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1

Estimating delayed density-dependent mortality in sockeye salmon ( Oncorhynchus nerka ): a meta-analytic approach  

Microsoft Academic Search

Delayed density-dependent mortality can be a cause of the cyclic patterns in abundance observed in many populations of sockeye salmon (Oncorhynchus nerka). We used a meta-analytical approach to test for delayed density dependence using 34 time series of sockeye data. We found no consistent evidence for delayed density-dependent mortality using spawner - spring fry or spawner-recruit data. We did find

Ransom A. Myers; Michael J. Bradford; Jessica M. Bridson; Gordon Mertz

1997-01-01

2

Optical absorption in B{sub 13} cluster: A time-dependent density functional approach  

SciTech Connect

The linear optical absorption spectra of three isomers of planar boron cluster B{sub 13} are calculated using time-dependent spin-polarized density functional approach. The geometries of these cluster are optimized at the B3LYP/6-311+G* level of theory. Even though the isomers are almost degenerate, the calculated spectra are quite different, indicating a strong structure-property relationship. Therefore, these computed spectra can be used in the photo-absorption experiments to distinguish between different isomers of a cluster.

Shinde, Ravindra [Department of Physics, Indian Institute of Technology Bombay, Mumbai 400076, Maharashtra (India); Tayade, Meenakshi [Department of Chemistry, Institute of Chemical Technology, Mumbai 400019, Maharashtra (India)

2013-02-05

3

Why Density Dependent Propulsion?  

NASA Technical Reports Server (NTRS)

In 2004 Khoury and Weltman produced a density dependent cosmology theory they call the Chameleon, as at its nature, it is hidden within known physics. The Chameleon theory has implications to dark matter/energy with universe acceleration properties, which implies a new force mechanism with ties to the far and local density environment. In this paper, the Chameleon Density Model is discussed in terms of propulsion toward new propellant-less engineering methods.

Robertson, Glen A.

2011-01-01

4

Orientation-Dependent Multiphoton Ionization of Diatomic Molecules: Many-Electron Time-Dependent Density Functional Approach  

Microsoft Academic Search

We present time-dependent density functional calculations of multiphoton ionization (MPI) of diatomic molecules N2, O2, and F2 with arbitrary orientation of the molecular axis by intense linearly-polarized laser pulses with the wavelength 800 nm. For the peak intensity 1x10^14 W\\/cm^2, the inner shells contributions to the MPI probability are quite significant for N2 molecule, particularly at intermediate angles, while for

Dmitry Telnov; Shih-I. Chu

2009-01-01

5

Linear optical response of current-carrying molecular junction: a nonequilibrium Green's function-time-dependent density functional theory approach.  

PubMed

We propose a scheme for calculation of linear optical response of current-carrying molecular junctions for the case when electronic tunneling through the junction is much faster than characteristic time of external laser field. We discuss relationships between nonequilibrium Green's function (NEGF) and time-dependent density functional theory (TDDFT) approaches and derive expressions for optical response and linear polarizability within NEGF-TDDFT scheme. Corresponding results for isolated molecule, derived within TDDFT approach previously, are reproduced when coupling to contacts is neglected. PMID:18376958

Galperin, Michael; Tretiak, Sergei

2008-03-28

6

A non-self-consistent range-separated time-dependent density functional approach for large-scale simulations  

NASA Astrophysics Data System (ADS)

We propose an efficient method for carrying out time-dependent density functional theory (TDDFT) calculations using range-separated hybrid exchange-correlation functionals. Based on a non-self-consistent range-separated Hamiltonian, the method affords large-scale simulations at a fraction of the computational time of conventional hybrid TDDFT approaches. For typical benchmark molecules including N2, CO, C6H6, H2CO and the C2H4-C2F4 dimer, the method possesses the same level of accuracy as the conventional approaches for the valence, Rydberg, and charge-transfer excitation energies when compared to the experimental results. The method is used to determine ? ? ?* excitations in both disordered and crystalline poly(3-hexylthiophene) (P3HT) conjugated polymers with more than six hundred atoms and it yields excitation energies and charge densities that are in excellent agreement with experiments. The simulation of the crystalline P3HT reveals that the phase of the wavefunctions could have an important effect on the excitation energy; a hypothesis based on ?-? stacking is proposed to explain this novel effect in conjugated polymers.

Zhang, Xu; Li, Zi; Lu, Gang

2012-05-01

7

Sampling variability and estimates of density dependence: a composite-likelihood approach.  

PubMed

It is well known that sampling variability, if not properly taken into account, affects various ecologically important analyses. Statistical inference for stochastic population dynamics models is difficult when, in addition to the process error, there is also sampling error. The standard maximum-likelihood approach suffers from large computational burden. In this paper, I discuss an application of the composite-likelihood method for estimation of the parameters of the Gompertz model in the presence of sampling variability. The main advantage of the method of composite likelihood is that it reduces the computational burden substantially with little loss of statistical efficiency. Missing observations are a common problem with many ecological time series. The method of composite likelihood can accommodate missing observations in a straightforward fashion. Environmental conditions also affect the parameters of stochastic population dynamics models. This method is shown to handle such nonstationary population dynamics processes as well. Many ecological time series are short, and statistical inferences based on such short time series tend to be less precise. However, spatial replications of short time series provide an opportunity to increase the effective sample size. Application of likelihood-based methods for spatial time-series data for population dynamics models is computationally prohibitive. The method of composite likelihood is shown to have significantly less computational burden, making it possible to analyze large spatial time-series data. After discussing the methodology in general terms, I illustrate its use by analyzing a time series of counts of American Redstart (Setophaga ruticilla) from the Breeding Bird Survey data, San Joaquin kit fox (Vulpes macrotis mutica) population abundance data, and spatial time series of Bull trout (Salvelinus confluentus) redds count data. PMID:16634310

Lele, Subhash R

2006-01-01

8

Efficient real-space approach to time-dependent density functional theory for the dielectric response of nonmetallic crystals  

Microsoft Academic Search

Time-dependent density functional theory has been used to calculate the static and frequency-dependent dielectric function ?(omega) of nonmetallic crystals. We show that a real-space description becomes feasible for crystals by using a combination of a lattice-periodic (microscopic) scalar potential with a uniform (macroscopic) electric field as perturbation in a periodic structure calculation. The induced density and microscopic potential can be

F. Kootstra; P. L. de Boeij; J. G. Snijders

2000-01-01

9

Efficient real-space approach to time-dependent density functional theory for the dielectric response of nonmetallic crystals  

Microsoft Academic Search

Time-dependent density functional theory has been used to calculate the static and frequency-dependent dielectric function &egr;(?) of nonmetallic crystals. We show that a real-space description becomes feasible for crystals by using a combination of a lattice-periodic (microscopic) scalar potential with a uniform (macroscopic) electric field as perturbation in a periodic structure calculation. The induced density and microscopic potential can be

F. Kootstra; P. L. de Boeij; J. G. Snijders

2000-01-01

10

Density dependence tests, are they?  

Microsoft Academic Search

A large number of time series of abundances of insects and birds from a variety of data sets were submitted to a new density dependence test. The results varied enormously between data sets, but the relation between the frequency of statistically significant density dependence (SSDD) and the length of the series was similar to that of the power curve of

Henk Wolda; Brian Dennis

1993-01-01

11

Time-dependent density functional approach for the calculation of inelastic x-ray scattering spectra of molecules  

SciTech Connect

We apply time-dependent density functional theory to study the valence electron excitations of molecules and generalize the typically used time-propagation scheme and Casida's method to calculate the full wavevector dependent response function. This allows the computational study of dipole-forbidden valence electron transitions and the dispersion of spectral weight as a function of the wavevector. The method provides a novel analysis tool for spectroscopic methods such as inelastic x-ray scattering and electron energy loss spectroscopy. We present results for benzene and CF{sub 3}Cl and make a comparison with experimental results.

Sakko, Arto; Hakala, Mikko; Haemaelaeinen, Keijo [Department of Physics, University of Helsinki, P.O. Box 64, Helsinki FI-00014 (Finland); Rubio, Angel [Nano-Bio Spectroscopy Group and ETSF Scientific Development Centre, Departamento Fisica de Materiales, Universidad del Pais Vasco, Centro de Fisica de Materiales CSIC-UPV/EHU-MPC and DIPC, Av. Tolosa 72, San Sebastian E-20018 (Spain)

2010-11-07

12

Tailoring High-Order Harmonics: A Computational Approach Based on Time-Dependent Density-Functional Theory  

NASA Astrophysics Data System (ADS)

Atoms and molecules react in complex manners when they are irradiated with high-intensity electromagnetic pulses: multi-photon, tunnelling and over-the-barrier ionisation, laser driven photo-induced isomerisations or fragmentations, and high harmonic generation are some of the non-linear effects that are observed. The so-called pulse shaping techniques can be used to design pulses that produce a desired effect. A technologically appealing possibility is to tailor the harmonic emission spectrum: enhancement of some given orders, supressions of others, etc. We have undertaken the task of exploring this possibility from a theoretical point of view, by making use of time-dependent density-functional theory to describe the electrons, a real-space numerical representation, and various optimization techniques.

Castro, Alberto; Akbari, Ali; Rubio, Angel; Gross, Eberhard

2009-03-01

13

Density Functional Approach Francesco Sottile  

E-print Network

Density Functional Approach Francesco Sottile Ecole Polytechnique, Palaiseau - France European Theoretical Spectroscopy Facility (ETSF) 22 October 2010 #12;Density Functional Theory 1. Any observable of a quantum system can be obtained from the density of the system alone. = O[n] Hohenberg, P. and W. Kohn

Botti, Silvana

14

Density-dependent covariant energy density functionals  

SciTech Connect

Relativistic nuclear energy density functionals are applied to the description of a variety of nuclear structure phenomena at and away fromstability line. Isoscalar monopole, isovector dipole and isoscalar quadrupole giant resonances are calculated using fully self-consistent relativistic quasiparticle randomphase approximation, based on the relativistic Hartree-Bogoliubovmodel. The impact of pairing correlations on the fission barriers in heavy and superheavy nuclei is examined. The role of pion in constructing desnity functionals is also investigated.

