Projector Augmented Wave Research Articles

Impurity Combination Effect on Oxygen Absorption in α2-Ti3Al

The effect of substitutional impurities of the transition metals of VB–VIIB groups on the oxygen absorption in the doped α2-Ti3Al alloy was studied by the projector-augmented wave method within the density functional theory. It is established that all considered impurities prefer to substitute for a Ti atom rather than an Al atom. Changes in the absorption energy due to impurities being in the first neighbors of the oxygen atom were estimated. It was demonstrated that the doping resulted in a decrease in the oxygen absorption energy, which is mainly caused by the chemical contribution to it. The interaction energy between impurity atoms was calculated in the dependence on the interatomic distance. It was shown that the configuration with the impurity atoms being in the second neighbors of each other was stable in comparison with other possible configurations. The influence of two impurity atoms being in the first neighbors of oxygen is additively enhanced. It was revealed that the effect of two impurity atoms on the oxygen absorption energy can be estimated as the sum of the effects of separate impurities with an accuracy of more than ~90%.

The structural, electronic, magnetic and optical properties of 4d transition metal adatoms (Tc, Ru, Rh and Cd) adsorbed on monolayer blue phosphorene

The structural, electronic, magnetic and optical properties of 4d transition metal adatoms (Tc, Ru, Rh and Cd) adsorbed on blue phosphorene (blue P) monolayer have been systematically investigated by using the first-principles projector augmented wave (PAW) potential within the generalized gradient approximation (GGA) as well as taking into account the on-site Coulomb repulsive interactions (GGA + U). The pristine blue P monolayer is a nonmagnetic semiconductor, but adatom-doping with these 4d transition metals is an effective way to modify its electronic, magnetic and optical properties and thus expands its applications in spintronics, magnetoelectronics and optoelectronics. Except for the close-filled Cd adatom is unadsorbed, the Tc and Ru adatom-adsorbed blue P monolayers change to bipolar magnetic semiconductor, while the Rh adatom-adsorbed blue P monolayer turns into magnetic spin-gapless semiconductor. The binding energy and optical absorption results confirm that, the Tc, Ru and Rh adatom-adsorbed blue P monolayers are not only fabricable experimentally but also have enhanced absorption for visible and infrared lights.

Cubic spline solver for generalized density functional treatments of atoms and generation of atomic datasets for use with exchange-correlation functionals including meta-GGA

Within generalized density functional theory, recent developments of functionals which depend upon the kinetic energy density, such as the r2SCAN form, show significant improvement in the computational representation of real material properties. In order to implement these forms within plane-wave codes, it is necessary to use pseudopotentials, ideally constructed with the same exchange-correlation functional as has been implemented for the projector augmented-wave (PAW) formalism in the atompaw code. This was accomplished with the help of an efficient solver for the self-consistent radial bound states of each atom, based on cubic spline interpolation. Details of the formalism are presented, and preliminary results for several simple materials are encouraging.

First principles calculations to investigate magnetic tetranuclear ferrous complexes

Spin-crossover has been intensively studied during the last past years due to the need to improve our understanding of the phenomenon and in the hope to exploit switching for data storage at the molecular level. Here we employ periodic spin polarized density functional theory in conjunction with the projector augmented wave method within the generalized gradient approximation, including a Hubbard interaction U at the iron site and a van der Waals correction, to investigate the structural, electronic and magnetic properties of the tetranuclear [Fe4(C23N6H15)4](BF4)4 molecular crystals. Two geometrical structures of the considered system are evidenced that correspond to two spin state configurations: HS-LS-HS-HS and HS-HS-HS-HS. We found that the two magnetic configurations seen experimentally can indeed by reproduced by our calculations, and that the magnetic centers are coupled ferromagnetically. These findings enable promising electronic and spintronic perspectives.

X-ray Absorption Near-Edge Structure (XANES) at the O K-Edge of Bulk Co3O4: Experimental and Theoretical Studies.

We combine theoretical and experimental X-ray absorption near-edge spectroscopy (XANES) to probe the local environment around cationic sites of bulk spinel cobalt tetraoxide (CoO). Specifically, we analyse the oxygen K-edge spectrum. We find an excellent agreement between our calculated spectra based on the density functional theory and the projector augmented wave method, previous calculations as well as with the experiment. The oxygen K-edge spectrum shows a strong pre-edge peak which can be ascribed to dipole transitions from O to O states hybridized with the unoccupied states of cobalt atoms. Also, since CoO contains two types of Co atoms, i.e., Co and Co, we find that contribution of Co ions to the pre-edge peak is solely due to single spin-polarized orbitals (, , and ) while that of Co ions is due to spin-up and spin-down polarized orbitals ( and ). Furthermore, we deduce the magnetic moments on the Co and Co to be zero and 3.00 respectively. This is consistent with an earlier experimental study which found that the magnetic structure of CoO consists of antiferromagnetically ordered Co spins, each of which is surrounded by four nearest neighbours of oppositely directed spins.

