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Density Approximation (density + approximation)
Kinds of Density Approximation Selected AbstractsElectronic structure of binary and ternary components of CdTe:O thin filmsPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue S1 2004E. Menéndez-Proupin Abstract We report first-principles calculations of the electronic structure of the simplest compounds that may be present in Cd,Te,O mixtures: CdTe, CdO, ,-TeO2, CdTeO3 and Cd3TeO6. The calculations are carried out in the Local Density Approximation (LDA) and predict the insulating character of these compounds, underestimating the optical bandgaps by nearly 1 eV, as usual for LDA. In the four oxides, the top valence bands originate mainly from the O 2p atomic levels. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Modeling the model organism Dictyostelium discoideumDEVELOPMENT GROWTH & DIFFERENTIATION, Issue 6 2000Seido Nagano The cellular slime mold Dictyostelium discoideum is a fascinating organism, not only for biologists, but also for physicists. Since the Belousov,Zhabotinskii reaction pattern, a well-known non-linear phenomenon in chemistry, was observed during aggregation of Dictyostelium amoebae, Dictyostelium has been one of the major subjects of non-linear dynamics studies. Macroscopic theory, such as continuous cell density approximation, has been a common approach to studying pattern formation since the pioneering work of Turing. Recently, promising microscopic approaches, such as the cellular dynamics method, have emerged. They have shown that Dictyostelium is useful as a model system in biology. The synchronization mechanism of oscillatory production of cyclic adenosine 3,,5,-monophosphate in Dictyostelium is discussed in detail to show how it is a universal feature that can explain synchronization in other organisms. [source] Time-dependent density functional theory calculation of van der Waals coefficient of potassium clustersINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 6 2009Arup Banerjee Abstract We employ all-electron ab initio time-dependent density functional theory (DFT)-based method to calculate the long range dipole,dipole dispersion coefficient (van der Waals coefficient) C6 of potassium atom clusters (Kn) containing even number of atoms, n ranging from 2 to 20. The dispersion coefficients are obtained via Casimir,Polder relation. The calculations are carried out with the asymptotically correct statistical average of orbital potential and compared with the results obtained using Vosko,Wilk,Nusair representation of exchange-correlation potential within local density approximation. We report the dispersion coefficients between clusters of sodium and potassium atoms as well. The present results have been compared with the available jellium-based model and other DFT results in the literature. We also study the relationship between volume of the cluster and the C6 for K clusters. It is observed that the C6 scales as square of the volume. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2009 [source] Theoretical study in [C2H4,Tl]+ and [C2H2,Tl]+ complexesINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 1 2007Fernando Mendizabal Abstract We studied the attraction between [C2Hn] and Tl(I) in the hypothetical [C2Hn,Tl]+ complexes (n = 2,4) using ab initio methodology. We found that the changes around the equilibrium distance C,Tl and in the interaction energies are sensitive to the electron correlation potential. We evaluated these effects using several levels of theory, including Hartree,Fock (HF), second-order Mřller,Plesset (MP2), MP4, coupled cluster singles and doubles CCSD(T), and local density approximation augmented by nonlocal corrections for exchange and correlation due to Becke and Perdew (LDA/BP). The obtained interaction energies differences at the equilibrium distance Re (C,Tl) range from 33 and 46 kJ/mol at the different levels used. These results indicate that the interaction between olefinic systems and Tl(I) are a real minimum on the potential energy surfaces (PES). We can predict that these new complexes are viable for synthesizing. At long distances, the behavior of the [C2Hn],Tl+ interaction may be related mainly to charge-induced dipole and dispersion terms, both involving the individual properties of the olefinic ,-system and thallium ion. However, the charge-induced dipole term (R,4) is found as the principal contribution in the stability at long and short distances. