Plane-wave Density Functional Calculations Research Articles

The elastic properties of disordered Fe–Ni alloys under high pressures from first-principles calculations

The elastic constants and the elastic anisotropy in hexagonal close-packed Fe–Ni alloys have been investigated at high pressures by plane-wave density functional calculations within the generalized gradient approximation and the Monte Carlo special quasi-random structure method. The comparisons with previous calculations and experiments are shown and discussed. We find that adding Ni to Fe has a certain influence on the structure and elasticity of hexagonal close-packed Fe–Ni alloys generally. In particular, the elastic constant C44 is seriously affected by the increasing Ni content. Furthermore, the elastic moduli and wave velocities yielded from disordered structures are slightly greater than those from the ordered structures in previous calculations at high pressures. Interestingly, the elastic anisotropies of the compressional wave are close to 1.1 and there are small changes with increasing pressure and Ni content, which indicates that the anisotropies are little affected by pressures and Ni content within disordered structure.

Halo-substituted azobenzenes adsorbed at Ag(111) and Au(111) interfaces: Structures and optical properties

The adsorption of azobenzene (AB), ortho fluoro-azobenzene (FAB) and ortho chlor-azobenzol (ClAB), in both the cis and trans isomers, at the Au(111) and Ag(111) surfaces is investigated using plane-wave density functional calculations with the revPBE-vdW-DF functional. The resulting adsorption energies and internal structures of AB adsorbed to both metal surfaces are in broad agreement with available experimental data. In the gas phase, FAB and ClAB feature a significant reduction in the energy difference between the two isomeric states, compared with AB. This relative reduction in the energy difference is still significant for the adsorbed form of FAB but is only weakly apparent for ClAB. The absorption spectra of the molecules have also been calculated, with the halogen substituents generating significant changes in the gas phase, but only a modest difference for the adsorbed molecules.

Sculpting the band gap: a computational approach.

Materials with optimized band gap are needed in many specialized applications. In this work, we demonstrate that Hellmann-Feynman forces associated with the gap states can be used to find atomic coordinates that yield desired electronic density of states. Using tight-binding models, we show that this approach may be used to arrive at electronically designed models of amorphous silicon and carbon. We provide a simple recipe to include a priori electronic information in the formation of computer models of materials, and prove that this information may have profound structural consequences. The models are validated with plane-wave density functional calculations.

Insights from arsenate adsorption on rutile (110): grazing-incidence X-ray absorption fine structure spectroscopy and DFT+U study.

Insights into the bonding of As(V) at the metal oxide/aqueous interface can further our understanding of its fate and transport in the environment. The motivation of this work is to explore the interfacial configuration of As(V) on single crystal rutile (110) using grazing-incidence X-ray absorption fine structure spectroscopy (GI-XAFS) and planewave density functional calculations with on-site repulsion (DFT+U). In contrast to the commonly considered corner-sharing bidentate binuclear structure, tetrahedral As(V) binds as an edge/corner-sharing tridentate binuclear complex on rutile (110), as evidenced by observation of three As-Ti distances at 2.83, 3.36, and 4.05 Å. In agreement with the GI-XAFS analysis, our DFT+U calculations for this configuration resulted in the lowest adsorption energy among five possible alternatives. In addition, the electron density difference further demonstrated the transfer of charge between surface Ti atoms and O atoms in AsO4. This charge transfer consequently induced the formation of a chemical bond, which is also confirmed by the partial density of states analysis. Our results may shed new light on coupling the GI-XAFS and DFT approaches to explore molecular-scale adsorption mechanisms on single crystal surfaces.

Effect of nitrogen doping of graphene oxide on hydrogen and hydroxyl adsorption

We investigate how nitrogen-doping affects the hydrogen (H) and the hydroxyl (OH) adsorption on graphene oxide (GO) and on nitrogen-doped GO (NGO) via pseudopotential plane wave density functional calculations within the local spin density approximation. We find that the nitrogendoping brings about drastic changes in the hydrogen and the hydroxyl adsorption energetics, but its effects depend sensitively on the nitrogen configuration in NGO. The H and the OH adsorption energies are comparable only for pyrrolic NGO. In GO and quarternary NGO, the H adsorption energy is greater than the OH adsorption energy while the trend is reversed in pyridinic NGO. Also, the OH adsorption process is less affected by nitrogen-doping than the H adsorption is.

