Local Spin Density Approximation Research Articles

Full Potential Study of Electronic and Optical properties of Laves phase HoFe2 Compound

In this work we presented optical properties and electronic structure of intermetallic compound. Our estimations are based on density functional theory (DFT) utilizing the Full Potential Linearized Augmented Plane Wave (FPLAPW) method with the inclusion of spin orbit coupling. Local spin density approximation with correlation energy (LSDA+U) was used for exchange-correlation potential. Band structure was estimated through calculation which shows the metallic character of HoFe2 compound. We found that majority spin d states of Ho and Fe mainly contribute to the valence band while Minority spin states Ho-4f, 5d dominate in the conduction band from EF to 4 eV. We again found decent agreement accompanying experiment for the diagonal optical conductivity using LSDA+U approximations. The results are examined considering band to band transitions.

Ab initio расчет зонной структуры и свойств модификаций соединения Ti-=SUB=-3-=/SUB=-Sb, допированного литием

Using the density functional theory (DFT) in the local electron spin density approximation (LSDA), 2×2×2 supercells based on the Ti3Sb compound have been studied. The supercells contained their own vacancies and doped lithium atoms replacing Ti and/or Sb. The DFT-LSDA method was used to calculate the structural, electronic, and magnetic properties, the enthalpy of formation, and the cohesion energy of supercells of two modifications of the Ti3Sb compound. We studied supercells with cubic system (A15 type structure; a = 5.217 Å) and tetragonal system (D8_m type structure; а = 10.457 Å, с = 5.258 Å). It has been established that the distribution of the density of states of the Ti3Sb cubic system has a more metallic character than for the D8_mTi3Sb modification. The enhancement of "metallicity" in the cubic modification A15 Ti3Sb is associated with an increase in the Ti–Sb interatomic distance in the crystal. Due to this, the degree of metallic bonding increases during the electronic interaction between Ti and Sb atoms near the Fermi level in Ti3Sb. In DFT calculations, the spin-polarized density of states was taken into account. It has been established that in both modifications (A15 and D8_m) of Ti3Sb near the Fermi level for s, p, d-states with spin "up" and spin "down" there is a spin imbalance in the population of energy levels. Defect-containing supercells based on Ti3Sb were studied by DFT-LSDA calculations. It is shown that Ti and/or Sb vacancies in the lattices of Ti3Sb crystals of both modifications (A15 and D8_m) increase the magnetic moment (M) as compared to the value of M (M_tot = 0.08 μ_B) of a “pure” Ti3Sb crystal. Considering that Al5 Ti3Sb is also interesting as a material for Li-ion batteries, we studied Ti3Sb–Li supercells doped with lithium. Li-doping and the creation of own Ti and/or Sb vacancies changes the distance between atoms in Ti3Sb. Correspondingly, the resulting local magnetic moments near the Ti or Sb vacancies also change. The enthalpy of formation and magnetic moment of Ti3Sb and Ti3Sb–Li based supercells are calculated. DFT calculations of the structural stability of binary compounds (phases) have been carried out, and the stability of conodes between phases in the Ti–Sb–Li system has been established. The isothermal section of the Ti–Sb–Li system was plotted at 298 K. It is shown that the introduction of various lithium concentrations (<6.25 at. % Li) into the Ti3Sb (Space group Pm3 ̅n; № 223; a = 5.217 Å) crystal lattice reduces the partial magnetic moment of Ti in the Ti3Sb–Li supercell.

Ab initio calculation of the band structure and properties of modifications of the Ti-=SUB=-3-=/SUB=-Sb compound doped with lithium

