Plane-wave Pseudopotential Calculations Research Articles

Improving the precision of work-function calculations within plane-wave density functional theory

Abstract Work function is a fundamental property of metals and is related to many surface-related phenomena of metals. Theoretically, it can be calculated with a metal slab supercell in density functional theory (DFT) calculations. In this paper, we discuss how the commensurability of atomic structure with the underlying fast Fourier transform (FFT) grid affects the accuracy of work function obtained from plane-wave pseudopotential DFT calculations. We show that the macroscopic average potential, which is an important property in work function calculations under the ‘bulk reference’ method, is more numerically stable when it is calculated with commensurate FFT grids than with incommensurate FFT grids. Due to the stability of the macroscopic average potential, work function calculated with commensurate FFT grids shows better convergence with respect to basis set size, vacuum length and slab thickness of a slab supercell. After we control the FFT grid commensurability issue in our work function calculations, we obtain well-converged work functions for Al, Pd, Au and Pt of (100), (110) and (111) surface orientations. For all the metals considered, the ordering of our calculated work functions of the three surface orientations agrees with experiment. Our findings reveal the importance of the FFT grid commensurability issue, which is usually neglected in practice, in obtaining accurate metal work functions, and are also meaningful to other DFT calculations which can be affected by the FFT grid commensurability issue.

Ab initio predictions of pressure-dependent structural, elastic, and thermodynamic properties of GaMF3 (M = Ca, and Sr) halide perovskites

The structural, elastic, and thermodynamic characteristics of the GaCaF3 and GaSrF3 halide perovskites have been predicted for the first time, using ab initio pseudopotential plane wave calculations. The equilibrium lattice parameters are in good agreement with previously documented results. Both compounds exhibit substantial thermodynamic and mechanical stability. The single-crystal and polycrystalline elastic properties and related properties have been examined. GaCaF3 exhibits higher hardness than GaSrF3. The temperature dependence of the lattice parameter, bulk modulus, volume thermal expansion coefficient, heat capacity, and Debye temperature, have been computed using ab initio calculations combined with the quasi-harmonic Debye model.

Spin-Orbit Coupling Electronic Structures of Organic-Group Functionalized Sb and Bi Topological Monolayers.

Electronic band-gap is a key factor in applying two-dimensional (2D) topological insulators into room-temperature quantum spin Hall effect (QSH) spintronic devices. Employing pseudopotential plane-wave first-principles calculations, we investigate spin-orbit coupling (SOC) electronic structures of the novel 2D topological insulator series of antimony (Sb) and bismuth (Bi) monolayers (isolated double atomic layers) functionalized by organic-groups (methyl, amino and hydroxyl). Cohesive energies and phonon frequency dispersion spectra indicate that these organic-group decorated Sb and Bi monolayers possess structural stability in both energetic statics and lattice dynamics. The giant electronic band-gaps adequate for room-temperature applications are attributed to the effective SOC enhancement of group functionalization. The nontrivial topology of these novel 2D monolayer materials is verified by the Z2 invariant derived from wave-function parity and edge-states of their nanoribbons, which is prospective for QSH spintronic devices. The chemical functional group changes the p-orbital component of Fermi level electrons, leading to strong intra-layer spin-orbit coupling, opening a large band-gap of approaching 1.4 eV at Dirac-cone point and resulting in a global indirect band-gap of 0.75 eV, which, even underestimated, is adequate for room-temperature operations. Sb and Bi monolayers functionalized by organic groups are also predicted to maintain stable nontrivial topology under in-layer biaxial strain, which is suitable for epitaxy technology to realize QSH spintronic devices.

