Pseudopotential Density Functional Theory Research Articles

First-principles study on the mechanism of Hf and Ti doping promoting hydrogen release in ZrCoH3

This paper reveals the mechanism of Hf and Ti doping promoting hydrogen release in ZrCoH3. Different from previous experimental methods, this study employs the plane wave pseudopotential method and density functional theory to predict the relationship between the formation energy and electronic structure of the ZrCoH3-Hf/Ti system, and uses theoretical calculations to guide the modification design of ZrCoH3. Firstly, by calculating the thermodynamic properties of the ZrCoH3-Hf/Ti system and combining the density of states theory, the bonding characteristics and dehydrogenation preference between H atoms and neighboring metal atoms in the ZrCoH3-Hf/Ti system are determined. Additionally, the electronic structure of the ZrCoH3-Hf/Ti system is calculated to elucidate the hybridization of bonding and antibonding orbitals between atoms in the ZrCoH3-Hf/Ti system. Moreover, DCo-d is utilized to measure the strength of antibonding interactions in the ZrCoH3-Hf/Ti system. The main cause of antibonding interactions in the ZrCoH3-Hf/Ti system is attributed to the variation in formation energy, and the change in antibonding interactions is sensitive to the variation in formation energy. Finally, a feasible method to improve the properties of ZrCoH3 is proposed, which involves modifying the electronic structure to increase the partial density of states at the Fermi energy level, making the formation energy of hydrides less negative and forcing the structure to be more unstable. This method has potential application value in the modification of ZrCoH3.

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.

Improving the precision of forces in real-space pseudopotential density functional theory.

The high-order finite difference real-space pseudopotential density functional theory (DFT) approach is a valuable method for large-scale, massively parallel DFT calculations. A significant challenge in the approach is the oscillating "egg-box" error introduced by aliasing associated with a coarse grid spacing. To address this issue while minimizing computational cost, we developed a finite difference interpolation (FDI) scheme [Roller et al., J. Chem. Theory Comput. 19, 3889 (2023)] as a means of exploiting the high resolution of the pseudopotential to reduce egg-box effects systematically. Here, we show an implementation of this method in the PARSEC code and examine the practical utility of the combination of FDI with additional methods for improving force precision and/or reducing its computational cost, including orbital-based forces, compensating charges (namely, adding and subtracting a judiciously chosen charge density such that the total density is unaltered), and a modified spatial domain in which the real-space grid is defined. Using selected small molecules, as well as metallic Li, as test cases, we show that a combination of all four aspects leads to a significant reduction in computational cost while retaining a high level of precision that supports accurate structures and vibrational spectra, as well as stable and accurate molecular dynamics runs.

Structural, Elastic, Mechanical, Electronic, and Optical Properties of a Novel Quaternary Chalcogenide Semiconductor Ba3GeTeS4

The structural, elastic, mechanical, electronic and optical properties of Ba3GeTeS4 have been studied by pseudopotential density functional theory static calculations. The results indicated that both the lattice constant and cell volume decrease with the increase of pressure, which match well with available previous values. The pressure has a more significant influence on the b direction than the a and c direction. The obtained elastic constants reveal that Ba3GeTeS4 is mechanically stable between 0 GPa and 20 GPa. The bulk modulus, shear modulus, and Young’s modulus are evaluated by Voigt-Reuss-Hill approximations. All these elastic moduli exhibit a monotonic feature as a function of pressure. The Poisson’s ratio, and Pugh’s criterion indicate that the ductility of this quaternary Ba3GeTeS4 compound is more and more prominent with increasing applied pressure. Meanwhile, the analysis of the electronic structures reveals that the states near the valence band top are derived from Te 5p, S 3p, and Ba 6 s orbitals, and the lowest conduction band is composed of Ge 4 s and S 3p orbitals. We expect that the findings predicted the physical properties of this compound will promote future experimental studies on Ba3GeTeS4.

Finite Difference Interpolation for Reduction of Grid-Related Errors in Real-Space Pseudopotential Density Functional Theory.