Using the density functional theory (DFT) in the local electron spin density approximation (LSDA), 2x2x2 supercells based on the Ti3Sb compound have been studied. The supercells contained their own vacancies and doped lithium atoms replacing Ti and/or Sb. The DFT-LSDA method was used to calculate the structural, electronic, and magnetic properties, the enthalpy of formation, and the cohesion energy of supercells of two modifications of the Ti3Sb compound. We studied supercells with cubic system (A15 type structure; a=5.217 Angstrem) and tetragonal system (D8m type structure; a=10.457 Angstrem, c=5.258 Angstrem). It has been established that the distribution of the density of states of the Ti3Sb cubic system has a more metallic character than for the D8m Ti3Sb modification. The enhancement of "metallicity" in the cubic modification A15 Ti3Sb is associated with an increase in the Ti-Sb interatomic distance in the crystal. Due to this, the degree of metallic bonding increases during the electronic interaction between Ti and Sb atoms near the Fermi level in Ti3Sb. In DFT calculations, the spin-polarized density of states was taken into account. It has been established that in both modifications (A15 and D8m) of Ti3Sb near the Fermi level for s-, p-, d-states with spin "up" and spin "down" there is a spin imbalance in the population of energy levels. Defect-containing supercells based on Ti3Sb were studied by DFT-LSDA calculations. It is shown that Ti and/or Sb vacancies in the lattices of Ti3Sb crystals of both modifications (A15 and D8m) increase the magnetic moment (M) as compared to the value of M (M=0.08 μB) of a "pure" Ti3Sb crystal. Considering that Al5 Ti3Sb is also interesting as a material for Li-ion batteries, we studied Ti3Sb-Li supercells doped with lithium. Li-doping and the creation of own Ti and/or Sb vacancies changes the distance between atoms in Ti3Sb. Correspondingly, the resulting local magnetic moments near the Ti or Sb vacancies also change. The enthalpy of formation and magnetic moment of Ti3Sb and Ti3Sb-Li based supercells are calculated. DFT calculations of the structural stability of binary compounds (phases) have been carried out, and the stability of conodes between phases in the Ti-Sb-Li system has been established. The isothermal section of the Ti-Sb-Li system was plotted at 298 K. It is shown that the introduction of various lithium concentrations (≤6.25 at.% Li) into the Ti3Sb (Space group Pm3 n; N 223; a=5.217 Angstrem) crystal lattice reduces the partial magnetic moment of Ti in the Ti3Sb-Li supercell. Keywords: Ti3Sb intermetallic compounds, A15 cubic modification, D8m tetragonal modification, doping with lithium, Ti3Sb-Li, DFT-LSDA calculations, supercell, electronic and magnetic properties, Ti-Sb-Li phase stability.

First principal calculations to investigate structural, electronic, optical, and magnetic properties of Fe3O4and Cd-doped Fe2O4

A theoretical study based on Density Function Theory (DFT) was accomplished to explore thestructural, electronic, optical and magnetic propertiesof Fe3O4 and Cd-doped Fe2O4 (cadmium ferrites) using PBE-GGA(Perdew-Burke-Ernzerhof-Generalized Gradient Approximation) for the exchange-correlation potentials. In the present study 10% cadmium (Cd) was doped as a metal ion into ferrites. The structure of Cd-doped Fe2O4 (cadmium ferrites)compound was optimized using the GGA, while the electronic optical and magnetic properties have been explored by employing the modified Beckie-Johnson local spin density approximation scheme(mBJ- LDA). PBE-GGA was performed to determine the structural properties where spin up and spin down configurations of Fe3O4wasobserved.By the comparing electronic properties ofFe3O4and CdFe2O4 in which to spin configuration showing that the material is having anti-ferromagnetic properties where the behavior of Fe atoms spins are oppositely magnetized. The band gap computed using PBE-GGA are improved using mBJ-LDA which is found to be 1.7 eV for CdFe2O4. The results show that band gap is improved by using mBJ-LDA which allows us to study Cd-doped Fe2O4. By doping the 10% of Cd, the bond length in Cd–O and in Fe–O was significantly decreased as compared to Fe3O4.Under the DFT scheme, cadmiumis consistent as dopants for reducing the bandgap of cadmium ferrites. These results indicate that Cd-doped Fe2O4ferritesare appropriateferromagnetic compoundfor promising photovoltaic device applications.

Comparative analysis on the structure, electronic and magnetic properties of SrRuO3 and PbRuO3

The structure, electronic and magnetic properties of SrRuO3 and PbRuO3 are studied by using density functional theory in the local spin density approximation. The lattice constants of orthorhombic SrRuO3 and PbRuO3 with a Pbnm space group have a > b, different from most of other ABO3 structures with a < b. We obtain two reasons about a > b for SrRuO3 and PbRuO3: One is that the horizontal middle plane of the oxygen octahedra deforms from square to rectangle, resulting in the side length of rectangle along x direction, aoct, is larger than that along y direction, boct; The other is the tilting angle of the oxygen octahedra having θ < arccos (boct/aoct). SrRuO3 and PbRuO3 with similar structure should have similar magnetic ground states. But SrRuO3 is ferromagnetic, while PbRuO3 is paramagnetic. Our results show that the orbital hybridization of Pb–O is much stronger than that of Sr–O, which lead to the broaden bandwidth near Fermi level. This is the main reason for the different magnetic ground states of SrRuO3 and PbRuO3. The calculations for PbRuO3 with Pbnm and Imma structures have considered the spin-orbit coupling effect, the calculated results show that the ground states of Pbnm and Imma PbRuO3 are both paramagnetic metals.