Lalazissis, G. A. [Physics Department, Aristotle University of Thessaloniki, GR-54124 (Greece)

2012-10-20

15

Density Gradient Dependent Helicon Modes  

NASA Astrophysics Data System (ADS)

Radially localized helicon modes have been proposed to provide a fuller description of helicon discharges over a wide span of operating conditions and gas types [1]. These plasma modes could be of vital importance to the VASIMR engine. They depend on a radial density gradient and appear to operate over a range of frequencies inaccessible to traditional helicon discharges. Our work focuses on confirming experimentally the existence and properties of these helicon modes in Argon, Helium, and Hydrogen. We investigate the density profile, power deposition, wavefields, and dispersion relation of the new helicon modes which differ substantially from the properties of the traditional helicon plasma. We are using a set of dual half-turn helical antennas driven at 13.56 MHz. Our diagnostics includes a system for monitoring the plasma impedance, a set of Langmuir probes, a set of magnetic probes, as well as sensors for monitoring the pressure and DC magnetic field. *Work supported in part by Advanced Space Propulsion Lab, Johnson Space Center, NASA [1] B. N. Breizman and A. V. Arefiev, Phys. Rev. 84, 3863 (2000)

Panevsky, Martin; Bengtson, Roger

2002-11-01

16

Simulation of resonance hyper-Rayleigh scattering of molecules and metal clusters using a time-dependent density functional theory approach  

NASA Astrophysics Data System (ADS)

Resonance hyper-Rayleigh scattering (HRS) of molecules and metal clusters have been simulated based on a time-dependent density functional theory approach. The resonance first-order hyperpolarizability (?) is obtained by implementing damped quadratic response theory using the (2n + 1) rule. To test this implementation, the prototypical dipolar molecule para-nitroaniline (p-NA) and the octupolar molecule crystal violet are used as benchmark systems. Moreover, small silver clusters {Ag_8} and {Ag_{20}} are tested with a focus on determining the two-photon resonant enhancement arising from the strong metal transition. Our results show that, on a per atom basis, the small silver clusters possess two-photon enhanced HRS comparable to that of larger nanoparticles. This finding indicates the potential interest of using small metal clusters for designing new nonlinear optical materials.

Hu, Zhongwei; Autschbach, Jochen; Jensen, Lasse

2014-09-01

17

Multistage density dependence in an amphibian  

Microsoft Academic Search

Density dependence is the major process keeping the sizes of natural populations within bounds. In organisms with complex life cycles, the stage at which density dependence occurs and whether it occurs in one or several life stages have important consequences for the dynamics of their populations. I manipulated density of pool frogs ( Rana lessonae) during the aquatic larval and

Res Altwegg

2003-01-01

18

Fusion using time-dependent density-constrained DFT  

E-print Network

We present results for calculating fusion cross-sections using a new microscopic approach based on a time-dependent density-constrained DFT calculations. The theory is implemented by using densities and other information obtained from TDDFT time-evolution of the nuclear system as constraint on the density for DFT calculations.

Keser, R; Oberacker, V E; Maruhn, J A; Reinhard, P -G

2014-01-01

19

Fusion using time-dependent density-constrained DFT  

E-print Network

We present results for calculating fusion cross-sections using a new microscopic approach based on a time-dependent density-constrained DFT calculations. The theory is implemented by using densities and other information obtained from TDDFT time-evolution of the nuclear system as constraint on the density for DFT calculations.

R. Keser; A. S. Umar; V. E. Oberacker; J. A. Maruhn; P. -G. Reinhard

2014-02-06

20

Density Dependence, Regulation and Variability in Animal Populations  

Microsoft Academic Search

This paper reviews a series of approaches to the study of density dependence, regulation and variability in terrestrial animals, by using single-species, multispecies and life table time series data. Special emphasis is given to the degree of density dependence in the level of variability, which is seldom discussed in this context, but which is conceptually related to population regulation. Broad

Ilkka Hanski

1990-01-01

21

Tail Density Archimedean and t Copulas Tail Densities of Vines Concluding Remarks A tail density approach in extremal  

E-print Network

Tail Density Archimedean and t Copulas Tail Densities of Vines Concluding Remarks A tail density of Mathematics Washington State University Munich, May 2011 Haijun Li A tail density approach in extremal dependence analysis for vine copulas Munich, May 2011 1 / 21 #12;Tail Density Archimedean and t Copulas Tail

Li, Haijun

22

turboTDDFT - A code for the simulation of molecular spectra using the Liouville-Lanczos approach to time-dependent density-functional perturbation theory  

NASA Astrophysics Data System (ADS)

We introduce turboTDDFT, an implementation of the Liouville-Lanczos approach to linearized time-dependent density-functional theory, designed to simulate the optical spectra of molecular systems made of up to several hundred atoms. turboTDDFT is open-source software distributed under the terms of the GPL as a component of QUANTUM ESPRESSO. As with other components, turboTDDFT is optimized to run on a variety of different platforms, from laptops to massively parallel architectures, using native mathematical libraries (LAPACK and FFTW) and a hierarchy of custom parallelization layers built on top of MPI. Program summaryProgram title: turboTDDFT Catalogue identifier: AEIX_v1_0 Program summary URL:http://cpc.cs.qub.ac.uk/summaries/AEIX_v1_0.html Program obtainable from: CPC Program Library, Queen's University, Belfast, N. Ireland Licensing provisions: GNU General Public License V 2.0 No. of lines in distributed program, including test data, etc.: 3 559 889 No. of bytes in distributed program, including test data, etc.: 254 283 981 Distribution format: tar.gz Programming language: Fortran 95 Computer: Any computer architecture Operating system: GNU/Linux, AIX, IRIX, Mac OS X, and other UNIX-like OS's Classification: 16.2, 16.6, 7.7 External routines: turboTDDFT is a tightly integrated component of the Quantum ESPRESSO distribution and requires the standard libraries linked by it: BLAS, LAPACK, FFTW, MPI. Nature of problem: Calculation of the optical absorption spectra of molecular systems. Solution method: The dynamical polarizability of a system is expressed in terms of the resolvent of its Liouvillian super-operator within time-dependent density-functional theory, and calculated using a non-Hermitean Lanczos method, whose implementation does not require the calculation of any virtual states. Pseudopotentials (both norm-conserving and ultrasoft) are used in conjunction with plane-wave basis sets. Restrictions: Spin-restricted formalism. Linear-response regime. No hybrid functionals. Adiabatic XC kernels only. Unusual features: No virtual orbitals are used, nor even calculated. A single Lanczos recursion gives access to the whole optical spectrum. Additional comments: !!!!! The distribution file for this program is over 254 Mbytes and therefore is not delivered directly when download or E-mail is requested. Instead a html file giving details of how the program can be obtained is sent. Running time: From a few minutes for small molecules on serial machines up to many hours on multiple processors for complex nanosystems with hundreds of atoms.

Malc?o?lu, Osman Bar??; Gebauer, Ralph; Rocca, Dario; Baroni, Stefano

2011-08-01

23

New insights into testing for density dependence  

Microsoft Academic Search

The reasons why tests for density dependence often differ in their results for a particular time-series were investigated using modelled time-series of 20 generations in lenght. The test of Pollard et al. (1987) is the most reliable; it had the greatest power with the three forms of density dependent data investigated (mean detection rates of 50.8–61.1%) and was least influenced

M. Holyoak

1993-01-01

24

Wildlife disease elimination and density dependence  

PubMed Central

Disease control by managers is a crucial response to emerging wildlife epidemics, yet the means of control may be limited by the method of disease transmission. In particular, it is widely held that population reduction, while effective for controlling diseases that are subject to density-dependent (DD) transmission, is ineffective for controlling diseases that are subject to frequency-dependent (FD) transmission. We investigate control for horizontally transmitted diseases with FD transmission where the control is via culling or harvest that is non-selective with respect to infection and the population can compensate through DD recruitment or survival. Using a mathematical model, we show that culling or harvesting can eradicate the disease, even when transmission dynamics are FD. Eradication can be achieved under FD transmission when DD birth or recruitment induces compensatory growth of new, healthy individuals, which has the net effect of reducing disease prevalence by dilution. We also show that if harvest is used simultaneously with vaccination, and there is high enough transmission coefficient, application of both controls may be less efficient than vaccination alone. We illustrate the effects of these control approaches on disease prevalence for chronic wasting disease in deer where the disease is transmitted directly among deer and through the environment. PMID:22593103

Potapov, Alex; Merrill, Evelyn; Lewis, Mark A.

2012-01-01

25

Time Dependent Density Functional Theory An introduction  

E-print Network

entries 8 bytes/entry 8 · 1021 bytes 5 · 109 bytes/DVD 1012 DVDs Francesco Sottile (ETSF) Time Dependent entries/coordinate 1021 entries 8 bytes/entry 8 · 1021 bytes 5 · 109 bytes/DVD 1012 DVDs Francesco · 109 bytes/DVD 1012 DVDs Francesco Sottile (ETSF) Time Dependent Density Functional Theory Palaiseau

Botti, Silvana

26

Local density dependent potential for compressible mesoparticles.  

PubMed

This work proposes a coarse grained description of molecular systems based on mesoparticles representing several molecules, where interactions between mesoparticles are modelled by an interparticle potential of molecular type. Since strong non-equilibrium situations over a wide range of pressure and density are targeted, the internal compressibility of the mesoparticles has to be considered. This is done by introducing a dependence of the potential on the local environment of the mesoparticles. To define local densities, we resort to a three-dimensional Voronoi tessellation instead of standard local, spherical averages. As an example, a local density dependent potential is fitted to reproduce the Hugoniot curve of a model of nitromethane over a large range of pressures and densities. PMID:24655170

Faure, Gérôme; Maillet, Jean-Bernard; Stoltz, Gabriel

2014-03-21

27

Local density dependent potential for compressible mesoparticles  

NASA Astrophysics Data System (ADS)

This work proposes a coarse grained description of molecular systems based on mesoparticles representing several molecules, where interactions between mesoparticles are modelled by an interparticle potential of molecular type. Since strong non-equilibrium situations over a wide range of pressure and density are targeted, the internal compressibility of the mesoparticles has to be considered. This is done by introducing a dependence of the potential on the local environment of the mesoparticles. To define local densities, we resort to a three-dimensional Voronoi tessellation instead of standard local, spherical averages. As an example, a local density dependent potential is fitted to reproduce the Hugoniot curve of a model of nitromethane over a large range of pressures and densities.

Faure, Gérôme; Maillet, Jean-Bernard; Stoltz, Gabriel

2014-03-01

28

The Cosmological Dependence of Cluster Density Profiles  

E-print Network

We use N-body simulations to study the shape of mean cluster density and velocity profiles formed via gravitational instability. The dependence of the final structure on both cosmology and initial density field is examined, using a grid of cosmologies and scale-free initial power spectra P\\propto k^n. For each model, we stack clusters to define an average density profile in the non-linear regime. The profiles are well fit by a power law over 99% of the cluster volume, with a clear trend toward steeper slopes with both increasing n and decreasing Omega_o. For models with a Omega_o = 0.2, the profile slopes are consistently higher than those for Omega-1.0. Cluster density profiles are thus potentially useful cosmological diagnostics. We find no evidence for a constant density core in any of the models, although the density profiles do tend to flatten at small radii. Much of the flattening is due to the force softening required by the simulations, and an attempt is made to recover the unsoftened profiles assuming angular momentum invariance. The recovered profiles in the Omega=1 cosmologies are consistent with a pure power law up to the highest density contrasts (10^6) accessible with our resolution. The low density models show significant deviations from a power law above density contrasts \\sim 10^5. We interpret this curvature as reflecting the non scale-invariant nature of the background cosmology in these models.

