6,800 research outputs found
Semilocal density functional theory with correct surface asymptotics
Semilocal density functional theory is the most used computational method for
electronic structure calculations in theoretical solid-state physics and
quantum chemistry of large systems, providing good accuracy with a very
attractive computational cost. Nevertheless, because of the non-locality of the
exchange-correlation hole outside a metal surface, it was always considered
inappropriate to describe the correct surface asymptotics. Here, we derive,
within the semilocal density functional theory formalism, an exact condition
for the image-like surface asymptotics of both the exchange-correlation energy
per particle and potential. We show that this condition can be easily
incorporated into a practical computational tool, at the simple
meta-generalized-gradient approximation level of theory. Using this tool, we
also show that the Airy-gas model exhibits asymptotic properties that are
closely related to the ones at metal surfaces. This result highlights the
relevance of the linear effective potential model to the metal surface
asymptotics.Comment: 6 pages, 4 figure
The highly ionized disk wind of GRO J1655-40
Aims: The galactic superluminal microquasar GRO J1655-40 started a new
outburst in February 2005, after seven years in quiescence, rising to a
high/soft state in March 2005. In this paper we study the X-ray spectra during
this rise. Methods: We observed GRO J1655-40 with XMM-Newton, on 27 February
2005, in the low/hard state, and on three consecutive days in March 2005,
during the rise of the source to its high/soft state. The EPIC-pn camera was
used in the fast-read Burst mode to avoid photon pile-up. Results: First, we
contributed to the improvement of the calibration of the EPIC-pn, since the
high flux received from the source required some refinements in the correction
of the Charge Transfer Efficiency of the camera.Second, we find that the X-ray
spectrum of GRO J1655-40 is dominated in the high/soft state by the thermal
emission from the accretion disk, with an inner radius of 13-14(D/3.2kpc)km and
a maximum temperature of 1.3 keV. Two absorption lines are detected in the
EPIC-pn spectra, at 6.7-6.8 and 7.8-8.0 keV, which can be identified either as
blended Fe XXV and Fe XXVI K-alpha and K-beta lines, or as blueshifted Fe XXV.
We find no orbital dependence on the X-ray properties, which provides an upper
limit for the inclination of the system of 73 degr. The RGS spectrometers
reveal interstellar absorption features at 17.2AA, 17.5AA (Fe L edges) and
23.54AA (OI K-alpha). Finally, while checking the interstellar origin of the OI
line, we find a general correlation of the OI K-alpha line equivalent width
with the hydrogen column density using several sources available in the
literature.Comment: 10 pages, 7 figures, 4 tables. Revised version with important change
Real-Time Description of the Electronic Dynamics for a Molecule close to a Plasmonic Nanoparticle
The optical properties of molecules close to plasmonic nanostructures greatly
differ from their isolated molecule counterparts. To theoretically investigate
such systems in a Quantum Chemistry perspective, one has to take into account
that the plasmonic nanostructure (e.g., a metal nanoparticle - NP) is often too
large to be treated atomistically. Therefore, a multiscale description, where
the molecule is treated by an ab initio approach and the metal NP by a lower
level description, is needed. Here we present an extension of one such
multiscale model [Corni, S.; Tomasi, J. {\it J. Chem. Phys.} {\bf 2001}, {\it
114}, 3739] originally inspired by the Polarizable Continuum Model, to a
real-time description of the electronic dynamics of the molecule and of the NP.
In particular, we adopt a Time-Dependent Configuration Interaction (TD CI)
approach for the molecule, the metal NP is described as a continuous dielectric
of complex shape characterized by a Drude-Lorentz dielectric function and the
molecule- NP electromagnetic coupling is treated by an equation-of-motion (EOM)
extension of the quasi-static Boundary Element Method (BEM). The model includes
the effects of both the mutual molecule- NP time-dependent polarization and the
modification of the probing electromagnetic field due to the plasmonic
resonances of the NP. Finally, such an approach is applied to the investigation
of the light absorption of a model chromophore, LiCN, in the presence of a
metal NP of complex shape.Comment: This is the final peer-reviewed manuscript accepted for publication
of an open access article published under an ACS AuthorChoice License, which
permits copying and redistribution of the article or any adaptations for
non-commercial purposes. Link to the original article:
http://pubs.acs.org/doi/abs/10.1021/acs.jpcc.6b1108
Free-carrier screening of polarization fields in wurtzite GaN/InGaN laser structures
The free-carrier screening of macroscopic polarization fields in wurtzite
GaN/InGaN quantum wells lasers is investigated via a self-consistent
tight-binding approach. We show that the high carrier concentrations found
experimentally in nitride laser structures effectively screen the built-in
spontaneous and piezoelectric polarization fields, thus inducing a
``field-free'' band profile. Our results explain some heretofore puzzling
experimental data on nitride lasers, such as the unusually high lasing
excitation thresholds and emission blue-shifts for increasing excitation
levels.Comment: RevTeX 4 pages, 4 figure
A single residue in the M2-M3 loop is a major determinant of coupling between binding and gating in neuronal nicotinic receptors.