Spherical vs non-spherical and symmetry-preserving vs symmetry-breaking densities of open-shell atoms in density functional theory.

The atomization energies of molecules from first-principles density functional approximations improve from the local spin-density approximation to the Perdew-Burke-Ernzerhof (PBE) generalized gradient approximation (GGA) to the strongly constrained and appropriately normed (SCAN) meta-GGA, and their sensitivities to non-spherical components of the density increase in the same order. Thus, these functional advances increase density sensitivity and imitate the exact constrained search over correlated wavefunctions better than that over ensembles. The diatomic molecules studied here, singlet C2 and F2 plus triplet B2 and O2, have cylindrically symmetric densities. Because the densities of the corresponding atoms are non-spherical, the approximate Kohn-Sham potentials for the atoms have a lower symmetry than that of the external (nuclear) potential so that the non-interacting wavefunctions are not eigenstates of the square of total orbital angular momentum, breaking a symmetry that yields a feature of the exact ground-state density. That spatial symmetry can be preserved by a non-self-consistent approach in which a self-consistent equilibrium-ensemble calculation is followed by integer re-occupation of the Kohn-Sham orbitals as the first of several steps. The symmetry-preserving approach is different from symmetry restoration based on projection. First-step space- (and space-spin-) symmetry preservation in atoms is shown to have a small effect on the atomization energies of molecules, quantifying earlier observations by Fertig and Kohn. Thus, the standard Kohn-Sham way of calculating atomization energies, with self-consistent symmetry breaking to minimize the energy, is justified at least for the common cases where the molecules cannot break symmetry. Unless symmetry breaking is allowed in the molecule, SCAN strongly underestimates the atomization energy of strongly correlated singlet C2.

Magnetic spin order in the honeycomb structured Pb6Co9(TeO6)5 compound

We present a comprehensive experimental and theoretical study of the structural, electronic, magnetic, and thermodynamic properties of a ${\mathrm{Pb}}_{6}{\mathrm{Co}}_{9}{({\mathrm{TeO}}_{6})}_{5}$ single crystal. The ${\mathrm{Pb}}_{6}{\mathrm{Co}}_{9}{({\mathrm{TeO}}_{6})}_{5}$ crystal has shown a unique type of magnetic spin-lattice coupling, in which the lattice structure consists of four different Co ions sites with distorted octahedral coordinations. The x-ray photoelectron spectroscopy (XPS) results confirmed the oxidation states of Pb, Co, Te, and O elements in the sample. Moreover, XPS spectra revealed the adsorbed oxygen in the defect/vacancy sites of the lattice structure. The dc magnetization measurements exhibited a complex magnetic behavior with ferrimagnetic (FIM) transition with Curie temperature ${T}_{C}$ at $\ensuremath{\sim}21$ K. At lower magnetic fields $H$, the zero-field-cooled and field-cooled curves showed a broad hump at $\ensuremath{\sim}10.8$ K and a shoulder peak at $\ensuremath{\sim}6.2$ K, which are suppressed at higher magnetic fields. The ac susceptibility data indicated spin-glass-like features. The heat capacity ${C}_{P}$ measurements confirmed the FIM transition at ${T}_{C}$ at $\ensuremath{\sim}21$ K, but without any trace of additional peaks at lower temperatures. The estimated Curie-Weiss constant ${\ensuremath{\theta}}_{\mathrm{CW}}$ showed a peculiar field-dependent behavior along the $H\ensuremath{\parallel}c$ direction of the single crystal, where ${\ensuremath{\theta}}_{\mathrm{CW}}$ is less field dependent for the $H\ensuremath{\perp}c$ direction. A large coercivity (13 kOe) is observed at 2 K for $H\ensuremath{\parallel}c$, whereas the magnetization curve of the single crystal is dominated by an antiferromagnetic feature for $H\ensuremath{\perp}c$. The behaviors indicate the anisotropy nature of the exchange interactions in the compound. The local spin density approximation $+ U$ total energy calculations were performed for various collinear spin configurations of a classical Heisenberg model in order to obtain the magnetic exchange interactions ${J}_{i}$ at different distances for different neighbors.