First-principles calculations of high-pressure physical properties anisotropy for magnesite

The first-principles calculations based on density functional theory with projector-augmented wave are used to study the anisotropy of elastic modulus, mechanical hardness, minimum thermal conductivity, acoustic velocity and thermal expansion of magnesite (MgCO3) under deep mantle pressure. The calculation results of the phase transition pressure, equation of state, elastic constants, elastic moduli, elastic wave velocities and thermal expansion coefficient are consistent with those determined experimentally. The research results show that the elastic moduli have strong anisotropy, the mechanical hardness gradually softens with increasing pressure, the conduction velocity of heat in the [100] direction is faster than that in the [001] direction, the plane wave velocity anisotropy first increases and then gradually decreases with increasing pressure, and the shear wave velocity anisotropy increases with the increase of pressure, the thermal expansion in the [100] direction is greater than that in the [001] direction. The research results are of great significance to people’s understanding of the high-pressure physical properties of carbonates in the deep mantle.

An Adaptive Planewave Method for Electronic Structure Calculations

We propose an adaptive planewave method for eigenvalue problems in electronic structure calculations. The method combines a priori convergence rates and accurate a posteriori error estimates into an effective way of updating the energy cut-off for planewave discretizations, for both linear and nonlinear eigenvalue problems. The method is error controllable for linear eigenvalue problems in the sense that for a given required accuracy, an energy cut-off for which the solution matches the target accuracy can be reached efficiently. Further, the method is particularly promising for nonlinear eigenvalue problems in electronic structure calculations as it shall reduce the cost of early iterations in self-consistent algorithms. We present some numerical experiments for both linear and nonlinear eigenvalue problems. In particular, we provide electronic structure calculations for some insulator and metallic systems simulated with Kohn--Sham density functional theory (DFT) and the projector augmented wave (PAW) method, illustrating the efficiency and potential of the algorithm.

Hirshfeld atom refinement based on projector augmented wave densities with periodic boundary conditions.

Hirshfeld atom refinement (HAR) is an X-ray diffraction refinement method that, in numerous publications, has been shown to give H-atom bond lengths in close agreement with neutron diffraction derived values. Presented here is a first evaluation of an approach using densities derived from projector augmented wave (PAW) densities with three-dimensional periodic boundary conditions for HAR. The results show an improvement over refinements that neglect the crystal environment or treat it classically, while being on a par with non-periodic approximations for treating the solid-state environment quantum mechanically. A suite of functionals were evaluated for this purpose, showing that the SCAN and revSCAN functionals are most suited to these types of calculation.

First-principles study of absolute XPS binding energy with PAW planewave pseudopotential method: application to tungsten disulfides

We propose an efficient theoretical method to take into account the core-hole spin density in the projector augmented wave method, combined with spin un-polarized pseudopotentials and the planewave basis set. We apply this method to the calculation of absolute core level X-ray photoelectron spectroscopy binding energies of WS2 and its related materials, and find the following points. First, inclusion of core-hole spin in the core-exited state is essential for accurate description of the binding energies, especially for light elements. Second, the calculated absolute binding energies show excellent agreement with experimental results. Finally, when oxygen atoms are incorporated in the basal plane of WS2 in the metallic phase, the O 1s binding energy is expected to appear at lower energy than the corresponding value in the semiconducting phase.

Gpaw-tools – higher-level user interaction scripts for GPAW calculations and interatomic potential based structure optimization

GPAW is a density-functional theory (DFT) code based on the projector-augmented wave (PAW) method written mainly in Python. It has high integration with atomic simulation environment (ASE) software and high interoperability with the Python language. However, this high interoperability requires Python language knowledge and therefore limits the user numbers of GPAW worldwide. For an end-user, who is not interested in the Python language itself and interested in pure material science, gpaw-tools scripts are free cross-platform Python scripts that aim to be a user interface (UI) and a Tcl/Tk based graphical UI (GUI) software. With gpaw-tools, the end-user can calculate, view, and save density of states (DOS), band structure, charge densities, and optical properties of the investigated structure without the need for Python language knowledge. The gpaw-tools scripts are written in Python language and distributed with an MIT license.

