11,955 research outputs found
Ab initio pseudopotential study of Fe, Co, and Ni employing the spin-polarized LAPW approach
The ground-state properties of Fe, Co, and Ni are studied with the
linear-augmented-plane-wave (LAPW) method and norm-conserving pseudopotentials.
The calculated lattice constant, bulk modulus, and magnetic moment with both
the local-spin-density approximation (LSDA) and the generalized gradient
approximation (GGA) are in good agreement with those of all-electron
calculations, respectively. The GGA results show a substantial improvement over
the LSDA results, i.e., better agreement with experiment. The accurate
treatment of the nonlinear core-valence exchange and correlation interaction is
found to be essential for the determination of the magnetic properties of 3d
transition metals. The present study demonstrates the successful application of
the LAPW pseudopotential approach to the calculation of ground-state properties
of magnetic 3d transition metals.Comment: RevTeX, 14 pages, 2 figures in uufiles for
Nonempirical Density Functionals Investigated for Jellium: Spin-Polarized Surfaces, Spherical Clusters, and Bulk Linear Response
Earlier tests show that the Tao-Perdew-Staroverov-Scuseria (TPSS)
nonempirical meta-generalized gradient approximation (meta-GGA) for the
exchange-correlation energy yields more accurate surface energies than the
local spin density (LSD) approximation for spin-unpolarized jellium. In this
study, work functions and surface energies of a jellium metal in the presence
of ``internal'' and external magnetic fields are calculated with LSD,
Perdew-Burke-Ernzerhof (PBE) GGA, and TPSS meta-GGA and its predecessor, the
nearly nonempirical Perdew-Kurth-Zupan-Blaha (PKZB) meta-GGA, using
self-consistent LSD orbitals and densities. The results show that: (i) For
normal bulk densities, the surface correlation energy is the same in TPSS as in
PBE, as it should be since TPSS strives to represent a self-correlation
correction to PBE; (ii) Normal surface density profiles can be scaled uniformly
to the low-density or strong-interaction limit, and TPSS provides an estimate
for that limit that is consistent with (but probably more accurate than) other
estimates; (iii) For both normal and low densities, TPSS provides the same
description of surface magnetism as PBE, suggesting that these approximations
may be generally equivalent for magnetism. The energies of jellium spheres with
up to 106 electrons are calculated using density functionals and compared to
those obtained with Diffusion Quantum Monte Carlo data, including our estimate
for the fixed-node correction. Finally we calculate the linear response of bulk
jellium using these density functionals, and find that not only LSD but also
PBE GGA and TPSS meta-GGA yield a linear-response in good agreement with that
of the Quantum Monte Carlo method, for wavevectors of the perturbing external
potential up to twice the Fermi wavevector.Comment: 14 pages, 9 figure
Wavevector analysis of the jellium exchange-correlation surface energy in the random-phase approximation: detailed support for nonempirical density functionals
We report the first three-dimensional wavevector analysis of the jellium
exchange-correlation (xc) surface energy in the random-phase approximation
(RPA). The RPA accurately describes long-range xc effects which are challenging
for semi-local approximations, since it includes the universal small-wavevector
behavior derived by Langreth and Perdew. We use these rigorous RPA calculations
for jellium slabs to test RPA versions of nonempirical semi-local
density-functional approximations for the xc energy. The local spin density
approximation (LSDA) displays cancelling errors in the small and intermediate
wavevector regions. The PBE GGA improves the analysis for intermediate
wavevectors, but remains too low for small wavevectors (implying too-low
jellium xc surface energies). The nonempirical meta-generalized gradient
approximation of Tao, Perdew, Staroverov, and Scuseria (TPSS meta-GGA) gives a
realistic wavevector analysis, even for small wavevectors or long-range
effects. We also study the effects of slab thickness and of short-range
corrections to RPA.Comment: 7 pages, 7 figures, to appear in Phys. Rev.
Applications of the generalized gradient approximation to ferroelectric perovskites
The Perdew-Burke-Ernzerhof generalized gradient approximation to the density
functional theory is tested with respect to sensitivity to the choice of the
value of the parameter , which is associated to the degree of
localization of the exchange-correlation hole. A study of structural and
dynamical properties of four selected ferroelectric perovskites is presented.
The originally proposed value of =0.804 %(best suited for atoms and
molecules) works well for some solids, whereas for the ABO perovskites it
must be decreased in order to predict equilibrium lattice parameters in good
agreement with experiments. The effects on the structural instabilities and
zone center phonon modes are examined. The need of varying from one
system to another reflects the fact that the localization of the
exchange-correlation hole is system dependent, and the sensitivity of the
structural properties to its actual value illustrates the necessity of finding
a universal function for .Comment: 15 pages, 2 figures, PRB in pres
Comment on "Diffusion Monte Carlo study of jellium surfaces: Electronic densities and pair correlation functions"
In a fixed-node diffusion Monte Carlo calculation of the total energy of
jellium slabs, Acioli and Ceperley [Phys. Rev. B {\bf 54}, 17199 (1996)]
reported jellium surface energies that at low electron densities were
significantly higher than those predicted in the local-density approximation
(LDA) of density-functional theory. Assuming that the fixed-node error in the
slab and the bulk calculations cancel out, we show that their data yield
surface energies that are considerably closer to the LDA and in reasonable
agreement with those obtained in the random-phase approximation.Comment: 3 pages, 2 figures, to appear in Phys. Rev.
