Local Density Approximation Method Research Articles

Self-Diffusion Coefficient of fcc Mg: First-Principles Calculations and Semi-Empirical Predictions

First-principles calculations and semi-empirical equations are employed to determine the self-diffusion mobility of fcc Mg. All factors entering the vacancy-mediated self-diffusion coefficient, which include the equilibrium lattice parameter, the enthalpy of vacancy formation and atom migration, and the vibrational entropy of vacancy formation as well as the effective frequency, are evaluated with the local density approximation (LDA) and generalized gradient approximation (GGA) in first-principles calculations. These computed quantities are then utilized to calculate the self-diffusion coefficient of fcc Mg. For comparison, four widely used semi-empirical equations are also used to estimate the self-diffusion coefficient of fcc Mg. The comparisons show that first-principles calculations and semi-empirical equations yield values close to the activation energy Q for self-diffusion of fcc Mg from the LDA calculation, but the diffusion prefactor D 0 predicted from the four semi-empirical equations are all about one order of magnitude larger than those from the first-principles calculations. Based on the comparison for the self-diffusion coefficients of fcc Al computed with the first-principles method and semi-empirical approaches, it is concluded that the self-diffusion coefficient of fcc Mg calculated with the LDA method is more accurate than the estimation from semi-empirical approaches.

High pressure, mechanical, and optical properties of ZrW2O8

In this paper, we present the high pressure studies of well known negative thermal expansion material ZrW2O8 on the basis of our density functional theory calculations. We reproduce the experimental α and γ crystal structures of ZrW2O8. We calculate the transition pressure (from α to γ) of this material to be ≈26 kbar. Our calculated positional and axial parameters are in an excellent agreement with the available experimental values for the both α and γ phases of this material. Then we study the mechanical properties of this material. We calculate the elastic constants, bulk, shear and Young’s moduli, Poisson’s ratio, and Debye temperature of α-ZrW2O8. In our study, we find that the generalized gradient approximation method fails to obtain the correct values while the local density approximation (LDA) method successfully reproduces the experimental bulk modulus of ZrW2O8. Our calculated values of the shear and Young’s moduli, Poisson’s ratio, and Debye temperature of α-ZrW2O8 with LDA method are also in good agreement with the experimental results. Finally, we predict and analyze the optical properties of this material.

Structural, electronic and thermodynamic properties of cubic Zn 3N 2 under high pressure from first-principles calculations

The structural, electronic and thermodynamic properties of cubic Zn 3N 2 under hydrostatic pressure up to 80 GPa are investigated using the local density approximation method with pseudopotentials of the ab initio norm-conserving full separable Troullier–Martin scheme in the frame of density functional theory. The structural parameters obtained at ambient pressure are in agreement with experimental data and other theoretical results. The change of bond lengths of two different types of Zn–N bond with pressure suggests that the tetrahedral Zn–N bond is slightly less compressible than the octahedral bond. By fitting the calculated band gap, the first and second order pressure coefficients for the direct band gap ofthe Zn 3N 2 were determined to be 1.18×10 −2 eV/GPa and −2.4×10 −4 eV/(GPa) 2, respectively. Based on the Mulliken population analysis, Zn 3N 2 was found to have a higher covalent character with increasing pressure. As temperature increases, heat capacity, enthalpy, product of temperature and entropy increase, whereas the Debye temperature and free energy decrease. The present study leads to a better understanding of how Zn 3N 2 materials respond to compression.

First Principle Study of Cubic SrMO<sub>3</sub> Perovskites (M = Ti, Zr, Mo, Rh, Ru)

Lattice parameter and corresponding total free energy have been computed for cubic SrMO3 perovskites (M = Ti, Zr, Mo, Rh, Ru) using the first principle approach within Density functional theory. The results have been calculated using local density approximation (LDA) method. It is found that the calculated lattice parameter for all transition metal oxides are in good agreement with the available experimental data. The total free energy corresponding to this lattice constant has been calculated along with different components of the total free energy. All these calculations have been carried out using ABINIT computer code.

