Full-potential Calculations Research Articles

H — P Finite element approximation for full-potential electronic structure calculations

The (continuous) finite element approximations of different orders for the computation of the solution to electronic structures was proposed in some papers and the performance of these approaches is becoming appreciable and is now well understood. In this publication, the author proposes to extend this discretization for full-potential electronic structure calculations by combining the refinement of the finite element mesh, where the solution is most singular with the increase of the degree of the polynomial approximations in the regions where the solution is mostly regular. This combination of increase of approximation properties, done in an a priori or a posteriori manner, is well-known to generally produce an optimal exponential type convergence rate with respect to the number of degrees of freedom even when the solution is singular. The analysis performed here sustains this property in the case of Hartree-Fock and Kohn-Sham problems.

GGA-FPKKR 法による Fe 基希薄合金中の点欠陥エネルギーの第一原理計算

GGA-FPKKR 法による Fe 基希薄合金中の点欠陥エネルギーの第一原理計算

Recent Advances in the Korringa-Kohn-Rostoker Green Function Method

The Korringa-Kohn-Rostoker (KKR) Green function (GF) method is a technique for all-electron full-potential density-functional calculations. Similar to the historical Wigner-Seitz cellular method, the KKR-GF method uses a partitioning of space into atomic Wigner-Seitz cells. However, the numerically demanding wave-function matching at the cell boundaries is avoided by use of an integral equation formalism based on the concept of reference Green functions. The advantage of this formalism will be illustrated by the recent progress made for very large systems with thousands of inequivalent atoms and for very accurate calculations of atomic forces and total energies.

Full-Potential KKR Calculations for Lattice Distortion Effect of Point Defect in bcc-Fe Dilute Alloys, Based on the Generalized-Gradient Approximation

Full-Potential KKR Calculations for Lattice Distortion Effect of Point Defect in bcc-Fe Dilute Alloys, Based on the Generalized-Gradient Approximation

Electronic structure of stacking faults in hexagonal graphite

We present results of self-consistent, full-potential electronic structure calculations for slabs of hexagonal graphite with stacking faults and for slabs with one displaced surface layer. There are two types of stacking faults, which differ qualitatively in their chemical bonding picture. We find that both types induce localized interface bands near the symmetry line $K$-$M$ in the Brillouin zone and a related peak in the local density of states (LDOS) very close to the Fermi energy, which should give rise to a dominating contribution of the interface bands to the local conductivity at the stacking faults. In contrast, a clean surface does not host any surface bands in the energy range of the $\ensuremath{\pi}$ and $\ensuremath{\sigma}$ bands, and the LDOS near the surface is even depleted. On the other hand, displacement of even one single surface layer induces a surface band near $K$-$M$. A special role is played by ${p}_{z}$-bonded dimers (directed perpendicular to the layers) in the vicinity of one type of stacking faults. They produce a half-filled pair of interface states or interface resonances. The formation energy of both types of stacking faults and the surface energy are estimated.

Errata: Graphene and Some of its Structural Analogues: Full-potential Density Functional Theory Calculations [WJOE 10(1) (2013) 39-48

List of Corrections: 1. Page No. 41, Left Column, Paragraph No.1, Line No. 11, Read “ 2 μμeV/ato.” as “ 2 μeV/atom.” 2. Page No. 41, Right Column, Paragraph No. 2, Line No. 11, Read “Mukhopadhyay, 2012)” as “Mukhopadhyay 2010)” 3. Page No. 43, Table 2, Second Row for MLBN, Read “183.91” as “181.91” 4. Page No. 44, Left Column, Read Equation No. 1 as “E± = ± VF hk” 5. Page 44, Left Column, Read Line No. 2 as “charge carriers in graphene and (h k) is the” 6. Page 45, Table 3, Row No. 8 and Column 2 for ML-SiC, Read “2.39 (K→M)” as “2.39” 7. Page 45, Table 3, Row No. 15 and Column 2 for ML-BN, Read “4.35 (K→Γ)” as “4.35” 8. Page 45, Table 3, Row Nos. 22-26 and Column 2 for ML-ZnO, ML-ZnS and MLZnSe, Read “(K→Γ)” as “(Γ→Γ)” 9. Page 46, Left Column, Paragraph 1 (References), Last Line, Read “Phys. Rev. Lett. 96, 66-102” as “Phys. Rev. Lett. 96, 066102.” Errata: Graphene and Some of its Structural Analogues: Full-potential Density Functional Theory Calculations [WJOE 10(1) (2013) 39-48]

Graphene and some of its structural analogues: full-potential density functional theory calculations

Using full-potential density functional calculations we have investigated the structural and electronic properties of graphene and some of its structural analogues, viz., monolayer (ML) of SiC, GeC, BN, AlN, GaN, ZnO, ZnS and ZnSe. While our calculations corroborate some of the reported results based on different methods, our results on ZnSe, the two dimensional bulk modulus of ML-GeC, ML-AlN, ML-GaN, ML-ZnO and ML-ZnS and the effective masses of the charge carriers in these binary mono-layers are something new. With the current progress in synthesis techniques, some of these new materials may be synthesized in near future for applications in nano-devices.

