173 research outputs found
Diffusive Nested Sampling
We introduce a general Monte Carlo method based on Nested Sampling (NS), for
sampling complex probability distributions and estimating the normalising
constant. The method uses one or more particles, which explore a mixture of
nested probability distributions, each successive distribution occupying ~e^-1
times the enclosed prior mass of the previous distribution. While NS
technically requires independent generation of particles, Markov Chain Monte
Carlo (MCMC) exploration fits naturally into this technique. We illustrate the
new method on a test problem and find that it can achieve four times the
accuracy of classic MCMC-based Nested Sampling, for the same computational
effort; equivalent to a factor of 16 speedup. An additional benefit is that
more samples and a more accurate evidence value can be obtained simply by
continuing the run for longer, as in standard MCMC.Comment: Accepted for publication in Statistics and Computing. C++ code
available at http://lindor.physics.ucsb.edu/DNes
Improved tensor-product expansions for the two-particle density matrix
We present a new density-matrix functional within the recently introduced
framework for tensor-product expansions of the two-particle density matrix. It
performs well both for the homogeneous electron gas as well as atoms. For the
homogeneous electron gas, it performs significantly better than all previous
density-matrix functionals, becoming very accurate for high densities and
outperforming Hartree-Fock at metallic valence electron densities. For isolated
atoms and ions, it is on a par with previous density-matrix functionals and
generalized gradient approximations to density-functional theory. We also
present analytic results for the correlation energy in the low density limit of
the free electron gas for a broad class of such functionals.Comment: 4 pages, 2 figure
Application of an electrified benthic frame trawl for sampling fish in a very large European river (the Danube River) – Is offshore monitoring necessary?
The organization of fish assemblages in offshore, deep channel habitats is poorly known in very largerivers compared with shoreline, littoral areas. We report on the parameters and testing of an electrifiedbenthic frame trawl (EBFT), developed for monitoring the distribution and abundance of benthic fishesin the Danube River, Hungary. We also compare the results of the benthic main channel survey witha shoreline electrofishing (SE) data set. Altogether 33 species were collected offshore during the 175trawling paths (500 m long each). Both sample based and individual based rarefaction showed that nighttime SE detected significantly more species with increasing sampling effort than day time trawling ofoffshore areas. However, offshore surveys detected sterlet Acipenser ruthenus, which could not be detectedby SE, even using extreme high sampling effort. Offshore trawling also proved the common occurrenceand high abundance of the strictly protected endemic Danube streber Zingel streber in the river, whichproved to be extremely rare in SE catches. The EBFT caught larger/older individuals of many species thanSE, and indicated diverse size/age structure for many species offshore. Our survey revealed that offshoreareas are intensively used by a variety of species, which occur relatively even, but with variable abundancein the Danube River. We suggest that even a relatively small (i.e. 2 m wide 1 m high) EBFT can be a veryuseful device for monitoring offshore fish assemblages in very large rivers and provide important datafor bioassessment and conservation purposes
Presence of Many Stable Nonhomogeneous States in an Inertial Car-Following Model
A new single lane car following model of traffic flow is presented. The model
is inertial and free of collisions. It demonstrates experimentally observed
features of traffic flow such as the existence of three regimes: free,
fluctuative (synchronized) and congested (jammed) flow; bistability of free and
fluctuative states in a certain range of densities, which causes the hysteresis
in transitions between these states; jumps in the density-flux plane in the
fluctuative regime and gradual spatial transition from synchronized to free
flow. Our model suggests that in the fluctuative regime there exist many stable
states with different wavelengths, and that the velocity fluctuations in the
congested flow regime decay approximately according to a power law in time.Comment: 4 pages, 4 figure
Critical behavior of a traffic flow model
The Nagel-Schreckenberg traffic flow model shows a transition from a free
flow regime to a jammed regime for increasing car density. The measurement of
the dynamical structure factor offers the chance to observe the evolution of
jams without the necessity to define a car to be jammed or not. Above the
jamming transition the dynamical structure factor exhibits for a given k-value
two maxima corresponding to the separation of the system into the free flow
phase and jammed phase. We obtain from a finite-size scaling analysis of the
smallest jam mode that approaching the transition long range correlations of
the jams occur.Comment: 5 pages, 7 figures, accepted for publication in Physical Review
Nested sampling for materials: the case of hard spheres
The recently introduced nested sampling algorithm allows the direct and
efficient calculation of the partition function of atomistic systems. We
demonstrate its applicability to condensed phase systems with periodic boundary
conditions by studying the three dimensional hard sphere model. Having obtained
the partition function, we show how easy it is to calculate the compressibility
and the free energy as functions of the packing fraction and local order,
verifying that the transition to crystallinity has a very small barrier, and
that the entropic contribution of jammed states to the free energy is
negligible for packing fractions above the phase transition. We quantify the
previously proposed schematic phase diagram and estimate the extent of the
region of jammed states. We find that within our samples, the maximally random
jammed configuration is surprisingly disordered
Variability of M giant stars based on Kepler photometry: general characteristics
M giants are among the longest-period pulsating stars which is why their
studies were traditionally restricted to analyses of low-precision visual
observations, and more recently, accurate ground-based data. Here we present an
overview of M giant variability on a wide range of time-scales (hours to
years), based on analysis of thirteen quarters of Kepler long-cadence
observations (one point per every 29.4 minutes), with a total time-span of over
1000 days. About two-thirds of the sample stars have been selected from the
ASAS-North survey of the Kepler field, with the rest supplemented from a
randomly chosen M giant control sample.
We first describe the correction of the light curves from different quarters,
which was found to be essential. We use Fourier analysis to calculate multiple
frequencies for all stars in the sample. Over 50 stars show a relatively strong
signal with a period equal to the Kepler-year and a characteristic phase
dependence across the whole field-of-view. We interpret this as a so far
unidentified systematic effect in the Kepler data. We discuss the presence of
regular patterns in the distribution of multiple periodicities and amplitudes.
In the period-amplitude plane we find that it is possible to distinguish
between solar-like oscillations and larger amplitude pulsations which are
characteristic for Mira/SR stars. This may indicate the region of the
transition between two types of oscillations as we move upward along the giant
branch.Comment: 12 pages, 13 figures, accepted for publication in MNRAS. The
normalized light curves are available upon reques
First-Principles Electronic Structure of Solid Picene
To explore the electronic structure of the first aromatic superconductor,
potassium-doped solid picene which has been recently discovered by Mitsuhashi
et al with the transition temperatures K, we have obtained a
first-principles electronic structure of solid picene as a first step toward
the elucidation of the mechanism of the superconductivity. The undoped crystal
is found to have four conduction bands, which are characterized in terms of the
maximally localized Wannier orbitals. We have revealed how the band structure
reflects the stacked arrangement of molecular orbitals for both undoped and
doped (Kpicene) cases, where the bands are not rigid. The Fermi surface for
Kpicene is a curious composite of a warped two-dimensional surface and a
three-dimensional one.Comment: 5 pages, 4 figure
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