The study of structure characteristics of GeTe and GeSe molecules under the external electric field

Equilibrium structures of the GeTe and GeSe ground state molecules are obtained by employing the local spin density approximation method with 6-311++G** basis sets for Ge and SDB-cc-pVTZ for Te and Se. Also obtained are the equilibrium geometry, the highest occupied molecular orbital(HOMO) energy level, the lowest unoccupied molecular orbital(LUMO)energy level, the energy gap, the harmonic frequency and the infrared intensity of GeTe and GeSe ground state molecules under different electric fields. On the basis of the above calculation, the excited states of GeTe and GeSe molecules under different electric fields are also investigated by using the single-excitation configuration interaction-local spin density approximation method. The results show that the equilibrium internuclear distance and the intensity of infrared are found to increase, but the total energy and harmonic frequency are proved to decrease with the increase of positive direction electric field. The HOMO energy EH of GeTe molecule is higher than that of GeSe molecule under electric fields ranging from 0 to 2.05691010 V m-1. For GeTe and GeSe molecules, their difference in EH gradually increases with the increase of positive direction electric field. The LUMO energy EL of GeTe molecule is lower than that of GeSe molecule, and their LUMO energies are found to increase with the increase of positive direction electric field. The energy gap of GeTe is low than that of GeSe, and their energy gaps always decrease with the increase the negative direction electric field. The magnitude and the direction of the external electric field have important effects on excitation energy, oscillator strength and wavelength.

Structural, elastic, and lattice dynamical properties of Germanium diiodide (GeI 2)

To investigate the structural, elastic, and lattice dynamical properties of the germanium diiodide, we have performed the first-principles calculations by using the local density approximation method based on density-functional theory. Some basic physical parameters such as lattice constant, bulk modulus and its first derivatives, elastic constants, shear modulus, Young’s modulus, and Poisson’s ratio are calculated. The phonon dispersion curves, electronic band-structures, and total and partial density of states have also been calculated for ground state C6 phase of GeI 2. Our results show that this structure has got 1.72 eV direct band gap. Our secondary results on the temperature-dependent behavior of thermodynamical properties such as entropy, heat capacity, internal energy, and free energy are also presented for the same compounds. The obtained results are in good agreement with the available experimental and other theoretical data.

Ionic layered BaFCl andBa1−xSrxFClcompounds: Physical- and chemical-pressure effects

The effect on crystal structure and vibrational frequencies of physical pressure in BaFCl and chemical pressure in ${\text{Ba}}_{1\ensuremath{-}x}{\text{Sr}}_{x}\text{FCl}$ solid solutions is studied using periodic density-functional theory (DFT) calculations performed within the local-density approximation (LDA) and the generalized gradient approximation (GGA). These results are compared with previously published experimental data for BaFCl in conjunction with new experimental data for ${\text{Ba}}_{1\ensuremath{-}x}{\text{Sr}}_{x}\text{FCl}$ and show overall a good agreement with experiment. The GGA method outperforms the LDA method for the description of BaFCl under pressure. However, the two DFT methods perform equally well for the description of the solid solutions, which have been studied within the virtual-crystal approximation. They also give consistent values of the energy of formation of ${\text{Ba}}_{1\ensuremath{-}x}{\text{Sr}}_{x}\text{FCl}$, which can be correlated with the experimentally observed melting points. The comparison of the calculated mode Gr\"uneisen parameters shows that, for the investigated systems, the effect of the chemical pressure and that of the physical pressure are not identical.

Study of Co2MnAl Heusler alloy as half metallic ferromagnet

We present the study of half metallacity of Co2MnAl as half-Heusler alloy in L21 structure which consists of four inter-penetrating fcc sub-lattices. Density functional theory based electronic structure calculations will be performed by using the full-potential linear augmented plane wave (FP-LAPW). We will use the general gradient approximation method (GGA) and local density approximation method called LDA+U. Density of states and band structure results will be presented in this paper.

