Local Density Approximation Method Research Articles

Studies of the defect structure for Mn 2+ in KTaO 3 crystal from the calculation of EPR zero-field splitting

There are several mistakes in the recent paper about the theoretical studies of defect structure for Mn 2+ ion at the 12-fold tetrakaidecahedral K + site in KTaO 3 by calculating the spin Hamiltonian (SH) parameters, so the calculated defect structure (which is smaller than those obtained from the local density approximation (LDA) method, density functional theory in the generalized gradient approximation (GGA) and the dipole moment study (DMS)) is doubtful. Therefore, we restudy the defect structure in this paper by using the reasonable expressions and parameters. The present result is in agreement with those based on LDA, GGA and DMS methods and can be regarded as reasonable. It appears that the reliability of the defect structure of impurity center determined from the calculation of SH parameter depends strongly upon reasonableness of the used expressions and parameters.

Band gap of Hg chalcogenides: Symmetry-reduction-induced band-gap opening of materials with inverted band structures

We have investigated the band structure of zinc-blende (ZB) Hg chalcogenides using a corrected local density approximation method. We find that the band gaps of HgS, HgSe, and HgTe are 0.30, $\ensuremath{-}0.24$, and $\ensuremath{-}0.31\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$, respectively. That is, HgS has a positive band gap, whereas HgSe and HgTe have inverted band structures. The chemical trend of the band gaps is explained by the atomic energy levels and sizes, as well as by the related deformation potentials for these compounds. We also show systematically how the band gap of the inverted band structure can open up when the ${T}_{d}$ symmetry of the ZB structure is reduced under strain or in the presence of a surface or interface.

Electronic structure of zinc-blendeAlxGa1−xN: Screened-exchange study

We present a first principle investigation of the electronic structure and the band gap bowing parameter of zinc-blende using both local density approximation and screened-exchange density functional method. The calculated screened exchange local density approximation (sX-LDA) band gaps for GaN and AlN are 95% and 90% of the experimentally observed values, respectively, while LDA underestimates the gaps to 62% and 70%. In contrast to the gap itself, the band gap bowing parameter is found to be very similar in sX-LDA and LDA. Because of the difference in the conduction band structure, the direct to indirect band gap crossover is predicted to occur at different Al concentration.

First-principles investigation of isolated band formation in half-metallicTixGa1−xP(x=0.3125–0.25)

Several alloy semiconductors containing a transition metal atom of the type $M{\mathrm{Ga}}_{4}{X}_{3}$ with $X=\mathrm{As}$ or P and $M=\mathrm{Sc}$ or Ti have been found previously to present an isolated partially filled intermediate band within the usual band gap of the host semiconductors and have been proposed as highly efficient photovoltaic materials. In this paper, we carry out an ab initio investigation of band structures and electronic properties for the more chemically stable ${\mathrm{Ti}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{P}$ compound as a candidate for isolated intermediate band formation. We have calculated the electronic structures using self-consistent density functional theory method in both local density approximation and generalized gradient approximation (GGA) approaches and compared the GGA results with those obtained with the exact exchange method that we have implemented in the code SIESTA. Using spin-polarized localized wave functions to represent the valence electrons states and nonlocal pseudopotentials for the core electrons, we have also studied in detail the ${\mathrm{Ti}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{P}$ compounds at different dilution levels of the Ti transition metal atom $(x=3.125%,6.25%,12.5%,25.0%)$ and for two (cubic and tetragonal) different crystal cells. Results at the different dilutions show in all cases a fully spin-polarized structure and, except for the case of immediate Ti neighbors, indicate a rather small spin coupling between Ti atoms and confirm the presence of the isolated narrow partially filled intermediate band for this compound. They also show the higher suitability of isotropic crystal structures for obtaining in these materials the intermediate band with the desired small band width.

