819 research outputs found
Spin Resolution of the Electron-Gas Correlation Energy: Positive same-spin contribution
The negative correlation energy per particle of a uniform electron gas of
density parameter and spin polarization is well known, but its
spin resolution into up-down, up-up, and down-down contributions is not.
Widely-used estimates are incorrect, and hamper the development of reliable
density functionals and pair distribution functions. For the spin resolution,
we present interpolations between high- and low-density limits that agree with
available Quantum Monte Carlo data. In the low-density limit for ,
we find that the same-spin correlation energy is unexpectedly positive, and we
explain why. We also estimate the up and down contributions to the kinetic
energy of correlation.Comment: new version, to appear in PRB Rapid Communicatio
Stabilized jellium model and structural relaxation effects on the fragmentation energies of ionized silver clusters
Using the stabilized jellium model in two schemes of `relaxed' and `rigid',
we have calculated the dissociation energies and the fission barrier heights
for the binary fragmentations of singly-ionized and doubly-ionized Ag clusters.
In the calculations, we have assumed spherical geometries for the clusters.
Comparison of the fragmentation energies in the two schemes show differences
which are significant in some cases. This result reveals the advantages of the
relaxed SJM over the rigid SJM in dynamical processes such as fragmentation.
Comparing the relaxed SJM results and axperimental data on fragmentation
energies, it is possible to predict the sizes of the clusters just before their
fragmentations.Comment: 9 pages, 12 JPG figure
Two-dimensional limit of exchange-correlation energy functional approximations in density functional theory
We investigate the behavior of three-dimensional (3D) exchange-correlation
energy functional approximations of density functional theory in anisotropic
systems with two-dimensional (2D) character. Using two simple models, quasi-2D
electron gas and two-electron quantum dot, we show a {\it fundamental
limitation} of the local density approximation (LDA), and its semi-local
extensions, generalized gradient approximation (GGA) and meta-GGA (MGGA), the
most widely used forms of which are worse than the LDA in the strong 2D limit.
The origin of these shortcomings is in the inability of the local (LDA) and
semi-local (GGA/MGGA) approximations to describe systems with 2D character in
which the nature of the exchange-correlation hole is very nonlocal. Nonlocal
functionals provide an alternative approach, and explicitly the average density
approximation (ADA) is shown to be remarkably accurate for the quasi-2D
electron gas system. Our study is not only relevant for understanding of the
functionals but also practical applications to semiconductor quantum structures
and materials such as graphite and metal surfaces. We also comment on the
implication of our findings to the practical device simulations based on the
(semi-)local density functional method.Comment: 21 pages including 9 figures, to be published in Phys. Rev.
Comparative study of density functional theories of the exchange-correlation hole and energy in silicon
We present a detailed study of the exchange-correlation hole and
exchange-correlation energy per particle in the Si crystal as calculated by the
Variational Monte Carlo method and predicted by various density functional
models. Nonlocal density averaging methods prove to be successful in correcting
severe errors in the local density approximation (LDA) at low densities where
the density changes dramatically over the correlation length of the LDA hole,
but fail to provide systematic improvements at higher densities where the
effects of density inhomogeneity are more subtle. Exchange and correlation
considered separately show a sensitivity to the nonlocal semiconductor crystal
environment, particularly within the Si bond, which is not predicted by the
nonlocal approaches based on density averaging. The exchange hole is well
described by a bonding orbital picture, while the correlation hole has a
significant component due to the polarization of the nearby bonds, which
partially screens out the anisotropy in the exchange hole.Comment: 16 pages, 5 figures, RevTeX, added conten
Magnetic phenomena in 5d transition metal nanowires
We have carried out fully relativistic full-potential, spin-polarized,
all-electron density-functional calculations for straight, monatomic nanowires
of the 5d transition and noble metals Os, Ir, Pt and Au. We find that, of these
metal nanowires, Os and Pt have mean-field magnetic moments for values of the
bond length at equilibrium. In the case of Au and Ir, the wires need to be
slightly stretched in order to spin polarize. An analysis of the band
structures of the wires indicate that the superparamagnetic state that our
calculations suggest will affect the conductance through the wires -- though
not by a large amount -- at least in the absence of magnetic domain walls. It
should thus lead to a characteristic temperature- and field dependent
conductance, and may also cause a significant spin polarization of the
transmitted current.Comment: 7 pages, 5 figure
The local magnetic moments and hyperfine magnetic fields in disordered metal-metalloid alloys
The local magnetic moments and hyperfine magnetic fields (HFF) in the ordered
alloys Fe_{15}Sn and Fe_{15}Si are calculated with the first-principles
full-potential linear augmented plane wave (FP LAPW) method. The results are
compared with the experimental data on Fe-M (M=Si, Sn) disordered alloys at
small metalloid concentration. The relaxation of the lattice around the
impurity and its influence on the quantities under consideration are studied.