Full potential calculations and atom in molecule analysis of the bonding properties of perovskites Borides XRh3B (X=Dy, Ho, Er)

ab initio calculations were performed for the cubic perovskites Borides XRh3B, (X=Dy, Ho, Er). In this work, we have used the augmented plane-wave plus local orbital method to compute the equilibrium structural parameters and electronic structure of densities of states, as well as for the first time, prediction of the thermo-elastic prop- erties of these crystals are presented. The chemical bonding of these compounds has been investigated by using of topological analyses grounded in the theory of atoms in molecules (AIM). All of the electron density critical points in the unit cell were system- atically calculated in order to calculate basins interaction of each atoms and give exact classification of the bonding character.

Full-Potential KKR Calculations for Point Defect Energies in Fe-Based Dilute Alloys, Based on the Generalized-Gradient Approximation

We present systematically ab-initio calculations for defect energies of 3d and 4sp impurities (Sc­Ge) in Fe. The calculations are based on the Generalized-Gradient-Approximation in the density-functional formalism and the full-potential Korringa-Kohn-Rostoker (FPKKR) Green’s function method. First we examine the distance dependence, from the 1st- to the 10th-neighbors, of the impurity-impurity (I­I; I = Sc­Ge) interaction energies (Eint) and show that for most cases, the 1st-neighboring I­I interaction energies (E 1) are dominant. We found that fundamental features of phase diagrams of Fe-based binary alloys, such as segregation, solid solution and order, known experimentally, may be classified by use of the sign and magnitude of E 1. Second we discuss the calculated results for the 1st- and 2nd-neighboring interaction energies of 3d and 4sp impurities with perturbed-angular-correlation (PAC)-probe Sn in Fe. The comparison of the calculated results with available experimental results shows that the observed attraction for Sn­Co, Sn­Ni and Sn­Cu may be understood by the 1st-neighboring interaction energies of these impurity pairs, while the obsreved repulsion for Sn­Ga, and Sn­Ge by the 2nd-neighboring interaction energies of these impurity pairs. We also discuss the magnetism of single impurities X (= Sc­Cu) in Fe. The anti-parallel coupling to the bulk magnetization of the neighboring Fe atoms is stable for Sc­Mn, while the parallel coupling for Fe­Cu. [doi:10.2320/matertrans.M2013176]

Erratum to: Full-Potential Calculation of Structural, Electronic, and Thermodynamic Properties of Fluoroperovskite $$\text{ CsMF}_{3}$$ (M = Be and Mg)

The structural and electronic properties of the cubic fluoroperoveskite $$\text{ CsBeF}_{3}$$ and $$\text{ CsMgF}_{3}$$ have been investigated using the full-potential-linearized augmented plane wave method within the density functional theory. The exchange-correlation potential was treated with the local density approximation and the generalized gradient approximation. The calculations of the electronic band structures show that $$\text{ CsBeF}_{3 }$$ has an indirect bandgap, whereas $$\text{ CsMgF}_{3}$$ has a direct bandgap. Through the quasi-harmonic Debye model, in which the phononic effects are considered, the effect of pressure $$P$$ and temperature $$T$$ on the lattice parameter, bulk modulus, thermal expansion coefficient, Debye temperature, and the heat capacity for $$\text{ CsBeF}_{3}$$ and $$\text{ CsMgF}_{3}$$ compounds are investigated for the first time.

Importance of anisotropic Coulomb interactions and exchange to the band gap and antiferromagnetism ofβ-MnO2from DFT+U