Density functional study of antiferromagnetic copper oxide

Cupric oxide CuO (tenorite) is examined computationally by employing the local density approximation (LDA) method including on-site Coulomb-interaction. The geometry of the antiferromagnetic (AF) unit cell was optimized to obtain a correctly equilibrated crystallographic structure. AF ordering and the band gap are found to be in agreement with the known experimental results. A study of different simple point and extended defects of the CuO is presented. An oxygen vacancy point defect was found to be energetically more favourable than a copper interstitial. These defects were found to suppress the magnetic moments of the nearby Cu atoms and cause the band gap to become diminished. A comparison of the different (001) and (100) twin boundary defects of CuO was also made. The lowest total energy was evaluated for the (001) twin boundary introduced, which shares copper atoms with the neighbouring components of the twins. It is indicated that copper enrichment at the twin boundary is possible.

Probabilistic electron density distribution in CdTe at RT and 200K

The bonding between the atoms in the II-VI compound semiconductors has always been a subject of rigorous research because of their tremendous applications in a variety of fields. The bonding and ionic character in CdTe at 300 and 200 K have been determined quantitatively as well as qualitatively using single crystal X-ray data sets and MEM (Maximum Entropy Method) as the tool for the reconstruction of the electron densities distributed within the unit cell. The ab-initio band calculation of the total and valence charge densities have been carried out theoretically by means of the local density approximation (LDA) method in support of the experimentally derived MEM maps. The difference density maps show fewer errors between the theoretical and experimental charge density and thus gives credence to the results accordingly. Along the bonding direction [111], the mid-bond electron densities are found to be 0.233 e/Å3 and 0.284 e/Å3 at 300 K and 200 K at distances 1.4026 Å and 1.4036 Å respectively. The densities along [100] and [110] show an increase in the charge concentration at the bond at lower temperatures. © 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

First-principles study on the electronic structure and the ferromagnetic properties of the organic–inorganic hybrid compound Cr[(H 3N–(CH 2) 2–PO 3)(Cl)(H 2O)

Using first-principles, namely density-functional theory (DFT) with generalized gradient approximation (GGA) method [Yu et al., Physica B 337 (2003) 102; Debernardiet al., Mater. Sci. Eng. C 23 (2003) 743; Charlotet al., J. Am. Chem. Soc 108 (1986) 2574; Andersen, Phys. Rev. B 12 (1975) 3060] and local spin density approximation (LSDA) method [Perdew, Wang, Phys. Rev. B 45 (1992) 13244], the electronic structure and the ferromagnetic properties of organic–inorganic hybrid chromium (II) organophosphonate Cr[(H 3N–(CH 2) 2–PO 3)(Cl)(H 2O)] are investigated. The density of states, the total energy of the cell, and the spin magnetic moment of Cr[(H 3N–(CH 2) 2–PO 3)(Cl)(H 2O)] were calculated. The calculations reveal that the compound Cr[(H 3N–(CH 2) 2–PO 3)(Cl)(H 2O)] has a stable metal-ferromagnetic ground state, and the spin magnetic moment per molecule is about 2.000 μ B, which comes mainly from Cr and O_{4}. The computation is in good agreement with experiment.

IONIZATION OFNa2BY HIGHLY CHARGED PARTICLES

Based on the time dependent of local density approximation method, the processes of excitation of sodium clusters by swift highly charged C6+, C4+, C2+, and C+are studied. The time dependent probabilities of escaped electrons are obtained and the ionization probabilities of the Na2are deduced. One finds that, the cluster is strongly ionized by C6+, and there is only a weak ionization by C+.

Insulator-metal transition in the doped3d1transition metal oxideLaTiO3

The doping induced insulator-metal transition in ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}\mathrm{Ti}{\mathrm{O}}_{3}$ is studied using the ab initio local density approximation (LDA) and DMFT method. Combining the LDA bandstructure for the actual, distorted structure found recently with multiorbital DMFT to treat electronic correlations, we find (i) ferro-orbital order in the Mott insulating state without orbital degeneracy, (ii) a continuous filling induced transition to the paramagnetic metal (PM) with $x$, and (iii) excellent quantitative agreement with published photoemission data for the case of 6% doping. Our results imply that this system can be described as a Mott-Hubbard system without orbital (liquid) degeneracy.

