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.

Structural, electronic and energetic properties of uranium-americium mixed oxides [Formula: see text] using DFT[Formula: see text] calculations.

In this paper, we determine for the first time the electronic, structural and energetic properties of [Formula: see text] mixed oxides in the entire range of Am content using the generalized gradient approximation (GGA)[Formula: see text] in combination with the special quasirandom structure (SQS) approach to reproduce chemical disorder. This study reveals that in [Formula: see text] oxides, Am cations act as electron acceptors, whereas U cations act as electron donors showing a fundamental difference with [Formula: see text] or [Formula: see text] in which there is no cation valence change in stoichiometric conditions compared to the pure oxides. We show for the first time that the lattice parameter of stoichiometric [Formula: see text] follows a linear evolution which is the structural signature of an ideal solid solution behavior. Finally, using two approaches (SQS and parametric), we show by assessing the enthalpy of mixing that there is no phase separation in the whole range of Am concentration.

First-principles calculations of momentum distributions of annihilating electron-positron pairs in defects in UO2.

We performed first-principles calculations of the momentum distributions of annihilating electron-positron pairs in vacancies in uranium dioxide. Full atomic relaxation effects (due to both electronic and positronic forces) were taken into account and self-consistent two-component density functional theory schemes were used. We present one-dimensional momentum distributions (Doppler-broadened annihilation radiation line shapes) along with line-shape parameters S and W. We studied the effect of the charge state of the defect on the Doppler spectra. The effect of krypton incorporation in the vacancy was also considered and it was shown that it should be possible to observe the fission gas incorporation in defects in UO2 using positron annihilation spectroscopy. We suggest that the Doppler broadening measurements can be especially useful for studying impurities and dopants in UO2 and of mixed actinide oxides.

DFT + U investigation of charged point defects and clusters in UO2.

We present a physically justified formalism for the calculation of point defects and cluster formation energies in UO2. The accessible ranges of chemical potentials of the two components U and O are calculated using the U-O experimental phase diagram and a constraint on the formation energies of vacancies. We then apply this formalism to the DFT + U investigation of the point defects and cluster defects in this material (including charged ones). The most stable charge states obtained for these defects near stoichiometry are consistent with a strongly ionic system. Calculations predict similarly low formation energies for V(U)(4)(-) and I(O)(2)(-) in hyperstoichiometric UO2. In stoichiometric UO2, V(O)(2)(+) and I(o)(@)(-) have the same formation energy in the middle of the gap and in hypostoichiometric UO2, V[Formula: see text] is the most stable defect.