Benchmarking of Density Functionals for Z-Azoarene Half-Lives via Automated Transition State Search

Molecular photoswitches use light to interconvert from a thermodynamically stable isomer into a metastable isomer. Photoswitches have been used in photopharmacology, catalysis, and molecular solar thermal (MOST) materials because of their spatiotemporal activation. Visible-light-absorbing photoswitches are especially attractive because low-energy light minimizes undesired photochemical reactions and enables biological applications. Ideal photoswitches require well-separated absorption spectra for both isomers and long-lived metastable states. However, predicting thermal half-lives with density functional theory is difficult because it requires locating transition structures and chosing an accurate model chemistry. We now report EZ-TS; by automatically calculating activation energies for the thermal Z → E isomerization. We used 28 density functionals [local spin density approximation, generalized gradient approximation, meta-GGA, hybrid GGA, and hybrid meta-GGA] and five basis sets [6-31G(d), 6-31+G(d,p), 6-311+G(d,p), cc-pVDZ, and aug-cc-pVDZ]. The hybrid GGA functionals performed the best among all tested functionals. We demonstrate that the mean absolute errors of 14 model chemistries approach chemical accuracy.

Effect of Edge Fluorination on Electronic and Transport Properties of Aarmchair Boron Nitride Nanoribbons: A First-Principles Study

We have systematically investigated the effect of edge fluorination on structural stability, electronic and transport properties of armchair boron nitride nanoribbons (ABNNRs), using first-principles calculations within the frame work of local spin-density approximation (LSDA). ABNNRs with F-passivation on only edge B atoms, regardless of their width, are found to be half-metallic and energetically most stable. For these ribbons, completely spin polarized charge transport is predicted across the Fermi level as a result of giant spin splitting. Transmission spectrum analysis also confirms the separation of spin-up and spin-down electronic channels. It is revealed that F-passivation of only edge N atoms transforms nonmagnetic bare ribbons into magnetic semiconductors whereas fluorination of both the edges does not affect the electronic and magnetic state of bare ribbons significantly. Predicted Half-metallicity projects these ABNNRs as potential candidates for inorganic spintronic devices.

Exchange interactions in hexagonal YMnO3 and LuMnO3 multiferroic compounds

In this work, spin density functional theory calculations are employed to obtain the exchange interactions in hexagonal YMnO3 and LuMnO3 multiferroic compounds, which have been a matter of debate in the literature. The exchange and correlation electronic effects are approximated using the local spin density approximation (LSDA) with the inclusion of the effective Hubbard U correction (Ueff). The intraplane superexchange interaction (Jnn) decreases with increasing Ueff. Moreover, the Jnn value with the best agreement with experiment is found for Ueff values between 2.55 and 4.0 ​eV. Without the addition of Ueff corrections, i.e., with standard LSDA calculations, the magnitude of Jnn is overestimated. The Bader topological analysis of the electron density at the bond critical point shows that the closed-shell interaction of the Mn–O3 and Mn–O4 bonds in both materials increases with increasing Ueff, which justifies the decrease of Jnn with increasing Ueff.. Additionally, both interplane and intraplane super-superexchange interactions are computed and compared with previous studies.

Advanced First-Principle Modeling of Relativistic Ruddlesden—Popper Strontium Iridates

In this review, we provide a survey of the application of advanced first-principle methods on the theoretical modeling and understanding of novel electronic, optical, and magnetic properties of the spin-orbit coupled Ruddlesden–Popper series of iridates Srn+1IrnO3n+1 (n = 1, 2, and ∞). After a brief description of the basic aspects of the adopted methods (noncollinear local spin density approximation plus an on-site Coulomb interaction (LSDA+U), constrained random phase approximation (cRPA), GW, and Bethe–Salpeter equation (BSE)), we present and discuss select results. We show that a detailed phase diagrams of the metal–insulator transition and magnetic phase transition can be constructed by inspecting the evolution of electronic and magnetic properties as a function of Hubbard U, spin–orbit coupling (SOC) strength, and dimensionality n, which provide clear evidence for the crucial role played by SOC and U in establishing a relativistic (Dirac) Mott–Hubbard insulating state in Sr2IrO4 and Sr3Ir2O7. To characterize the ground-state phases, we quantify the most relevant energy scales fully ab initio—crystal field energy, Hubbard U, and SOC constant of three compounds—and discuss the quasiparticle band structures in detail by comparing GW and LSDA+U data. We examine the different magnetic ground states of structurally similar n = 1 and n = 2 compounds and clarify that the origin of the in-plane canted antiferromagnetic (AFM) state of Sr2IrO4 arises from competition between isotropic exchange and Dzyaloshinskii–Moriya (DM) interactions whereas the collinear AFM state of Sr3Ir2O7 is due to strong interlayer magnetic coupling. Finally, we report the dimensionality controlled metal–insulator transition across the series by computing their optical transitions and conductivity spectra at the GW+BSE level from the the quasi two-dimensional insulating n = 1 and 2 phases to the three-dimensional metallic n=∞ phase.