Mary M. Crone; August E. Evrard; Douglas O. Richstone

1994-04-15

29

Density-dependent pairing in nuclei far from stability  

Microsoft Academic Search

An approach based on the local energy-density functional method with the Green's function formalism for describing the ground-state properties of superfluid nuclei is presented. The Gor'kov equations are treated exactly in the coordinate-space representation. The method is used to calculate the odd-even mass differences and odd-even effects in charge radii which occur to be very sensitive to the density dependence

S. A. Fayans; S. V. Tolokonnikov; E. L. Trykov; D. Zawischa

1998-01-01

30

Charge transfer optical absorption and fluorescence emission of 4-(9-acridyl)julolidine from long-range-corrected time dependent density functional theory in polarizable continuum approach  

NASA Astrophysics Data System (ADS)

A long-range-corrected time-dependent density functional theory (LC-TDDFT) in combination with polarizable continuum model (PCM) have been applied to study charge transfer (CT) optical absorption and fluorescence emission energies basing on parameterized LC-BLYP xc-potential. The molecule of 4-(9-acridyl)julolidine selected for this study represents typical CT donor-acceptor dye with strongly solvent dependent optical absorption and fluorescence emission spectra. The result of calculations are compared with experimental spectra reported in the literature to derive an optimal value of the model screening parameter ?. The first absorption band appears to be quite well predictable within DFT/TDDFT/PCM with the screening parameter ? to be solvent independent (??0.245 Bohr-1) whereas the fluorescence emission exhibits a strong dependence on the range separation with ?-value varying on a rising solvent polarity from about 0.225 to 0.151 Bohr-1. Dipolar properties of the initial state participating in the electronic transition have crucial impact on the effective screening.

Kityk, A. V.

2014-07-01

31

Effective pairing interactions with isospin density dependence  

SciTech Connect

We perform Hartree-Fock-Bogoliubov (HFB) calculations for semi-magic calcium, nickel, tin, and lead isotopes and N=20,28,50, and 82 isotones using density-dependent pairing interactions recently derived from a microscopic nucleon-nucleon interaction. These interactions have an isovector component so that the pairing gaps in symmetric and neutron matter are reproduced. Our calculations well account for the experimental data for the neutron number dependence of binding energy, two-neutron separation energy, and odd-even mass staggering of these isotopes. This result suggests that by introducing the isovector term in the pairing interaction, one can construct a global effective pairing interaction that is applicable to nuclei in a wide range of the nuclear chart. It is also shown with the local density approximation that the pairing field deduced from the pairing gaps in infinite matter reproduces qualitatively well the pairing field for finite nuclei obtained with the HFB method.

Margueron, J. [Institut de Physique Nucleaire, IN2P3-CNRS and Universite Paris-Sud, F-91406 Orsay Cedex (France); Center for Mathematical Sciences, University of Aizu, Aizu-Wakamatsu, 965-8580 Fukushima (Japan); Sagawa, H. [Center for Mathematical Sciences, University of Aizu, Aizu-Wakamatsu, 965-8580 Fukushima (Japan); Hagino, K. [Department of Physics, Tohoku University, Sendai, 980-8578 (Japan)

2008-05-15

32

Sublinear scaling for time-dependent stochastic density functional theory  

E-print Network

A stochastic approach to time-dependent density functional theory (TDDFT) is developed for computing the absorption cross section and the random phase approximation (RPA) correlation energy. The core idea of the approach involves time-propagation of a small set of stochastic orbitals which are fi?rst projected on the occupied space and then propagated in time according to the time-dependent Kohn-Sham equations. The evolving electron density is exactly represented when the number of random orbitals is in?finite, but even a small number (? 16) of such orbitals is enough to obtain meaningful results for absorption spectrum and the RPA correlation energy per electron. We implement the approach for silicon nanocrystals (NCs) using real-space grids and ?find that the overall scaling of the algorithm is sublinear with computational time and memory.

Gao, Yi; Baer, Roi; Rabani, Eran

2014-01-01

33

Shell Model Approach to Nuclear Level Density  

NASA Astrophysics Data System (ADS)

Nuclear level densities (NLD) are traditionally estimated using variations of Fermi Gas Formula (FGF) or combinatoric techniques. Recent investigations using Monte Carlo Shell Model (MCSM) techniques indicate that a shell model description of NLD may be an accurate and stable approach. Full shell model calculations of NLD are very difficult. We calculated the NLD for all nuclei in the sd shell and show that the results can be described by a single particle combinatoric model, which depends on two parameters similar to FGF. We further investigated other models and find that a sum of gaussians with means and variances given by French and Ratcliff averages (Phys. Rev. C 3, 94(1971)) is able to accurately describe shell model NLD, even when shell effects are present. The contribution of the spurious center-of-mass motion to the shell model NLD is also discussed.

Horoi, Mihai

2000-04-01

34

Density-dependence in parasite transmission dynamics.  

PubMed

The transmission of vector-borne parasites is complex, yet to a large extent this complexity can be unravelled through the insights gained from simple mathematical models of the transmission system. The principle is simple because the key question is merely "what is the rate of increase in numbers of hosts affected?" Clearly, if this rate of increase is greater than unity then the infection can spread, while if it is less than unity it will decline. Ronald Ross in 1911 was the first to formulate this idea for malarial(1) and malaria transmission has since attracted most attention from modellers of parasitic diseases(2-4). But although it is implicitly recognized that nothing- not even parasitic transmission - can increase indefinitely, the importance of some degree of density-dependence in regulating the system tends to be neglected (see Box 1). In this article, Klaus Dietz explores some classical ideas of modelling parasitic disease transmission, emphasizing not only the importance of density dependence but also the importance of knowing exactly where such effects operate in the system. PMID:15463054

Dietz, K

1988-04-01

35

Selective fishing induces density-dependent growth.  

PubMed

Over the last decades, views on fisheries management have oscillated between alarm and trust in management progress. The predominant policy for remedying the world fishing crisis aims at maximum sustainable yield (MSY) by adjusting gear selectivity and fishing effort. Here we report a case study on how striving for higher yields from the Eastern Baltic cod stock by increasing selectivity has become exceedingly detrimental for its productivity. Although there is a successive increase in numbers of undersized fish, growth potential is severely reduced, and fishing mortality in fishable size has increased. Once density-dependent growth is introduced, the process is self-enforcing as long as the recruitment remains stable. Our findings suggest that policies focusing on maximum yield while targeting greater sizes are risky and should instead prioritize catch rates over yield. Disregarding the underlying population structure may jeopardize stock productivity, with dire consequences for the fishing industry and ecosystem structure and function. PMID:24920387

Svedäng, Henrik; Hornborg, Sara

2014-01-01

36

Density-dependent diversification in North American wood warblers  

E-print Network

Density-dependent diversification in North American wood warblers Daniel L. Rabosky1,2,* and Irby J diversification. This model predicts density-dependent declines in diversification rates, but has not been that distinguishes density dependence from alternative processes that also produce temporally declining

Rabosky, Daniel L.

37

Relativistic Coulomb excitation within Time Dependent Superfluid Local Density Approximation  

E-print Network

Within the framework of the unrestricted time-dependent density functional theory, we present for the first time an analysis of the relativistic Coulomb excitation of the heavy deformed open shell nucleus $^{238}$U. The approach is based on Superfluid Local Density Approximation (SLDA) formulated on a spatial lattice that can take into account coupling to the continuum, enabling self-consistent studies of superfluid dynamics of any nuclear shape. We have computed the energy deposited in the target nucleus as a function of the impact parameter, finding it to be significantly larger than the estimate using the Goldhaber-Teller model. The isovector giant dipole resonance, the dipole pygmy resonance and giant quadrupole modes were excited during the process. The one body dissipation of collective dipole modes is shown to lead a damping width $\\Gamma_\\downarrow \\approx 0.4$ MeV and the number of pre-equilibrium neutrons emitted has been quantified.

I. Stetcu; C. Bertulani; A. Bulgac; P. Magierski; K. J. Roche

2014-03-11

38

Empirical Evidence of Density Dependence in Populations of  

E-print Network

. . . . . . . . . . . . . . . . . . . . . . . . . 318 III. Strength and Prevalence of DensityDependence in Demographic Parameters. Density Effects on AgeSpecific Survival Rates . . . . . . . . . . . . . . . 329 C. Dispersal Rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 333 A. The Confounding Effects of Age . . . . . . . . . . . . . . . . . . . . . . . . . . 333 B

Festa-Bianchet, Marco

39

Parameter-free calculation of response functions in time dependent density functional theory  

E-print Network

- Quasiparticle approach - Density functional approach Mapping theory to describe spectra of solids in TDDFTParameter-free calculation of response functions in time dependent density functional theory Towards an efficient (fast) theory Conclusions #12;Dielectric function of the material Vtot = -1 Vext Non

Botti, Silvana

40

New Formula for Spin-Dependent Level Density  

Microsoft Academic Search

The first realistic calculations of spin-dependent level densities using the Gaussian polynomial generating function method (GFM) are performed for 114Cd and 244Am. Contrary to the results of previous combinatorial calculations, the curvature of the logarithmic energy dependence of the calculated total level density is consistent with the Bethe formula. The GFM results support the assumption that the spin-dependent level density

V. Paar; S. Brant; D. K. Sunko; M. G. Mustafa; R. G. Lanier

1991-01-01

41

Quasiparticle density-matrix representation of nonlinear time-dependent density-functional response functions  

E-print Network

Quasiparticle density-matrix representation of nonlinear time-dependent density-functional response s : 31.15.Ew, 42.65. k, 31.15.Lc, 71.15.Mb I. INTRODUCTION Time-dependent density-functional theory TDDFT . A closed algebra of quasiparticles which satisfy a somewhat was established unusual scalar product, which

Mukamel, Shaul

42

Sensitivity and Uncertainty Analysis for Functionals of the Time-Dependent Nuclide Density Field.  

National Technical Information Service (NTIS)

An approach to extend the present ORNL sensitivity program to include functionals of the time-dependent nuclide density field is developed. An adjoint equation for the nuclide field was derived previously by using generalized perturbation theory; the pres...

M. L. Williams, C. R. Weisbin

1978-01-01

43

Density-dependent growth in invasive Lionfish (Pterois volitans).  

PubMed

Direct demographic density dependence is necessary for population regulation and is a central concept in ecology, yet has not been studied in many invasive species, including any invasive marine fish. The red lionfish (Pterois volitans) is an invasive predatory marine fish that is undergoing exponential population growth throughout the tropical western Atlantic. Invasive lionfish threaten coral-reef ecosystems, but there is currently no evidence of any natural population control. Therefore, a manipulative field experiment was conducted to test for density dependence in lionfish. Juvenile lionfish densities were adjusted on small reefs and several demographic rates (growth, recruitment, immigration, and loss) were measured throughout an 8-week period. Invasive lionfish exhibited direct density dependence in individual growth rates, as lionfish grew slower at higher densities throughout the study. Individual growth in length declined linearly with increasing lionfish density, while growth in mass declined exponentially with increasing density. There was no evidence, however, for density dependence in recruitment, immigration, or loss (mortality plus emigration) of invasive lionfish. The observed density-dependent growth rates may have implications for which native species are susceptible to lionfish predation, as the size and type of prey that lionfish consume is directly related to their body size. The absence of density-dependent loss, however, contrasts with many native coral-reef fish species and suggests that for the foreseeable future manual removals may be the only effective local control of this invasion. PMID:23825604

Benkwitt, Cassandra E

2013-01-01

44

Statistical inference for density dependent Markovian forestry models  

E-print Network

Statistical inference for density dependent Markovian forestry models Abstract A stochastic forestry model with a density-dependence structure is studied. The population evolves in discrete roughly speaking, becomes large. From the perspective of the analysis of forestry data and predict

Paris-Sud XI, Université de

45

Information density and dependency length as complementary cognitive models.  