Study of the electronic and the structural properties of iron monosilicide via plane wave density functional theory calculations

We studied the electronic and the structural properties of iron monosilicide in low-pressure ɛ-phase structure and in the high-pressure CsCl structure via pseudopotential plane wave density functional calculations within the local spin density approximation (LSDA) and the generalized gradient approximation (GGA). The relaxation of the internal parameters of ɛ-FeSi opens up the band gap in the LSDA while it only increases the magnitude of the band gap in the GGA The structural transition from the ɛ-phase to the CsCl-phase is estimated to occur at about 19 GPa in GGA, in good agreement with the synthesis of the CsCl-phase of FeSi at 24 GPa.

The effect of nickel on the properties of iron at the conditions of Earth's inner core: Ab initio calculations of seismic wave velocities of Fe–Ni alloys

We have performed athermal periodic plane-wave density functional calculations within the generalised gradient approximation on the bcc, fcc and hcp structures of Fe1−XNiX alloys (X=0, 0.0625, 0.125, 0.25, and 1) in order to obtain their relative stability and elastic properties at 360GPa and 0K. For the hcp structure, using ab initio molecular dynamics, we have also calculated the elastic properties and wave velocities for X=0, 0.0625, and 0.125, at 360GPa and 5500K, with further calculations for X=0, and 0.125 at 360GPa and 2000K. At 0K, the hcp structure is the most stable for X=0, 0.0625, 0.125, and 0.25, with the fcc structure becoming the most stable above X∼0.45; the bcc structure is not the most stable phase for any composition. At 0K, compressional and shear wave velocities are structure dependent; in the case of fcc the velocities are very similar to pure Fe, but for the hcp structure the addition of Ni strongly reduces VS. Ni also reduced velocities in fcc iron, but to a lesser extent. However, at 5500K and 360GPa, Ni has little effect on the wave velocities of the hcp structure, which remain similar to those of pure iron throughout the range of compositions studied and, in the case of VS, >30% greater than that from seismological models. The effect of temperature on Fe–Ni alloys is, therefore, very significant, indicating that conclusions based on the extrapolation of results obtained at much lower temperatures must be treated with great caution. The significance of temperature is confirmed by the additional simulation at 2000K for X=0, and 0.125 which reveals a remarkably linear temperature dependence of the change in VS relative to that of pure iron. At 0K, the maximum anisotropy in VP is found to be only very weakly dependent on nickel content, but dependent on structure, being ∼15% for fcc and ∼8% for hcp. For the hcp structure at 2000 and 5500K, the maximum anisotropy in VP is also ∼8% and almost independent of the Ni content. We conclude that Ni can safely be ignored when considering its effect on the seismic properties of hcp-Fe under core pressures and temperatures and that the negligible effect of nickel on the physical properties of iron in the core arises not because of the chemical similarities between iron and nickel, but because of the high temperature of the system.

Enhancing dissociative chemisorption of H 2 O on Cu(111) via vibrational excitation

The dissociative chemisorption of water is an important step in many heterogeneous catalytic processes. Here, the mode selectivity of this process was examined quantum mechanically on a realistic potential energy surface determined by fitting planewave density functional calculations spanning a large configuration space. The quantum dynamics of the surface reaction were characterized by a six-dimensional model including all important internal coordinates of H(2)O and its distance to the surface. It was found that excitations in all three vibrational modes are capable of enhancing reactivity more effectively than increasing translational energy, consistent with the "late" transition state in the reaction path.

A theoretical study for thorium monocarbide (ThC)

The structural, mechanical, electronic and thermodynamic properties of thorium monocarbide (ThC) with NaCl-type structure have been investigated by using first-principles plane wave density functional calculations with GGA, LDA and LDA+U functionals. It is shown that calculated equilibrium structural parameters of ThC are in agreement with the experimental results. It is seen from calculated single-crystal elastic constants that ThC with NaCl-type structure is mechanically stable. And from calculated density of states and band structure, it is observed that ThC is metallic. After the properties at 0GPa are clarified, pressure dependency of the structural parameters, the elastic properties and related mechanical properties, density of states (DOS) and hardness are studied. Furthermore, the thermodynamic properties of ThC are obtained from the quasi-harmonic Debye model (QHM) over high pressure and temperature ranges for three functionals. The results are compared to each other, and the available experimental and theoretical data.