Ab initio studies of the electronic structure induced by the CO and N2 adsorptions on graphene and on graphite slab

In this paper, we present an Ab initio study on the electronic properties of graphene and graphite slabs, using band structure and density of states calculations. The calculations are based on the use of GPAW (Grid-based Projector-Augmented Wave) and ASE (Atomistic Simulation Environment) methods. Our study also focuses on the adsorption mechanism of CO and N2 molecules, deposited on adsorbents such as graphene and graphite slabs. The main results of our work are the optimization of the energies of CO (optimal energy equals −14.67 eV) and N2 (optimal energy equals −15.80 eV) molecules, as well as the calculation of the optimal adsorption distance of these molecules on the adsorbents namely d(C0-graphene) = 4.69 Å, d(N2-graphene) = 6.03 Å, d(C0-graphite) = 4.69 Å and d(N2-graphite) = 5.36 Å. The calculated changes in adsorption energies and density of state of the adsorbent could be exploited as an application for gas sensors. Simulated surface/gas tunneling microscopy (STM) observations are also reported. They allow to highlight the change of electronic structure of the surface when the gas is absorbed. All results contribute to the optimization of the potential realization of a gas sensor. Our paper provides interesting information for the theoretical and experimental communities working on this research area. It contributes to the debate on the adsorption of CO and N2 molecules on graphene and on a graphite slab.

Surface energies and relaxation of NiCoCr and NiFeX (X = Cu, Co or Cr) equiatomic multiprincipal element alloys from first principles calculations

First principles calculations of the energies and relaxation of unreconstructed low-index surfaces, i.e. (001), (011) and (111) surfaces, in NiCoCr and NiFeX (X = Cu, Co or Cr) equiatomic multi-principal element alloys (MPEAs) are presented. The calculations were conducted for 12-layer slabs represented by special quasi-random supercells using the projector augmented wave method within the generalized gradient approximation. While experimental predictions are unavailable for comparison, the calculated surface energies agree fairly well with those from thermodynamic modeling and a bond-cutting model. In addition, the calculations unveil an important surface structure, namely, that the topmost surface layer is in contraction except for the (001) surface of NiFeCr alloy, the next layer below is in extension, and the bulk spacing is gradually recovered from the subsequent layers down. Additionally, the surface contraction is the most pronounced on the (011) plane, being about 4%–10% relative to the bulk spacings. The results presented here can provide an understanding of surface-controlled phenomena such as corrosion, catalytic activities and fracture properties in these equiatomic MPEAs.

Structure, mechanical and phonon stability of the Th-Sn system from ab initio

The first-principles projector augmented-wave (PAW) method within the framework of density functional theory (DFT) is applied to study the equilibrium lattice parameters, mechanical stability, tensile and shear properties, lattice dynamical stability and vibration modes of the Th-Sn system. The optimized equilibrium lattice parameters and equilibrium cell volume reproduce perfectly the available experimental data. The individual elastic constants and polycrystalline aggregates of Pm-3m-ThSn3, Cmcm-ThSn2, P63/mcm-Th5Sn3, and P63/mcm-Th5Sn4 have been obtained, the results show that four Th-Sn systems are mechanically stable according to the elastic stability criteria, also, the calculated elastic constants have allowed us to evaluate their elastic anisotropy. Additionally, the Density Functional Perturbation Theory (DFPT) is used to determine the lattice dynamical stability and phonon modes of the series of the Th-Sn compounds, by the vibrational frequencies, it is predicted that four Th-Sn systems are also dynamically stable. Pm-3m-ThSn3 is mechanical and dynamical stable up to 35GPa. The microscopic symmetry properties for optical modes at the center of the Brillouin zone of ThSn3, ThSn2, Th5Sn3, and Th5Sn4 have also been estimated by using group theory.

First-principles estimation of the superconducting transition temperature of a metallic hydrogen liquid

We present a full density-functional theory-based implementation of the stochastic path-integral approach proposed by Liu et al. [H. Liu, Y. Yuan, D. Liu, X.-Z. Li, and J. Shi, Phys. Rev. Research 2, 013340 (2020)] for estimating the superconducting transition temperature (${T}_{c}$) of a liquid. The implementation is based on the all-electron projector augmented-wave (PAW) method. We generalize Liu et al.'s formalism to accommodate the pseudodescription of electron states in the PAW method. A formula for constructing the overlap operator of the PAW method is proposed to eliminate errors due to the incompleteness of a pseudobasis set. We apply the implementation to estimate ${T}_{c}$'s of metallic hydrogen liquids. It confirms Liu et al.'s prediction that metallic hydrogen could form a superconducting liquid.