PubMed

Certain English constructions permit two syntactic alternations. (1) a. I looked up the number. b. I looked the number up. (2) a. He is often at the office. b. He often is at the office. This study investigates the relationship between syntactic alternations and processing difficulty. What cognitive mechanisms are responsible for our attraction to some alternations and our aversion to others?This article reviews three psycholinguistic models of the relationship between syntactic alternations and processing: Maximum Per Word Surprisal (building on the ideas of Hale, in Proceedings of the 2nd Meeting of the North American chapter of the association for computational linguistics. Association for Computational Linguistics, Pittsburgh, PA, pp 159-166, 2001), Uniform Information Density (UID) (Levy and Jaeger in Adv Neural Inf Process Syst 19:849-856, 2007; inter alia), and Dependency Length Minimization (DLM) (Gildea and Temperley in Cognit Sci 34:286-310, 2010). Each theory makes predictions about which alternations native speakers should favor. Subjects were recruited using Amazon Mechanical Turk and asked to judge which of two competing syntactic alternations sounded more natural. Logistic regression analysis on the resulting data suggests that both UID and DLM are powerful predictors of human preferences. We conclude that alternations that approach uniform information density and minimize dependency length are easier to process than those that do not. PMID:24077911

Collins, Michael Xavier

2014-10-01

46

Density dependence of nuclear symmetry energy constrained by mean-field calculations  

E-print Network

We establish a correlation for the symmetry energy at saturation density $S_{0}$, slope parameter $L$ and curvature parameter $K_{\\text{sym}}$ based on widely different mean field interactions. With the help of this correlation and available empirical and theoretical information, the density dependent behavior around the saturation density is determined. We compare the results obtained with the present approach with those by other analyses. With this obtained density dependent behavior of the symmetry energy, the neutron skin thickness of $^{208}$Pb and some properties of neutron stars are investigated. In addition, it is found that the expression $S(\\rho)=S_{0}(\\rho /\\rho_{0})^{\\gamma}$ or $S(\\rho)=12.5(\\rho /\\rho_{0}) ^{2/3}+C_{p}(\\rho /\\rho_{0}) ^{\\gamma}$ does not reproduce the density dependence of the symmetry energy as predicted by the mean-field approach around nuclear saturation density.

Jianmin Dong; Wei Zuo; Jianzhong Gu; Umberto Lombardo

2012-09-09

47

Density dependence of nuclear symmetry energy constrained by mean-field calculations  

E-print Network

We establish a correlation for the symmetry energy at saturation density $S_{0}$, slope parameter $L$ and curvature parameter $K_{\\text{sym}}$ based on widely different mean field interactions. With the help of this correlation and available empirical and theoretical information, the density dependent behavior around the saturation density is determined. We compare the results obtained with the present approach with those by other analyses. With this obtained density dependent behavior of the symmetry energy, the neutron skin thickness of $^{208}$Pb and some properties of neutron stars are investigated. In addition, it is found that the expression $S(\\rho)=S_{0}(\\rho /\\rho_{0})^{\\gamma}$ or $S(\\rho)=12.5(\\rho /\\rho_{0}) ^{2/3}+C_{p}(\\rho /\\rho_{0}) ^{\\gamma}$ does not reproduce the density dependence of the symmetry energy as predicted by the mean-field approach around nuclear saturation density.

Dong, Jianmin; Gu, Jianzhong; Lombardo, Umberto; 10.1103/PhysRevC.85.034308

2012-01-01

48

Heavy-ion Collisions: Direct and indirect probes of the density and temperature dependence of Esym  

E-print Network

Heavy-ion collisions provide a versatile terrestrial probe of the nuclear equation of state through the formation of nuclear matter at a wide variety of temperatures, densities, and pressures. Direct and indirect approaches for constraining the density dependence of the symmetry energy using heavy-ion collisions have been developed. The direct approach relies on scaling methods which attempt to connect isotopic fragment distributions to the symmetry energy. Using the indirect approach constraints on the equation of state are extracted from comparison of experimental results and theoretical transport calculations which utilize effective nucleon-nucleon interactions. Besides exploring the density dependence of the equation of state, heavy-ion collisions are simultaneously probing different temperature gradients of nuclear matter allowing for the temperature dependence of the symmetry energy to be examined. The current progress and open questions related to constraining the density and temperature dependence of the symmetry energy with heavy-ion collisions are discussed in the review.

Z. Kohley; S. J. Yennello

2014-01-22

49

Exact density-functional potentials for time-dependent quasiparticles.  

PubMed

We calculate the exact Kohn-Sham potential that describes, within time-dependent density-functional theory, the propagation of an electron quasiparticle wave packet of nonzero crystal momentum added to a ground-state model semiconductor. The potential is observed to have a highly nonlocal functional dependence on the charge density, in both space and time, giving rise to features entirely lacking in local or adiabatic approximations. The dependence of the nonequilibrium part of the Kohn-Sham electric field on the local current and charge density is identified as a key element of the correct Kohn-Sham functional. PMID:22861876

Ramsden, J D; Godby, R W

2012-07-20

50

Seasonality, density dependence, and population cycles in Hokkaido voles  

PubMed Central

Voles and lemmings show extensive variation in population dynamics regulated across and within species. In an attempt to develop and test generic hypotheses explaining these differences, we studied 84 populations of the gray-sided vole (Clethrionomys rufocanus) in Hokkaido, Japan. We show that these populations are limited by a combination of density-independent factors (such as climate) and density-dependent processes (such as specialist predators). We show that density-dependent regulation primarily occurs in winter months, so that populations experiencing longer winters tend to have a stronger delayed density-dependence and, as a result, exhibit regular density cycles. Altogether, we demonstrate that seasonality plays a key role in determining whether a vole population is cyclic or not. PMID:14504382

Stenseth, Nils Chr.; Viljugrein, Hildegunn; Saitoh, Takashi; Hansen, Thomas F.; Kittilsen, Marte O.; B?lviken, Erik; Glockner, Fredrik

2003-01-01

51

Influence of density dependence of symmetry energy on fragmentation  

E-print Network

The fragmentation of projectile and spectator is studied at the different incident energies using isospin dependent QMD model with reduced isospin dependent cross-section. Different systems have been used for the analysis of fragment production(IMF). We have used enhanced constant isospin dependent cross-section to explain the experimental findings which is valid for soft equation of state. In addition to that we have tried to study the influence of density dependent symmetry energy on fragment production.

Karan Singh Vinayak; Mohinder Singh; Suneel Kumar

2011-02-09

52

Prefrontal and temporal gray matter density decreases in opiate dependence  

Microsoft Academic Search

Rationale  There have been only a few structural brain-imaging studies, with varied findings, of opiate-dependent subjects. Voxel-based\\u000a morphometry (VBM) is suitable for studying whole brain-wise structural brain changes in opiate-dependent subjects.\\u000a \\u000a \\u000a \\u000a Objectives  The objective of the current study is to explore gray matter density in opiate-dependent subjects.\\u000a \\u000a \\u000a \\u000a Methods  Gray matter density in 63 opiate-dependent subjects and 46 age- and sex-matched healthy comparison subjects

In Kyoon Lyoo; Mark H. Pollack; Marisa M. Silveri; Kyung Heup Ahn; Claudia I. Diaz; Jaeuk Hwang; Seog Ju Kim; Deborah A. Yurgelun-Todd; Marc J. Kaufman; Perry F. Renshaw

2006-01-01

53

Density-dependent recruitment in grassland small mammals  

Microsoft Academic Search

Summary 1. Density dependence has an important influence on the dynamics of many species of small mam- mals. To regulate population growth, density must affect negatively a vital rate (e.g. fecundity); however, little is known about which vital rates are most affected by density. 2. We used a long-term data set for five species of rodents from north-eastern Kansas, USA

Aaron W. Reed; Norman A. Slade

2008-01-01

54

Testing for density dependence allowing for weather effects  

Microsoft Academic Search

A test for density dependence in time-series data allowing for weather effects is presented. The test is based on a discrete\\u000a time autoregressive model for changes in population density with a covariate for the effects of weather. The distribution\\u000a of the test statistic on the null hypothesis of density independence is obtained by parametric bootstrapping. A computer simulation\\u000a exercise is

Peter Rothery; Ian Newton; Lois Dale; Tomasz Wesolowski

1997-01-01

55

Density-dependent recruitment in grassland small mammals.  

PubMed

1. Density dependence has an important influence on the dynamics of many species of small mammals. To regulate population growth, density must affect negatively a vital rate (e.g. fecundity); however, little is known about which vital rates are most affected by density. 2. We used a long-term data set for five species of rodents from north-eastern Kansas, USA to test for relationships between density and the proportion of pregnant females, per capita fecundity and recruitment. We estimated proportion of pregnant females using data collected in the field and fecundity using data on survival, proportion of pregnant females and literature-based density-dependent litter size for each species. We used reverse capture histories to estimate per-capita recruitment. 3. The proportion of pregnant females was related positively to density in most species. Fecundity was related negatively to density in the cotton rat (Sigmodon hispidus Say & Ord, 1825) and not related to density in the four remaining species. Recruitment was related negatively to density in all five species, although not all relationships were statistically significant. 4. The signature of density-dependent recruitment was strongest in the prairie vole [Microtus ochrogaster (Wagner, 1842)] and cotton rat and less so in the remaining species. 5. Our analyses indicate that density affects recruitment negatively in grassland small mammals either through a reduction in immigration or reduced survival of nestlings. Models that seek to include empirical estimates of density dependence may need to include immigration in addition to survival and fecundity. PMID:18039321

Reed, Aaron W; Slade, Norman A

2008-01-01

56

Floquet formulation of time-dependent density functional theory  

Microsoft Academic Search

We present a generalized Floquet formulation of time-dependent density functional theory for nonperturbative treatment of both bound-bound and bound-free multiphoton transitions of many-electron systems in intense monochromatic laser fields. It is shown that the time-dependent Kohn-Sham equations can be converted into an equivalent time-independent infinite-dimensional non-Hermitian Floquet Hamiltonian eigenvalue problem. We introduce the notion of complex density and present a

Dmitry A. Telnov; Shih-I Chu

1997-01-01

57

Density dependence of symmetry free energy of hot nuclei  

E-print Network

The density and excitation energy dependence of symmetry energy and symmetry free energy for finite nuclei are calculated microscopically in a microcanonical framework taking into account thermal and expansion effects. A finite-range momentum and density dependent two-body effective interaction is employed for this purpose. The role of mass, isospin and equation of state (EoS) on these quantities is also investigated; our calculated results are in consonance with the available experimental data.