The real-space pseudopotential approach is a well-known method for large-scale density functional theory (DFT) calculations. One of its main limitations, however, is the introduction of errors associated with the positioning of the underlying real-space grid, a phenomenon usually known as the "egg-box" effect. The effect can be controlled by using a finer grid, but this raises the cost of the calculations or even undermines their feasibility altogether. Therefore, there is ongoing interest in the reduction of the effect per a given real-space grid. Here, we present a finite difference interpolation of electron orbitals as a means of exploiting the high resolution of the pseudopotential to reduce egg-box effects systematically. We implement the method in PARSEC, a finite difference real-space pseudopotential DFT code, and demonstrate error mitigation and improved convergence at a low additional computational cost.

Band unfolding with a general transformation matrix: From code implementation to interpretation of photoemission spectra

Unfolding of a supercell band structure into a primitive Brillouin zone is important for understanding implications of structural distortions, disorder, defects, and solid solutions on materials electronic structure. The necessity of band unfolding is also recognized in the interpretation of angle-resolved photoemission spectroscopy (ARPES) measurements. We describe an extension of the fold2Bloch package by implementing an arbitrary transformation matrix used to establish a relation between the primitive cell and supercell. This development allows us to overcome limitations of supercells constructed exclusively by scaling of primitive cell lattice vectors. It becomes possible to transform between primitive and conventional cells as well as include rotations. The fold2Bloch is publicly available from a GitHub repository as a FORTRAN code. It interfaces with the all-electron full-potential WIEN2k and the pseudopotential VASP density functional theory packages. The fold2Bloch is supplemented by additional pre- and post-processing utilities that aid in generating k points in the supercell (such that they later fall onto a desired path in the primitive Brillouin zone after unfolding) and plotting the unfolded band structure. We selected ▪ as an illustrative example and, for the first time, present its properly unfolded band structure in direct comparison with ARPES measurements. In addition, critical importance of the band unfolding for interpretation of ▪ ARPES data is illustrated and discussed as a perspective.

C and N effect on magnetic coupling and vibrational properties of bcc and fcc-FeMnCr alloys: DFT study

In this work we report, magnetic and thermal properties of the bcc-Fe0.52MnxCr0.28-xCu0.06Mo0.10C0.012N0.014 (xMn = 5.77, 11.51 and 17.22 wt%) and the fcc-Fe0.63MnxCr0.26-xCu0.03Mo0.05C0.006N0.007 (xMn = 8.98, 11.95 and 14.92 wt%) alloys, applying the pseudo-potential Density Functional Theory (PP-DFT). The obtained results show that magnetic properties of these alloys are governed by Mn concentrations and C, N additions. The magnetic moments at each atom depend strongly on Mn concentrations as well as on the presence of C, N atoms. The C, N or both CN additions into octahedral sites do not affect the magnetic coupling between atoms. The quasi-harmonic Debye model is successfully applied to predict thermal properties of these alloys. The bulk modulus, volume expansion and Gibbs free energy are significantly affected by temperature, Mn concentrations and C, N additions in both bcc and fcc alloys. Whereas, vibrational entropy is more affected by bcc–fcc phase transition than C, N addition and Mn concentration.

Structural and optical properties of GaAs and InAs for doping Sb under the effect of pressure and temperature: DFT and EPM investigations

We have developed first-principles calculations utilizing the empirical pseudopotential method and density functional theory to examine the basic behaviors of the GaAs and InAs semiconductor compounds for doped Sb. The electronic and optical responses of these compounds were calculated. We examined the impacts of the temperature and pressure with the energy band forbidden and parameters of optical parameters (static dielectric constant) of the considered compounds. The literature hasn’t fully analyzed the electronic and optical characteristics of the compounds with doping Sb under the impact of pressure and temperature for fixed composition (x = 0.5). So, we concentrated on the study of these properties.