Excitons in ZnO Quantum Dots: The Role of Dielectric Confinement

Quantum dots (QDs) grown by chemical synthesis methods are relatively unstable and present difficulties in dispersion and preservation. The most common way of surpassing the stability issues is to embed (and passivate) the QDs in a matrix material. However, some fundamental questions concerning the influence of the dielectric confinement caused by the matrix environment on the exciton energy and exciton binding energy of the QDs have not yet been properly addressed. Here we explore the exciton fine structure of wurtzite ZnO QDs by means of plane-wave million-atom atomistic pseudopotential calculations and a configuration interaction approach taking into account the dielectric confinement from the surrounding material. Our results indicate that the exciton energy increases and the exciton binding energy decreases as the dielectric constant of the surrounding material increases. The behavior of the exciton binding energy is caused by the presence of self-polarization potential induced by the dielectric mismatch in the surface of the QD. This mainly alters the localization of the hole charge density and thus reduces the electron–hole overlap and inevitably the exciton binding energy.

211-MAX borides: The stable boron-substituted 211-MAX compounds by first-principles

Applying the first-principles density functional pseudopotential plane-wave calculations, the energetic, mechanical, and dynamic stabilities of boron-substituted M2AlB (M=Sc, Ti, V, Cr, Mn, Y, Zr, Nb, Mo, Tc, Ru, Hf, Ta, W, Re, Os, Ir, Pt, Ag, and Au) compounds as the members of 211-MAX borides with Cr2AlC-type crystal structure were investigated. After well-converged geometry optimizations, the cohesive energy, formation enthalpy, single-crystal elastic constants, and phonon dispersion curves were calculated to explain the stabilities of the compounds. Except for M=Y, Ag, and Au compounds, the others are thermodynamically stable due to the negative formation enthalpies. M=Sc, Y, Zr, Hf, Os, Ir, and Pt compounds are dynamically unstable because of the negative phonon frequencies, and M=Ag and Au compounds are mechanically unstable. Beyond the detailed stability analysis, the structural, elastic, and mechanical properties, anisotropy factors, exfoliation states, and specifically hardness nature of the stable compounds were examined. A new modified hardness method, which includes the metallicity, is proposed to characterize the softness nature of MAX-phases. The relevant quantities, such as bond electronegativity, ionicity, and metallicity, were debated. The presented data were compared with available theoretical and experimental literature.

Density Functional Theory Study on the Electronic Properties of Doped-Cobalt Oxide (CoO)

Cobalt oxide (CoO) has been widely studied for photocatalyst of water splitting and displaying a high-efficiency material. This paper reports a Density Functional Theory (DFT) study on the electronic properties of rock-salt CoO and analyzes effects of cations (Ni and Fe) and anions (N and F) dopants on the electronic properties. For this purpose, CASTEP software used for first principles plane-wave pseudo-potential calculations at different functional, i.e: GGA-PW91 and LDA. The electronic calculations of the CoO optimized structure showed a metallic structure if without considering spin-orbital interactions. After considering the spin-orbital interaction calculation, the CoO band structure possessed indirect and direct band gaps. The direct bandgap by GGA-PW91 calculation is 2.10 eV, it was agreed to the experimentally reported value of approximately 1.9-2.6 eV. Meanwhile, Ni, Fe, and F-doped CoO, demonstrating decreased CoO direct band gaps to 1.70 eV, 1.80 eV, and 1.73 eV, respectively. While N-doped CoO increased the CoO direct bandgap to 3.05 eV. All dopants shifted the conduction and valence bands position, where Ni-doped CoO band edges keep straddle to the redox potential of water splitting. Among other elements in this study, Ni is a more desirable dopant of CoO to enhance photoelectrochemical hydrogen production.