A natural orbital functional for the many-electron problem
The exchange-correlation energy in Kohn-Sham density functional theory is
expressed as a functional of the electronic density and the Kohn-Sham orbitals.
An alternative to Kohn-Sham theory is to express the energy as a functional of
the reduced first-order density matrix or equivalently the natural orbitals. In
the former approach the unknown part of the functional contains both a kinetic
and a potential contribution whereas in the latter approach it contains only a
potential energy and consequently has simpler scaling properties. We present an
approximate, simple and parameter-free functional of the natural orbitals,
based solely on scaling arguments and the near satisfaction of a sum rule. Our
tests on atoms show that it yields on average more accurate energies and charge
densities than the Hartree Fock method, the local density approximation and the
generalized gradient approximations
Evaluation of Exchange-Correlation Energy, Potential, and Stress
We describe a method for calculating the exchange and correlation (XC)
contributions to the total energy, effective potential, and stress tensor in
the generalized gradient approximation. We avoid using the analytical
expressions for the functional derivatives of E_xc*rho, which depend on
discontinuous second-order derivatives of the electron density rho. Instead, we
first approximate E_xc by its integral in a real space grid, and then we
evaluate its partial derivatives with respect to the density at the grid
points. This ensures the exact consistency between the calculated total energy,
potential, and stress, and it avoids the need of second-order derivatives. We
show a few applications of the method, which requires only the value of the
(spin) electron density in a grid (possibly nonuniform) and returns a
conventional (local) XC potential.Comment: 7 pages, 3 figure
Electronic Fine Structure in the Electron-Hole Plasma in SrB6
Electron-hole mixing-induced fine structure in alkaline earth hexaborides
leads to lower energy (temperature) scales, and thus stronger tendency toward
an excitonic instability, than in their doped counterparts (viz.
Ca(1-x)La(x)B(6), x=0.005), which are high Curie temperature, small moment
ferromagnets. Comparison of Fermi surfaces and spectral distributions with de
Haas - van Alphen (dHvA), optical, transport, and tunneling data indicates that
SrB6 remains a fermionic semimetal down to (at least) 5 K, rather than forming
an excitonic condensate. For the doped system the Curie temperature is higher
than the degeneracy temperature.Comment: Four two-column pages, three postscript figures. Phys. Rev. Lett.
(April 2000, in press
Effects of 3-d and 4-d-transition metal substitutional impurities on the electronic properties of CrO2
We present first-principles based density functional theory calculations of
the electronic and magnetic structure of CrO2 with 3d (Ti through Cu) and 4d
(Zr through Ag) substitutional impurities. We find that the half-metallicity of
CrO2 remains intact for all of the calculated substitutions. We also observe
two periodic trends as a function of the number of valence electrons: if the
substituted atom has six or fewer valence electrons (Ti-Cr or Zr-Mo), the
number of down spin electrons associated with the impurity ion is zero,
resulting in ferromagnetic (FM) alignment of the impurity magnetic moment with
the magnetization of the CrO2 host. For substituent atoms with eight to ten
(Fe-Ni or Ru-Pd with the exception of Ni), the number of down spin electrons
contributed by the impurity ion remains fixed at three as the number
contributed to the majority increases from one to three resulting in
antiferromagnetic (AFM) alignment between impurity moment and host
magnetization. The origin of this variation is the grouping of the impurity
states into 3 states with approximate "t2g" symmetry and 2 states with
approximate "eg" symmetry. Ni is an exception to the rule because a
Jahn-Teller-like distortion causes a splitting of the Ni eg states. For Mn and
Tc, which have 8 valence electrons, the zero down spin and 3 down spin
configurations are very close in energy. For Cu and Ag atoms, which have 11
valence electrons, the energy is minimized when the substituent ion contributes
5 Abstract down-spin electrons. We find that the interatomic exchange
interactions are reduced for all substitutions except for the case of Fe for
which a modest enhancement is calculated for interactions along certain
crystallographic directions.Comment: 26 pages, 10 figures, 2 table
A local density functional for the short-range part of the electron-electron interaction
Motivated by recent suggestions --to split the electron-electron interaction
into a short-range part, to be treated within the density functional theory,
and a long-range part, to be handled by other techniques-- we compute, with a
diffusion Monte Carlo method, the ground-state energy of a uniform electron gas
with a modified, short-range-only electron-electron interaction \erfc(\mu
r)/r, for different values of the cutoff parameter and of the electron
density. After deriving some exact limits, we propose an analytic
representation of the correlation energy which accurately fits our Monte Carlo
data and also includes, by construction, these exact limits, thus providing a
reliable ``short-range local-density functional''.Comment: 7 pages, 3 figure
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