Search results for "Local-density approximation"
showing 10 items of 48 documents
Towards nonlocal density functionals by explicit modelling of the exchange-correlation hole in inhomogeneous systems
2013
We put forward new approach for the development of a non-local density functional by a direct modeling of the shape of exchange-correlation (xc) hole in inhomogeneous systems. The functional is aimed at giving an accurate xc-energy and an accurate corresponding xc-potential even in difficult near-degeneracy situations such as molecular bond breaking. In particular we demand that: (1) the xc hole properly contains -1 electron, (2) the xc-potential has the asymptotic -1/r behavior outside finite systems and (3) the xc-potential has the correct step structure related to the derivative discontinuities of the xc-energy functional. None of the currently existing functionals satisfies all these re…
Magnetic and electronic properties of double perovskites and estimation of their Curie temperatures byab initiocalculations
2008
First principles electronic structure calculations have been carried out on ordered double perovskites Sr_2B'B"O_6 (for B' = Cr or Fe and B" 4d and 5d transition metal elements) with increasing number of valence electrons at the B-sites, and on Ba_2MnReO_6 as well as Ba_2FeMoO_6. The Curie temperatures are estimated ab initio from the electronic structures obtained with the local spin-density functional approximation, full-potential generalized gradient approximation and/or the LDA+U method (U - Hubbard parameter). Frozen spin-spirals are used to model the excited states needed to evaluate the spherical approximation for the Curie temperatures. In cases, where the induced moments on the oxy…
Coupled theoretical and experimental studies for the radiation hardening of silica-based optical fibers
2014
International audience; We applied theoretical and experimental spectroscopy tools to ad hoc silica-based "canonical" samples to characterize the influence of several dopants and of some drawing process parameters on their radiation sensitivities. We present in this paper, the recent advances and results occurring from our coupled approach. On the experimental side, we studied the doping influence on the response of optical fibers and showed that changing the drawing parameters has a negligible influence on the fiber response in the case of specialty fibers. We focus mainly on the ${rm SiE}^prime$ defect that is observed through Electron Paramagnetic Resonance (EPR) measurements in all cano…
A comparative study of the atomic and electronic structure of F centers in ferroelectric KNbO3: Ab initio and semi-empirical calculations
1998
Abstract The linear muffin-tin-orbital method combined with density functional theory (in a local density approximation) and the semi-empirical method of the intermediate neglect of the differential overlap (INDO) based on the Hartree-Fock formalism are used for the supercell study of the F centers (O vacancy with two electrons) in cubic and orthorhombic ferroelectric KNbO3 crystals. The two electrons are found to be considerably delocalized even in the ground state of the defect. Their wave functions extend over the two Nb atoms closest to the O vacancy and over other nearby atoms. Thus, the F center in KNbO3 resembles much more electron defects in the partly covalent SiO2 crystal (the so-…
First-principles calculations of perovskite thin films
2002
Abstract The results of the electronic structure calculations for different surface terminations of SrTiO3 (1 0 0) perovskite thin films are discussed. These calculations are based on ab initio Hartree–Fock method with a posteriori electron correlation corrections and density functional theory with a number of different exchange-correlation functionals, including hybrid (B3PW, B3LYP) exchange techniques. Results are compared with previous ab initio plane-wave local density approximation and classical shell model calculations. Calculated considerable increase of the Ti–O chemical bond covalency nearby the surface is confirmed by experimental data. We predict also the band-gap reduction, espe…
Semiempirical pseudopotential approach for nitride-based nanostructures and {\it ab initio} based passivation of free surfaces
2013
We present a semiempirical pseudopotential method based on screened atomic pseudopotentials and derived from \textit{ab initio} calculations. This approach is motivated by the demand for pseudopotentials able to address nanostructures, where \textit{ab initio} methods are both too costly and insufficiently accurate at the level of the local-density approximation, while mesoscopic effective-mass approaches are inapplicable due to the small size of the structures along, at least, one dimension. In this work we improve the traditional pseudopotential method by a two-step process: First, we invert a set of self-consistently determined screened {\it ab initio} potentials in wurtzite GaN for a ra…
Nuclear medium modification of the structure function
2011
Abstract We study the nuclear effects in the electromagnetic structure function F 2 ( x , Q 2 ) in the deep inelastic lepton–nucleus scattering process by taking into account Fermi motion, binding, pion and rho meson cloud contributions. Calculations have been done in a local density approximation using relativistic nuclear spectral functions which include nucleon correlations. The ratios R F 2 A ( x , Q 2 ) = 2 F 2 A ( x , Q 2 ) A F 2 D ( x , Q 2 ) are obtained and compared with recent JLab results for light nuclei with special attention to the slope of the x distributions. This magnitude shows a non-trivial A dependence and it is insensitive to possible normalization uncertainties. The re…
Lattice dynamics of CuAlO2 under high pressure fromab initio calculations
2007
The density functional perturbation theory is employed to study the vibrational properties of CuAlO 2 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.
Models of Metal Clusters and Quantum Dots
2007
The electronic structure of simple metal clusters and quantum dots is studied on the basis of the density functional theory and simple models. It is demonstrated that single-particle models explain well the gross features of deformation and magnetism in small clusters, nuclei and quantum dots and that the local density approximation can give valuable information of the internal structure of the manybody state.
Correlation effects in the total energy, the bulk modulus, and the lattice constant of a transition metal: Combined local-density approximation and d…
2009
We present an accurate implementation of total-energy calculations into the local-density approximation plus dynamical mean-field theory $(\text{LDA}+\text{DMFT})$ method. The electronic structure problem is solved through the full-potential linear muffin-tin orbital and Korringa-Kohn-Rostoker methods with a perturbative solver for the effective impurity suitable for moderately correlated systems. We have tested the method in detail for the case of Ni, and investigated the sensitivity of the results to the computational scheme and to the complete self-consistency. It is demonstrated that the $\text{LDA}+\text{DMFT}$ method can resolve a long-standing controversy between the LDA/generalized …