The structure and stability of the low-index surfaces of D8m-Mo5Si3 by first-principles calculations

Atomic relaxations, surface energies and electronic structures of the (100), (001) and (110) surfaces for D8m-Mo5Si3 are investigated to analyze the surface stability by DFT (Density Functional Theory) based first-principles plane-wave pseudo-potential calculations. The atomic relaxation and surface energy results reflect minimum relaxations of the (ns-8Mo4Si)-terminated (001), (ns-4Si)-terminated (110) and (ns-2Si-2)-terminated (100) surfaces. Moreover, the (ns-8Mo4Si)-terminated (001) and (ns-4Si)-terminated (110) surfaces are most stable under Si-poor and Si-rich conditions, respectively. The density of states indicates that the surface electronic structures are mainly influenced by the atomic relaxations. The calculated charge densities show dependence of the surface relaxation mainly on dangling bonds. According to the calculated surface energies, the predicted crystal shapes show the octagonal prism and a structure which is close to cuboid in the μSislab-μSibulk range from −1.077 eV to −0.546 eV and −0.546 eV–0 eV, respectively. These calculation results are compared with the previous reports.

Temperature-dependent properties of magnetic CuFeS2 from first-principles calculations: Structure, mechanics, and thermodynamics

Chalcopyrite (CuFeS2) is an antiferromagnetic semiconductor with promising magnetic and electrical properties, although these properties are not yet completely understood. The structural, magnetic, and electronic properties of bulk CuFeS2 were studied via first-principles plane-wave pseudopotential calculations based on density functional theory (DFT) using DFT+U and hybrid functional B3LYP methodology. The temperature-dependent structural, thermal, and mechanical properties of tetragonal CuFeS2 were also investigated via density functional perturbation theory. Furthermore, the structural parameters, elastic constants, bulk and shear moduli, volume expansion, and specific heats as a function of temperature were evaluated. Tetragonal CuFeS2 was found to exhibit negative thermal expansion behavior at temperatures lower than 100 K. A comprehensive comparison of the various calculated parameters with earlier published studies is also presented along with available experimental data and used as a basis to critically discuss the various properties of CuFeS2.

First-principles study of solid solution strengthening in Mg–X (X=Al, Er) alloys

To study the solid solution strengthening effect on magnesium (Mg)–X (X = Al, Er) alloys, supercell models of Mg, \(\hbox {Mg}_{35}\hbox {Er}\) and \(\hbox {Mg}_{35}\hbox {Al}\) are established to perform the first-principles pseudopotential plane wave calculations based on density functional theory. The calculated cohesive energy of \(\hbox {Mg}_{35}\hbox {Er}\) is lower than that of \(\hbox {Mg}_{35}\hbox {Al}\). This indicates that \(\hbox {Mg}_{35}\hbox {Er}\) has better structural stability than \(\hbox {Mg}_{35}\hbox {Al}\). The bulk modulus, Young’s modulus and shear modulus of the solid solutions increases simultaneously when Al and Er are doped into the Mg matrix. Moreover, the solid solution strengthening of Er is much higher than the Al containing alloy. The order of toughness of the three solutions from the highest to the lowest is Mg, \(\hbox {Mg}_{35}\hbox {Er}\) and \(\hbox {Mg}_{35}\hbox {Al}\), while the order of increasing elastic anisotropy is in the reverse order. The number of bonding electrons of \(\hbox {Mg}_{35}\hbox {Er}\) in the low-energy region of the Fermi level is much higher than that of \(\hbox {Mg}_{35}\hbox {Al}\), and the density of states of \(\hbox {Mg}_{35}\hbox {Er}\) at the Fermi level is higher than that of \(\hbox {Mg}_{35}\hbox {Al}\). Compared with Al atoms, Er atoms share more electric charges with Mg atoms, which leads to an increasingly uniform charge distribution around Er atoms.