A systematic DFT study of (Ti3/2RE1/2)AlC alloys: A new database for adjustable mechanical and electronic properties

In this study, ab initio calculations based on Pseudo-Potential Density Functional Theory (PP-DFT) method are carried out in order to highlight the partial substitution effect of Rare Earth (RE) elements in the well-known 211-MAX phase of Ti2AlC. The considered elements are Y, Sc and RE = La, Ce, Pr, Nd, Sm, Eu, Gd leading to (Ti3/2RE1/2)AlC alloys. According to the obtained results, the (Ti3/2RE1/2)AlC alloys are significantly less compressible under uniaxial stress along x and z axes. They exhibit high resistance to shearing along <001> direction. In addition, the calculated heat capacity for (Ti3/2RE1/2)AlC alloys increases with respect to the temperature, a maximum is found in the temperature range 200–300 K. Localized states occur in (Ti3/2RE1/2)AlC alloys due to the f states filling of the rare earth elements. The magnetic moment of (Ti3/2RE1/2)AlC compounds increases according to 4fn(n=2 for Ce to n=7 for Gd) filling. Our findings provide a theoretical database for new tunable properties of (Ti3/2RE1/2)AlC alloys.

First principle study on modulating of Schottky barrier at metal/4H-SiC interface by graphene intercalation

In the production of SiC electronic devices, one of the main challenges is the fabrication of good Ohmic contacts due to the difficulty in finding the metals with low Schottky barriers of wide band gap SiC. Therefore, reducing the Schottky barrier height (SBH) at the metal/SiC interface is of great importance. In this paper, the effects of graphene intercalation on the SBH in different metals (Ag, Ti, Cu, Pd, Ni, Pt)/4H-SiC interfaces are studied by combining the average electrostatic potential and local density of states calculation methods based on first-principles plane wave pseudopotential density functional theory. The calculation results show that single-layer graphene intercalation can reduce the SBH of metal/4H-SiC contact. When the two layers of graphene are inserted, the SBH are further reduced. Especially, the contact between Ni and Ti exhibits negative SBH values, inferring that good Ohmic contacts are formed. When layers of graphene continue to increase, the SBH no longer changes obviously. By analyzing the differential charge density and the local density of states of the interface, the mechanism of SBH reduction may be that the dangling bonds on the SiC surface are saturated by the graphene C atoms and the influence of the metal-induced energy gap state at the interface is reduced, thereby reducing the interface state density. In addition, graphene and the corresponding new phases at the interface have low work functions. Moreover, an interfacial electric dipole layer may be formed at the SiC/graphene interface which also contributes to barrier reduction.

Effect of electronic interactions and coordination spheres on ionic diffusion in LaxSr1-xGayMg1-yO3-δ

The effect of the partial covalent character of cations on their ionic diffusivity in LaxSr1-xGayMg1-yO3-δ, a perovskite with AxA'x-1ByB'y-1O3-δ general formula, is determined computationally using plane wave, periodic, pseudopotential density functional theory. The diffusion of ions is assumed to take place by a vacancy-mediated transport mechanism. Using Vineyard's transition state theory, diffusion coefficients for each ion are obtained. The results of the calculations show that at temperature of 1000 K the diffusivity of the oxide ion is greater than that of the A-site cations by at least ten orders of magnitude, and the diffusivities of the A-site cations are greater then the diffusivities of B-site cations by at least eight orders of magnitude. Those results are related to local ionic mobilities and are not representative for macroscopic ionic diffusion. The diffusivities of rather covalent, polarizable, cations (La and Ga) were found to be lower than those of more ionic, hard, cations (Sr and Mg) despite the fact that La and Ga have smaller ionic radii than Sr and Mg, respectively. The migration pathway of the B-site cations is curved for different possible locations of the necessary oxygen vacancies and the calculations revealed that the curved pathway is not caused by steric hindrance of an intervening oxide ion but rather the driving force to maximize the electronic interactions of the B cation with surrounding oxide ions along the entire migration pathway.

Screening in Graphene: Response to External Static Electric Field and an Image-Potential Problem.