Intermolecular Interaction Potentials of CH4–NeComplex Calculated with Local Density Approximation Methods

The intermolecular interactions potentials for twoconfigurations of CH4–Ne complex are calculated with localdensity approximation methods in the frame of density functionaltheory. It is found that the calculated potentials have twominima when the distance between the carbon atom of CH4 and theNe atom takes R = 5.80 a.u. and 6.20 a.u. for both the twoconfigurations. For the edge configuration, the corresponding depth ofthe potential is 0.0669536 eV and 0.0671416 eV. For the faceconfiguration, the corresponding depth of the potential is 0.0737956 eVand 0.0645506 eV. The global minimum occurs at R = 5.80 a.u. forthe face configuration with a depth of the potential 0.0737956 eV. Thedepths of our calculation are in better agreement with the experimentaldata than the quantum chemical calculation approach, while the positionof minimum potential for our calculation is underestimated.

Screened-exchange determination of the optical properties of large gap insulators: CaF2

Optical measurements have provided an extremely difficult challenge to existing electronic band structure calculations. Although CaF2, an important large gap insulator, has been intensively investigated, no parameter-free first-principles calculations have been done due to the well-known failure of the local density approximation (LDA) in treating excited states. Here, we present results of fully first-principles calculations of the electronic structure and optical properties of CaF2 with the self-consistent screened-exchange LDA method implemented in the highly precise full-potential linearized augmented plane wave approach. The calculated optical energy gap, 12.05 eV, is in excellent agreement with experiment (12.0±0.1 eV) and so greatly improves the LDA result (7.23 eV). The optical properties, including the imaginary part of the dielectric function and the reflectance determined ab initio with full matrix elements and no parameters, are found to be in good agreement with experiment.

Study of the Interaction of Pollutant Nitro Polycyclic Aromatic Hydrocarbons with Different Metallic Surfaces by Surface-Enhanced Vibrational Spectroscopy (SERS and SEIR)

In this work, we have applied surface-enhanced Raman scattering (SERS) and surface-enhanced infrared spectroscopy (SEIR) to study the interaction of nitro polycyclic aromatic hydrocarbons (NPAHs) with metal surfaces. The nitro group confers the polycyclic aromatic hydrocarbons (PAHs) moiety additional electronic and structural characteristics which verify NPAHs−surface interactions. SERS and SEIR spectra suggest that the NPAHs 1-nitropyrene (1NP) and 2-nitrofluorene (2NF) at concentrations of <10-6 M (∼200 ppb) interact with metal surfaces through the nitro group, adopting a perpendicular orientation onto the surface. In both cases, the interaction seems to occur through the O atoms of the nitro groups, which are coplanar to the PAHs fragment. The most efficient interaction was obtained using silver metal colloidal surfaces. The mechanism of interaction with the metal surface is related to the structure of NPAHs: although a monodentate interaction is deduced in the case of 1NP, a bidentate one seems to occur in the case of 2NF. To complete this study, theoretical Raman vibrational spectra of both free 2NF and their complexes with silver metal clusters were obtained using the local spin density approximation (LSDA) methods with the LAN12DZ basis set. The theoretical calculations were compared to the experimental results, obtaining a good agreement with the SERS and SEIR data.

