3,862 research outputs found
Performance of a non-empirical meta-GGA density functional for excitation energies
It is known that the adiabatic approximation in time-dependent density
functional theory usually provides a good description of low-lying excitations
of molecules. In the present work, the capability of the adiabatic nonempirical
meta-generalized gradient approximation (meta-GGA) of Tao, Perdew, Staroverov,
and Scuseria (TPSS) to describe atomic and molecular excitations is tested. The
adiabatic (one-parameter) hybrid version of the TPSS meta-GGA and the adiabatic
GGA of Perdew, Burke, and Ernzerhof (PBE) are also included in the test. The
results are compared to experiments and to two well-established hybrid
functionals PBE0 and B3LYP. Calculations show that both adiabatic TPSS and
TPSSh functionals produce excitation energies in fairly good agreement with
experiments, and improve upon the adiabatic local spin density approximation
and, in particular, the adiabatic PBE GGA. This further confirms that TPSS is
indeed a reliable nonhybrid universal functional which can serve as the
starting point from which higher-level approximations can be constructed. The
systematic underestimate of the low-lying vertical excitation energies of
molecules with time-dependent density functionals within the adiabatic
approximation suggests that further improvement can be made with nonadiabatic
corrections.Comment: 7 page
Management of Acute Stroke in the Older Person.
The majority of people who suffer a stroke are older adults. The last two decades have brought major progress in the diagnosis and management of stroke, which has led to significant reductions in mortality, long-term disability, and the need for institutional care. However, acute, interventional and preventative treatments have mostly been trialled in younger age groups. In this article we will provide an overview of the evidence for acute stroke treatments in relation to age, discuss special considerations in the older person, and contemplate patient choice, quality of life, and end-of-life-decisions
The WTO Cotton Case and US Domestic Policy
Crop Production/Industries, International Relations/Trade,
Effect of planting dates on the population dynamics of Cylas puncticollis and sweet potato storage roots damage in South Western Cameroon
The sweetpotato Ipomoea batatas L. (Convolvulaceae) is one of the most important food crops in Africa and the world. The weevil, Cylas puncticollis (Fabricius), is the most destructive field and storage insect pest of sweet potato in Africa. Because of the cryptic feeding nature of the very destructive larval stages, chemical control is often not effective. A field study was therefore designed to determine the effects of different planting dates on infestation and damage of various sweet potato cultivars by the weevils in South Western Cameroon. Ten sweet potato cultivars were planted in different months i.e April and July for the wet season and October and January for the dry season in 2012 and 2013. The vines and storage roots were observed for C. puncticollis damage. Results showed a significance difference on the percentage infestation and yield (P< 0.05) amongst the various planting periods with the least infestation registered for the July (0.3%) while the highest infestation was realized in the January planting (9.22%). The highest yield was obtained from the April planting (7.22 tons/Ha) and the lowest was recorded for the October planting (4.92 tons/Ha). Differences in vine damage amongst the planting periods were not significant at (P > 0.05). Delayed harvesting during the rainy season led to only slight (9.1%) increase in C. puncticollis infestation at 135 DAP while the infestation was high during the dry season at harvest and further increased as harvesting was delayed more than 90 DAP. Sex ratio of weevils throughout the wet and dry seasons showed a higher ratio of females.Key words: Sweetpotato Cylas puncticollis, Planting dates, Percentage infestation
Spin gaps and spin-flip energies in density-functional theory
Energy gaps are crucial aspects of the electronic structure of finite and
extended systems. Whereas much is known about how to define and calculate
charge gaps in density-functional theory (DFT), and about the relation between
these gaps and derivative discontinuities of the exchange-correlation
functional, much less is know about spin gaps. In this paper we give
density-functional definitions of spin-conserving gaps, spin-flip gaps and the
spin stiffness in terms of many-body energies and in terms of single-particle
(Kohn-Sham) energies. Our definitions are as analogous as possible to those
commonly made in the charge case, but important differences between spin and
charge gaps emerge already on the single-particle level because unlike the
fundamental charge gap spin gaps involve excited-state energies. Kohn-Sham and
many-body spin gaps are predicted to differ, and the difference is related to
derivative discontinuities that are similar to, but distinct from, those
usually considered in the case of charge gaps. Both ensemble DFT and
time-dependent DFT (TDDFT) can be used to calculate these spin discontinuities
from a suitable functional. We illustrate our findings by evaluating our
definitions for the Lithium atom, for which we calculate spin gaps and spin
discontinuities by making use of near-exact Kohn-Sham eigenvalues and,
independently, from the single-pole approximation to TDDFT. The many-body
corrections to the Kohn-Sham spin gaps are found to be negative, i.e., single
particle calculations tend to overestimate spin gaps while they underestimate
charge gaps.Comment: 11 pages, 1 figure, 3 table
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
On the effect of Ti on Oxidation Behaviour of a Polycrystalline Nickel-based Superalloy
Titanium is commonly added to nickel superalloys but has a well-documented
detrimental effect on oxidation resistance. The present work constitutes the
first atomistic-scale quantitative measurements of grain boundary and bulk
compositions in the oxide scale of a current generation polycrystalline nickel
superalloy performed through atom probe tomography. Titanium was found to be
particularly detrimental to oxide scale growth through grain boundary
diffusion
Particle-Number Restoration within the Energy Density Functional Formalism
We give a detailed analysis of the origin of spurious divergences and finite
steps that have been recently identified in particle-number restoration
calculations within the nuclear energy density functional framework. We isolate
two distinct levels of spurious contributions to the energy. The first one is
encoded in the definition of the basic energy density functional itself whereas
the second one relates to the canonical procedure followed to extend the use of
the energy density functional to multi-reference calculations. The first level
of spuriosity relates to the long-known self-interaction problem and to the
newly discussed self-pairing interaction process which might appear when
describing paired systems with energy functional methods using auxiliary
reference states of Bogoliubov or BCS type. A minimal correction to the second
level of spuriosity to the multi-reference nuclear energy density functional
proposed in [D. Lacroix, T. Duguet, M. Bender, arXiv:0809.2041] is shown to
remove completely the anomalies encountered in particle-number restored
calculations. In particular, it restores sum-rules over (positive) particle
numbers that are to be fulfilled by the particle-number-restored formalism. The
correction is found to be on the order of several hundreds of keVs up to about
1 MeV in realistic calculations, which is small compared to the total binding
energy, but often accounts for a substantial percentage of the energy gain from
particle-number restoration and is on the same energy scale as the excitations
one addresses with multi-reference energy density functional methods.Comment: 37 pages, 14 figures, accepted for publication in PR
Universal trend of the information entropy of a fermion in a mean field
We calculate the information entropy of single-particle states in
position-space and momentum-space for a nucleon in a nucleus, a
particle in a hypernucleus and an electron in an atomic cluster. It
is seen that and obey the same approximate functional form as
functions of the number of particles, ({\rm or}
in all of the above many-body systems in position- and momentum- space
separately. The net information content is a slowly varying
function of of the same form as above. The entropy sum is
invariant to uniform scaling of coordinates and a characteristic of the
single-particle states of a specific system. The order of single-particle
states according to is the same as their classification according to
energy keeping the quantum number constant. The spin-orbit splitting is
reproduced correctly. It is also seen that enhances with
excitation of a fermion in a quantum-mechanical system. Finally, we establish a
relationship of with the energy of the corresponding single-particle
state i.e. . This relation holds for all the
systems under consideration.Comment: 9 pages, latex, 6 figure
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