Nature of the magnetic moment of cobalt in ordered FeCo alloy

The magnets are typically classified into Stoner and Heisenberg type, depending on the itinerant or localized nature of the constituent magnetic moments. In this work, we investigate theoretically the behaviour of the magnetic moments of iron and cobalt in their B2-ordered alloy. The results based on local spin density approximation for the density functional theory (DFT) suggest that the Co magnetic moment strongly depends on the directions of the surrounding magnetic moments, which usually indicates the Stoner-type mechanism of magnetism. This is consistent with the disordered local moment picture of the paramagnetic state, where the magnetic moment of cobalt gets substantially suppressed. We argue that this is due to the lack of strong on-site electron correlations, which we take into account by employing a combination of DFT and dynamical mean-field theory (DMFT). Within LDA + DMFT, we find a substantial quasiparticle mass renormalization and a non Fermi-liquid behaviour of Fe–3d orbitals. The resulting spectral functions are in very good agreement with measured spin-resolved photoemission spectra. Our results suggest that local correlations play an essential role in stabilizing a robust local moment on Co in the absence of magnetic order at high temperatures.

Self-interaction correction in water-ion clusters.

We study the importance of self-interaction errors in density functional approximations for various water-ion clusters. We have employed the Fermi-Löwdin orbital self-interaction correction (FLOSIC) method in conjunction with the local spin-density approximation, Perdew-Burke-Ernzerhof (PBE) generalized gradient approximation (GGA), and strongly constrained and appropriately normed (SCAN) meta-GGA to describe binding energies of hydrogen-bonded water-ion clusters, i.e., water-hydronium, water-hydroxide, water-halide, and non-hydrogen-bonded water-alkali clusters. In the hydrogen-bonded water-ion clusters, the building blocks are linked by hydrogen atoms, although the links are much stronger and longer-ranged than the normal hydrogen bonds between water molecules because the monopole on the ion interacts with both permanent and induced dipoles on the water molecules. We find that self-interaction errors overbind the hydrogen-bonded water-ion clusters and that FLOSIC reduces the error and brings the binding energies into closer agreement with higher-level calculations. The non-hydrogen-bonded water-alkali clusters are not significantly affected by self-interaction errors. Self-interaction corrected PBE predicts the lowest mean unsigned error in binding energies (≤50 meV/H2O) for hydrogen-bonded water-ion clusters. Self-interaction errors are also largely dependent on the cluster size, and FLOSIC does not accurately capture the subtle variation in all clusters, indicating the need for further refinement.

Exploring and enhancing the accuracy of interior-scaled Perdew-Zunger self-interaction correction.

The Perdew-Zunger self-interaction correction (PZ-SIC) improves the performance of density functional approximations for the properties that involve significant self-interaction error (SIE), as in stretched bond situations, but overcorrects for equilibrium properties where SIE is insignificant. This overcorrection is often reduced by local scaling self-interaction correction (LSIC) of the PZ-SIC to the local spin density approximation (LSDA). Here, we propose a new scaling factor to use in an LSIC-like approach that satisfies an additional important constraint: the correct coefficient of the atomic number Z in the asymptotic expansion of the exchange-correlation (xc) energy for atoms. LSIC and LSIC+ are scaled by functions of the iso-orbital indicator zσ, which distinguishes one-electron regions from many-electron regions. LSIC+ applied to the LSDA works better for many equilibrium properties than LSDA-LSIC and the Perdew, Burke, and Ernzerhof generalized gradient approximation (GGA), and almost close to the strongly constrained and appropriately normed (SCAN) meta-GGA. LSDA-LSIC and LSDA-LSIC+, however, fail to predict interaction energies involving weaker bonds, in sharp contrast to their earlier successes. It is found that more than one set of localized SIC orbitals can yield a nearly degenerate energetic description of the same multiple covalent bond, suggesting that a consistent chemical interpretation of the localized orbitals requires a new way to choose their Fermi orbital descriptors. To make a locally scaled down SIC to functionals beyond the LSDA requires a gauge transformation of the functional's energy density. The resulting SCAN-sdSIC, evaluated on SCAN-SIC total and localized orbital densities, leads to an acceptable description of many equilibrium properties including the dissociation energies of weak bonds.

