1,721,006 research outputs found
Electronic Excitations: Density-Functional versus Many-Body Green's Function Approaches
No full textElectronic excitations lie at the origin of most of the commonly measured spectra. However, the first-principles computation of excited states requires a larger effort than ground-state calculations, which can be very efficiently carried out within density-functional theory. On the other hand, two theoretical and computational tools have come to prominence for the description of electronic excitations. One of them, many-body perturbation theory, is based on a set of Green's-function equations, starting with a one-electron propagator and considering the electron-hole Green's function for the response. Key ingredients are the electron's self-energy Sigma and the electron-hole interaction. A good approximation for Sigma is obtained with Hedin's GW approach, using density-functional theory as a zero-order solution. First-principles GW calculations for real systems have been successfully carried out since the 1980s. Similarly, the electron-hole interaction is well described by the Bethe-Salpeter equation, via a functional derivative of Sigma . An alternative approach to calculating electronic excitations is the time-dependent density-functional theory (TDDFT), which offers the important practical advantage of a dependence on density rather than on multivariable Green's functions. This approach leads to a screening equation similar to the Bethe-Salpeter one, but with a two-point, rather than a four-point, interaction kernel. At present, the simple adiabatic local-density approximation has given promising results for finite systems, but has significant deficiencies in the description of absorption spectra in solids, leading to wrong excitation energies, the absence of bound excitonic states, and appreciable distortions of the spectral line shapes. The search for improved TDDFT potentials and kernels is hence a subject of increasing interest. It can be addressed within the framework of many-body perturbation theory: in fact, both the Green's functions and the TDDFT approaches profit from mutual insight. This review compares the theoretical and practical aspects of the two approaches and their specific numerical implementations, and presents an overview of accomplishments and work in progress
First-principles approach to the calculation of electronic spectra in clusters
No full textWe discuss a method for first-principles calculations of photoemission spectra in small clusters, going well beyond a standard density functional theory-local density approximation (DFT-LDA) approach. Starting with a DFT-LDA calculation, we evaluate self-energy contributions to the quasiparticle energies of an electron or hole in the GW scheme, where the self-energy Sigma = GW is constructed from the one-particle Green's function G and the RPA screened Coulomb interaction W. The contributions of structural relaxation are taken into account. We show the importance of these effects at the example of the photoemission spectrum of SiH(4). We also briefly discuss results for longer hydrogenated silicon chains, and address the problem of optical absorption. Copyright (C) 1998 Elsevier Science B.V
Electronic structure of tin oxides
No full textStannic oxide SnO2 is a technologically important material which is frequently obtained by the oxidation of SnO. The tin oxides both have a tetragonal structure which differ essentially by the insertion of an oxygen plane between two tin planes in the layered SnO crystal. In order to well understand this structural evolution, it is crucial to have a precise description of the atomic and electronic structure of the two oxides. Preliminary results of calculations performed within Density Functional Theory in the Local Density Approximation (DFT-LDA) have already shown the relation existing between the electronic and geometric configurations of the two oxides. The gap calculated for SnO2 was in good agreement with the experimental value, but the calculations did not reproduce with a very good accuracy the experimental structure of SnO. We present ab-initio (DFT-LDA) study of the electronic structure of SnO, in comparison with SnO2. The charge density distribution of each oxide is analyzed with a special emphasis on low-charge-density contributions. Particular problems in the calculation of the equilibrium structure due to the pseudopotential of tin are put into evidence. We discuss the origin of these problems, and a possible solution
Electronic structure of stannous oxide
No full textWe present an ab initio study of the electronic structure of SnO. Density functional theory in the local density approximation (DFT-LDA) is used in conjunction with carefully tested smooth pseudopotentials. Total energies and charge densities are calculated and analysed as a function of the atomic geometry, with a particular emphasis on the importance of low-charge-density contributions to the interlayer cohesion. SnO2 has already been studied in the past and is used for comparison, Copyright (C) 1998 Elsevier Science B.V
Ab initio calculation of excitonic effects in realistic materials
No full textAb initio calculations, based on the density functional theory (DFT) in the local density approximation (LDA), allow for the description of the ground state properties of a wide class of materials. Also one-quasiparticle excitations can be obtained with good precision by adding self-energy corrections to the DFT-LDA eigenvalues. A realistic description of two-particle excitations, like the creation of electron-hole pairs in absorption experiments, is hardly feasible for systems where the electron and the hole interact. In this work we show how such excitonic effects can be included in ab initio electronic structure calculations, via the solution of an effective two-particle equation. Results for different systems are presented. Copyright (C) 1998 Elsevier Science B.V
Computing optical absorption spectra from first principles: Self-energy and electron-hole interaction effects
No full textA method for the inclusion of self-energy and excitonic effects in first-principles calculations of absorption spectra, within the state-of-the-art plane-wave pseudopotential approach, is discussed. Self-energy effects are computed within GW; and the electron-hole interaction is treated solving an effective tyro-particle equation which is derived from the relevant Bethe-Salpeter equation. We review numerical results for three systems: a small sodium cluster, the lithium oxyde insulating crystal, and bulk silicon, the prototype semiconductor. In the case of silicon, we present new results obtained considering additional approximations intended to reduce the computational effort and generally employed in Wannier-Mott exciton calculations, and discuss their reliability
Ab initio calculation of the quasiparticle spectrum and excitonic effects in Li2O
No full textWe report an ab initio calculation of the binding energies and the nature of the excitonic states in the near-gap absorption spectrum of a real solid, Li2O. We calculate the ground-state properties using density-functional theory together with soft pseudopotentials. Applying Hedin's GW approximation for the self-energy corrections to the band structure, we determine the minimal gap about 1 eV above the measured absorption onset. Finally, we obtain agreement with experiment by solving an effective two-particle Schrodinger equation for the electron-hole pairs
Elimination of unoccupied state summations in it ab initio self-energy calculations for large supercells
Full text linkWe present a new method for the computation of self-energy corrections in large supercells. It eliminates the explicit summation over unoccupied states, and uses an iterative scheme based on an expansion of the Green's function around a set of reference energies. This improves the scaling of the computational time from the fourth to the third power of the number of atoms for both the inverse dielectric matrix and the self-energy, yielding improved efficiency for 8 or more silicon atoms per unit cell
Efficient calculation of the polarizability: a simplified effective-energy technique
No full textIn a recent publication [J.A. Berger, L. Reining, F. Sottile, Phys. Rev. B 82, 041103(R) (2010)] we introduced the effective-energy technique to calculate in an accurate and numerically efficient manner the GW self-energy as well as the polarizability, which is required to evaluate the screened Coulomb interaction W. In this work we show that the effective-energy technique can be used to further simplify the expression for the polarizability without a significant loss of accuracy. In contrast to standard sum-over-state methods where huge summations over empty states are required, our approach only requires summations over occupied states. The three simplest approximations we obtain for the polarizability are explicit functionals of an independent- or quasi-particle one-body reduced density matrix. We provide evidence of the numerical accuracy of this simplified effective-energy technique as well as an analysis of our method
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