Structure, metal-insulator transitions, and magnetic properties of FeO at high pressures

The high-pressure behavior of rocksalt-structured FeO has been investigated using the LDA + U method, a first-principles computational technique that allows treatment of correlated electrons with strong localized repulsions. Within the local density approximation (LDA) FeO is predicted to be a metal, but with LDA + U, an insulating state is obtained at zero pressure. Electronic and magnetic behavior, the equation of state, and lattice strain are determined for three values of the Coulomb repulsion U. We find two self-consistent solutions, one with rhombohedral and one with monoclinic electronic symmetry. For U = 4.6 eV, the monoclinic solution becomes more stable than the rhombohedral solution at 110 GPa, leading to an insulator-metal transition; with increasing U, metallization occurs at higher pressures. Results from the LDA + U calculation suggest that the high-spin magnetic state should persist to pressures greater than 300 GPa. The method gives improved agreement with experiments for ground state properties as compared to LDA and GGA methods that do not explicitly include a local Coulomb repulsion.

Electronic structure of several polytypes of SiC: a study of band dispersion from a semi-empirical approach

A study of the most common polytypes of silicon carbide, namely 3C, 2H, 4H, 6H and 15R is performed, by using the extended Hückel tight binding approximation. Special account is taken in describing optical properties of the material. New basis sets and empirical parameters are suggested which contain excited s* orbitals for 3C, 4H, 6H and 15R, and p* orbitals for 2H. The new basis sets reproduce most important features of the topology of valence and conduction bands given in the literature from ab initio, pseudopotential and local density approximation methods, in special the appearance of an indirect gap of the correct order in each polytype. It is argued that the present results indicate that one-electron methods are suitable for describing optical properties in more complex polar semiconductors, such as the polytypes of SiC.

Electronic structure and optical properties of SrCu2O2

The electronic structure of SrCu2O2, a wide gap (∼3.3 eV) p-type oxide semiconductor, was examined by photoelectron and optical spectroscopy. The spectroscopy results were compared with the energy band structure calculated by the local density approximation method to clarify the origins of p-type conductivity in this material. Despite the significant difference in band gap energy, the basic electronic structure around the band gap region was found to be quite similar to that of Cu2O. Thus, the 3d−4sp orbital of Cu+ ion is hybridized with the 2p orbital of ligand O2− ions due to the covalency of Cu–O bonds, to form states near the valence band maximum; the conduction band minimum is predominantly composed of the hybridized orbital of Cu 4sp and O 2p, forming the direct band gap at Γ point. A sharp absorption band observed near the fundamental absorption edge is likely attributable to an exciton. Although the corresponding exciton emission was not observed near the absorption edge, a blue-green emission band (Stokes shift of ∼1 eV) was observed at ∼2.47 eV. The emission is presumably attributable to intra-atomic transitions of Cu+, partially allowed by p-orbital mixing into s and d orbitals.

Van der Waals binding energies in graphitic structures

Two types of methods are commonly used to describe the van der Waals cohesive properties of graphitic systems: one is based on density functional theory and the other on empirical model potentials. This paper examines the relation between the two and finds that, when properly done, both methods give the same results. The local density approximation (LDA) method can describe cohesion when graphitic molecules are close together, but must be supplemented with the theory of dispersion forces when the intermolecular distance increases. It is found that LDA dispersion force calculations reproduce the empirical potentials, which are thereby validated by fundamental theory. A recent disparity between two types of calculations in determining binding energy of ${\mathrm{C}}_{60}$ molecules inside a (10,10) nanotube is also examined.

Hybrid LSDA/Diffusion Quantum Monte-Carlo Method for Spin Sequences in Vertical Quantum Dots

We present an new hybrid Diffusion Quantum Monte-Carlo (DQMC)/Local Spin Density Approximation (LSDA) method, to compute the electronic structure of vertical quantum dots (VQD). The exact many-body electronic configuration is computed with a realistic confining potential. Our model confirms the atomic-like model of 2D shell structures obeying Hund's rule already predicted by LSDA.