Structural, electronic and elastic properties of wurtzite-structured TlxAl1−xN alloys from first principles

Structural, electronic and elastic properties of wurtzite-structured thallium aluminum nitride (TlxAl1−xN) across the entire alloy composition range have been studied using first-principles pesudopotential plane-wave density functional calculations. The alloy's lattice constants obey Vegard's law well. Band-gap bowing coefficients show very strong composition dependence. Elastic constants and their aggregate elastic modulus change continuously with Tl-concentrations varying from 0 up to 100%. A transition from brittle fracture to plastic flow occurs in TlxAl1−xN alloys at about x=0.33. Further, elastic wave velocity and Debye temperature all monotonically decrease with increasing Tl concentration. Our calculated results for the binary cases, aluminum nitride (AlN) and thallium nitride (TlN), are in good agreement with previously reported data.

Phonon and Elastic Instabilities in Zincblende TlN under Hydrostatic Pressure from First Principles Calculations

The lattice dynamic and elastic instabilities of zincblende (ZB) thallium nitride (TlN) under hydrostatic pressure are extensively studied to reveal the physically driven mechanism of phase transition from the ZB to a rocksalt structure using pseudopotential plane-wave density functional calculations within the local density approximation. Our calculated results shows that both transverse acoustic phonon mode softening behavior and elastic instability are responsible for the pressure-induced structural phase transition in ZB TlN.

A model bismuth oxide intergranular thin film in a ZnO twist grain boundary

The electronic properties of a model bismuth oxide intergranular film in ZnO wereinvestigated using density functional plane wave calculations. It was found that oxygenexcess plays a fundamental role in the appearance of electrical activity. The introduction byoxygen interstitials or zinc vacancies results in depletion of the charge in deep gap statesintroduced by the bismuth impurities. This makes the boundary less metallic and promotesthe formation of acceptor states localized to the boundary core, resulting in Schottkybarrier enhancement. The results indicate that the origin of electrical activity in thinintergranular bismuth oxide films is probably not distinct from that in decorated ZnOboundaries.

Computational challenges of large-scale, long-time, first-principles molecular dynamics

Plane wave density functional calculations have traditionally been able to use the largest available supercomputing resources. We analyze the scalability of modern projector-augmented wave implementations to identify the challenges in performing molecular dynamics calculations of large systems containing many thousands of electrons. Benchmark calculations on the Cray XT4 demonstrate that global linear-algebra operations are the primary reason for limited parallel scalability. Plane-wave related operations can be made sufficiently scalable. Improving parallel linear-algebra performance is an essential step to reaching longer timescales in future large-scale molecular dynamics calculations.

Density Functional Calculations of Pd Nanoparticles Using a Plane-Wave Method

We deal with usage of plane-wave density functional calculations of crystallites formed of 100-200 transition metal atoms to mimic larger experimentally treated particles. A series of model Pd clusters containing up to 225 atoms is chosen as an example. We focused on the description of size-dependent geometric parameters and binding energies of these clusters as compared with previous benchmark calculations; evolution of the particle electronic structure with increasing size has also been addressed. The high performance of the plane-wave calculations for transition-metal nanoparticles has been documented. Implications of this work on broadening opportunities to design and study realistic models of catalytic systems are outlined.

Density functional studies of coinage metal nanoparticles: scalability of their properties to bulk

Density functional plane-wave calculations have been carried out for series of Cun,Agn and Aun particles containing up to 146 (Cu, Ag) and 225 (Au) atoms. Full geometry optimization has been performed for all particles starting from the structures created by cuts from the bulk. In line with previous studies, calculated average nearest-neighbour distances and cohesive energies of the particles linearly depend on such size-derived parameters as the average coordination number of metal atoms and the inverse of the mean particle radius, respectively. Rather accurate linear extrapolation of the observables under scrutiny to the bulk values has been achieved. However, we show that the scalability for particles made of various elements of the same d10s1 electron configuration differs, e.g. for bond lengths in Aun species it is noticeably less perfect than that for Cun and Agn ones. Implications of encountered structural peculiarities of the nanoparticles for their reactivity are outlined.

Density functional studies of model cerium oxide nanoparticles

Density functional plane-wave calculations have been performed to investigate a series of ceria nanoparticles (CeO2-x)(n), n <or= 85. Strong correlation effects of the Ce f-electron introduced upon Ce4+ --> Ce3+ reduction have been accounted for through the use of an effective on-site Coulomb repulsive interaction within the so-called DFT+U approach. Twelve nanoparticles of up to 2 nm in diameter and of both cuboctahedral and octahedral forms are chosen as representative model systems. Energetic and structural effects of oxygen vacancy formation in these nanoparticles are discussed with respect to those in the bulk and on extended surfaces. We show that the average interatomic distances of the nanoparticles are most significantly affected by the creation of oxygen vacancies. The formation energies of non-stoichiometric nanoparticles (CeO2-x)(n) are found to scale linearly with the average coordination number of Ce atoms; where x < 0 species, containing partially reduced O atoms, are less stable. The stability of octahedral ceria particles at small sizes, and the predicted strong propensity of Ce cations to acquire a reduced state at lower coordinated sites, is supported by interatomic potential-based global optimisations probing the low energy isomers of the Ce19O32 nanoparticle.