The mechanism of the local magnetic moment formation is described. It is proved
that the main distinction between these alloys is connected with the different
lattice parameters. Three contributions to the HFF are discussed: the
contributions of the core and valence electron polarization to the
Fermi-contact part, and the contibution from the orbital magnetic moment.Comment: 3 pages, 3 figures, submitted to Phys. Rev.
Compressibility and Electronic Structure of MgB2 up to 8 GPa
The lattice parameters of MgB2 up to pressures of 8 GPa were determined using
high-resolution x-ray powder diffraction in a diamond anvil cell. The bulk
modulus, B0, was determined to be 151 +-5 GPa. Both experimental and
first-principles calculations indicate nearly isotropic mechanical behavior
under pressure. This small anisotropy is in contrast to the 2 dimensional
nature of the boron pi states. The pressure dependence of the density of states
at the Fermi level and a reasonable value for the average phonon frequency
account within the context of BCS theory for the reduction of Tc under
pressure.Comment: REVTeX file. 4 pages, 4 figure
Exact-exchange density-functional calculations for noble-gas solids
The electronic structure of noble-gas solids is calculated within density
functional theory's exact-exchange method (EXX) and compared with the results
from the local-density approximation (LDA). It is shown that the EXX method
does not reproduce the fundamental energy gaps as well as has been reported for
semiconductors. However, the EXX-Kohn-Sham energy gaps for these materials
reproduce about 80 % of the experimental optical gaps. The structural
properties of noble-gas solids are described by the EXX method as poorly as by
the LDA one. This is due to missing Van der Waals interactions in both, LDA and
EXX functionals.Comment: 4 Fig
Metal Surface Energy: Persistent Cancellation of Short-Range Correlation Effects beyond the Random-Phase Approximation
The role that non-local short-range correlation plays at metal surfaces is
investigated by analyzing the correlation surface energy into contributions
from dynamical density fluctuations of various two-dimensional wave vectors.
Although short-range correlation is known to yield considerable correction to
the ground-state energy of both uniform and non-uniform systems, short-range
correlation effects on intermediate and short-wavelength contributions to the
surface formation energy are found to compensate one another. As a result, our
calculated surface energies, which are based on a non-local
exchange-correlation kernel that provides accurate total energies of a uniform
electron gas, are found to be very close to those obtained in the random-phase
approximation and support the conclusion that the error introduced by the
local-density approximation is small.Comment: 5 pages, 1 figure, to appear in Phys. Rev.
Use of the Generalized Gradient Approximation in Pseudopotential Calculations of Solids
We present a study of the equilibrium properties of -bonded solids within
the pseudopotential approach, employing recently proposed generalized gradient
approximation (GGA) exchange correlation functionals. We analyze the effects of
the gradient corrections on the behavior of the pseudopotentials and discuss
possible approaches for constructing pseudopotentials self-consistently in the
context of gradient corrected functionals. The calculated equilibrium
properties of solids using the GGA functionals are compared to the ones
obtained through the local density approximation (LDA) and to experimental
data. A significant improvement over the LDA results is achieved with the use
of the GGA functionals for cohesive energies. For the lattice constant, the
same accuracy as in LDA can be obtained when the nonlinear coupling between
core and valence electrons introduced by the exchange correlation functionals
is properly taken into account. However, GGA functionals give bulk moduli that
are too small compared to experiment.Comment: 15 pages, latex, no figure
- …