Electronic Structure and Characteristics of Chemical Bonds in CuInSe2, CuGaSe2, and CuAlSe2

Electronic structural calculations of chalcopyrite-type CuInSe2 and its related compounds, CuGaSe2 and CuAlSe2, were performed by a recently proposed screened-exchange local density approximation (sX-LDA) method. The theoretical band gaps of CuInSe2, CuGaSe2, and CuAlSe2, obtained by the conventional calculation method using a generalized gradient approximation (GGA) functional, were 0.04, 0.14, and 1.11 eV, respectively. These values were considerably underestimated in comparison with their experimental values of 1.04, 1.68, and 2.67 eV because the exchange–correlation energy was not precisely calculated. Therefore, electronic structural calculations of chalcopyrite-type CuInSe2 and related compounds were performed with an sX-LDA functional to obtain accurate electronic structure. The present sX-LDA calculation successfully reproduced the band gaps of CuInSe2 (0.96 eV), CuGaSe2 (1.36 eV), and CuAlSe2 (2.22 eV). The obtained electronic structures and band gap energies of CuInSe2, CuGaSe2, and CuAlSe2 are discussed on the basis of schematic molecular orbital diagrams of tetrahedral CuSe47-, InSe45-, GaSe45-, and AlSe45- clusters.

Theory of the insulator ground state of the plutonium monochalcogenides: An LDA+DMFT investigation

The electronic structures and Kondo insulator state of the Pu monochalcogenides are computationally investigated, using the local density approximation (LDA), the LDA+U and dynamical mean field theory (LDA+DMFT) methods. We show that an enhanced hybridization of the Pu-5f state with the conduction bands occurs through the dynamical effect in the DMFT, which leads to the effective formation of a small energy gap around the Fermi level. Our LDA+DMFT calculations explain quantitatively the anomalous energy gap enhancement under pressure, observed for PuTe. The peculiar change of the energy gap of PuTe with pressure is found to result from a subtle crossover in the k-dispersion of the lowest excited states.

Flexible Polar Nanowires of Cs5BiP4Se12 from Weak Interactions between Coordination Complexes: Strong Nonlinear Optical Second Harmonic Generation

The Cs(5)BiP(4)Se(12) salt grows naturally as nanowires that crystallize in the polar space group Pmc2(1), with a = 7.5357(2) A, b = 13.7783(6) A, c = 28.0807(8) A, and Z = 4 at 293(2) K. The compound features octahedral [Bi(P(2)Se(6))(2)](5-) coordination complexes that stack via weak intermolecular Se...Se interactions to form long, flexible fibers and nanowires. The Cs(5)BiP(4)Se(12) fibers are transparent in the near- and mid-IR ranges and were found to exhibit a nonlinear optical second harmonic generation response at 1 microm that is approximately twice that of the benchmark material AgGaSe(2). The material has a nearly direct band gap of 1.85 eV and melts congruently at 590 degrees C. Ab initio electronic structure calculations performed with the full-potential linearized augmented plane wave (FLAPW) method show that the band gap increases from its local density approximation (LDA) spin-orbit coupling value of 1.15 eV to the higher value of 2.0 eV when the screened-exchange LDA method is invoked and explain how the long nanowire nature of Cs(5)BiP(4)Se(12) emerges.

Electronic structure, optical and X‐ray emission spectra in FeS2

AbstractThe electronic structure, optical spectra, X‐ray emission (XE) and X‐ray absorption (XA) spectra of FeS2 were investigated theoretically from first principles, using the fully relativistic Dirac LMTO band structure method in local density approximation (LDA). Theory produces correct semiconductor ground state in FeS2 with the energy gap of Eg = 0.9 eV in good agreement with the experimental data. Densities of valence states are analyzed and discussed. The origin of the optical, XA and XE spectra in the FeS2 compound is examined. The calculated results are compared with available experimental data. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Comparative study of electron- and hole-doped high-T c compounds in pseudogap regime: LDA+DMFT+Σ k approach

Pseudogap regime for the prototype high-T c compounds hole-doped Bi 2Sr 2CaCu 2O 8− x (Bi2212) and electron-doped Nd 2− x Ce x CuO 4 (NCCO) is described by means of novel generalized LDA+DMFT+Σ k approach. Here, conventional dynamical mean-field theory (DMFT) equations are supplied with additional (momentum dependent) self-energy Σ k . In the present case, Σ k describes nonlocal dynamical correlations induced by short-ranged collective Heisenberg-like antiferromagnetic spin fluctuations. Material-specific model parameters of two neighboring CuO 2 layers of Bi2212 and single CuO 2 layer of NCCO were obtained within local density approximation (LDA) and constrained LDA method. We show that Fermi surface in presence of the pseudogap fluctuations have perfectly visible “hot-spots” for NCCO, while in Bi2212 there is just a rather broad region with strong antiferromagnetic scattering. Results obtained are in good agreement with recent ARPES and optical experiments.