S. K. Samaddar; J. N. De; X. Vinas; M. Centelles

2008-04-15

58

Site-site memory equation approach in study of density/pressure dependence of translational diffusion coefficient and rotational relaxation time of polar molecular solutions: acetonitrile in water, methanol in water, and methanol in acetonitrile  

E-print Network

We present results of theoretical study and numerical calculation of the dynamics of molecular liquids based on combination of the memory equation formalism and the reference interaction site model - RISM. Memory equations for the site-site intermediate scattering functions are studied in the mode-coupling approximation for the first order memory kernels, while equilibrium properties such as site-site static structure factors are deduced from RISM. The results include the temperature-density(pressure) dependence of translational diffusion coefficients D and orientational relaxation times t for acetonitrile in water, methanol in water and methanol in acetonitrile, all in the limit of infinite dilution. Calculations are performed over the range of temperatures and densities employing the SPC/E model for water and optimized site-site potentials for acetonitrile and methanol. The theory is able to reproduce qualitatively all main features of temperature and density dependences of D and t observed in real and comp...

Kobryn, A E; Hirata, F

2005-01-01

59

Female elk contacts are neither frequency nor density dependent.  

PubMed

Identifying drivers of contact rates among individuals is critical to understanding disease dynamics and implementing targeted control measures. We studied the interaction patterns of 149 female elk (Cervus canadensis) distributed across five different regions of western Wyoming over three years, defining a contact as an approach within one body length (-2 min). Using hierarchical models that account for correlations within individuals, pairs, and groups, we found that pairwise contact rates within a group declined by a factor of three as group sizes increased 33-fold. Per capita contact rates, however, increased with group size according to a power function, such that female elk contact rates fell in between the predictions of density- or frequency-dependent disease models. We found similar patterns for the duration of contacts. Our results suggest that larger elk groups are likely to play a disproportionate role in the disease dynamics of directly transmitted infections in elk. Supplemental feeding of elk had a limited impact on pairwise interaction rates and durations, but per capita rates were more than two times higher on feeding grounds. Our statistical approach decomposes the variation in contact rate into individual, dyadic, and environmental effects, and provides insight into factors that may be targeted by disease control programs. In particular, female elk contact patterns were driven more by environmental factors such as group size than by either individual or dyad effects. PMID:24279278

Cross, P C; Creech, T G; Ebinger, M R; Manlove, K; Irvine, K; Henningsen, J; Rogerson, J; Scurlock, B M; Creel, S

2013-09-01

60

Transverse momentum dependent quark densities from Lattice QCD  

SciTech Connect

We study transverse momentum dependent parton distribution functions (TMDs) with non-local operators in lattice QCD, using MILC/LHPC lattices. We discuss the basic concepts of the method, including renormalization of the gauge link. Results obtained with a simplified operator geometry show visible dipole deformations of spin-dependent quark momentum densities.

Bernhard Musch,Philipp Hagler,John Negele,Andreas Schafer

2011-10-01

61

Transverse momentum dependent quark densities from Lattice QCD  

SciTech Connect

We study transverse momentum dependent parton distribution functions (TMDs) with non-local operators in lattice QCD, using MILC/LHPC lattices. Results obtained with a simpli?ed operator geometry show visible dipole de- formations of spin-dependent quark momentum densities. We discuss the basic concepts of the method, including renormalization of the gauge link, and an ex- tension to a more elaborate operator geometry that would allow us to analyze process-dependent TMDs such as the Sivers-function.

Bernhard Musch,Philipp Hagler,John Negele,Andreas Schafer

2011-02-01

62

THE TIMING AND SIGNIFICANCE OF DENSITY-DEPENDENT AND DENSITy..INDEPENDENT MORTALITY OF AMERICAN SHAD,  

E-print Network

-dependent and density-independent mortality rates at the prejuvenile (age 1-100 days) and posijuvenile (age 101 days and early larval mortality rates and the strength of 1979-87 year classes in the adult stock, whereas mortality rates of late larvae and juveniles were independent ofyear-class strength. Density

63

Intercohort density dependence drives brown trout habitat selection  

NASA Astrophysics Data System (ADS)

Habitat selection can be viewed as an emergent property of the quality and availability of habitat but also of the number of individuals and the way they compete for its use. Consequently, habitat selection can change across years due to fluctuating resources or to changes in population numbers. However, habitat selection predictive models often do not account for ecological dynamics, especially density dependent processes. In stage-structured population, the strength of density dependent interactions between individuals of different age classes can exert a profound influence on population trajectories and evolutionary processes. In this study, we aimed to assess the effects of fluctuating densities of both older and younger competing life stages on the habitat selection patterns (described as univariate and multivariate resource selection functions) of young-of-the-year, juvenile and adult brown trout Salmo trutta. We observed all age classes were selective in habitat choice but changed their selection patterns across years consistently with variations in the densities of older but not of younger age classes. Trout of an age increased selectivity for positions highly selected by older individuals when their density decreased, but this pattern did not hold when the density of younger age classes varied. It suggests that younger individuals are dominated by older ones but can expand their range of selected habitats when density of competitors decreases, while older trout do not seem to consider the density of younger individuals when distributing themselves even though they can negatively affect their final performance. Since these results may entail critical implications for conservation and management practices based on habitat selection models, further research should involve a wider range of river typologies and/or longer time frames to fully understand the patterns of and the mechanisms underlying the operation of density dependence on brown trout habitat selection.

Ayllón, Daniel; Nicola, Graciela G.; Parra, Irene; Elvira, Benigno; Almodóvar, Ana

2013-01-01

64

Estimates of leaf vein density are scale dependent.  

PubMed

Leaf vein density (LVD) has garnered considerable attention of late, with numerous studies linking it to the physiology, ecology, and evolution of land plants. Despite this increased attention, little consideration has been given to the effects of measurement methods on estimation of LVD. Here, we focus on the relationship between measurement methods and estimates of LVD. We examine the dependence of LVD on magnification, field of view (FOV), and image resolution. We first show that estimates of LVD increase with increasing image magnification and resolution. We then demonstrate that estimates of LVD are higher with higher variance at small FOV, approaching asymptotic values as the FOV increases. We demonstrate that these effects arise due to three primary factors: (1) the tradeoff between FOV and magnification; (2) geometric effects of lattices at small scales; and; (3) the hierarchical nature of leaf vein networks. Our results help to explain differences in previously published studies and highlight the importance of using consistent magnification and scale, when possible, when comparing LVD and other quantitative measures of venation structure across leaves. PMID:24259686

Price, Charles A; Munro, Peter R T; Weitz, Joshua S

2014-01-01

65

Fitness and density-dependent population growth in Drosophila melanogaster  

SciTech Connect

The density-dependent rates of population growth were determined for 26 populations of Drosophila melanogaster maintained in the serial transfer system. Twenty-five populations were homozygous for an entire chromosome 2 sampled from nature; the other was a random heterozygous population. Rates of population growth around the carrying capacity cannot explain the large fitness depression of these lines. However, the homozygous lines show large differences in rates of population growth at low densities relative to the random heterozygous standard. The average relative fitness of the homozygous lines, as determined from the growth rates at the lowest density, is 0.51.

Mueller, L.D.; Ayala, F.J.

1981-03-01

66

Evidence for positive density-dependent emigration in butterfly metapopulations.  

PubMed

A positive effect of (meta)population density on emigration has been predicted by many theoretical models and confirmed empirically in various organisms. However, in butterflies, the most popular species for dispersal studies, the evidence for its existence has so far been equivocal, with negative relationships between density and emigration being reported more frequently. We analysed dispersal in sympatric metapopulations of two Maculinea butterflies, intensively surveyed with mark-release-recapture methods for 7 years. Dispersal parameters, derived using the virtual migration model, were assessed against butterfly densities, which fluctuated strongly over the study period. Emigration was positively correlated with density, and this effect was particularly strong at densities above carrying capacity, when emigration increased up to threefold in females and twofold in males compared with the normal levels. In turn, density had little impact on other dispersal parameters analysed. Our findings provide good evidence for positive density-dependence of emigration in butterflies. Emigrating at high densities is particularly beneficial for females, because it gives them a chance to lay part of their egg-load in less crowded patches, where offspring survival is higher due to lower intraspecific competition. Even though the rise in emigration becomes considerable at densities exceeding carrying capacity, i.e. relatively infrequently, it still has serious implications for many ecological phenomena, such as species range expansions, gene flow, and metapopulation persistence. Consequently, instead of treating emigration as a fixed trait, it is worth allowing for its density-dependence in applications such as population viability analyses, genetic models or metapopulation models. PMID:21625981

Nowicki, Piotr; Vrabec, Vladimir

2011-11-01

67

Evaporation Prescription for Time-Dependent Density Functional Calculations  

E-print Network

Collisions between $^{248}$Cm and $^{48}$Ca are systematically calculated by time-dependent density functional calculations with evaporation prescription. Depending on the incident energy and impact parameter, fusion, fusion-fission, and quasi-fission events are expected to appear. In this paper, the evaporation prescription is introduced, which is expected to be rather important to heavy-ion reactions producing superheavy nuclei, where the heavier total mass can be related to the higher total excitation energy.

Yoritaka Iwata; Sophia Heinz

2012-08-30

68

Molecular correlation energy: Density functional and quantum field approaches  

NASA Astrophysics Data System (ADS)

The theory of Local Correlation Functions has been analyzed in the context of Density Functional Theory, and compared to the related g-Hartree Theory. Both theories have been shown to relate to the Generalized (Exchange) Local Spin Density Theories, which have been extended, by introducing new, orbital dependent correlation factors, to give, as an alternative, the Local Correlation Spin Density Theory. A novel approach to the electron correlation problem has been consequently formulated, on the basis of all three classes of Functional Theories, and its feasibility has been demonstrated. Next, the Second Quantization formalism, and generalized coherent states have been used to give, through the path integral formalism, an expansion of electronic interaction which leads to the Dyson equation. The mass operator has been connected with the time-dependent fluctuation fields, which consequently yield an effective potential that differs from the 'classical' single-well potential, and originates from spontaneous symmetry breaking. This is responsible for ground state correlations. The Hartree-Fock-Bogolyubov approximation has been shown suitable to describe the physics, where the particle-hole interactions account for the electron correlation energy correction.

Suba, Slaven L.

69

Density-functional fidelity approach to quantum phase transitions  

E-print Network

We propose a new approach to quantum phase transitions in terms of the density-functional fidelity, which measures the similarity between density distributions of two ground states in parameter space. The key feature of the approach, as we will show, is that the density-functional fidelity can be measured easily in experiments. Both the validity and versatility of the approach are checked by the Lipkin-Meshkov-Glick model and the one-dimensional Hubbard model.

Shi-Jian Gu

2008-09-23

70

Cuticular Antifungals in Spiders: Density- and Condition Dependence  

PubMed Central

Animals living in groups face a high risk of disease contagion. In many arthropod species, cuticular antimicrobials constitute the first protective barrier that prevents infections. Here we report that group-living spiders produce cuticular chemicals which inhibit fungal growth. Given that cuticular antifungals may be costly to produce, we explored whether they can be modulated according to the risk of contagion (i.e. under high densities). For this purpose, we quantified cuticular antifungal activity in the subsocial crab spider Diaea ergandros in both natural nests and experimentally manipulated nests of varying density. We quantified the body-condition of spiders to test whether antifungal activity is condition dependent, as well as the effect of spider density on body-condition. We predicted cuticular antifungal activity to increase and body-condition to decrease with high spider densities, and that antifungal activity would be inversely related to body-condition. Contrary to our predictions, antifungal activity was neither density- nor condition-dependent. However, body-condition decreased with density in natural nests, but increased in experimental nests. We suggest that pathogen pressure is so important in nature that it maintains high levels of cuticular antifungal activity in spiders, impacting negatively on individual energetic condition. Future studies should identify the chemical structure of the isolated antifungal compounds in order to understand the physiological basis of a trade-off between disease prevention and energetic condition caused by group living, and its consequences in the evolution of sociality in spiders. PMID:24637563

Gonzalez-Tokman, Daniel; Ruch, Jasmin; Pulpitel, Tamara; Ponton, Fleur

2014-01-01

71

Cuticular antifungals in spiders: density- and condition dependence.  

