525 research outputs found
Construction of transferable spherically-averaged electron potentials
A new scheme for constructing approximate effective electron potentials
within density-functional theory is proposed. The scheme consists of
calculating the effective potential for a series of reference systems, and then
using these potentials to construct the potential of a general system. To make
contact to the reference system the neutral-sphere radius of each atom is used.
The scheme can simplify calculations with partial wave methods in the
atomic-sphere or muffin-tin approximation, since potential parameters can be
precalculated and then for a general system obtained through simple
interpolation formulas. We have applied the scheme to construct electron
potentials of phonons, surfaces, and different crystal structures of silicon
and aluminum atoms, and found excellent agreement with the self-consistent
effective potential. By using an approximate total electron density obtained
from a superposition of atom-based densities, the energy zero of the
corresponding effective potential can be found and the energy shifts in the
mean potential between inequivalent atoms can therefore be directly estimated.
This approach is shown to work well for surfaces and phonons of silicon.Comment: 8 pages (3 uuencoded Postscript figures appended), LaTeX,
CAMP-090594-
First-principle Wannier functions and effective lattice fermion models for narrow-band compounds
We propose a systematic procedure for constructing effective lattice fermion
models for narrow-band compounds on the basis of first-principles electronic
structure calculations. The method is illustrated for the series of
transition-metal (TM) oxides: SrVO, YTiO, VO, and
YMoO. It consists of three parts, starting from LDA. (i)
construction of the kinetic energy Hamiltonian using downfolding method. (ii)
solution of an inverse problem and construction of the Wannier functions (WFs)
for the given kinetic energy Hamiltonian. (iii) calculation of screened Coulomb
interactions in the basis of \textit{auxiliary} WFs, for which the
kinetic-energy term is set to be zero. The last step is necessary in order to
avoid the double counting of the kinetic-energy term, which is included
explicitly into the model. The screened Coulomb interactions are calculated in
a hybrid scheme. First, we evaluate the screening caused by the change of
occupation numbers and the relaxation of the LMTO basis functions, using the
conventional constraint-LDA approach, where all matrix elements of
hybridization involving the TM orbitals are set to be zero. Then, we switch
on the hybridization and evaluate the screening associated with the change of
this hybridization in RPA. The second channel of screening is very important,
and results in a relatively small value of the effective Coulomb interaction
for isolated bands. We discuss details of this screening and consider
its band-filling dependence, frequency dependence, influence of the lattice
distortion, proximity of other bands, and the dimensionality of the model
Hamiltonian.Comment: 35 pages, 25 figure
LDA+DMFT computation of the electronic spectrum of NiO
The electronic spectrum, energy gap and local magnetic moment of paramagnetic
NiO are computed by using the local density approximation plus dynamical
mean-field theory (LDA+DMFT). To this end the noninteracting Hamiltonian
obtained within the local density approximation (LDA) is expressed in Wannier
functions basis, with only the five anti-bonding bands with mainly Ni 3d
character taken into account. Complementing it by local Coulomb interactions
one arrives at a material-specific many-body Hamiltonian which is solved by
DMFT together with quantum Monte-Carlo (QMC) simulations. The large insulating
gap in NiO is found to be a result of the strong electronic correlations in the
paramagnetic state. In the vicinity of the gap region, the shape of the
electronic spectrum calculated in this way is in good agreement with the
experimental x-ray-photoemission and bremsstrahlung-isochromat-spectroscopy
results of Sawatzky and Allen. The value of the local magnetic moment computed
in the paramagnetic phase (PM) agrees well with that measured in the
antiferromagnetic (AFM) phase. Our results for the electronic spectrum and the
local magnetic moment in the PM phase are in accordance with the experimental
finding that AFM long-range order has no significant influence on the
electronic structure of NiO.Comment: 15 pages, 6 figures, 1 table; published versio
Calculation of the work function with a local basis set
Electronic structure codes usually allow to calculate the work function as a
part of the theoretical description of surfaces and processes such as
adsorption thereon. This requires a proper calculation of the electrostatic
potential in all regions of space, which is apparently straightforward to
achieve with plane wave basis sets, but more difficult with local basis sets.