Structural and electronic properties of tin clathrate materials

Using an ab initio local density approximation (LDA) method, we have studied the structural properties and the electronic band structures of several tin-based clathrate materials. We find that the pure type-I $({\mathrm{Sn}}_{46})$ and type-II $({\mathrm{Sn}}_{136})$ clathrate frameworks are metastable, expanded-volume phases of Sn and that they are semiconductors. We further find that the Cs encapsulated type-I binary clathrate is metallic if full stoichiometry $({\mathrm{Cs}}_{8}{\mathrm{Sn}}_{46})$ is assumed. However, our calculations suggest that vacancy formation is energetically favored, and that the stable vacancy compound ${\mathrm{Cs}}_{8}{\mathrm{Sn}}_{44}{\ensuremath{\square}}_{2}$ is a narrow gap semiconductor. We have also studied the ternary clathrate compounds ${\mathrm{Cs}}_{8}{\mathrm{Ga}}_{8}{\mathrm{Sn}}_{38}$ and ${\mathrm{Cs}}_{8}{\mathrm{Zn}}_{4}{\mathrm{Sn}}_{42},$ which have been suggested as potential thermoelectrics, and we find that these are semiconductors. The trends in the structural and electronic properties of these Sn-based clathrates are discussed, and our results are compared with experiment where possible.

Structure and Stability of Boron Nitrides: The Crossover between Rings and Cages

A previous study on B12N12 showed that the Hartree−Fock (HF) method, local density approximation (LDA), and gradient-corrected density functional theory (specifically B3LYP) all agree in predicting that the cage isomer of B12N12 is the most stable. If the cage is the most stable (BN)x for x = 12, then the energetic crossover between rings and cages must take place at some x < 12. Rings would be most stable for small molecules, and cages would be most stable for larger molecules. The current study explores the question of whether rings or cages are more stable at sizes smaller than B12N12, with the goal of defining a crossover point between rings and cages. These issues are examined by theoretical calculations using the HF, LDA, and B3LYP methods, along with second-order perturbation theory (MP2). In particular, an energetic comparison is carried out between the ring and cage isomers of (BN)x for x = 8−11. The major result is that boron nitride cages are more stable than rings if at least two of the six fo...

Excitation energies of dissociating H2: A problematic case for the adiabatic approximation of time-dependent density functional theory

Time-dependent density functional theory (TDDFT) is applied for calculation of the excitation energies of the dissociating H2 molecule. The standard TDDFT method of adiabatic local density approximation (ALDA) totally fails to reproduce the potential curve for the lowest excited singlet Σu+1 state of H2. Analysis of the eigenvalue problem for the excitation energies as well as direct derivation of the exchange-correlation (xc) kernel fxc(r,r′,ω) shows that ALDA fails due to breakdown of its simple spatially local approximation for the kernel. The analysis indicates a complex structure of the function fxc(r,r′,ω), which is revealed in a different behavior of the various matrix elements K1c,1cxc (between the highest occupied Kohn–Sham molecular orbital ψ1 and virtual MOs ψc) as a function of the bond distance R(H–H). The effect of nonlocality of fxc(r,r′) is modeled by using different expressions for the corresponding matrix elements of different orbitals. Asymptotically corrected ALDA (ALDA-AC) expressions for the matrix elements K12,12xc(στ) are proposed, while for other matrix elements the standard ALDA expressions are retained. This approach provides substantial improvement over the standard ALDA. In particular, the ALDA-AC curve for the lowest singlet excitation qualitatively reproduces the shape of the exact curve. It displays a minimum and approaches a relatively large positive energy at large R(H–H). ALDA-AC also produces a substantial improvement for the calculated lowest triplet excitation, which is known to suffer from the triplet instability problem of the restricted KS ground state. Failure of the ALDA for the excitation energies is related to the failure of the local density as well as generalized gradient approximations to reproduce correctly the polarizability of dissociating H2. The expression for the response function χ is derived to show the origin of the field-counteracting term in the xc potential, which is lacking in the local density and generalized gradient approximations and which is required to obtain a correct polarizability.