PubMed

Animals living in groups face a high risk of disease contagion. In many arthropod species, cuticular antimicrobials constitute the first protective barrier that prevents infections. Here we report that group-living spiders produce cuticular chemicals which inhibit fungal growth. Given that cuticular antifungals may be costly to produce, we explored whether they can be modulated according to the risk of contagion (i.e. under high densities). For this purpose, we quantified cuticular antifungal activity in the subsocial crab spider Diaea ergandros in both natural nests and experimentally manipulated nests of varying density. We quantified the body-condition of spiders to test whether antifungal activity is condition dependent, as well as the effect of spider density on body-condition. We predicted cuticular antifungal activity to increase and body-condition to decrease with high spider densities, and that antifungal activity would be inversely related to body-condition. Contrary to our predictions, antifungal activity was neither density- nor condition-dependent. However, body-condition decreased with density in natural nests, but increased in experimental nests. We suggest that pathogen pressure is so important in nature that it maintains high levels of cuticular antifungal activity in spiders, impacting negatively on individual energetic condition. Future studies should identify the chemical structure of the isolated antifungal compounds in order to understand the physiological basis of a trade-off between disease prevention and energetic condition caused by group living, and its consequences in the evolution of sociality in spiders. PMID:24637563

González-Tokman, Daniel; Ruch, Jasmin; Pulpitel, Tamara; Ponton, Fleur

2014-01-01

72

FITNESS AND DENSITY-DEPENDENT POPULATION GROWTH IN DROSOPHILA MELANOGASTER  

E-print Network

FITNESS AND DENSITY-DEPENDENT POPULATION GROWTH IN DROSOPHILA MELANOGASTER LAURENCE D. MUELLER of population growth were determined for 26 populations of Drosophila melanogaster maintained in the serial for genotypesof Drosophila melanogaster homo- zygous for whole second chromosomes sampled from nature. The net

Rose, Michael R.

73

The nest site lottery: how selectively neutral density dependent growth suppression induces frequency dependent selection.  

PubMed

Modern developments in population dynamics emphasize the role of the turnover of individuals. In the new approaches stable population size is a dynamic equilibrium between different mortality and fecundity factors instead of an arbitrary fixed carrying capacity. The latest replicator dynamics models assume that regulation of the population size acts through feedback driven by density dependent juvenile mortality. Here, we consider a simplified model to extract the properties of this approach. We show that at the stable population size, the structure of the frequency dependent evolutionary game emerges. Turnover of individuals induces a lottery mechanism where for each nest site released by a dead adult individual a single newborn is drawn from the pool of newborn candidates. This frequency dependent selection leads towards the strategy maximizing the number of newborns per adult death. However, multiple strategies can maximize this value. Among them, the strategy with the greatest mortality (which implies the greatest instantaneous growth rate) is selected. This result is important for the discussion about universal fitness measures and which parameters are maximized by natural selection. This is related to the fitness measures R0 and r, because the number of newborns per single dead individual equals the lifetime production of newborn R0 in models without aging. We thus have a two-stage procedure, instead of a single fitness measure, which is a combination of R0 and r. According to the nest site lottery mechanism, at stable population size, selection favors strategies with the greatest r, i.e. those with the highest turnover, from those with the greatest R0. PMID:24071631

Argasinski, K; Broom, M

2013-12-01

74

Angular momentum dependence of the nuclear level density parameter  

SciTech Connect

Dependence of nuclear level density parameter on the angular momentum and temperature is investigated in a theoretical framework using the statistical theory of hot rotating nuclei. The structural effects are incorporated by including shell correction, shape, and deformation. The nuclei around Zapprox =50 with an excitation energy range of 30 to 40 MeV are considered. The calculations are in good agreement with the experimentally deduced inverse level density parameter values especially for {sup 109}In, {sup 113}Sb, {sup 122}Te, {sup 123}I, and {sup 127}Cs nuclei.

Aggarwal, Mamta [UM-DAE Centre for Excellence in Basic Sciences, University of Mumbai-Kalina Campus, Mumbai 400 098 (India); Kailas, S. [Nuclear Physics Division, Bhabha Atomic Research Centre, Mumbai 400 085 (India)

2010-04-15

75

Density Versions of Plunnecke Inequality Epsilon-Delta Approach  

E-print Network

Density Versions of Pl¨unnecke Inequality ­ Epsilon-Delta Approach Renling Jin Abstract We discuss whether Pl¨unnecke's inequality for Shnirel'man density with respect to Shnirel'man basis can be generalized to other densities with respect to other concepts of basis. We show behavioral disparities between

Jin, Renling

76

Density Versions of Plunnecke Inequality Epsilon-Delta Approach  

E-print Network

Density Versions of Pl¨unnecke Inequality ­ Epsilon-Delta Approach Renling Jin Abstract We discuss whether Pl¨unnecke's inequality for Shnirel'man density with respect to Shnirel'man basis can be generalized to other densities with respect to other con- cepts of basis. We show behavioral disparities

Jin, Renling

77

Density dependence of isospin observables in spinodal decomposition  

SciTech Connect

Isotopic fluctuations in fragment formation are investigated in a quasianalytical description of the spinodal decomposition scenario. By exploiting the fluctuation-dissipation relations the covariance matrix of density fluctuations is derived as a function of the wave vector k for nuclear matter at given values of density, charge asymmetry, temperature, and the time that the system spends in the instability region. Then density fluctuations in ordinary space are implemented with a Fourier transform performed in a finite cubic lattice. Inside this box, domains with different density coexist, from which clusters of nucleons eventually emerge. Within our approach, the isotopic distributions are determined by the N/Z ratio of the leading unstable isoscalar-like modes and by isovector-like fluctuations present in the matter undergoing the spinodal decomposition. Hence the average value of the N/Z ratio of clusters and the width of the relative distribution reflect the properties of the symmetry energy. By generating a large number of events, these calculations allow a careful investigation of the cluster isotopic content as a function of the cluster density. A uniform decrease of the average charge asymmetry and of the width of the isotopic distributions with increasing density is observed. Finally, we remark that the results essentially refer to the early breakup of the system.

Colonna, M.; Matera, F. [Laboratori Nazionali del Sud, Via S. Sofia 62, I-95123 Catania (Italy); Dipartimento di Fisica, Universita degli Studi di Firenze and Istituto Nazionale di Fisica Nucleare, Sezione di Firenze, Via G. Sansone 1, I-50019 Sesto Fiorentino (Firenze) (Italy)

2008-06-15

78

A brief compendium of time-dependent density-functional theory  

E-print Network

Time-dependent density-functional theory (TDDFT) is a formally exact approach to the time-dependent electronic many-body problem which is widely used for calculating excitation energies. We present a survey of the fundamental framework, practical aspects, and applications of TDDFT. This paper is mainly intended for non-experts (students or researchers in other areas) who would like to learn about the present state of TDDFT without going too deeply into formal details.

Ullrich, Carsten A

2014-01-01

79

Cycles, stochasticity and density dependence in pink salmon population dynamics.  

PubMed

Complex dynamics of animal populations often involve deterministic and stochastic components. A fascinating example is the variation in magnitude of 2-year cycles in abundances of pink salmon (Oncorhynchus gorbuscha) stocks along the North Pacific rim. Pink salmon have a 2-year anadromous and semelparous life cycle, resulting in odd- and even-year lineages that occupy the same habitats but are reproductively isolated in time. One lineage is often much more abundant than the other in a given river, and there are phase switches in dominance between odd- and even-year lines. In some regions, the weak line is absent and in others both lines are abundant. Our analysis of 33 stocks indicates that these patterns probably result from stochastic perturbations of damped oscillations owing to density-dependent mortality caused by interactions between lineages. Possible mechanisms are cannibalism, disease transmission, food depletion and habitat degradation by which one lineage affects the other, although no mechanism has been well-studied. Our results provide comprehensive empirical estimates of lagged density-dependent mortality in salmon populations and suggest that a combination of stochasticity and density dependence drives cyclical dynamics of pink salmon stocks. PMID:21147806

Krkosek, Martin; Hilborn, Ray; Peterman, Randall M; Quinn, Thomas P

2011-07-01

80

Exploration of a modified density dependence in the Skyrme functional  

SciTech Connect

A variant of the basic Skyrme-Hartree-Fock functional is considered dealing with a new form of density dependence. It employs only integer powers and thus will allow a more sound basis for projection schemes (particle number, angular momentum). We optimize the new functional with exactly the same adjustment strategy as used in an earlier study with a standard Skyrme functional. This allows direct comparisons of the performance of the new functional relative to the standard one. We discuss various observables: bulk properties of finite nuclei, nuclear matter, giant resonances, superheavy elements, and energy systematics. The new functional performs at least as well as the standard one, but offers a wider range of applicability (e.g., for projection) and more flexibility in the regime of high densities.

Erler, J.; Reinhard, P.-G. [Institut fuer Theoretische Physik II, Universitaet Erlangen-Nuernberg, Staudtstrasse 7, D-91058 Erlangen (Germany); Kluepfel, P. [Science Institute, University of Iceland, Dunhaga 3, IS-107 Reykjavik (Iceland)

2010-10-15

81

Role of density dependent symmetry energy in nuclear stopping  

E-print Network

Information about the nuclear matter under the extreme conditions of temperature and density and the role of symmetry energy under these conditions is still a topic of crucial importance in the present day nuclear physics research. The multifragmentation, collective flow and the nuclear stopping is among the various rare phenomenon which can be observed in heavy-ion collisions at intermediate energies. The nuclear stopping, which is sensitive towards the symmetry energy has gained a lot of interest because it provides the possibility to examine the degree of thermalization or equilibration in the matter. Aim of the present study is to pin down the nuclear stopping for the different forms of density dependent symmetry energy

Karan Singh Vinayak; Suneel Kumar

2011-07-27

82

Limit cycles in Norwegian lemmings: tensions between phase-dependence and density-dependence  

PubMed Central

Ever since Elton, the 3–5 year density cycles in lemmings (and other microtines) in Fennoscandia have troubled scientists. Explanations have involved intrinsic regulation and trophic interactions. We have analysed yearly changes in fall abundances for lemmings over 25 years from two local mountain sites in South Norway. These time series appear to have an underlying nonlinear structure of order two. Fitting a piece-wise linear threshold model of maximum order two, the most parsimonious model was, however, of first order for both series. The resulting dynamics from this model is a limit cycle. Reformulating the model in terms of abundances yields a model which combines (delayed) density-dependent effects and the influence of the cyclic phase. The delayed density-dependence of one part of the model is consistent with an effect of specialist predators during the peak and crash phases of the cycle, although other trophic interactions cannot be excluded.