To overcome this, a relatively simple scheme is proposed to accurately compute
the work function when a local basis set is used, by having some additional
basis functions in the vacuum. Tests on various surfaces demonstrate that a
very good agreement with experimental and other theoretical data can be
achieved.Comment: to appear in Surf. Sci. Let
Self-consistency over the charge-density in dynamical mean-field theory: a linear muffin-tin implementation and some physical implications
We present a simple implementation of the dynamical mean-field theory
approach to the electronic structure of strongly correlated materials. This
implementation achieves full self-consistency over the charge density, taking
into account correlation-induced changes to the total charge density and
effective Kohn-Sham Hamiltonian. A linear muffin-tin orbital basis-set is used,
and the charge density is computed from moments of the many body
momentum-distribution matrix. The calculation of the total energy is also
considered, with a proper treatment of high-frequency tails of the Green's
function and self-energy. The method is illustrated on two materials with
well-localized 4f electrons, insulating cerium sesquioxide Ce2O3 and the
gamma-phase of metallic cerium, using the Hubbard-I approximation to the
dynamical mean-field self-energy. The momentum-integrated spectral function and
momentum-resolved dispersion of the Hubbard bands are calculated, as well as
the volume-dependence of the total energy. We show that full self-consistency
over the charge density, taking into account its modification by strong
correlations, can be important for the computation of both thermodynamical and
spectral properties, particularly in the case of the oxide material.Comment: 20 pages, 6 figures (submitted in The Physical Review B
A Density Functional Study of Atomic Hydrogen and Oxygen Chemisorption on the Relaxed (0001) Surface of Double Hexagonal Close Packed Americium
Ab initio total energy calculations within the framework of density
functional theory have been performed for atomic hydrogen and oxygen
chemisorption on the (0001) surface of double hexagonal packed americium using
a full-potential all-electron linearized augmented plane wave plus local
orbitals method. Chemisorption energies were optimized with respect to the
distance of the adatom from the relaxed surface for three adsorption sites,
namely top, bridge, and hollow hcp sites, the adlayer structure corresponding
to coverage of a 0.25 monolayer in all cases. Chemisorption energies were
computed at the scalar-relativistic level (no spin-orbit coupling NSOC) and at
the fully relativistic level (with spin-orbit coupling SOC). The two-fold
bridge adsorption site was found to be the most stable site for O at both the
NSOC and SOC theoretical levels with chemisorption energies of 8.204 eV and
8.368 eV respectively, while the three-fold hollow hcp adsorption site was
found to be the most stable site for H with chemisorption energies of 3.136 eV
at the NSOC level and 3.217 eV at the SOC level. The respective distances of
the H and O adatoms from the surface were found to be 1.196 Ang. and 1.164 Ang.
Overall our calculations indicate that chemisorption energies in cases with SOC
are slightly more stable than the cases with NSOC in the 0.049-0.238 eV range.
The work functions and net magnetic moments respectively increased and
decreased in all cases compared with the corresponding quantities of bare dhcp
Am (0001) surface. The partial charges inside the muffin-tins, difference
charge density distributions, and the local density of states have been used to
analyze the Am-adatom bond interactions in detail. The implications of
chemisorption on Am 5f electron localization-delocalization are also discussed.Comment: 9 Tables, 5 figure
Pareto-optimal alloys
Large databases that can be used in the search for new materials with
specific properties remain an elusive goal in materials science. The search
problem is complicated by the fact that the optimal material for a given
application is usually a compromise between a number of materials properties
and the price. In this letter we present a database consisting of the lattice
parameters, bulk moduli, and heats of formation for over 64,000 ordered
metallic alloys, which has been established by direct first-principles
density-functional-theory calculations. Furthermore, we use a concept from
economic theory, the Pareto-optimal set, to determine optimal alloy solutions
for the compromise between low compressibility, high stability and price.Comment: 5 pages, 2 figures, To be published in Appl. Phys. Let
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