Synthesis of Single Crystals of ε-Iron and Direct Measurements of Its Elastic Constants.

Seismology finds that Earth's solid inner core behaves anisotropically. Interpretation of this requires a knowledge of crystalline elastic anisotropy of its constituents-the major phase being most likely ε-Fe, stable only under high pressure. Here, single crystals of this phase are synthesized, and its full elasticity tensor is measured between 15 and 33 GPa at 300 K. It is calculated under the same conditions, using the combination of density functional theory and dynamical mean field theory, which describes explicitly electronic correlation effects. The predictive power of this scheme is checked by comparison with measurements; it is then used to evaluate the crystalline anisotropy in ε-Fe under higher density. This anisotropy remains of the same amplitude up to densities typical of Earth's inner core.

ABINIT: Overview and focus on selected capabilities.

abinit is probably the first electronic-structure package to have been released under an open-source license about 20 years ago. It implements density functional theory, density-functional perturbation theory (DFPT), many-body perturbation theory (GW approximation and Bethe-Salpeter equation), and more specific or advanced formalisms, such as dynamical mean-field theory (DMFT) and the "temperature-dependent effective potential" approach for anharmonic effects. Relying on planewaves for the representation of wavefunctions, density, and other space-dependent quantities, with pseudopotentials or projector-augmented waves (PAWs), it is well suited for the study of periodic materials, although nanostructures and molecules can be treated with the supercell technique. The present article starts with a brief description of the project, a summary of the theories upon which abinit relies, and a list of the associated capabilities. It then focuses on selected capabilities that might not be present in the majority of electronic structure packages either among planewave codes or, in general, treatment of strongly correlated materials using DMFT; materials under finite electric fields; properties at nuclei (electric field gradient, Mössbauer shifts, and orbital magnetization); positron annihilation; Raman intensities and electro-optic effect; and DFPT calculations of response to strain perturbation (elastic constants and piezoelectricity), spatial dispersion (flexoelectricity), electronic mobility, temperature dependence of the gap, and spin-magnetic-field perturbation. The abinit DFPT implementation is very general, including systems with van der Waals interaction or with noncollinear magnetism. Community projects are also described: generation of pseudopotential and PAW datasets, high-throughput calculations (databases of phonon band structure, second-harmonic generation, and GW computations of bandgaps), and the library libpaw. abinit has strong links with many other software projects that are briefly mentioned.

A unified and efficient theory for the structural properties of actinides and phases of plutonium.

We show that a calculation using density functional theory (DFT) in the generalized gradient approximation (GGA) supplemented by an explicit Coulomb interaction term between correlated electrons (GGA+U), can accurately describe structural properties of (1) the room temperature phases of U, Np, Pu, Am and Cm, and (2) the α, ß, γ, δ and ϵ phases of plutonium, as does the combination of GGA with dynamical mean field theory (DMFT). It thus changes the view on the role of electronic interaction in these systems and opens the way to fast calculations of structural properties in actinides metallic system. We use ab initio values of effective Coulomb interactions and underline that Hund's exchange and spin-orbit coupling are of utmost importance in these calculations. Secondly, we show that phonons properties in δ plutonium are impacted by strong interactions. The GGA+DMFT results exhibits a lattice instability for the transverse (1 1 1) phonon mode. Moreover the amplitude of this lattice instability is consistent with the experimental temperature of stability of this phase. Our calculation thus shows that when the δ phase is thermodynamically unstable (at 0 K), it is also dynamically unstable.