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007 [source] Model density approach to the Kohn,Sham problem: Efficient extension of the density fitting techniqueINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 5 2005Uwe Birkenheuer Abstract We present a novel procedure for treating the exchange-correlation contributions in the Kohn,Sham procedure. The approach proposed is fully variational and closely related to the so-called "fitting functions" method for the Coulomb Hartree problem; in fact, the method consistently uses this auxiliary representation of the electron density to determine the exchange-correlation contributions. The exchange-correlation potential and its matrix elements in a basis set of localized (atomic) orbitals can be evaluated by reusing the three-center Coulomb integrals involving fitting functions, while the computational cost of the remaining numerical integration is significantly reduced and scales only linearly with the size of the auxiliary basis. We tested the approach extensively for a large set of atoms and small molecules as well as for transition-metal carbonyls and clusters, by comparing total energies, atomization energies, structure parameters, and vibrational frequencies at the local density approximation and generalized gradient approximation levels of theory. The method requires a sufficiently flexible auxiliary basis set. We propose a minimal extension of the conventional auxiliary basis set, which yields essentially the same accuracy for the quantities just mentioned as the standard approach. The new method allows one to achieve substantial savings compared with a fully numerical integration of the exchange-correlation contributions. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005 [source] Relativistic effects in the optical response of HgSe by time-dependent density functionals theoryINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 4-5 2001P. L. de Boeij Abstract We treat the dominant relativistic effects in the optical response properties of mercury selenide using time-dependent density functional theory (TDDFT). The scalar relativistic effects have been included within the zeroth-order regular approximation (ZORA) in both the ground-state DFT calculations and in the time-dependent response calculations. Within this approximation the HgSe crystal is found to be a semimetal. In a previous study [J Chem Phys 2001, 114, 1860] we have shown that TDDFT/ZORA can be applied successfully to narrow-gap semiconductors, such as indium antimonide, that become semimetallic within the local density approximation when scalar relativistic effects are included. Results are given for the band structure, the static dielectric constant ,,, and the dielectric function ,(,) of HgSe, and these results are compared with the similar ones for InSb. We find considerably improved results for the dielectric function of HgSe when relativistic effects are included. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001 [source] Calculation of quasiparticle energy of molecular systems by the GW methodINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 3 2001Y. Ohta Abstract The quasiparticle energy of the H2 molecule is calculated by using the GW method, in which the self-energy operator fully depends on the frequency. The initial Green function G0 is constructed from the wave function obtained by the Hartree,Fock approximation (HFA) and local density approximation (LDA) in the framework of the density functional theory (DFT). From the results obtained we have shown that the wave function from the DFT,LDA is more effective than that from the HFA for G0. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem 84: 348,353, 2001 [source] Interesting properties of Thomas,Fermi kinetic and Parr electron,electron-repulsion DFT energy functional generated compact one-electron density approximation for ground-state electronic energy of molecular systemsJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 9 2009Sandor Kristyan Abstract The reduction of the electronic Schrodinger equation or its calculating algorithm from 4N -dimensions to a (nonlinear, approximate) density functional of three spatial dimension one-electron density for an N -electron system, which is tractable in the practice, is a long desired goal in electronic structure calculation. If the Thomas-Fermi kinetic energy (,,,5/3dr1) and Parr electron,electron repulsion energy (,,,4/3dr1) main-term functionals are accepted, and they should, the later described, compact one-electron density approximation for calculating ground state electronic energy from the 2nd Hohenberg,Kohn theorem is also noticeable, because it is a certain consequence of the aforementioned two basic functionals. Its two parameters have been fitted to neutral and ionic atoms, which are transferable to molecules when one uses it for estimating ground-state electronic energy. The convergence is proportional to the number of nuclei (M) needing low disc space usage and numerical integration. Its properties are discussed and compared with known ab initio methods, and for energy differences (here atomic ionization potentials) it is comparable or sometimes gives better result than those. It does not reach the chemical accuracy for total electronic energy, but beside its amusing simplicity, it is interesting in theoretical point of view, and can serve as generator function for more accurate one-electron density models. © 2008 Wiley