Framstad, E.; Stenseth, N. C.; Bjørnstad, O. N.; Falck, W.

1997-01-01

83

Time-dependent density functional theory for many-electron systems interacting with cavity photons.  

PubMed

Time-dependent (current) density functional theory for many-electron systems strongly coupled to quantized electromagnetic modes of a microcavity is proposed. It is shown that the electron-photon wave function is a unique functional of the electronic (current) density and the expectation values of photonic coordinates. The Kohn-Sham system is constructed, which allows us to calculate the above basic variables by solving self-consistent equations for noninteracting particles. We suggest possible approximations for the exchange-correlation potentials and discuss implications of this approach for the theory of open quantum systems. In particular we show that it naturally leads to time-dependent density functional theory for systems coupled to the Caldeira-Leggett bath. PMID:25167487

Tokatly, I V

2013-06-01

84

Current density partitioning in time-dependent current density functional theory  

NASA Astrophysics Data System (ADS)

We adapt time-dependent current density functional theory to allow for a fragment-based solution of the many-electron problem of molecules in the presence of time-dependent electric and magnetic fields. Regarding a molecule as a set of non-interacting subsystems that individually evolve under the influence of an auxiliary external electromagnetic vector-scalar potential pair, the partition 4-potential, we show that there are one-to-one mappings between this auxiliary potential, a sharply-defined set of fragment current densities, and the total current density of the system. The partition electromagnetic (EM) 4-potential is expressed in terms of the real EM 4-potential of the system and a gluing EM 4-potential that accounts for exchange-correlation effects and mutual interaction forces between fragments that are required to yield the correct electron dynamics. We prove the zero-force theorem for the fragmented system, establish a variational formulation in terms of action functionals, and provide a simple illustration for a charged particle in a ring.

Mosquera, Martín A.; Wasserman, Adam

2014-05-01

85

Intraspecific variation in the strength of density dependence in aphid populations  

Microsoft Academic Search

Experimental evidence is presented for positive, negative, and no density dependence from 32 independent density manipulations of milkweed aphids (Aphis nerii) in laboratory and field experiments. This substantial variation in intra- specific density dependence is associated with temperature and host-plant species. 2. It is reported that as population growth rate increases, density dependence becomes more strongly negative, suggesting that the

ANURAG A. A GRAWAL; NORA U NDERWOOD

2004-01-01

86

Time-dependent quasiparticle current-density-functional theory of x-ray nonlinear response functions  

E-print Network

Time-dependent quasiparticle current-density-functional theory of x-ray nonlinear response.45.Gm, 31.15.Lc I. INTRODUCTION Time-dependent current-density-functional theory TDCDFT offers density alone, as in standard time-dependent density-functional-theory TDDFT .2 Another reason

Mukamel, Shaul

87

Non-Linear Density Dependence in a Stochastic Wild Turkey Harvest Model.  

E-print Network

??Current eastern wild turkey (Meleagris gallopavo silvestris) harvest models assume density-independent population dynamics despite indications that populations are subject to a form of density dependence.… (more)

McGhee, Jay D.

2006-01-01

88

Time-dependent density functional theory employing optimized effective potentials  

NASA Astrophysics Data System (ADS)

Exchange-only ab initio (parameter-free) time-dependent density functional calculations for the vertical excitation energies of atoms and polyatomic molecules are performed by employing optimized effective potentials (OEP's) and their corresponding adiabatic exchange kernels for the first time. Accurate OEP's are obtained by a novel linear-combination-of-atomic-orbital (LCAO) algorithm [R. Colle and R. K. Nesbet, J. Phys. B 34, 2475 (2001)] in which a potential is represented as a sum of a seed potential having the correct -1/r asymptotic behavior and a small and rapidly decaying correction, the latter being approximated accurately by a linear combination of Gaussian functions. The time-dependent OEP (TDOEP) methods with and without the Tamm-Dancoff approximation are implemented by using a trial-vector algorithm, which allows us to avoid the storage or manipulation of transformed two-electron integrals or the diagonalization of large matrices. No approximation is made to TDOEP, besides the adiabatic approximation to the exchange kernel, the LCAO expansion of the orbitals and potentials, and occasionally the Tamm-Dancoff approximation. The vertical excitation energies of the beryllium atom and the nitrogen and water molecules calculated by TDOEP are compared with those obtained from time-dependent density functional theory (TDDFT) employing conventional local or gradient-corrected functionals, configuration interaction singles (CIS), time-dependent Hartree-Fock (TDHF) theory, similarity-transformed equation-of-motion coupled-cluster with single and double substitutions, and experiments. TDOEP, which neglects electron correlation while treating the exchange contribution rigorously within the Kohn-Sham DFT framework, performs equally well as, or even appreciably better than, CIS or TDHF. The slightly better performance of TDOEP might be attributed to the local nature of the exchange potentials that allows the bare orbital energy differences to approximate excitation energies well. Nevertheless, TDDFT employing local or gradient-corrected functionals outperforms TDOEP for low-lying valence excited states, implying that the former somehow accounts for electron correlation effectively, whereas for high-lying and Rydberg excited states, the latter performs better than the former. By combining the desirable features of OEP and local or gradient-corrected exchange-correlation potentials, we arrive at a simple asymptotic correction scheme to the latter. TDDFT with the asymptotic correction yields uniformly accurate excitation energies for both valence and Rydberg excited states.

Hirata, So; Ivanov, Stanislav; Grabowski, Ireneusz; Bartlett, Rodney J.

2002-04-01

89

Bone density in transfusion dependent thalassemia patients in Urmia, Iran  

PubMed Central

Background Patients with thalassemia major and intermedia are susceptible to osteopenia and osteoporosis. The mechanism of osteoporosis in these patients is multifactorial. Transfusion related iron overload in endocrine organs leads to impaired growth hormone secretion, diabetes mellitus, hypothyroidism, hypoparathyroidism, lack of sex steroids and vitamin D deficiency that contribute to impairment in achieving an adequate bone mass .The aim of this study was assessment of frequency of bone loss in patients with thalassemia major and intermedia in Urmia City of West Azerbaijan, Iran Materials and Methods In this cross sectional descriptive study,10 patients (lower than 18 y/o)with transfusion dependent thalassemia attending to Motahari and Emam Khomeini hospitals in Urmia city of Iran were enrolled and scanned for Bone Mineral Density (BMD) starting at around 10 years old. Results Tenatients (6 male and 4 female) with transfusion dependent thalassemia (?-thalassemia major and intermedia) aged 13to 17 years in Urmia city of Iran were enrolled. Mean age of patients was 15.1±.37year old. Among them, 8 patients (80%)had low BMD and2 of them (20%) had normal BMD in lumbar spine. Only 30% of patients had low BMD in the neck of femur. Conclusion We should perform annual BMD in patients with thalassemia major and intermedia and hemoglobin H disease in age of higher than 8 year old and treat low BMD with administration of bisphosphonate, calcium and vitamin D supplements. Medical consultation with a rheumatologist and /or an endocrinologist should be performed in these patients. Changing lifestyle with mild daily exercise, adequate calcium containing foods, avoiding heavy activities, stop smoking, iron chelation therapy in adequate dosage, early diagnosis and treatment of endocrine insufficiency and regular blood transfusions can help to achieve an optimal bone density in these patients. PMID:25002928

Valizadeh, N; Farrokhi, F; Alinejad, V; Said Mardani, SM; Valizadeh, N; Hejazi, S; Noroozi, M

2014-01-01

90

He?2++ molecular ion in a strong time-dependent magnetic field: a current-density functional study.  

PubMed

The He?2++ molecular ion exposed to a strong ultrashort time-dependent (TD) magnetic field of the order of 10(9) G is investigated through a quantum fluid dynamics (QFD) and current-density functional theory (CDFT) based approach using vector exchange-correlation (XC) potential and energy density functional that depend not only on the electronic charge-density but also on the current density. The TD-QFD-CDFT computations are performed in a parallel internuclear-axis and magnetic field-axis configuration at the field-free equilibrium internuclear separation R = 1.3 au with the field-strength varying between 0 and 10(11) G. The TD behavior of the exchange- and correlation energy of the He?2++ is analyzed and compared with that obtained using a [B-TD-QFD-density functional theory (DFT)] approach based on the conventional TD-DFT under similar computational constraints but using only scalar XC potential and energy density functional dependent on the electronic charge-density alone. The CDFT based approach yields TD exchange- and correlation energy and TD electronic charge-density significantly different from that obtained using the conventional TD-DFT based approach, particularly, at typical magnetic field strengths and during a typical time period of the TD field. This peculiar behavior of the CDFT-based approach is traced to the TD current-density dependent vector XC potential, which can induce nonadiabatic effects causing retardation of the oscillating electronic charge density. Such dissipative electron dynamics of the He?2++ molecular ion is elucidated by treating electronic charge density as an electron-"fluid" in the terminology of QFD. PMID:21598275

Vikas

2011-08-01

91

Unified approach for molecular dynamics and density-functional theory  

Microsoft Academic Search

We present a unified scheme that, by combining molecular dynamics and density-functional theory, profoundly extends the range of both concepts. Our approach extends molecular dynamics beyond the usual pair-potential approximation, thereby making possible the simulation of both covalently bonded and metallic systems. In addition it permits the application of density-functional theory to much larger systems than previously feasible. The new

R. Car; M. Parrinello

1985-01-01

92

Density-dependent mortality in an oceanic copepod population.  

PubMed

Planktonic copepods are primary consumers in the ocean and are perhaps the most numerous metazoans on earth. Secondary production by these zooplankton supports most food webs of the open sea, directly affecting pelagic fish populations and the biological pump of carbon into the deep ocean. Models of marine ecosystems are quite sensitive to the formulation of the term for zooplankton mortality, although there are few data available to constrain mortality rates in such models. Here we present the first evidence for nonlinear, density-dependent mortality rates of open-ocean zooplankton. A high-frequency time series reveals that per capita mortality rates of eggs of Calanus finmarchicus Gunnerus are a function of the abundance of adult females and juveniles. The temporal dynamics of zooplankton populations can be influenced as much by time-dependent mortality rates as by variations in 'bottom up' forcing. The functional form and rates chosen for zooplankton mortality in ecosystem models can alter the balance of pelagic ecosystems, modify elemental fluxes into the ocean's interior, and modulate interannual variability in pelagic ecosystems. PMID:11493921

Ohman, M D; Hirche, H J

2001-08-01

93

From patterns to processes: Phase and density dependencies in the Canadian lynx cycle  

PubMed Central

Across the boreal forest of North America, lynx populations undergo 10-year cycles. Analysis of 21 time series from 1821 to the present demonstrates that these fluctuations are generated by nonlinear processes with regulatory delays. Trophic interactions between lynx and hares cause delayed density-dependent regulation of lynx population growth. The nonlinearity, in contrast, appears to arise from phase dependencies in hunting success by lynx through the cycle. Using a combined approach of empirical, statistical, and mathematical modeling, we highlight how shifts in trophic interactions between the lynx and the hare generate the nonlinear process primarily by shifting functional response curves during the increase and the decrease phases. PMID:9860985

Stenseth, Nils C.; Falck, Wilhelm; Chan, Kung-Sik; Bj?rnstad, Ottar N.; O'Donoghue, Mark; Tong, Howell; Boonstra, Rudy; Boutin, Stan; Krebs, Charles J.; Yoccoz, Nigel G.