XMM-Newton and INTEGRAL observations of the black hole candidate XTE J1817-330
The galactic black hole candidate XTE J1817-330 was discovered in outburst by
RXTE in January 2006. We present here the results of an XMM-Newton Target of
opportunity observation (TOO), performed on 13 March 2006 (44 days after the
maximum), and an INTEGRAL observation performed on 15-18 February 2006 (18 days
after the maximum). The EPIC-pn camera on-board XMM-Newton was used in the fast
read-out Burst mode to avoid photon pile-up, while the RGSs were used in
Spectroscopy high count-rate mode. We fit both the XMM-Newton and the INTEGRAL
spectra with a two-component model consisting of a thermal accretion disk and a
comptonizing hot corona. The soft X-ray spectrum is dominated by an accretion
disk component, with a maximum temperature decreasing from 0.96+/-0.04 keV at
the time of the INTEGRAL observation to 0.70+/-m0.01 keV on 13 March. The
Optical Monitors on board INTEGRAL and XMM-Newton showed the source with
magnitudes V: 11.3-11.4, U:15.0-15.1 and UVW1:14.7-14.8. The soft X-ray
spectrum, together with the optical and UV data, show a low hydrogen column
density towards the source, and several absorption lines, most likely of
interstellar origin, are detected in the RGS spectrum: OI K-alpha, OI K-beta,
OII, OIII and OVII, which trace both cold and hot components of the ISM. The
soft X-ray spectrum indicates the presence of a black hole, with an estimate
for the upper limit of the mass of 6.0(+4.0/-2.5) Msun.Comment: Accepted for publication in A&
X-ray monitoring of classical novae in the central region of M31. III. Autumn and winter 2009/10, 2010/11 and 2011/12
[Abridged] Classical novae (CNe) represent the major class of supersoft X-ray
sources (SSSs) in the central region of our neighbouring galaxy M31. We
performed a dedicated monitoring of the M31 central region, aimed to detect SSS
counterparts of CNe, with XMM-Newton and Chandra between Nov and Mar of the
years 2009/10, 2010/11 and 2011/12. In total we detected 24 novae in X-rays.
Seven of these sources were known from previous observations, including the M31
nova with the longest SSS phase, M31N~1996-08b, which was found to fade below
our X-ray detection limit 13.8 yr after outburst. Of the new discoveries
several novae exhibit significant variability in their short-term X-ray light
curves with one object showing a suspected period of about 1.3 h. We studied
the SSS state of the most recent outburst of a recurrent nova which had
previously shown the shortest time ever observed between two outbursts (about 5
yr). The total number of M31 novae with X-ray counterpart was increased to 79
and we subjected this extended catalogue to detailed statistical studies. Four
previously indicated correlations between optical and X-ray parameters could be
confirmed and improved. We found indications that the multi-dimensional
parameter space of nova properties might be dominated by a single physical
parameter. We discuss evidence for a different X-ray behaviour of novae in the
M31 bulge and disk. Exploration of the multi-wavelength parameter space of
optical and X-ray measurements is shown to be a powerful tool for examining
properties of extragalactic nova populations. While there are hints that the
different stellar populations of M31 (bulge vs disk) produce dissimilar nova
outbursts, there is also growing evidence that the overall behaviour of an
average nova might be understood in surprisingly simple terms.Comment: 39 pages (half of them for 9 tables), 14 figures, accepted for
publication in A&A; updated after language editing stag
Recent developments of MCViNE and its applications at SNS
MCViNE is an open source, object-oriented Monte Carlo neutron ray-tracing simulation software package. Its design allows for flexible, hierarchical representations of sophisticated instrument components such as detector systems, and samples with a variety of shapes and scattering kernels. Recently this flexible design has enabled several applications of MCViNE simulations at the Spallation Neutron Source (SNS) at Oak Ridge National Lab, including assisting design of neutron instruments at the second target station and design of novel sample environments, as well as studying effects of instrument resolution and multiple scattering. Here we provide an overview of the recent developments and new features of MCViNE since its initial introduction (Jiao et al 2016 Nucl. Instrum. Methods Phys. Res., Sect. A 810, 86–99), and some example applications
SQG-Differential Evolution for difficult optimization problems under a tight function evaluation budget
In the context of industrial engineering, it is important to integrate
efficient computational optimization methods in the product development
process. Some of the most challenging simulation-based engineering design
optimization problems are characterized by: a large number of design variables,
the absence of analytical gradients, highly non-linear objectives and a limited
function evaluation budget. Although a huge variety of different optimization
algorithms is available, the development and selection of efficient algorithms
for problems with these industrial relevant characteristics, remains a
challenge. In this communication, a hybrid variant of Differential Evolution
(DE) is introduced which combines aspects of Stochastic Quasi-Gradient (SQG)
methods within the framework of DE, in order to improve optimization efficiency
on problems with the previously mentioned characteristics. The performance of
the resulting derivative-free algorithm is compared with other state-of-the-art
DE variants on 25 commonly used benchmark functions, under tight function
evaluation budget constraints of 1000 evaluations. The experimental results
indicate that the new algorithm performs excellent on the 'difficult' (high
dimensional, multi-modal, inseparable) test functions. The operations used in
the proposed mutation scheme, are computationally inexpensive, and can be
easily implemented in existing differential evolution variants or other
population-based optimization algorithms by a few lines of program code as an
non-invasive optional setting. Besides the applicability of the presented
algorithm by itself, the described concepts can serve as a useful and
interesting addition to the algorithmic operators in the frameworks of
heuristics and evolutionary optimization and computing
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