Realization of linearly polarized exciton emission in wurtzite zinc oxide quantum dots

Linearly polarized exciton emission from a quantum dot is very appealing for polarized light emitting diodes and quantum information science and technologies. However, it has been very difficult to access it, because conventional wurtzite quantum dots (QDs) usually favor circularly polarized exciton emission originating from a single electron bound to a hole with dominant heavy hole ($A$ band) character. Here we explore the exciton fine structure of wurtzite ZnO QDs by means of plane-wave million-atom atomistic pseudopotential calculations and a configuration interaction approach. We demonstrate that linearly polarized emission could be realized in these QDs by (i) enhancing the quantum confinement effects and (ii) applying hydrostatic pressure. Such an exciton has a polarization exclusively along the $c$-axis direction of the QDs, which derives from a single electron bounded to a hole state with a split-off hole ($C$-band) character. This work paves the way for the exploration of the fundamental properties of wurtzite ZnO QDs and their potential applications in optoelectronic devices and quantum technologies.

Ab-initio study of electronic properties of Si(C) honeycomb structures

In this work, the electronic structures of pure and concentrated graphene and Silicene have been studied by performing first-principles pseudo potential plane-wave calculations. The concentrated structures have been obtained by the substitution of Si(C) atoms in the graphene (silicene), respectively. Firstly, the calculations are performed for pure graphene and continued for its concentrations. The concentrated graphene is obtained by substitution of Si atoms (with: 12.5, 25, 37.5 and 50 mol percentage) at different positions in the unit cell of graphene. Similar to graphene, the same calculations are performed for pure silicene as well as for silicene after substitution of C atoms. We have modeled the lattice constant, the band structure and its directivity, while the position and mole fractions of the substituted atoms are changed in the unit cell of the studied compound. Our results showed that: the total energy, the density of States (DOS), the charge density (CD), the opening of the band gap and its directivity are strongly dependent both on the position and mole fraction of the substituted Si(C) atoms. As an interesting result, we found an indirect open band gap, as large as 2.53 eV for silicon doped graphene. Also, it was found that both the elemental concentration and unit cell geometry could offer remarkable advantages for band splitting and band gap opening in these graphene like structures, which have known as ideal structures with many promising potential applications in the electronic, optoelectronic and spintronic.

Band structure interpolation using optimized local orbitals from linear-scaling density functional theory

Several approaches to linear-scaling density functional theory (LS-DFT) that seek to achieve accuracy equivalent to plane-wave methods do so by optimizing in situ a set of local orbitals in terms of which the density matrix can be accurately expressed. These local orbitals, which can also accurately represent the canonical Kohn-Sham orbitals, qualitatively resemble the maximally localized Wannier functions employed in band structure interpolation. As LS-DFT methods are increasingly being used in real-world applications demanding accurate band structures, it is natural to question the extent to which these optimized local orbitals can provide sufficient accuracy. In this paper, we present and compare, in principle and in practice, two methods for obtaining band structures. We apply these to a (10, 0) carbon nanotube as an example. By comparing with the results from a traditional plane-wave pseudopotential calculation, the optimized local orbitals are found to provide an excellent description of the occupied bands and some low-lying unoccupied bands, with consistent agreement across the Brillouin zone. However free-electron-like states derived from weakly bound states independent of the $\ensuremath{\sigma}$ and $\ensuremath{\pi}$ orbitals can only be found if additional local orbitals are included.

Assessment of quality and reliability of band structures from exact-exchange-only Kohn–Sham, hybrid, and GW methods

Band gaps for a large number of semiconductors obtained with the exact-exchange(EXX)-only Kohn–Sham method are compared to those from various generalized Kohn–Sham methods based on hybrid functionals and with experimental values. EXX band gaps are shown to be of superior quality than the band gaps from most of the considered hybrid methods, band gaps obtained with the hybrid functional HSE06 have a similar quality as EXX ones. The EXX method is found to yield d -electron binding energies that are clearly superior to those determined within the local density approximation and to be comparable to d -electron binding energies from GW approaches. EXX plane-wave pseudopotential calculations and EXX all-electron calculations are shown to yield very similar KS transition energies demonstrating that the plane-wave pseudopotential approach is well-suited for EXX methods. Previous discrepancies between pseudopotential and all-electron calculations, most likely, have to be attributed to technical problems in early all-electron EXX calculations.