FIRST-PRINCIPLES STUDY OF GAMMA-Si3N4 WITH GENERALIZED GRADIENT APPROXIMATION AND LOCAL DENSITY APPROXIMATION

The electronic structure, and optical and mechanical properties of gamma- Si 3 N 4 are calculated by means of the plane-wave pseudo-potential method (PWP) with generalized gradient approximation (GGA) and local density approximation (LDA). The calculated values are in good agreement with experimental results. A comparative investigation reveals that the band gap calculated by GGA agrees better with the experimental value than by the other methods, while the calculated bulk modulus by LDA is comparable with that by the method of orthogonalized linear combinations of atomic orbitals (OLCAO). Our results indicate that a satisfactory calculation of the structures and properties of gamma- Si 3 N 4 could be achieved by combining GGA and LDA methods.

Magnetic properties, electronic structure, and optical properties of the filled skutterudite BaFe 4Sb 12

The magnetic properties, electronic structure, and optical properties of the filled skutterudite BaFe 4Sb 12 are calculated by the first-principles full-potential linearized augmented plane wave (FPLAPW) plus local orbital method. It is found that the local spin density approximation (LSDA) method appears more accurate than the generalized gradient approximation (GGA) method in calculating the electronic structures and optical properties of this compound. Furthermore, our calculated lattice constant and spin magnetic moments with the LSDA method are in overall better agreement with experiment. In contrast with recent experiment, our calculations are in good agreement with experimental reflectivity spectra and optical conductivity spectrum.

Elastic properties of the high-pressure rock-salt phase of ZnO

A pseudopotential and plane-wave based on the density functional theory (DFT) and carried within the exchange-correlation potential in the local density approximation (LDA) method were applied to investigate the relative stability and the elastic properties of ZnO under different hydrostatic pressures. The structural phase transition from the wurtzite to rock-salt structures and the elastic constants were all investigated. The calculated transition pressure and the elastic constants at zero pressure for the two structures are all in excellent agreement with their corresponding experimental and theoretical values. Moreover, a phase transition from rock-salt to caesium chloride structure is predicted to occur at 268 GPa according to the elastic mechanical instability. The pressure dependence of the bulk modulus, K, the shear modulus, G, the compressional wave velocity, VP , the shear wave velocity, VS , and the elastic anisotropy factor, A, for the rock-salt structure of ZnO are all successfully obtained.

Electronic Structure and Ferromagnetic Properties of Organic-Inorganic Hybrid Compound [Cu(<EM>μ</EM>-cbdca)(H<SUB>2</SUB>O)]<SUB><EM>n</EM></SUB>

The electronic structure and magnetic properties of the moleculer-based magnet [Cu(mu-cbdca)(H(2)O)](n) (cbdca=cyclobutanedicarboxylate) compound with copper ions as the metallic magnetic center were studied using the FP_LAPW(first-principle full-potential linearized augmented plane wave) method of first-principles, based on density functional theory (DFT) with generalized gradient approximation (GGA) method and local spin density approximation (LSDA) method. The total energies of the ferromagnetic, antiferromagnetic. and non-magnetic phases of organic-inorganic metal phosphonoacetate [Cu(mu-cbdca)(H(2)O)](n) were calculated. The calculations revealed that the compound [Cu(mu-cbdca)(H(2)O)](n) had a stable metal-ferromagnetic ground state, which was in agreement with the experimental result. There were large and positive spin populations on copper (II) ions, small and positive populations on the oxygen and carbon atoms of the bidentate ligand, which connected to the copper ions.