First principles density functional theory (DFT) is used to investigate the electronic structure of \beta-MnO2. From collinear spin polarized calculations we find that DFT+U_Eff predicts a gapless ferromagnet in contrast with experiment which indicates an insulating antiferromagnet. The inclusion of anisotropic Coulomb and exchange interactions in the DFT+U approach, defining U and J explicitly, corrects these errors and leads to an antiferromagnetic ground state with a fundamental gap of 0.8 eV consistent with low temperature experiments. To our knowledge, this work on \beta-MnO2 represents the first demonstration of a case in which the application of fully anisotropic interactions in DFT+U determines the magnetic order and consequent band gap, while the more commonly used effective U approach fails. Such effects are argued to be of importance in many insulating materials. The mechanism leading to an increase in band gap due to anisotropic interactions is highlighted by analytical calculation of DFT+U d-orbital eigenvalues obtained within a Kanamori-type model. Magnetic coupling constants obtained by the fitting of a Heisenberg spin Hamiltonian to the energies of a range of magnetic states assist in rationalizing the finding that anisotropic interactions enhance the stability of the experimentally observed helical antiferromagnetic order. The plane wave PAW method yields poorer results for the exchange couplings than full-potential LAPW calculations. Finally, we compare the DFT+U results with exchange couplings obtained from hybrid functionals. It is argued that anisotropic interactions should be included in DFT+U if the results are to be properly compared with those from hybrid functionals.

NMR compared to band structure calculations of the quaternary superconductor La3Ni2B2N3−x

We measured susceptibility and NMR spectra, spin-lattice, and spin-spin relaxation of ${}^{11}$B and ${}^{139}$La in the quaternary superconductor La${}_{3}$Ni${}_{2}$B${}_{2}$N${}_{3\ensuremath{-}x}$ and compare these experimental results to full potential density functional theory band structure calculations. The electric field gradients (EFGs) at B and La sites clearly show that the N vacancies are restricted to the N(2$b$) sublattice. Temperature-dependent susceptibility, Knight shift, and nuclear spin-lattice relaxation ${T}_{1}$ are successfully described within the Fermi-liquid model. The Hebel-Slichter peak is suppressed in the mixed state and in the presence of paramagnetic impurities.

Insulator-metal transition in TiGePt: A combined photoelectron spectroscopy, x-ray absorption spectroscopy, and band structure study

We present a combined experimental and theoretical study of the electronic structure of the intermetallic compound TiGePt by means of photoelectron spectroscopy, x-ray absorption spectroscopy, and full potential band structure calculations. It was recently shown [S.-V. Ackerbauer et al., Chemistry - a European Journal 18, 6272 (2012)] that TiGePt undergoes a structural phase transition by heating which is accompanied by a large volume contraction and a drastic change of physical properties, in particular a large decrease of the electrical resistivity. The present study revealed substantial differences in the electronic structure for the two TiGePt modifications, although they have the same nominal composition and show similar electron counts for particular valence band states. Our photoemission experiments and band structure calculations establish that an insulator-to-metal transition occurs with an appreciable band broadening and closing of the band gap.

Full potential x-ray absorption calculations using time dependent density functional theory

We report the implementation of a fully relativistic time dependent density functional theory (TDDFT) method for carrying out x-ray absorption spectroscopy calculations for extended systems. This is the first time that a TDDFT simulation of x-ray absorption in extended systems has featured a full potential ground state calculation. We prove that this unusual feature of the TDDFT implementation unequivocally yields improvement over the previous muffin-tin calculation methods.

Strain-tunable band gap in graphene/h-BN hetero-bilayer

Using full-potential density functional calculations within local density approximation (LDA), we predict that mechanically tunable band-gap and quasi-particle-effective-mass are realizable in graphene/hexagonal-BN hetero-bilayer (C/h-BN HBL) by application of in-plane homogeneous biaxial strain. While providing one of the possible reasons for the experimentally observed gap-less pristine-graphene-like electronic properties of C/h-BN HBL, which theoretically has a narrow band-gap, we suggest a schematic experiment for verification of our results which may find applications in nano-electromechanical systems (NEMS), nano opto-mechanical systems (NOMS) and other nano-devices based on C/h-BN HBL.

Structural, thermodynamic, electronic, and optical properties of NaH from first-principles calculations

The structural stability, thermodynamic, electronic, and optical properties of NaH with rock salt (B1) structure and cesium chloride (B2) structure under high pressure are investigated by first-principles calculations using norm-conserving pseudopotential applying a generalized gradient approximation (GGA) for exchange and correlation. Through the analysis of energy–volume variation, we find the phase transition of NaH from B1 to B2 structure occurs at 32.3GPa, which in good agreement with the diamond-anvil-cell high-pressure experimental value of 29.3±0.9GPa [Phys. Rev. B 36 (1987) 7664]. By using the quasi-harmonic Debye model, the thermodynamic properties including the Debye temperature ΘD, heat capacity CV, thermal expansion coefficient α, and Grüneisen parameter γ are successfully obtained in the temperature range from 0 to 700K and pressure ranges from 0 to 32GPa and 33 to 100GPa for NaH B1 and B2 phases, respectively. Analysis of band structures suggests that the NaH has an indirect band gap that the valence band maximum is at the W point and the conduction minimum locates at L point. The calculated energy gaps is very close to that value obtained in recent full potential augmented plane wave calculations. The optical properties including dielectric function ɛ(ω), absorption coefficient α(ω), reflectivity coefficient R(ω), and refractive index n(ω) are also calculated and analyzed.

