2,171,473 research outputs found
Local-spin-density functional for multideterminant density functional theory
Based on exact limits and quantum Monte Carlo simulations, we obtain, at any
density and spin polarization, an accurate estimate for the energy of a
modified homogeneous electron gas where electrons repel each other only with a
long-range coulombic tail. This allows us to construct an analytic
local-spin-density exchange-correlation functional appropriate to new,
multideterminantal versions of the density functional theory, where quantum
chemistry and approximate exchange-correlation functionals are combined to
optimally describe both long- and short-range electron correlations.Comment: revised version, ti appear in PR
A Local Density-Based Approach for Local Outlier Detection
This paper presents a simple but effective density-based outlier detection
approach with the local kernel density estimation (KDE). A Relative
Density-based Outlier Score (RDOS) is introduced to measure the local
outlierness of objects, in which the density distribution at the location of an
object is estimated with a local KDE method based on extended nearest neighbors
of the object. Instead of using only nearest neighbors, we further consider
reverse nearest neighbors and shared nearest neighbors of an object for density
distribution estimation. Some theoretical properties of the proposed RDOS
including its expected value and false alarm probability are derived. A
comprehensive experimental study on both synthetic and real-life data sets
demonstrates that our approach is more effective than state-of-the-art outlier
detection methods.Comment: 22 pages, 14 figures, submitted to Pattern Recognition Letter
Local-Density Driven Clustered Star Formation
A positive power-law trend between the local surface densities of molecular
gas, , and young stellar objects, , in molecular
clouds of the Solar Neighbourhood has been identified by Gutermuth et al. How
it relates to the properties of embedded clusters, in particular to the
recently established radius-density relation, has so far not been investigated.
In this paper, we model the development of the stellar component of molecular
clumps as a function of time and initial local volume density so as to provide
a coherent framework able to explain both the molecular-cloud and
embedded-cluster relations quoted above. To do so, we associate the observed
volume density gradient of molecular clumps to a density-dependent free-fall
time. The molecular clump star formation history is obtained by applying a
constant SFE per free-fall time, \eff.
For volume density profiles typical of observed molecular clumps (i.e.
power-law slope ), our model gives a star-gas surface-density
relation , in very good agreement with
the Gutermuth et al relation. Taking the case of a molecular clump of mass and radius experiencing star formation during
2 Myr, we derive what SFE per free-fall time matches best the normalizations of
the observed and predicted (, ) relations. We
find \eff \simeq 0.1. We show that the observed growth of embedded clusters,
embodied by their radius-density relation, corresponds to a surface density
threshold being applied to developing star-forming regions. The consequences of
our model in terms of cluster survivability after residual star-forming gas
expulsion are that due to the locally high SFE in the inner part of
star-forming regions, global SFE as low as 10% can lead to the formation of
bound gas-free star clusters.Comment: 16 pages, 15 figures, Astronomy & Astrophysics, in pres
Measuring the local dark matter density
We examine systematic problems in determining the local matter density from
the vertical motion of stars, i.e. the 'Oort limit'. Using collisionless
simulations and a Monte Carlo Markov Chain technique, we determine the data
quality required to detect local dark matter at its expected density. We find
that systematic errors are more important than observational errors and apply
our technique to Hipparcos data to reassign realistic error bars to the local
dark matter density.Comment: 3 pages, 1 figure, to be published in "Hunting for the Dark: The
Hidden Side of Galaxy Formation", Malta, 19-23 Oct. 2009, eds. V.P.
Debattista & C.C. Popescu, AIP Conf. Se
Time-Dependent Superfluid Local Density Approximation
The time-dependent superfluid local density approximation (TDSLDA) is an
extension of the Hohenberg-Kohn density functional theory (DFT) to
time-dependent phenomena in superfluid fermionic systems. Unlike linear
response theory, which is only valid for weak external fields, the (TDSLDA)
approach allows one to study non-linear excitations in fermionic superfluids,
including large amplitude collective modes, and the response to strong external
probes. Even in the case of weak external fields, the (TDSLDA) approach is
technically easier to implement. We will illustrate the implementation of the
(TDSLDA) for the unitary Fermi gas, where dimensional arguments and Galilean
invariance simplify the form of the functional, and ab initio input from (QMC)
simulations fix the coefficients to quite high precision.Comment: 6 pages, 1 figure. Unedited version of chapter to appear in Quantum
Gases: Finite Temperature and Non-Equilibrium Dynamics (Vol. 1 Cold Atoms
Series). N.P. Proukakis, S.A. Gardiner, M.J. Davis and M.H. Szymanska, eds.
Imperial College Press, London, 2013 (in press). See
http://www.icpress.co.uk/physics/p817.htm
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