which is consistent with several seismological signatures of the Earth’s inner core, particularly the positive correlation between P wave velocity and attenuation. This finding also has implications for nucleation and growth of the inner core and suggests the exotic state and properties of iron need to be considered in future geophysical and planetary models.