3,937 research outputs found
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
Calculating response functions in time domain with non-orthonormal basis sets
We extend the recently proposed order-N algorithms (cond-mat/9703224) for
calculating linear- and nonlinear-response functions in time domain to the
systems described by nonorthonormal basis sets.Comment: 4 pages, no figure
First-Principles Studies of Hydrogenated Si(111)--77
The relaxed geometries and electronic properties of the hydrogenated phases
of the Si(111)-77 surface are studied using first-principles molecular
dynamics. A monohydride phase, with one H per dangling bond adsorbed on the
bare surface is found to be energetically favorable. Another phase where 43
hydrogens saturate the dangling bonds created by the removal of the adatoms
from the clean surface is found to be nearly equivalent energetically.
Experimental STM and differential reflectance characteristics of the
hydrogenated surfaces agree well with the calculated features.Comment: REVTEX manuscript with 3 postscript figures, all included in uu file.
Also available at http://www.phy.ohiou.edu/~ulloa/ulloa.htm
First principles study of the origin and nature of ferromagnetism in (Ga,Mn)As
The properties of diluted GaMnAs are calculated for a wide range
of Mn concentrations within the local spin density approximation of density
functional theory. M\"ulliken population analyses and orbital-resolved
densities of states show that the configuration of Mn in GaAs is compatible
with either 3d or 3d, however the occupation is not integer due to the
large - hybridization between the Mn states and the valence band of
GaAs. The spin splitting of the conduction band of GaAs has a mean field-like
linear variation with the Mn concentration and indicates ferromagnetic coupling
with the Mn ions. In contrast the valence band is antiferromagnetically coupled
with the Mn impurities and the spin splitting is not linearly dependent on the
Mn concentration. This suggests that the mean field approximation breaks down
in the case of Mn-doped GaAs and corrections due to multiple scattering must be
considered. We calculate these corrections within a simple free electron model
and find good agreement with our {\it ab initio} results if a large exchange
constant (eV) is assumed.Comment: 15 pages, 14 figure
Angle-resolved photoemission study and first principles calculation of the electronic structure of GaTe
The electronic band structure of GaTe has been calculated by numerical atomic
orbitals density-functional theory, in the local density approximation. In
addition, the valence-band dispersion along various directions of the GaTe
Brillouin zone has been determined experimentally by angle-resolved
photoelectron spectroscopy. Along these directions, the calculated valence-band
structure is in good concordance with the valence-band dispersion obtained by
these measurements. It has been established that GaTe is a direct-gap
semiconductor with the band gap located at the Z point, that is, at Brillouin
zone border in the direction perpendicular to the layers. The valence-band
maximum shows a marked \textit{p}-like behavior, with a pronounced anion
contribution. The conduction band minimum arises from states with a comparable
\textit{s}- \textit{p}-cation and \textit{p}-anion orbital contribution.
Spin-orbit interaction appears to specially alter dispersion and binding energy
of states of the topmost valence bands lying at . By spin-orbit, it is
favored hybridization of the topmost \textit{p}-valence band with deeper
and flatter \textit{p}-\textit{p} bands and the valence-band minimum at
is raised towards the Fermi level since it appears to be determined by
the shifted up \textit{p}-\textit{p} bands.Comment: 7 text pages, 6 eps figures, submitted to PR
A practical guide to medical ethics for intensivists: Part 2
This series of articles provides a practical guide to dealing with the complex ethical and legal challenges we face in the provision of Intensive Care. They explain the implications of recent legal rulings and cases (such as "Montgomery" and the tragic case of Charlie Gard), and include discussion of clinical scenarios with which we are all familiar. Each article is written by those who are not only experts in their fields, but who also deal with these issues on a day-to-day basis
Neorealism: unifying cognition and environment
APA ProofsRecent trends in psychology have revealed increasing discontent toward representational explanations of behavior. However, whether these trends have the conceptual resources to address the full range of cognitive phenomena remains unclear. Here, I defend neorealism (Holt, 1914) as the missing philosophical link between radically embodied cognitive science and a more encompassing psychology. Neorealism identifies cognitive contents, however subjective or unreal in appearance, with portions or cross sections of the objective environment. These portions are extended in time and they sustain causal and
historical relations to current behavior. I sketch how neorealist concepts can help psychologists to address phenomena such as dreams and imagery in fully environmental terms
Fast algorithm for calculating two-photon absorption spectra
We report a numerical calculation of the two-photon absorption coefficient of
electrons in a binding potential using the real-time real-space higher-order
difference method. By introducing random vector averaging for the intermediate
state, the task of evaluating the two-dimensional time integral is reduced to
calculating two one-dimensional integrals. This allows the reduction of the
computation load down to the same order as that for the linear response
function. The relative advantage of the method compared to the straightforward
multi-dimensional time integration is greater for the calculation of non-linear
response functions of higher order at higher energy resolution.Comment: 4 pages, 2 figures. It will be published in Phys. Rev. E on 1, March,
199
Bonding, Moment Formation, and Magnetic Interactions in Ca14MnBi11 and Ba14MnBi11
The ``14-1-11'' phase compounds based on magnetic Mn ions and typified by
Ca14MnBi11 and Ba14MnBi11 show unusual magnetic behavior, but the large number
(104) of atoms in the primitive cell has precluded any previous full electronic
structure study. Using an efficient, local orbital based method within the
local spin density approximation to study the electronic structure, we find a
gap between a bonding valence band complex and an antibonding conduction band
continuum. The bonding bands lack one electron per formula unit of being
filled, making them low carrier density p-type metals. The hole resides in the
MnBi4 tetrahedral unit and partially compensates the high spin d^5 Mn moment,
leaving a net spin near 4 \mu_B that is consistent with experiment. These
manganites are composed of two disjoint but interpenetrating `jungle gym'
networks of spin 4/2 MnBi4^{9-} units with ferromagnetic interactions within
the same network, and weaker couplings between the networks whose sign and
magnitude is sensitive to materials parameters. Ca14MnBi11 is calculated to be
ferromagnetic as observed, while for Ba14MnBi11 (which is antiferromagnetic)
the ferro- and antiferromagnetic states are calculated to be essentially
degenerate. The band structure of the ferromagnetic states is very close to
half metallic.Comment: 17 pages, containing 10 postscript figures and 5 tables. Two
additional figures (Fig.8 and 11 of the paper) are provided in JPG format in
separate files. Submitted to Phys. Rev. B on September 20th 200
Ab-initio structural, elastic, and vibrational properties of carbon nanotubes
A study based on ab initio calculations is presented on the estructural,
elastic, and vibrational properties of single-wall carbon nanotubes with
different radii and chiralities. We use SIESTA, an implementation of
pseudopotential-density-functional theory which allows calculations on systems
with a large number of atoms per cell. Different quantities like bond
distances, Young moduli, Poisson ratio and the frequencies of different phonon
branches are monitored versus tube radius. The validity of expectations based
on graphite is explored down to small radii, where some deviations appear
related to the curvature effects. For the phonon spectra, the results are
compared with the predictions of the simple zone-folding approximation. Except
for the known defficiencies of this approximation in the low-frequency
vibrational regions, it offers quite accurate results, even for relatively
small radii.Comment: 13 pages, 7 figures, submitted to Phys. Rev. B (11 Nov. 98
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