3,321 research outputs found
Density-density functionals and effective potentials in many-body electronic structure calculations
We demonstrate the existence of different density-density functionals
designed to retain selected properties of the many-body ground state in a
non-interacting solution starting from the standard density functional theory
ground state. We focus on diffusion quantum Monte Carlo applications that
require trial wave functions with optimal Fermion nodes. The theory is
extensible and can be used to understand current practices in several
electronic structure methods within a generalized density functional framework.
The theory justifies and stimulates the search of optimal empirical density
functionals and effective potentials for accurate calculations of the
properties of real materials, but also cautions on the limits of their
applicability. The concepts are tested and validated with a near-analytic
model.Comment: five figure
Magnetosubband and edge state structure in cleaved-edge overgrown quantum wires
We provide a systematic quantitative description of the structure of edge
states and magnetosubband evolution in hard wall quantum wires in the integer
quantum Hall regime. Our calculations are based on the self-consistent Green's
function technique where the electron- and spin interactions are included
within the density functional theory in the local spin density approximation.
We analyze the evolution of the magnetosubband structure as magnetic field
varies and show that it exhibits different features as compared to the case of
a smooth confinement. In particularly, in the hard-wall wire a deep and narrow
triangular potential well (of the width of magnetic length ) is formed in
the vicinity of the wire boundary. The wave functions are strongly localized in
this well which leads to the increase of the electron density near the edges.
Because of the presence of this well, the subbands start to depopulate from the
central region of the wire and remain pinned in the well region until they are
eventually pushed up by increasing magnetic field. We also demonstrate that the
spin polarization of electron density as a function of magnetic field shows a
pronounced double-loop pattern that can be related to the successive
depopulation of the magnetosubbands. In contrast to the case of a smooth
confinement, in hard-wall wires the compressible strips do not form in the
vicinity of wire boundaries and spatial spin separation between spin-up and
spin-down states near edges is absent.Comment: 9 pages, submitted to Phys. Rev.
Ground state of two electrons on concentric spheres
We extend our analysis of two electrons on a sphere [Phys. Rev. A {\bf 79},
062517 (2009); Phys. Rev. Lett. {\bf 103}, 123008 (2009)] to electrons on
concentric spheres with different radii. The strengths and weaknesses of
several electronic structure models are analyzed, ranging from the mean-field
approximation (restricted and unrestricted Hartree-Fock solutions) to
configuration interaction expansion, leading to near-exact wave functions and
energies. The M{\o}ller-Plesset energy corrections (up to third-order) and the
asymptotic expansion for the large-spheres regime are also considered. We also
study the position intracules derived from approximate and exact wave
functions. We find evidence for the existence of a long-range Coulomb hole in
the large-spheres regime, and infer that unrestricted Hartree-Fock theory
over-localizes the electrons.Comment: 10 pages, 10 figure
The Global Competitiveness of the North American Livestock Industry
Livestock Production/Industries, F14, Q17,
Exchange and correlation near the nucleus in density functional theory
The near nucleus behavior of the exchange-correlation potential in Hohenberg-Kohn-Sham density functional theory is investigated. It is
shown that near the nucleus the linear term of of the spherically
averaged exchange-correlation potential is nonzero, and that
it arises purely from the difference between the kinetic energy density at the
nucleus of the interacting system and the noninteracting Kohn-Sham system. An
analytical expression for the linear term is derived. Similar results for the
exchange and correlation potentials are also
obtained separately. It is further pointed out that the linear term in
arising mainly from is rather small, and
therefore has a nearly quadratic structure near the nucleus.
Implications of the results for the construction of the Kohn-Sham system are
discussed with examples.Comment: 10 page
Extended Huckel theory for bandstructure, chemistry, and transport. II. Silicon
In this second paper, we develop transferable semi-empirical parameters for
the technologically important material, silicon, using Extended Huckel Theory
(EHT) to calculate its electronic structure. The EHT-parameters areoptimized to
experimental target values of the band dispersion of bulk-silicon. We obtain a
very good quantitative match to the bandstructure characteristics such as
bandedges and effective masses, which are competitive with the values obtained
within an orthogonal-tight binding model for silicon. The
transferability of the parameters is investigated applying them to different
physical and chemical environments by calculating the bandstructure of two
reconstructed surfaces with different orientations: Si(100) (2x1) and Si(111)
(2x1). The reproduced - and -surface bands agree in part
quantitatively with DFT-GW calculations and PES/IPES experiments demonstrating
their robustness to environmental changes. We further apply the silicon
parameters to describe the 1D band dispersion of a unrelaxed rectangular
silicon nanowire (SiNW) and demonstrate the EHT-approach of surface passivation
using hydrogen. Our EHT-parameters thus provide a quantitative model of
bulk-silicon and silicon-based materials such as contacts and surfaces, which
are essential ingredients towards a quantitative quantum transport simulation
through silicon-based heterostructures.Comment: 9 pages, 9 figure
Degenerate ground states and nonunique potentials: breakdown and restoration of density functionals
The Hohenberg-Kohn (HK) theorem is one of the most fundamental theorems of
quantum mechanics, and constitutes the basis for the very successful
density-functional approach to inhomogeneous interacting many-particle systems.
Here we show that in formulations of density-functional theory (DFT) that
employ more than one density variable, applied to systems with a degenerate
ground state, there is a subtle loophole in the HK theorem, as all mappings
between densities, wave functions and potentials can break down. Two weaker
theorems which we prove here, the joint-degeneracy theorem and the
internal-energy theorem, restore the internal, total and exchange-correlation
energy functionals to the extent needed in applications of DFT to atomic,
molecular and solid-state physics and quantum chemistry. The joint-degeneracy
theorem constrains the nature of possible degeneracies in general many-body
systems
Model for the on-site matrix elements of the tight-binding hamiltonian of a strained crystal: Application to silicon, germanium and their alloys
We discuss a model for the on-site matrix elements of the sp3d5s*
tight-binding hamiltonian of a strained diamond or zinc-blende crystal or
nanostructure. This model features on-site, off-diagonal couplings between the
s, p and d orbitals, and is able to reproduce the effects of arbitrary strains
on the band energies and effective masses in the full Brillouin zone. It
introduces only a few additional parameters and is free from any ambiguities
that might arise from the definition of the macroscopic strains as a function
of the atomic positions. We apply this model to silicon, germanium and their
alloys as an illustration. In particular, we make a detailed comparison of
tight-binding and ab initio data on strained Si, Ge and SiGe.Comment: Submitted to Phys. Rev.
Density Functional Theory for the Photoionization Dynamics of Uracil
Photoionization dynamics of the RNA base Uracil is studied in the framework
of Density Functional Theory (DFT). The photoionization calculations take
advantage of a newly developed parallel version of a multicentric approach to
the calculation of the electronic continuum spectrum which uses a set of
B-spline radial basis functions and a Kohn-Sham density functional hamiltonian.
Both valence and core ionizations are considered. Scattering resonances in
selected single-particle ionization channels are classified by the symmetry of
the resonant state and the peak energy position in the photoelectron kinetic
energy scale; the present results highlight once more the site specificity of
core ionization processes. We further suggest that the resonant structures
previously characterized in low-energy electron collision experiments are
partly shifted below threshold by the photoionization processes. A critical
evaluation of the theoretical results providing a guide for future experimental
work on similar biosystems
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