We present a detailed first-principles investigation of the response of a free-standing graphene sheet to an external perpendicular static electric field E. The charge density distribution in the vicinity of the graphene monolayer that is caused by E was determined using the pseudopotential density-functional theory approach. Different geometries were considered. The centroid of this extra density induced by an external electric field was determined as = 1.048 Å at vanishing E, and its dependence on E has been obtained. The thus determined was employed to construct the hybrid one-electron potential which generates a new set of energies for the image-potential states.

The structures, phase transition and elastic and thermodynamic properties of ZnS from first-principles calculations

The phase transition of zinc sulfide (ZnS) from Zinc-blende (ZB) to a rocksalt (RS) structure and the elastic, thermodynamic properties of the two structures under high temperature and pressure are investigated by first-principles study based on the pseudo-potential plane-wave density functional theory (DFT) combined with the quasi-harmonic Debye model. The lattice constant [Formula: see text], bulk modulus [Formula: see text] and the pressure derivative of bulk modulus [Formula: see text]’ of the two structures are calculated. The results are in good agreement with experimental results and the other theoretical data. From the energy–volume curve, enthalpy equal principle and mechanical stability criterion, the transition pressures from the ZB to the RS structure are 16.83, 16.96 and 16.61 GPa, respectively. The three results and the experimental values 14.7–18.1, 16 GPa are very close to each other. Then the elastic properties are also calculated under the pressure ranging from 0 to 30 GPa. Finally, through the quasi-harmonic Debye model, the thermodynamic properties dependence of temperature and pressure in the ranges between 0–1600 K and 0–30 GPa are obtained successfully.

Ab initio study of structural, electronic and thermodynamic properties of Be12V under high temperature

The structure, electronic, elasticity and thermodynamic properties, phonon spectra of Be12V alloy under high temperatures and pressure are investigated by using first-principles calculations based on pseudo-potential plane-wave density functional theory method within using the generalized gradient approximation (GGA) and quasi-harmonic Debye model. The results show that the calculated equilibrium structural parameters of Be12V are in good agreement with the experiments. Under high pressure, the elastic constant of Be12V alloy increases with the increase of pressure. The all elastic constants satisfy the Born's mechanical criterion, indicating that Be12V crystal is dynamic stable under the applied pressure (0 GPã50 GPa), and the pressure has little effect on the electronic properties of Be12V. The phonon spectra and phonon density of states under pressure from 0 GPa to 50 GPa are presented for the Be12V and discussed the effects of pressure on some properties. By the quasi-harmonic Debye model, the thermodynamic properties of Be12V under high temperature and pressure were predicted, and the relationship of heat capacity, thermal expansion coefficient with temperature and pressure was obtained in the range of 0–1300 K and 0–20 GPa, respectively.

Structural and electronic properties of ZnO: A first-principles density-functional theory study within LDA(GGA) and LDA(GGA)+U methods

In this article, the structural and electronic properties of bulk ZnO have been studied using the plane-wave-based pseudopotential density functional theory (DFT). The structural parameters, band structure (BS), and density of states (DOS) of ZnO wurtzite structure have been investigated by Quantum Wise within LDA (GGA) and LDA (GGA)+U methods by Fritz-Haber-Institute (FHI) pseudopotentials. The calculated band gaps using Hubbard U semiempirical corrections are in agreement with previous experimental works and shows both the valence band maximum and conduction band minimum located at the Γ point of the Brillouin zone (BZ).

Regulated microarchitecture, spin polarization state, and observed charge transfers for cerium boride CeB6 under electrical Field

The thermal stabilities, microarchitectures as well as the electronic states of the cubic CeB6 under electrical field have been systematically investigated in detail within the framework of plane wave pseudo-potential density functional theory calculational method. Specifically, theCeB6 under electrical field of 0.0002 V/Å has lower total energy and enthalpy of formation. The cell of theCeB6 under electrical field is slightly expanded and the lattice expansion is isotropic, along with microarchitecture regulations. The distances between B are 2.523 Å, 1.707 Å and 1.784 Å, showing covalent bonds. There are 5 sub-bands for CeB6, the energy widths of bands are regulated with the conduction band and the first valence band moving downwards under electrical field. There are 3 band valleys and 5 band peaks for the CeB6, with metallic transition and decreased band gaps under electrical field. The middle conduction band and the first valence band are formed by f, d and p orbital electrons, respectively. The Ce s and the B orbital electrons tend to polarize under electrical field. The Ce f, Ce s and B orbital electrons are responsible for the ferrimagnetic magnetism under electrical field. There are charge transfers among these orbitals for CeB6 under electrical field.

