Ab initio modeling of oxygen impurity atom incorporation into uranium mononitride surface and subsurface vacancies

The incorporation of oxygen atoms has been simulated into either nitrogen or uranium vacancy at the UN(001) surface, sub-surface or central layers. For calculations on the corresponding slab models both the relativistic pseudopotentials and the method of projector augmented-waves (PAW) as implemented in the VASP computer code have been used. The energies of O atom incorporation and solution within the defective UN surface have been calculated and discussed. For different configurations of oxygen ions at vacancies within the UN(001) slab, the calculated density of states and electronic charge re-distribution was analyzed. Considerable energetic preference of O atom incorporation into the N-vacancy as compared to U-vacancy indicates that the observed oxidation of UN is determined mainly by the interaction of oxygen atoms with the surface and sub-surface N vacancies resulting in their capture by the vacancies and formation of O-U bonds with the nearest uranium atoms. Keywords: Density functional calculations, uranium mononitride, surface, defects, N and U vacancie

Reliable First-Principles Alloy Thermodynamics via Truncated Cluster Expansions

In alloys cluster expansions (CE) are increasingly used to combine first-principles electronic-structure and Monte Carlo methods to predict thermodynamic properties. As a basis-set expansion in terms of lattice geometrical clusters and effective cluster interactions, the CE is exact if infinite, but is tractable only if truncated. Yet until now a truncation procedure was not well-defined and did not guarantee a reliable truncated CE. We present an optimal truncation procedure for CE basis sets that provides reliable thermodynamics. We then exemplify its importance in Ni$_3$V, where the CE has failed unpredictably, and now show agreement to a range of measured values, predict new low-energy structures, and explain the cause of previous failures.Comment: 4 pages, 2 figure

Room temperature dynamic correlation between methylammonium molecules in lead-iodine based perovskites: An ab-initio molecular dynamics perspective

The high efficiency of lead organo-metal-halide perovskite solar cells has raised many questions about the role of the methylammonium (MA) molecules in the Pb-I framework. Experiments indicate that the MA molecules are able to 'freely' spin around at room temperature even though they carry an intrinsic dipole moment. We have performed large supercell (2592 atoms) finite temperature ab-initio molecular dynamics calculations to study the correlation between the molecules in the framework. An underlying long range anti-ferroelectric ordering of the molecular dipoles is observed. The dynamical correlation between neighboring molecules shows a maximum around room temperature in the mid-temperature phase. In this phase, the rotations are slow enough to (partially) couple to neighbors via the Pb-I cage. This results in a collective motion of neighboring molecules in which the cage acts as the mediator. At lower and higher temperatures the motions are less correlated

Comparison of the full-potential and frozen-core approximation approaches to density-functional calculations of surfaces

We scrutinize the accuracy of the pseudopotential approximation in density-functional theory (DFT) calculations of surfaces by systematically comparing to results obtained within a full-potential setup. As model system we choose the CO oxidation at a RuO2(110) surface and focus in particular on the adsorbate binding energies and reaction barriers as target quantities for the comparison. Rather surprisingly, the major reason for discrepancy does not result from the neglected semi-core state relaxation in the frozen-core approximation, but from an inadequate description of the local part of the Ru pseudopotential, responsible for the scattering of f like waves. Tiny, seemingly irrelevant, imprecisions appearing in these properties can have a noticeable influence on the surface energetics. At least for the present example, we obtain excellent agreement between both approaches, if the pseudopotential describes these scattering properties accurately.Comment: 8 pages including 3 figures; related publications can be found at http://www.fhi-berlin.mpg.de/th/th.htm

Relativistic GWGW+BSE study of the optical properties of Ruddlesden-Popper iridates