Magnetic model forA2CuP2O7(A=Na, Li): One-dimensional versus two-dimensional behavior

We report magnetization measurements, full-potential band structure calculations, and microscopic modeling for the spin-1/2 Heisenberg magnets A2CuP2O7 (A = Na, Li). Based on a quantitative evaluation of the leading exchange integrals and the subsequent quantum Monte-Carlo simulations, we propose a quasi-one-dimensional magnetic model for both compounds, in contrast to earlier studies that conjectured on the two-dimensional scenario. The one-dimensional nature of A2CuP2O7 is unambiguously verified by magnetization isotherms measured in fields up to 50 T. The saturation fields of about 40 T for both Li and Na compounds are in excellent agreement with the intrachain exchange J1 ~ 27 K extracted from the magnetic susceptibility data. The proposed magnetic structure entails spin chains with the dominating antiferromagnetic nearest-neighbor interaction J1 and two inequivalent, nonfrustrated antiferromagnetic interchain couplings of about 0.01*J1 each. A possible long-range magnetic ordering is discussed in comparison with the available experimental information.

Band structure and optical properties of hexagonal In-rich InxAl1−xN alloys

Full potential linear augmented plane wave calculations have been performed to study the electronic and optical properties of In-rich InxAl1−xN alloys in the hexagonal wurtzite structure. Compositions of x = 0.9375, 0.8125 and 0.6875 are considered which follow from replacing one, three and five In atoms by Al in the 32-atom supercell. The new form of exchange correlation, i.e. Engel–Vosko’s generalized gradient approximation within density functional theory, is employed. The calculations yield the band structure and total density of states as well as the imaginary part ε2(ω) of the ordinary and extraordinary dielectric function. The calculated dependence of the bandgap on the composition is in good agreement with recent experimental studies. A reversal of the valence band ordering is found between x = 0.8125 and 0.6875. The absorption features in the high-energy range of ε2(ω) are related to critical points of the band structure. The transition energies for these van Hove singularities are determined and their bowing parameters are discussed.

Structural properties and bonding nature of 3-methyl-4-phenyl-5-(2-pyridyl)-1,2,4-triazole single crystal

The atomic positions of 3-methyl-4-phenyl-5-(2-pyridyl)-1,2,4-triazole single crystal taken from the X-ray diffraction data obtained by Liu et al. [1] were optimized by minimization of the forces acting on the atoms using a full potential linear augmented plane wave method within density-functional theory. The local density approximation (LDA) by Ceperley–Alder (CA) exchange–correlation potential was applied. The full potential calculations show that the valence band maximum (VBM) and conduction band minimum (CBM) are located at Γ point of the Brillouin zone resulting in a direct energy gap of 3.4 eV for the original experimental structure and 3.2 eV for the optimized structure. The upper VB is formed mainly by C, H, and N atoms while the lower CB has mainly C and H atoms. We present an analysis of the partial densities of states which gives useful information on hybridization and the orbital character of the states. We have calculated the bond lengths and angles, a good agreement with the experimental data were found. The origin of the chemical bonding and the electronic charge density contour in the (1 0 0) and (1 1 0) planes were analyzed.

Full potential calculation of electronic properties of rutile RO 2 (R=Si, Ge, Sn and Pb) compounds via modified Becke Johnson potential

The electronic properties of RO 2 (R=Si, Ge, Sn and Pb; a group IVA element) compounds in rutile structure have been calculated using WIEN2k implementation of full potential linearized augmented plane wave (FPLAPW) method. The exchange and correlation (XC) effects are taken into account by an orbital independent modified Becke Johnson (MBJ) potential as coupled with Local Density Approximation (LDA) for all the compounds except for PbO 2 where only Generalized Gradient Approximation (GGA) is considered for the same. We predict a direct band gap in all these compounds with continuous decrease as the atomic size of IVA element increases such that there is an appearance of semimetallic band structure for the last compound, PbO 2. The largest band gap (7.66 eV) has been found for SiO 2, which governs its insulating nature. We observe that MBJLDA results for band gaps of these compounds are far better than those obtained using GGA and Engel–Vosko's GGA (EV-GGA). A very good agreement is observed between MBJLDA band gaps with corresponding experimental values as compared to other calculations. The electronic band structures are also analyzed in terms of contributions from various electrons.