Computer Simulation of Oxygen Vacancy Formation in YFeO&lt;sub&gt;3&lt;/sub&gt; Perovskite

The pseudopotential method and density functional theory with Hubbard correction were used to study changes in the atomic and electronic structure of yttrium orthoferrite (YFeO3) during vacancy formation. Depending on the value of non-stoichiometry in YFeO3−δ(δ = 0.0625 and 0.25), the energy gain of one of the two types of vacancy decreases from 0.3 to 0.1 eV. So it have been shown that high concentrations of oxygen vacancies make more insignificant the difference in the type of formed vacancies.

Stability, rigidity and thermal vacancies evolution in Fe–Cr–Mn alloys with C and N additions: DFT and Wagner-Schottky model investigations

The pseudo-potential Density Functional Theory (PP-DFT) combined with the statistical Wagner-Schottky model, are applied to study the Fe-xMn-(26-x)Cr–5Mo–3Cu-0.6C-0.7 N austenitic alloys (x = 6, 9, 12 and 15 wt%). The obtained results show that the rigidity and the thermal vacancy behavior is very sensitive to the system composition. The overall system stability is found to be governed by the presence of nitrogen. Both carbon and nitrogen favor the vacancy creation at T∼1200K and prevent the thermal vacancy activation at T∼1600K. Interestingly, beyond 1273K, carbon atom could migrate toward the substitution sites while nitrogen atoms do not leave their favorable octahedral sites.

Validation of Pseudopotential Calculations for the Electronic Band Gap of Solids.

Nowadays pseudopotential (PP) density functional theory calculations constitute the standard approach to tackle solid-state electronic problems. These rely on distributed PP tables that were built from all-electron atomic calculations using few popular semilocal exchange-correlation functionals, while PPs based on more modern functionals, such as meta-generalized gradient approximation and hybrid functionals, or for many-body methods, such as GW, are often not available. Because of this, employing PPs created with inconsistent exchange-correlation functionals has become a common practice. Our aim is to quantify systematically the error in the determination of the electronic band gap when cross-functional PP calculations are performed. To this end, we compare band gaps obtained with norm-conserving PPs or the projector-augmented wave method with all-electron calculations for a large data set of 473 solids. We focus, in particular, on density functionals that were designed specifically for band gap calculations. On average, the absolute error is about 0.1 eV, yielding absolute relative errors in the 5–10% range. Considering that typical errors stemming from the choice of the functional are usually larger, we conclude that the effect of choosing an inconsistent PP is rather harmless for most applications. However, we find specific cases where absolute errors can be larger than 1 eV or others where relative errors can amount to a large fraction of the band gap.

Structural stability, mechanical, electronic and thermal behaviour of Ru2CrZ (Z=Sb, Si, Pb, Ge) Heusler alloys

In this paper, the structural, elastic, electronic properties of Ru2CrZ (Z=Si, Ge, Pb, Sn) are explored using the generalized gradient approximation based on ab initio plane-wave pseudopotential density functional theory. With the help of the quasi-harmonic Debye model, we also investigate the variation of normalized volume V/V0, the heat capacities CV and CP, thermal expansivity, and Debye temperature of Ru2CrZ (Z=Si, Ge, Pb, Sn). Results show that the Cu2MnAl type structure is more stable then Hg2CuTi type structure. The four compounds in the ground state are predicted to be nearly half-metal behavior with total magnetic moment near to the integer value. To provide a comparative and complementary study to future researches, we investigated the elastic and thermodynamic properties.