Incorporating Sulfur Atoms into Palladium Catalysts by Reactive Metal–Support Interaction for Selective Hydrogenation

Incorporating Sulfur Atoms into Palladium Catalysts by Reactive Metal–Support Interaction for Selective Hydrogenation

Efficient Electronic Modulation of g-C 3 N 4 Photocatalyst by Implanting Atomically Dispersed Ag 1 -N 3 for Extremely High Hydrogen Evolution Rates

Efficient Electronic Modulation of g-C ₃ N ₄ Photocatalyst by Implanting Atomically Dispersed Ag ₁ -N ₃ for Extremely High Hydrogen Evolution Rates

Effect of the projector augmented wave potentials on the simulation of thermodynamic properties of vanadium

We report significant differences in high-pressure properties of vanadium at zero temperature and finite temperature when different projector augmented wave (PAW) potentials are used in simulations based on density functional theory. When a PAW potential with only five electrons taken as valence electrons is used, the cold pressures in the high-pressure region are seriously underestimated, and an abnormality occurs in the melting curve of vanadium at about 400 GPa. We show that the reason for these discrepancies lies in the differences in the descriptions of the interatomic force, electron dispersion, and anisotropy of electron bonding obtained from different PAW potentials at high pressure, which lead to striking differences in the mechanical stability of the system. We propose a procedure for selecting PAW potentials suitable for simulations at high temperature and high pressure. Our results provide valuable guidance for future simulations of thermodynamic properties under extreme conditions.

Boosting the Rate Performance of Li–S Batteries via Highly Dispersed Cobalt Nanoparticles Embedded into Nitrogen-Doped Hierarchical Porous Carbon

Boosting the Rate Performance of Li–S Batteries via Highly Dispersed Cobalt Nanoparticles Embedded into Nitrogen-Doped Hierarchical Porous Carbon

Refined Structures of O-Phospho-l-serine and Its Calcium Salt by New Multinuclear Solid-State NMR Crystallography Methods.

O-phospho-l-serine (Pser) and its Ca salt, Ca[O-phospho-l-serine]·H2O (CaPser), play important roles for bone mineralization and were recently also proposed to account for the markedly improved bone-adhesive properties of Pser-doped calcium phosphate-based cements for biomedical implants. However, the hitherto few proposed structural models of Pser and CaPser were obtained by X-ray diffraction, thereby leaving the proton positions poorly defined. Herein, we refine the Pser and CaPser structures by density functional theory (DFT) calculations and contrast them with direct interatomic-distance constraints from two-dimensional (2D) nuclear magnetic resonance (NMR) correlation experimentation at fast magic-angle spinning (MAS), encompassing double-quantum–single-quantum (2Q–1Q) 1H NMR along with heteronuclear 13C{1H} and 31P{1H} correlation NMR experiments. The Pser and CaPser structures before and after refinements by DFT were validated against sets of NMR-derived effective 1H–1H, 1H–31P, and 1H–13C distances, which confirmed the improved accuracy of the refined structures. Each distance set was derived from one sole 2D NMR experiment applied to a powder without isotopic enrichment. The distances were extracted without invoking numerical spin-dynamics simulations or approximate phenomenological models. We highlight the advantages and limitations of the new distance-extraction procedure. Isotropic 1H, 13C, and 31P chemical shifts obtained by DFT calculations using the gauge including projector augmented wave (GIPAW) method agreed very well with the experimental results. We discuss the isotropic and anisotropic 13C and 31P chemical-shift parameters in relation to the previous literature, where most data on CaPser are reported herein for the first time.

Signature of linear-in- k Dresselhaus splitting in the spin relaxation of X -valley electrons in indirect band gap AlGaAs

GaAs/AlGaAs quantum well (QW) system is utilized to investigate the electron spin relaxation in the satellite X-valley of indirect band gap Al0.63Ga0.37As epitaxial layers through polarization resolved photo-luminescence excitation spectroscopy. Solving the rate equations, steady state electronic distribution in various valleys of AlxGa1-xAs is estimated against continues photo carrier generation and an expression for the degree of circular polarization (DCP) of photoluminescence coming from the adjacent quantum well (QW) is derived. Amalgamating the experimental results with analytical expressions, the X-valley electron spin relaxation time ({\tau}_S^X) is determined to be 2.7 +/- 0.1 ps at 10 K. To crosscheck its validity, theoretical calculations are performed based on Density Functional Theory within the framework of the projector augmented wave method, which support the experimental result quite well. Further, temperature dependence of {\tau}_S^X is studied over 10-80 K range, which is explained by considering the intra-valley scattering of carriers in the X-valley of indirect band gap AlGaAs barrier layer. It is learnt that the strain induced modification of band structure lifts the degeneracy in X-valley, which dominates the electron spin relaxation beyond 50 K. Furthermore, the DCP spectra of hot electrons in indirect band gap AlGaAs layers is found to be significantly different compared to that of direct bandgap AlGaAs. It is understood as a consequence of linear k dependent Dresselhaus spin splitting and faster energy relaxation procedure in the X-valley. Findings of this work could provide a new horizon for the realization of next generation spin-photonic devices which are less sensitive to Joule heating.