Full potential study of electronic and thermoelectric properties of GdNiSb compound

In the present work we have investigated electronic and thermoelectric properties of GdNiSb compound. The calculations have been performed using local spin density approximation (LSDA) with Hubbard parameter (U) included for exchange correlation effect. The calculated results of a cubic half-Heusler phase of GdNiSb compound shows semiconducting behaviour due to a narrow band gap (0.279 eV) is opened. Thermoelectric properties such as electrical conductivity (σ/τ), Seebeck coefficient (S) and figure of merit (ZT) was calculated. Values of thermoelectric parameters show that GdNiSb compound could be used as potential material for thermoelectric applications.

A first principle study of structural, opto-electronic and thermoelectric properties of Sm filled skutterudite SmRu4Sb12

Structural, elastic, electronic, magnetic, thermodynamic, optical and thermoelectric properties of a filled skutterudite SmRu4Sb12 has been studied using full-potential linearized augmented plane wave (FP-LAPW) method based on density functional theory (DFT). Local spin density approximation (LSDA) is used for the exchange–correlation potential. The material is found to be harder than the isostructural Ce filled skutterudites. Bulk modulus and Debye temperature are nearly constant up to 100 K. The spin-orbit interaction has significant influence on the band structures. It causes the splitting of the f-band and results in an effective magnetic moment of 4.88 μB indicating its ferromagnetic nature. Optical constants including the dielectric function, optical reflectivity, refractive index, electron energy loss and optical conductivity have also been calculated over a wide range of photon energy. The material has high reflectivity and low absorption near the Fermi energy and signature of surface plasmon effect is also visible at higher photon energy. Due to high concentration of flat bands near Fermi level the material shows high Seebeck coefficient and moderately high thermoelectric figure of merit (ZT).

Band structure of topological insulator Li(Na)AuS

Half-Heusler semiconductors exhibit similar properties: the differences among their properties lie only in the fact that in ternary compositions the zinc-blende binary substructure does not provide the required 18 electrons, but this is improved by adding an extra transition metal, which restores the electronic balance. Half-Heusler ternary compound with 18 valence electrons under an appropriate uniaxial strain is a topological insulating phase. Most importantly, it is proposed that in the half-Heusler family, the topological insulator should allow the incorporating of superconductivity and magnetism. Using the first-principle full-potential linearized augmented wave method we study the band structure of a series of Li(Na)AuS topological insulators. The electronic and magnetic properties of Heusler alloys are investigated by the WIEN2k package. The exchange-correlations are treated within the generalized gradient approximation of PerdeweBurke and Ernzerhof (GGA), the local spin density approximation (LSDA), by using the modified Becke-Johnson exchange potential and the correlation potential of the local-density approximation (MBJ). Spin-orbit coupling is treated by means of the second variational procedure with the scalar-relativistic calculation as basis. We first determine the equilibrium lattice constants by calculating the total energy. The theoretical lattice constant of LiAuS full-potential GGA is 6.02 Å, which is somewhat greater than the result of pseudopotential(5.99 Å). The calculated equilibrium lattice parameter is 5.86 Å for LSDA. Most of the half-Heusler compounds have band inversion, and open the nature band gaps, but the gap of MBJ is not very good. Smaller uniaxial stress damages the cubic structure and also such a natural band gap of topological insulators. By applying uniaxial tensile stress until the equilibrium position is reached in all directions of the structure, the system band gap value is about 0.2 eV, which is consistent with the result obtained from the band gap of cubic structure equilibrium position. When uniaxial tensile stress is 41%, the system turns into a tetragonal structure, the equilibrium lattice constant is &lt;i&gt;a &lt;/i&gt;= 5.2477 Å and &lt;i&gt;c&lt;/i&gt;/&lt;i&gt;a&lt;/i&gt; = 1.41. We use the method of substitution of homologous elements to ensure the properties of topological insulator of materials without changing the cubic structure, and open the bandgap of materials under the equilibrium lattice constant of the system, thereby improving the feasibility of experimental synthesis of topological insulator materials. Our results for the doping suggest that epitaxial strain encountered during experiment can result in electronic topological transition. We hope that the results presented here conduce to further experimental investigation of the electronic topological transition in half-Heusler compounds.