Prediction on elastic and thermal properties of defective L12–Al3Li intermetallics

The effects of Al vacancy, Li vacancy, Al anti-site and Li anti-site point defects on the elastic and thermal properties of L12–Al3Li intermetallic compound were investigated by the first-principle pseudopotential plane-wave calculations. The results show that Li anti-site and Al anti-site can easily form in L12–Al3Li. Li vacancy slightly increases hardness and the Debye temperature, whereas Li anti-site, Al anti-site, and Al vacancy impose the opposite influence. All these point defects have no obvious impact on the brittleness of L12–Al3Li. Under the Debye temperature, the specific heat of L12–Al3Li weakly decreases with the content of Li vacancy, but increases with those of Li anti-site, Li anti-site, Al anti-site, and Al vacancy.

Exact exchange plane-wave-pseudopotential calculations for slabs: Extending the width of the vacuum

Density functional calculations with the exact exchange (EXX) could potentially lead to an improved description of the electronic structure of surfaces and slabs. As soon as EXX is utilized, plane-wave pseudopotential (PWPP) calculations for slabs suffer from limitations of the plane-wave representation of the Kohn-Sham states in the vacuum between the replicas of the slab. Far outside the slab, however, the PWPP states can be extended by the analytically known asymptotic states of a single slab in a rather robust fashion. The resulting real-space states allow us to perform EXX calculations with the Krieger-Li-Iafrate approximation for very wide vacua.

<i>Ab initio</i> calculation of the structural, mechanical and thermodynamic properties of Beryllium Sulphide (BeS)

An ab initio plane-wave Pseudopotential calculations using the density functional theory (DFT) implementing the generalised gradient approximation (GGA) to study the structural, elastic constants, phonon dispersion curves, density of state and thermal properties of BeS. Also we calculated the shear modulus, Young’s modulus, Poisson’s ratio, and the Zener’s anisotropic factors. The calculated properties are agreement with previous experimental and theoretical results. The quasi-harmonic approximation is applied to determine the thermal properties, and these properties are in good agreement with available literatures. The major results of the properties determined were discussed.Keywords: Poisson’s ratio, Plane-Wave Pseudopotential, Quasi-Harmonic Approximation, Lattice Parameter

Study of the Non-Linearity on TiO2(0 0 1) Surface with Oxygen Defects: A First-Principles Study

First-principles plane-wave pseudopotential calculations were performed to study the energetics and electronic structures of oxygen defects on rutile TiO2(0 0 1). The influence of the material thickness on non-linearity (NL) was studied. With the increase in the thickness, the NL became stronger. Calculating the site-projected density of states by applying an external electric field showed that the NL of the bulk is due to the exchange of electrons between O 2p orbitals and Ti 3d orbitals. Finally, the influence of oxygen defects — oxygen vacancies (Vo), oxygen interstitials (Oi), and oxygen vacancies/oxygen interstitial (Vo[Formula: see text]Oi) pairs (Frenkel pair defects) — on the NL of TiO2 was studied. These results demonstrate that the band gap ([Formula: see text] of TiO2 became gradually narrower as the electric field increased. The Stark effect and defects can lead to the splitting of degenerate energy levels. Stronger electric fields increase the band splitting and reduce [Formula: see text]. With the increase in the Vo concentration, the decrease in the splitting amplitude and width of the energy level lead to weakening of the transfer of electrons between O and Ti atoms and optimizing the NL of TiO2. Therefore, the incorporation of Vo plays a significant role in improving the NL of TiO2.