Interface electronic structure, two-dimensional metallicity, and possible interface superconductivity inCuCl∕Sisuperlattices

Results of highly precise all-electron full-potential linearized augmented plane wave density functional calculations demonstrate (i) two-dimensional metallicity, and (ii) possible superconductivity in $\mathrm{Cu}\mathrm{Cl}∕\mathrm{Si}$ superlattices for [111] and [001] directions. The feature of two-dimensional (2D) metallicity arises from the charge transfer at the interfaces between CuCl and Si, where there is the valence charge imbalance. The 2D features are well evidenced by band structure, Fermi surfaces, and charge densities. Furthermore, to investigate possible superconductivity, we use McMillan [Phys. Rev. 167, 331 (1968)] formula to estimate ${T}_{C}$, and evaluate the electron-phonon coupling constant, $\ensuremath{\lambda}$, using the rigid muffin-tin approximation. From our calculations, it is the interfaces that contribute to all intriguing physics: 2D metallicity, charge transfer, nonvanishing Hopfield [Phys. Rev. 186, 443 (1969)] parameter, and electron-phonon coupling. Moreover, our ${T}_{C}$ estimations $(0.03--4.40\phantom{\rule{0.3em}{0ex}}\mathrm{K})$ reveal that the electron-phonon coupling is too weak to account for the high ${T}_{C}$ $(60--150\phantom{\rule{0.3em}{0ex}}\mathrm{K})$ suggested in early experiments.

Density functional calculations of the properties of silicon-substituted hydroxyapatite

Ab initio density functional plane-wave calculations are performed on silicon-substituted hydroxyapatite (SiHA). Formation energies are obtained for the substitution of a phosphorus atom by a silicon atom in each of the six phosphate groups of the unit cell in turn. It is found that the co-removal of a hydroxyl group to maintain charge neutrality is energetically favourable and the calculated unit cell volumes for the single silicon substitutions agree extremely well with experimental observation. The substitution of a second silicon atom in the unit cell is found to be almost as energetically favourable as the first (and on one site more favourable) and there can be an attractive interaction between the two Si substituents when they are closely separated. However, experimental observation suggests that for this concentration of silicon a phase transformation to a different structure occurs which, because of the imposed boundary conditions, could not be accessed in the calculations. The density of states of the SiHA indicates that new states are introduced deep into the valence band and the band gap decreases by 1.6 eV compared to phase-pure HA. No new states are introduced into the band gap indicating that the Si incorporation does not make the material inherently electrically active. Furthermore a population analysis shows that the Si impurity has only a small effect on the neighbouring ionic charge.

Implementation of linear‐scaling plane wave density functional theory on parallel computers

AbstractWe describe the algorithms we have developed for linear‐scaling plane wave density functional calculations on parallel computers as implemented in the onetep program. We outline how onetep achieves plane wave accuracy with a computational cost which increases only linearly with the number of atoms by optimising directly the single‐particle density matrix expressed in a psinc basis set. We describe in detail the novel algorithms we have developed for computing with the psinc basis set the quantities needed in the evaluation and optimisation of the total energy within our approach. For our parallel computations we use the general Message Passing Interface (MPI) library of subroutines to exchange data between processors. Accordingly, we have developed efficient schemes for distributing data and computational load to processors in a balanced manner. We describe these schemes in detail and in relation to our algorithms for computations with a psinc basis. Results of tests on different materials show that onetep is an efficient parallel code that should be able to take advantage of a wide range of parallel computer architectures. (© 2006 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Electronic Structure Study of the High‐Pressure Vibrational Spectrum of FeS2 Pyrite.

Plane-wave density functional calculations are used to investigate the pressure dependence of the geometry and Gamma-point phonons of FeS2 pyrite up to 150 GPa. The linear response method is employed to calculate the vibrational properties. Raman-active modes are in excellent agreement with the experimental data available up to 50 GPa,(1) and we predict the evolution with pressure of the IR-active modes for which no high-pressure spectroscopic data have been reported so far. Over the wide pressure range investigated here, all vibrational frequencies depend nonlinearly on pressure; their pressure dependence is quantified by determining the full set of mode Gruneisen parameters and their pressure derivatives.