Pseudogap in normal underdoped phase of Bi2212: LDA + DMFT + Σk

Pseudogap phenomena are observed for normal underdoped phase of different high- T c cuprates. Among others Bi 2Sr 2CaCu 2O 8− δ (Bi2212) compound is one of the most studied experimentally [A. Damascelli, Z. Hussain, Z.-X. Shen, Rev. Mod. Phys. 75 (2003) 473; J.C. Campuzano, M.R. Norman, M. Randeria, in: K.H. Bennemann, J.B. Ketterson (Eds.), Physics of Superconductors, vol. 2, Springer, Berlin, 2004, p. 167; J. Fink et al., cond-mat/0512307; X.J. Zhou et al., cond-mat/0604284]. To describe pseudogap regime in Bi2212, we employ novel generalized DMFT + Σ k approach [E.Z. Kuchinskii, I.A. Nekrasov, M.V. Sadovskii, JETP Lett. 82 (2005) 198; M.V. Sadovskii et al., Phys. Rev. B 72 (2005) 155105, and these proceedings, doi:10.1016/j.physc.2007.03.367]. This approach gives possibility to preserve conventional dynamical mean-field theory (DMFT) equations [A. Georges et al., Rev. Mod. Phys. 68 (1996) 13] and include an additional (momentum dependent) self-energy Σ k . In the present case, Σ k describes non-local dynamical correlations induced by short-ranged collective Heisenberg-like antiferromagnetic spin fluctuations [M.V. Sadovskii, Physics-Uspekhi 44 (2001) 515, cond-mat/0408489]. The effective single impurity problem in the DMFT + Σ k is solved by numerical renormalization group (NRG) [R. Bulla, A.C. Hewson, Th. Pruschke, J. Phys. Cond. Mat. 10 (1998) 8365; R. Bulla, Phys. Rev. Lett. 83 (1999) 136]. To take into account material specific properties of two neighboring CuO 2 layers of Bi2212 we employ local density approximation (LDA) to calculate necessary model parameters, e.g. the values of intra- and interlayer hopping integrals between Cu-sites. Onsite Coulomb interaction U for x 2– y 2 orbital was calculated in constrained LDA method [O. Gunnarsson et al., Phys. Rev. B 39 (1989) 1708]. The value of pseudogap potential Δ was obtained within DMFT(NRG) [E.Z. Kuchinskii, I.A. Nekrasov, M.V. Sadovskii, JETP Lett. 82 (2005) 198; M.V. Sadovskii et al., Phys. Rev. B 72 (2005) 155105, and these proceedings, doi:10.1016/j.physc.2007.03.367]. Here, we report theoretical LDA + DMFT + Σ k quasiparticle bands dispersion, Fermi surface (FS) and angular resolved photoemission (ARPES) spectra accounting for pseudogap and bilayer splitting effects for normal underdoped Bi2212 ( δ = 0.15). We show that LDA-calculated value of bilayer splitting (BS) is too small to describe experimentally observed peak-dip-hump structure. Fermi surface in presence of the pseudogap fluctuations is almost insensitive to the BS value. Results obtained are in good agreement with recent ARPES experiments.

Band gap bowing and electron localization of GaxIn1−xN

The band gap bowing and the electron localization of GaxIn1−xN are calculated using both the local density approximation (LDA) and screened-exchange local density functional (sX-LDA) methods. The calculated sX-LDA band gaps are in good agreement with the experimentally observed values, with errors of −0.26 and 0.09eV for bulk GaN and InN, respectively. The LDA band gap errors are 1.33 and 0.81eV for GaN and InN, in order. In contrast to the gap itself, the band gap bowing parameter is found to be very similar in sX-LDA and LDA. We identify the localization of hole states in GaxIn1−xN alloys along In–N–In chains. The predicted localization is stronger in sX-LDA.

Calculations of HubbardUfrom first-principles

The Hubbard $U$ of the $3d$ transition metal series as well as $\mathrm{Sr}\mathrm{V}{\mathrm{O}}_{3}$, $\mathrm{Y}\mathrm{Ti}{\mathrm{O}}_{3}$, Ce, and Gd has been estimated using a recently proposed scheme based on the random-phase approximation. The values obtained are generally in good accord with the values often used in model calculations but for some cases the estimated values are somewhat smaller than those used in the literature. We have also calculated the frequency-dependent $U$ for some of the materials. The strong frequency dependence of $U$ in some of the cases considered in this paper suggests that the static value of $U$ may not be the most appropriate one to use in model calculations. We have also made comparison with the constrained local density approximation (LDA) method and found some discrepancies in a number of cases. We emphasize that our scheme and the constrained local density approximation LDA method theoretically ought to give similar results and the discrepancies may be attributed to technical difficulties in performing calculations based on currently implemented constrained LDA schemes.