Periodicals, Inc. J Comput Chem 2009 [source] Energy landscapes of nucleophilic substitution reactions: A comparison of density functional theory and coupled cluster methodsJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 9 2007Marcel Swart Abstract We have carried out a detailed evaluation of the performance of all classes of density functional theory (DFT) for describing the potential energy surface (PES) of a wide range of nucleophilic substitution (SN2) reactions involving, amongst others, nucleophilic attack at carbon, nitrogen, silicon, and sulfur. In particular, we investigate the ability of the local density approximation (LDA), generalized gradient approximation (GGA), meta-GGA as well as hybrid DFT to reproduce high-level coupled cluster (CCSD(T)) benchmarks that are close to the basis set limit. The most accurate GGA, meta-GGA, and hybrid functionals yield mean absolute deviations of about 2 kcal/mol relative to the coupled cluster data, for reactant complexation, central barriers, overall barriers as well as reaction energies. For the three nonlocal DFT classes, the best functionals are found to be OPBE (GGA), OLAP3 (meta-GGA), and mPBE0KCIS (hybrid DFT). The popular B3LYP functional is not bad but performs significantly worse than the best GGA functionals. Furthermore, we have compared the geometries from several density functionals with the reference CCSD(T) data. The same GGA functionals that perform best for the energies (OPBE, OLYP), also perform best for the geometries with average absolute deviations in bond lengths of 0.06 Ĺ and 0.6°, even better than the best meta-GGA and hybrid functionals. In view of the reduced computational effort of GGAs with respect to meta-GGAs and hybrid functionals, let alone coupled cluster, we recommend the use of accurate GGAs such as OPBE or OLYP for the study of SN2 reactions. © 2007 Wiley Periodicals, Inc. J Comput Chem, 2007 [source] Optical properties of pure and transition metal-doped indium oxidePHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 5 2009H. A. Rahnamaye Aliabad Abstract The band structure, the dielectric function, the reflectivity, the refractive index and the oscillator strength sum rule were calculated for pure In2O3 and alloyed In1.5T0.5O3 (where T represents Sc, Y, La and Ac) using density functional theory (DFT). The full potential linearized augmented plane wave (FP-LAPW) method was used with the local density approximation (LDA + U). Calculations of the optical spectra were performed for the energy range 0,30 eV. The calculated results indicate that the upper valance bands of In2O3 show a small dispersion and the value of the band gap increases for Sc and Y dopants and decreases for Ac and La dopants. The calculations indicate that there are two band gaps for In2O3. The first shows a strong optical absorption, as a direct band gap occurs from a 0.81 eV energy level below the top of valence band. The second shows a much weaker absorption from the top of the valence band to the bottom of the conduction band. The refractive index for In2O3 is 1.69 nm at 800 nm, near the visible region. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Electronic structure of Fe4Si4,xGex (x = 0,4) compounds: ab initio calculationPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 1 2009G. I. Ameereh Abstract The structural and electronic properties of Fe4Si4,xGex (x = 0,4) with a cubic B20-type structure are investigated by density functional theory using an ab initio method. The calculations are based on a plane-wave expansion of the electronic wave functions and performed using the local density approximation. It is found that these compounds are narrow-gap semiconductors in the non-magnetic state. The band gap is found to decrease with increasing x, and the B20 FeSi is a small-gap semiconductor with sharp density of states features near the top of the valence band. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Elasticity, electronic structure, and dielectric property of cubic SrHfO3 from first-principlesPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 1 2009Z. F. Hou Abstract Recently, SrHfO3 compound was proposed as a potential gate dielectric to fabricate metal,oxide,semiconductor field-effect transistors (MOSFET) with equivalent oxide thickness (EOT) below 1 nm. Here we report the elasticity, electronic structure, and dielectric property of cubic SrHfO3 from first-principle study based on the plane-wave pseudopotential method within the local density approximation (LDA). The independent elastic constants of cubic SrHfO3 are derived from the derivative of total energy as a function of lattice strain. The elastic modulus is predicted from Voight-Hill bounds. The Born effective charges, electronic dielectric tensors, long wavelength phonon frequencies, and LO,TO splitting of cubic SrHfO3 are computed by linear response with density functional perturbation theory (DFPT). The calculated lattice constant and bulk modulus of cubic SrHfO3 are in good agreement with the available experimental data and other theoretical results. Our results show cubic SrHfO3 is a ductile insulator with an indirect band gap of 3.74 eV (LDA value) and electric dielectric tensor of 4.43, Hf 5d states and O 2p states exhibit a strong hybridization, and cubic SrHfO3 can be mechanically stable. In addition, the phonon frequency of ,soft mode' at zone-center also agrees well with previous theoretical value. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Theoretical study of the electronic structure and the totally symmetric vibrations of selected CoMoCat carbon nanotubesPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 10 2008Kürti Abstract In situ Raman spectroelectrochemical studies of CoMo-Cat single-walled carbon nanotubes enriched in (6,5) tubes have been carried out recently. We performed calculations on the density functional level using local density approximation for the electronic and vibrational properties of the most abundant tubes in these samples. The following chiral semiconducting tubes were investigated: (6,4), (7,3), (6,5), (9,1), (8,3) and (7,5). The calculated and the measured frequencies of the RBM and G, modes agree within several wave numbers. The calculated E11, E22 transition energies -after 30% and 20% upscaling, respectively- are comparable with the experimental values. The quenching of the RBM band with p- and n-doping can be interpreted within the rigid band approximation. The validity of the rigid band approximation was shown by calculating the density of states for neutral and charged tubes. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] The effects of inhomogeneous isotope distribution on the vibrational properties of isotope enriched double walled carbon nanotubesPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 11 2007V. Zólyomi Abstract The radial breathing mode in the Raman spectrum of 13C isotope enriched single walled carbon nanotubes is inhomogeneously broadened due to the random distribution of isotopes. We study this effect theoretically using density functional theory within the local density approximation and compare the result with experiments on isotope engineered double walled carbon nanotubes in which the inner tubes were grown from a mixture of 13C enriched fullerenes and natural fullerenes. As explained by the calculations, this synthesis procedure leads to an increased inhomogeneity compared to a case when only enriched fullerenes are used. The good agreement between the measurements and calculations shows the absence of carbon diffusion along the tube axis during inner tube growth, and presents a strong support of the theory that inner tube growth is governed by Stone,Wales transformations following the interconnection of fullerenes. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Lattice dynamics of CuAlO2 under high pressure from ab initio calculationsPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 1 2007P. Rodríguez-Hernández Abstract The density functional perturbation theory is employed to study the vibrational properties of CuAlO2 under pressure. The calculations are preformed using the pseudopotential wave method and the local density approximation for the exchange-correlation (XC) potential. The d electrons of Cu are treated as valence states. We present the phonon dispersion curves. Our results are in good agreement with the available experimental Raman scattering experiments. Ab initio calculations show the presence of a dynamical instability, possibly related with the experimentally observed phase transition. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] How do electronic properties of conventional III,V semiconductors hold for the III,V boron bismuth BBi compound?PHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 14 2005D. Madouri Abstract We have performed ab-initio self-consistent calculations using the full potential linear augmented plane wave method to investigate the structural and the electronic properties of the boron bismuth III,V compound BBi. Our calculations provide the first available information about the structural and electronic ground-state properties of BBi. Total energy calculations of the cubic zinc-blende, wurtzite, rock-salt, cesium chloride and orthorhombic Cmcm phases are made. The zinc-blende structure is found to be the ground-state phase of BBi; within the generalized gradient approximation (local density approximation), we found a lattice constant of 5.529 Ĺ (5.416 Ĺ) and a bulk modulus of 72.20 GPa (86.27 GPa). We found that, contrary to other boron compounds, the band gap of BBi is direct at the , point. The relativistic contraction of the 6s orbital of Bi has a strong influence on the bands and bonds of BBi. Consequently, the electronic properties of BBi are shown to differ considerably from those of common group III,V semiconductors (e.g. GaAs); in particular, we found an unusually strong p,p mixing of the valence-band maximum relative to most of the other III,V compounds. Furthermore, the calculated valence charge density shows an anomalous behavior, characterized by a charge transfer towards the ,cation' B atom, further illustrating the rich behavior of boron bismuth compounds. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] First-principles calculations of the structural, electronic and optical properties of IIA,IV antifluorite compoundsPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 10 2005O. Benhelal Abstract The structural, electronic and optical properties of the antifluorite semiconductors Mg2X (X = Si, Ge, Sn) have been studied using the full potential linearized augmented plane wave (FPLAPW) method. For treating the exchange-correlation term, the local density approximation (LDA) has been chosen. The total energy approach is used to determine the equilibrium volume. The energy gap is found to be indirect for all the compounds. Results for electronic properties such as band structures, density of states and electronic charge densities are presented. The optical properties are derived and interpreted. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Electronic and structural properties of strontium chalcogenides SrS, SrSe and SrTePHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 11 2004D. Rached Abstract We present the results of a first-principles study of the electronic and structural properties of strontium chalcogenides, SrS, SrSe and SrTe. The computational method is based on the full-potential linear muffin-tin orbitals method (FP-LMTO) augmented by a plane-wave basis (PLW). The exchange and correlation energy is described in the local density approximation (LDA) using the Perdew,Wang parameterization including a generalized gradient approximation (GGA). The calculated results of the structural properties are given for the NaCl (B1) and CsCl (B2) structures. We have also carried out band-structure calculations for the three considered compound, but only for the NaCl (B1) structure. A reasonable agreement is found from the comparison of our results with other theoretical calculations and experimental data. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Calculated high pressure crystal structure transformations for phosphorusPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 2 2003Rajeev Ahuja Abstract In this paper we have studied the structural stability of the sp bonded element, P, by means of the first principles calculations. The theoretical calculations made use of a full potential linear muffin-tin orbital (FPLMTO) method adopting the local density approximation to the density functional theory. We reproduce the observed crystallographic phase stability of P as a function of compression. Our results confirm the recent experimental finding of Akahama et al. We have also proposed a new structure for an experimentally reported unidentified intermediate phase in between simple cubic and simple hexagonal phase. This new structure is similar to what has been observed for Si. We have explained the stability of different phases under pressure using our calculated density of states (DOS). [source] Ab initio determination of structural and dynamical properties of the InP(110)-S interfacePHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 11 2004H. M. Tütüncü Abstract We have presented an ab initio pseudopotential study within the local density approximation of the structural properties of the S-treated InP(110) surface for an exchange model. The calculated structural parameters for this model are in good agreement with previous ab initio calculations. With these structural papameters, we have used an ab initio linear-response approach based on the density functional theory to investigate the dynamical properties of this surface. We discuss the similarities and contrast the differences in the phonon dispersion curves of the S-treated InP(110) and the clean InP(110) surfaces. We have found that the rotational phonon mode predicted for the clean surface can be also identified for the S-treated InP(110) surface. In addition to this, two new surface-localized phonon states appear in the acoustic-optical gap range due to the adsorption of S atoms. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Optimal predictive densities and fractional momentsAPPLIED STOCHASTIC MODELS IN BUSINESS AND INDUSTRY, Issue 1 2009Emanuele Taufer Abstract The maximum entropy approach used together with fractional moments has proven to be a flexible and powerful tool for density approximation of a positive random variable. In this paper we consider an optimality criterion based on the Kullback,Leibler distance in order to select appropriate fractional moments. We discuss the properties of the proposed procedure when all the available information comes from a sample of observations. The method is applied to the size distribution of the U.S. family income. Copyright © 2008 John Wiley & Sons, Ltd. [source] | ||||||||||||||||