On the Accuracy of the Direct Method to Calculate pKa from Electronic Structure Calculations.

The direct method (HA(soln) ⇌ A(soln)– + H(soln)+) for calculating pKa of monoprotic acids is as efficient as thermodynamic cycles. A selective adjustment of proton free energy in solution was used with experimental pKa data. The procedure was analyzed at different levels of theory. The solvent was described by the solvation model density (SMD) model, including or not explicit water molecules, and three training sets were tested. The best performance under any condition was obtained by the G4CEP method with a mean absolute error close to 0.5 units of pKa and an uncertainty around ±1 unit of pKa for any training set including or excluding explicit solvent molecules. PM6 and AM1 performed very well with average absolute errors below 0.75 units of pKa but with uncertainties up to ±2 units of pKa, using only the SMD solvent model. Density functional theory (DFT) results were highly dependent on the basis functions and explicit water molecules. The best performance was observed for the local spin density approximation (LSDA) functional in almost all calculations and under certain conditions, as high as those obtained by G4CEP. Basis set complexity and explicit solvent molecules were important factors to control DFT calculations. The training set molecules should consider the diversity of compounds.

Electronic Structure and Thermoelectric Performance of Narrow‐Gap GdNiSb Half Heusler Compound: Density Functional Calculations

The rare earth‐based half Heusler GdNiSb compound is investigated theoretically for electronic and thermoelectric (TE) performance. The most efficient method to treat the rare earth metals, Coulomb‐corrected local spin density approximation (LSDA + U), is used. The calculated total density of states (DOS) of GdNiSb shows that the maximum contribution near Fermi energy (EF) is due to the spin‐up/down Ni‐3d and spin‐down Gd‐4f states. A narrow energy gap of 0.279 eV is opened which is consistent with other reported values. TE properties are investigated using classical transport theory. The total Seebeck coefficient (S) increases linearly with temperature and attains a maximum value of 80 μV K−1 at 800 K. The S value of 60 μV K−1 at 380 K is in good agreement with the experimental value, 58 μV K−1, at the same temperature. The maximum value of the power factor (P) is as high as 825 × 109 W m−1 K−2 s−1 at 650 K in the spin‐down channel. The TE figure of merit (ZT) reaches the maximum value of ≈0.475 at 800 K in the spin‐down configuration. All the values of the TE parameters of GdNiSb indicate a good candidate for TE applications.

Structural Stability and Magnetic Ordering in BiFeO3 Perovskite Oxide: A Comparative Study GGA+U vs L(S)DA+U

Abstract Ab initio calculations of BiFeO3 magnetic perovskite are carried. Accurate density functional theory calculations were performed considering a U-Hubbard correction (DFT+U) to account for on-site Coulomb interactions of the 3d-Fe states. We have applied the Full-potential linearized augmented plane waves (FP-LAPW) method. Exchange-correlation effects are treated using the Local Spin Density approximation (L(S)DA+U) vs generalized gradient approximations (GGA+U). Equilibrium lattices agree very well with other theoretical and experimental data. The magnetization energy differences between Spin Up and Spin Dn states are small. Spin effect and magnetic moment obtained from subsequent (L(S)DA+U) and (GGA+U) calculations are also discussed in different magnetic configurations: The Ferromagnetic cubic phase (Pm-3m), The A-type Antiferromagnetic (P4/mmc) and The G-type Antiferromagnetic (Fm-3m). The nature of magnetism arises mainly from the Fe-site exhibiting a G-type antiferromagnetic ordering. The electronic structure shows that BiFeO3 has a metallic band gap. This multiferroic exhibit strong hybridization of the 3d-Fe and 2p-O orbitals. Therefore, the Multiferroic BiFeO3 perovskite has driven significant research interest due to their promising technological potential. It’s a good candidate for potential applications in spintronic, and to aid the development of the next generation of data storage and multi-functional technological devices.