N吸附SrTiO<sub>3</sub>(001)表面电子结构的第一性原理研究

N掺杂SrTiO3材料已成功用于研制蓝光波段发光二极管、高透明导电薄膜和钙钛矿氧化物光电子器件。本文采用第一性原理平面波赝势方法对N吸附SrTiO3(001)表面的原子构型、化学键特征和电子结构进行了计算研究并解释了In掺杂SrTiO3导电薄膜的相关实验现象。通过吸附能、置换能和态密度的比较分析发现，SrTiO3(001)表面吸附N原子的稳定性与N原子相对表面层的位置密切相关。 It is well known that N doped SrTiO3 has been successfully used to develop semiconductor diodes in a blue-light region, transparent conductive films and perovskite-oxides hetero-junction optical- electric devices. The first-principles Plane-Wave PseudoPotential (PWPP) calculations have been performed to investigate the atomic configuration, chemical bond characteristics, and electronic structure of N atomic adsorption on SrTiO3(001) surface with the explanations of the related ex-periment phenomena in the present study. These results indicated that the stability of the ad-sorbed N on the surface depends on the relative position of N atom to the surface by analysis of adsorption energy, substitution energy and density of states.

Linear-scaling density functional theory using the projector augmented wave method

Quantum mechanical simulation of realistic models of nanostructured systems, such as nanocrystals and crystalline interfaces, demands computational methods combining high-accuracy with low-order scaling with system size. Blöchl’s projector augmented wave (PAW) approach enables all-electron (AE) calculations with the efficiency and systematic accuracy of plane-wave pseudopotential calculations. Meanwhile, linear-scaling (LS) approaches to density functional theory (DFT) allow for simulation of thousands of atoms in feasible computational effort. This article describes an adaptation of PAW for use in the LS-DFT framework provided by the ONETEP LS-DFT package. ONETEP uses optimisation of the density matrix through in situ-optimised local orbitals rather than the direct calculation of eigenstates as in traditional PAW approaches. The method is shown to be comparably accurate to both PAW and AE approaches and to exhibit improved convergence properties compared to norm-conserving pseudopotential methods.

Electron-phonon superconductivity in the ternary phosphidesBaM2P2(M=Ni,Rh,and Ir)

Ab initio plane-wave pseudopotential calculations of electronic and vibrational properties have been carried out for the ternary phosphides $\mathrm{Ba}{M}_{2}{\mathrm{P}}_{2}$ $(M=\text{Ni},\text{Rh}\text{ and Ir})$ with a ${\mathrm{ThCr}}_{2}{\mathrm{Si}}_{2}$-type structure. The calculated electronic results show the metallic character of $\mathrm{Ba}{M}_{2}{\mathrm{P}}_{2}$, and the plots of total and partial density of states of $\mathrm{Ba}{M}_{2}{\mathrm{P}}_{2}$ exhibit strong hybridization between the $d$ states of the $M$ atom and the $p$ states of the P atom below the Fermi energy. Differences in the phonon spectrum and density of states both in the acoustical and optical ranges for these compounds are presented and discussed. The Eliashberg spectral function for these compounds has been calculated by using a linear response approach based on the density functional theory. By integrating the Eliashberg spectral function, the average electron-phonon coupling parameter $(\ensuremath{\lambda})$ is determined to be 0.61 for ${\mathrm{BaNi}}_{2}{\mathrm{P}}_{2}$, 0.55 for ${\mathrm{BaIr}}_{2}{\mathrm{P}}_{2}$, and 0.43 for ${\mathrm{BaRh}}_{2}{\mathrm{P}}_{2}$. Using the calculated values of $\ensuremath{\lambda}$ and the logarithmically averaged phonon frequency ${\ensuremath{\omega}}_{ln}$ the superconducting critical temperature $({T}_{c})$ values for ${\mathrm{BaNi}}_{2}{\mathrm{P}}_{2},{\mathrm{BaIr}}_{2}{\mathrm{P}}_{2}$, and ${\mathrm{BaRh}}_{2}{\mathrm{P}}_{2}$ are obtained to be 2.80, 1.97, and 0.70 K, respectively, which compare very well with their experimental values of 3.0, 2.1, and 1.0 K.