249 research outputs found
Two Avenues to Self-Interaction Correction within Kohn-Sham Theory: Unitary Invariance is the Shortcut
The most widely-used density functionals for the exchange-correlation energy
are inexact for one-electron systems. Their self-interaction errors can be
severe in some applications. The problem is not only to correct the
self-interaction error, but to do so in a way that will not violate
size-consistency and will not go outside the standard Kohn-Sham density
functional theory. The solution via the optimized effective potential (OEP)
method will be discussed, first for the Perdew-Zunger self-interaction
correction (whose performance for molecules is briefly summarized) and then for
the more modern self-interaction corrections based upon unitarily-invariant
indicators of iso-orbital regions. For the latter approaches, the OEP
construction is greatly simplified. The kinetic-energy-based iso-orbital
indicator \tau^W_\sigma(\re)/\tau_\sigma(\re) will be discussed and plotted,
along with an alternative exchange-based indicator
Matching the BRIC equity premium: A structural approach
The equity risk premium (ERP) in BRIC markets is, on average, significantly higher than that in the US market. This paper employs an endowment economy with recursive preferences and long-run risk to explain the ERP generated by a portfolio of BRIC equity indices. The combination of recursive preferences and long-run risk partially explains the BRIC ERP. It turns out that there is a puzzle with respect to BRIC data as well. This holds even if we account for high levels of aversion to consumption and utility risk and for the empirically observed autoregressive structure of US consumption and BRIC dividend growth. (C) 2014 Elsevier B.V. All rights reserved
Microscopic modeling of photoluminescence of strongly disordered semiconductors
A microscopic theory for the luminescence of ordered semiconductors is
modified to describe photoluminescence of strongly disordered semiconductors.
The approach includes both diagonal disorder and the many-body Coulomb
interaction. As a case study, the light emission of a correlated plasma is
investigated numerically for a one-dimensional two-band tight-binding model.
The band structure of the underlying ordered system is assumed to correspond to
either a direct or an indirect semiconductor. In particular, luminescence and
absorption spectra are computed for various levels of disorder and sample
temperature to determine thermodynamic relations, the Stokes shift, and the
radiative lifetime distribution.Comment: 35 pages, 14 figure
Exact exchange-correlation potential of a ionic Hubbard model with a free surface
We use Lanczos exact diagonalization to compute the exact
exchange-correlation (xc) potential of a Hubbard chain with large binding
energy ("the bulk") followed by a chain with zero binding energy ("the
vacuum"). Several results of density functional theory in the continuum
(sometimes controversial) are verified in the lattice. In particular we show
explicitly that the fundamental gap is given by the gap in the Kohn-Sham
spectrum plus a contribution due to the jump of the xc-potential when a
particle is added. The presence of a staggered potential and a nearest-neighbor
interaction V allows to simulate a ionic solid. We show that in the ionic
regime in the small hopping amplitude limit the xc-contribution to the gap
equals V, while in the Mott regime it is determined by the Hubbard U
interaction. In addition we show that correlations generates a new potential
barrier at the surface
Many-body perturbation theory calculations using the yambo code
International audienceyambo is an open source project aimed at studying excited state properties of condensed matter systems from first principles using many-body methods. As input, yambo requires ground state electronic structure data as computed by density functional theory codes such as quantum-espresso and abinit. yambo's capabilities include the calculation of linear response quantities (both independent-particle and including electron-hole interactions), quasi-particle corrections based on the GW formalism, optical absorption, and other spectroscopic quantities. Here we describe recent developments ranging from the inclusion of important but oft-neglected physical effects such as electron-phonon interactions to the implementation of a real-time propagation scheme for simulating linear and non-linear optical properties. Improvements to numerical algorithms and the user interface are outlined. Particular emphasis is given to the new and efficient parallel structure that makes it possible to exploit modern high performance computing architectures. Finally, we demonstrate the possibility to automate workflows by interfacing with the yambopy and AiiDA software tools
The RNA workbench: best practices for RNA and high-throughput sequencing bioinformatics in Galaxy
RNA-based regulation has become a major research topic in molecular biology. The analysis of epigenetic and expression data is therefore incomplete if RNA-based regulation is not taken into account. Thus, it is increasingly important but not yet standard to combine RNA-centric data and analysis tools with other types of experimental data such as RNA-seq or ChIP-seq. Here, we present the RNA workbench, a comprehensive set of analysis tools and consolidated workflows that enable the researcher to combine these two worlds. Based on the Galaxy framework the workbench guarantees simple access, easy extension, flexible adaption to personal and security needs, and sophisticated analyses that are independent of command-line knowledge. Currently, it includes more than 50 bioinformatics tools that are dedicated to different research areas of RNA biology including RNA structure analysis, RNA alignment, RNA annotation, RNA-protein interaction, ribosome profiling, RNA-seq analysis and RNA target prediction. The workbench is developed and maintained by experts in RNA bioinformatics and the Galaxy framework. Together with the growing community evolving around this workbench, we are committed to keep the workbench up-to-date for future standards and needs, providing researchers with a reliable and robust framework for RNA data analysis. Availability: The RNA workbench is available at https://github.com/bgruening/galaxy-rna-workbench
Improved Slater approximation to SIC-OEP
We propose a simplification of the Optimized Effective Potential (OEP)
applied to the Self Interaction Correction (SIC) scheme of Density Functional
Theory (DFT). The new scheme fulfills several key formal properties and turns
out to be both simple and accurate. We show examples of applications on model
molecules in terms of observables known to be especially sensitive to details
of the SIC-OEP approach.Comment: 3 figure
Random-phase approximation and its applications in computational chemistry and materials science
The random-phase approximation (RPA) as an approach for computing the
electronic correlation energy is reviewed. After a brief account of its basic
concept and historical development, the paper is devoted to the theoretical
formulations of RPA, and its applications to realistic systems. With several
illustrating applications, we discuss the implications of RPA for computational
chemistry and materials science. The computational cost of RPA is also
addressed which is critical for its widespread use in future applications. In
addition, current correction schemes going beyond RPA and directions of further
development will be discussed.Comment: 25 pages, 11 figures, published online in J. Mater. Sci. (2012
Assessment of correlation energies based on the random-phase approximation
The random-phase approximation to the ground state correlation energy (RPA)
in combination with exact exchange (EX) has brought Kohn-Sham (KS) density
functional theory one step closer towards a universal, "general purpose first
principles method". In an effort to systematically assess the influence of
several correlation energy contributions beyond RPA, this work presents
dissociation energies of small molecules and solids, activation energies for
hydrogen transfer and non-hydrogen transfer reactions, as well as reaction
energies for a number of common test sets. We benchmark EX+RPA and several
flavors of energy functionals going beyond it: second-order screened exchange
(SOSEX), single excitation (SE) corrections, renormalized single excitation
(rSE) corrections, as well as their combinations. Both the single excitation
correction as well as the SOSEX contribution to the correlation energy
significantly improve upon the notorious tendency of EX+RPA to underbind.
Surprisingly, activation energies obtained using EX+RPA based on a KS reference
alone are remarkably accurate. RPA+SOSEX+rSE provides an equal level of
accuracy for reaction as well as activation energies and overall gives the most
balanced performance, which makes it applicable to a wide range of systems and
chemical reactions.Comment: 14 pages, 5 figures, full articl
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