We study the optical properties of the Ruddlesden-Popper series of iridates Sr$_{n+1}$Ir$_n$O$_{3n+1}$ ($n$=1, 2 and $\infty$) by solving the Bethe-Salpeter equation (BSE), where the quasiparticle (QP) energies and screened interactions $W$ are obtained by the $GW$ approximation including spin-orbit coupling. The computed optical conductivity spectra show strong excitonic effects and reproduce very well the experimentally observed double-peak structure, in particular for the spin-orbital Mott insulators Sr$_2$IrO$_4$ and Sr$_3$Ir$_2$O$_7$. However, $GW$ does not account well for the correlated metallic state of SrIrO$_3$ owing to a much too small band renormalization, and this affects the overall quality of the optical conductivity. Our analysis describes well the progressive redshift of the main optical peaks as a function of dimensionality ($n$), which is correlated with the gradual decrease of the electronic correlation (quantified by the constrained random phase approximation) towards the metallic $n=\infty$ limit. We have also assessed the quality of a computationally cheaper BSE approach that is based on a model dielectric function and conducted on top of DFT+$U$ one-electron energies. Unfortunately, this model BSE approach does not accurately reproduce the outcome of the full $GW$+BSE method and leads to larger deviations to the measured spectra.Comment: 13 pages, 8 figure

Stability of gold nanowires at large Au-Au separations

The unusual structural stability of gold nanowires at large separations of gold atoms is explained from first-principles quantum mechanical calculations. We show that undetected light atoms, in particular hydrogen, stabilize the experimentally observed structures, which would be unstable in pure gold wires. The enhanced cohesion is due to the partial charge transfer from gold to the light atoms. This finding should resolve a long-standing controversy between theoretical predictions and experimental observations.Comment: 7 pages, 3 figure

The structural analysis of Cu(111)-Te (√3 × √3) R30° and (2√3 × 2√3)R30° surface phases by quantitative LEED and DFT,

The chemisorption of tellurium on atomically clean Cu(111) surface has been studied under ultra-high vacuum conditions. At room temperature, the initial stage of growth was an ordered 23×23R30° phase (0.08 ML). An ordered 3×3R30° phase is formed at 0.33 ML coverage of Te. The adsorption sites of the Te atoms on the Cu(111) surface at 0.08 ML and 0.33 ML coverages are explored by quantitative low energy electron diffraction (LEED) and density functional theory (DFT). Our results indicate that substitutional surface alloy formation starts at very low coverages

Quasi-chemical study of Be2+^{2+}(aq) speciation

Be$^{2+}$(aq) hydrolysis can to lead to the formation of multi-beryllium clusters, but the thermodynamics of this process has not been resolved theoretically. We study the hydration state of an isolated Be$^{2+}$ ion using both the quasi-chemical theory of solutions and ab initio molecular dynamics. These studies confirm that Be$^{2+}$(aq) is tetra-hydrated. The quasi-chemical approach is then applied to then the deprotonation of Be(H_2O)_4^{2+}} to give BeOH(H_2O)_3{}^{+}}. The calculated pK$_a$ of 3.8 is in good agreement with the experimentally suggested value around 3.5. The calculated energetics for the formation of BeOHBe$^{3+}$ are then obtained in fair agreement with experiments.Comment: 11 pages, 3 figure

Calculation of a Deuterium Double Shock Hugoniot from Ab initio Simulations

We calculate the equation of state of dense deuterium with two ab initio simulations techniques, path integral Monte Carlo and density functional theory molecular dynamics, in the density range of 0.67 < rho < 1.60 g/cc. We derive the double shock Hugoniot and compare with the recent laser-driven double shock wave experiments by Mostovych et al. [1]. We find excellent agreement between the two types of microscopic simulations but a significant discrepancy with the laser-driven shock measurements.Comment: accept for publication in Phys. Rev. Lett., Nov. 2001, 4 pages, 4 figure

First principles simulations of direct coexistence of solid and liquid aluminium

First principles calculations based on density functional theory, with generalised gradient corrections and ultrasoft pseudopotentials, have been used to simulate solid and liquid aluminium in direct coexistence at zero pressure. Simulations have been carried out on systems containing up to 1000 atoms for 15 ps. The points on the melting curve extracted from these simulations are in very good agreement with previous calculations, which employed the same electronic structure method but used an approach based on the explicit calculation of free energies [L. Vo\v{c}adlo and D. Alf\`e, Phys. Rev. B, {\bf 65}, 214105 (2002).]Comment: To appear in Phys. Rev.