1998-01-01

94

Density-dependent foraging effort of Deer Mice (Peromyscus maniculatus)  

Microsoft Academic Search

Summary 1. Little is known about how population density affects the foraging behaviour of individuals. Simple models are developed to predict the net effect of density on the quitting-harvest rates of optimal foragers. The theory was tested with experiments that measured the foraging behaviour of free-ranging Deer Mice under control and reduced densities. 2. An increased density of conspecifics may

D. L. Davidson; D. W. Morris

2001-01-01

95

The density of states approach to dense quantum systems  

E-print Network

We develop a first-principle generalised density of state method for studying numerically quantum field theories with a complex action. As a proof of concept, we show that with our approach we can solve numerically the strong sign problem of the $Z_3$ spin model at finite density. Our results are confirmed by standard simulations of the theory dual to the considered model, which is free from a sign problem. Our method opens new perspectives on ab initio simulations of cold dense quantum systems, and in particular of Yang-Mills theories with matter at finite densities, for which Monte Carlo based importance sampling are unable to produce sufficiently accurate results.

Kurt Langfeld; Biagio Lucini

2014-04-28

96

Density-dependence vs. density-independence - linking reproductive allocation to population abundance and vegetation greenness.  

PubMed

1. Recent studies have shown that optimal reproductive allocation depends on both climatic conditions and population density. We tested this hypothesis using six years of demographic data from eight reindeer (Rangifer tarandus) populations coupled with data on population abundance and vegetation greenness [measured using the Enhanced Vegetation Index (EVI)]. 2. Female spring body mass positively affected summer body mass gain, and lactating females were unable to compensate for harsh winters as efficiently as barren ones. Female spring body mass was highly sensitive to changes in population abundance and vegetation greenness and less dependent on previous autumn body mass and reproductive status. Lactating females were larger than barren females in the spring. Moreover, female autumn body mass was positively related to female autumn body mass and reproductive success and was not very sensitive to changes in vegetation greenness and population abundance. 3. Offspring autumn body mass was positively related to both maternal spring and autumn body mass, and as predicted from theory, offspring were more sensitive to changes in vegetation greenness and population abundance than adult females. A lagged cost of reproduction was present as larger females who were barren, the previous year produced larger offspring than equally sized females that successfully reproduced the previous year. 4. Reproductive success was negatively related to female autumn body mass and positively related to female spring body mass. Moreover, females who successfully reproduced the previous year experienced the highest reproductive success. The fact that negative density-dependence was only present for females that had successfully reproduced the previous year further support the hypothesis that reproduction is costly. 5. This study shows that female reindeer buffer their reproductive allocation according to expected winter conditions and that their buffering abilities were limited by population abundance and a lagged cost of reproduction and enhanced by vegetation greenness. PMID:21985598

Bårdsen, Bård-Jørgen; Tveraa, Torkild

2012-03-01

97

Density-dependent dispersal suggests a genetic measure of habitat suitability  

Microsoft Academic Search

Recent research shows that density dependence should result in predictable movements between habitats of different suitability, depending on whether population densities are increasing or decreasing. When population densities are increasing, habitats become filled in order of their suitability, resulting in a net flow from high suitability to low suitability. When populations decrease in density, the reverse can happen. These patterns

Denis Carr; Jeff Bowman; Paul J. Wilson

2007-01-01

98

On a Density-of-States Approach to Bohmian Mechanics  

E-print Network

We propose the idea that in Bohmian mechanics the wavefunction is related to a density of states and explore some of its consequences. Specifically, it allows a maximum-entropy interpretation of quantum probabilities, which creates a stronger link between it and statistical mechanics. The proposed approach also allows a range of extensions of the guidance condition in Bohmian mechanics.

Guy Potvin

2005-04-15

99

Density dependence in insect performance within individual plants: induced resistance to Spodoptera exigua in tomato  

E-print Network

1 Density dependence in insect performance within individual plants: induced resistance, Florida State Univ., Tallahassee, FL 32306-4295, USA. Net intraspecific density dependence experienced of this density dependence can influence the effects of plants on herbivore population dynamics. This study

Underwood, Nora

100

Time-dependent density functional theory quantum transport simulation in non-orthogonal basis  

SciTech Connect

Basing on the earlier works on the hierarchical equations of motion for quantum transport, we present in this paper a first principles scheme for time-dependent quantum transport by combining time-dependent density functional theory (TDDFT) and Keldysh's non-equilibrium Green's function formalism. This scheme is beyond the wide band limit approximation and is directly applicable to the case of non-orthogonal basis without the need of basis transformation. The overlap between the basis in the lead and the device region is treated properly by including it in the self-energy and it can be shown that this approach is equivalent to a lead-device orthogonalization. This scheme has been implemented at both TDDFT and density functional tight-binding level. Simulation results are presented to demonstrate our method and comparison with wide band limit approximation is made. Finally, the sparsity of the matrices and computational complexity of this method are analyzed.

Kwok, Yan Ho; Xie, Hang; Yam, Chi Yung; Chen, Guan Hua, E-mail: ghc@everest.hku.hk [Department of Chemistry, The University of Hong Kong, Pokfulam Road (Hong Kong); Zheng, Xiao [Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei, Anhui 230026 (China)] [Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei, Anhui 230026 (China)

2013-12-14

101

Time-dependent density functional theory of extreme environments  

NASA Astrophysics Data System (ADS)

We describe the challenges involved when using time-dependent density functional theory (TDDFT) to describe warm dense matter (WDM) within a plane-wave, real-time formulation. WDM occurs under conditions of temperature and pressure (over 1000 K and 1 Mbar) where plasma physics meets condensed matter physics. TDDFT is especially important in this regime as it can describe ions and electrons strongly out of equilibrium. Several theoretical challenges must be overcome including assignment of initial state orbitals, choice of time-propogation scheme, treatment of PAW potentials, and inclusion of non-adiabatic effects in the potential energy surfaces. The results of these simulations are critical in several applications. For example, we will explain how the TDDFT calculation can resolve modeling inconsistencies in X-ray Thompson cross-sections, thereby improving an important temperature diagnostic in experiments. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.

Shulenburger, Luke; Desjarlais, Michael; Magyar, Rudolph

2013-04-01

102

Time-dependent density functional theory of extreme environments  

NASA Astrophysics Data System (ADS)

We describe the challenges involved when using time-dependent density functional theory (TDDFT) to describe warm dense matter (WDM) within a plane-wave, real-time formulation. WDM occurs under conditions of temperature and pressure (over 1000 K and 1 Mbar) where plasma physics meets condensed matter physics. TDDFT is especially important in this regime as it can describe ions and electrons strongly out of equilibrium. Several theoretical challenges must be overcome including assignment of initial state orbitals, choice of time-propogation scheme, treatment of PAW potentials, and inclusion of non-adiabatic effects in the potential energy surfaces. The results of these simulations are critical in several applications. For example, we will explain how the TDDFT calculation can resolve modeling inconsistencies in X-ray Thompson cross-sections, thereby improving an important temperature diagnostic in experiments. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.

Magyar, Rudolph; Shulenburger, Luke; Desjarlais, Michael

2013-03-01

103

Further studies of density-dependent interactions for the excitation of collective states  

NASA Astrophysics Data System (ADS)

The density-dependent DDM3Y effective nucleon-nucleon interaction was used in a double-folding model with macroscopic transition densities to explore further density-dependent effects on the inelastic scattering of ?-particles and protons. In particular, we investigate whether transition densities with a node near the nuclear surface, such as occurs for the giant monopole (breathing mode) resonance, are particularly sensitive to density dependence in the interaction. We also study the imposition of a consistency condition on the use of density dependence for inelastic scattering. In both situations, the effects are small (?20%) for ?-particles but large for protons. The consistency condition is discussed further in an appendix.

Farid, M. El-Azab; Satchler, G. R.

1988-05-01

104

Quantifying correlations between isovector observables and the density dependence of the nuclear symmetry energy away from saturation density  

NASA Astrophysics Data System (ADS)

According to the Hugenholtz-Van Hove theorem, the nuclear symmetry energy S (?) and its slope L (?) at arbitrary densities can be decomposed in terms of the density and momentum dependence of the single-nucleon potentials in isospin-asymmetric nuclear matter. We quantify the correlations between several well-known isovector observables and L (?) to locate the density range in which each isovector observable is most sensitive to the density dependence of the S (?). We then study the correlation coefficients between those isovector observables and all the components of the L (?). The neutron skin thickness of 208Pb is found to be strongly correlated with the L (?) at a subsaturation density of ? =0.59?0 through the density dependence of the first-order symmetry potential. Neutron star radii are found to be strongly correlated with the L (?) over a wide range of suprasaturation densities mainly through both the density and momentum dependence of the first-order symmetry potential. Finally, we find that although the crust-core transition pressure has a complex correlation with the L (?), it is strongly correlated with the momentum derivative of the first-order symmetry potential and the density dependence of the second-order symmetry potential.

Fattoyev, F. J.; Newton, W. G.; Li, Bao-An

2014-08-01

105

Density of states approach to dense quantum systems  

NASA Astrophysics Data System (ADS)

We develop a first-principle generalized density-of-states method for numerically studying quantum field theories with a complex action. As a proof of concept, we show that with our approach we can numerically solve the strong sign problem of the Z3 spin model at finite density. Our results are confirmed by standard simulations of the theory dual to the considered model, which is free from a sign problem. Our method opens new perspectives on ab initio simulations of cold dense quantum systems, and in particular of Yang-Mills theories with matter at finite densities, for which Monte Carlo-based importance sampling is unable to produce sufficiently accurate results.

Langfeld, Kurt; Lucini, Biagio

2014-11-01

106

Structure, thermodynamics, and position-dependent diffusivity in fluids with sinusoidal density variations.  

PubMed

Molecular dynamics simulations and a stochastic method based on the Fokker-Planck equation are used to explore the consequences of inhomogeneous density profiles on the thermodynamic and dynamic properties of the hard-sphere fluid and supercooled liquid water. Effects of the inhomogeneity length scale are systematically considered via the imposition of sinusoidal density profiles of various wavelengths. For long-wavelength density profiles, bulk-like relationships between local structure, thermodynamics, and diffusivity are observed as expected. However, for both systems, a crossover in behavior occurs as a function of wavelength, with qualitatively different correlations between the local static and dynamic quantities emerging as density variations approach the scale of a particle diameter. Irrespective of the density variation wavelength, average diffusivities of hard-sphere fluids in the inhomogeneous and homogeneous directions are coupled and approximately correlate with the volume available for insertion of another particle. Unfortunately, a quantitatively reliable static predictor of position-dependent dynamics has yet to be identified for even the simplest of inhomogeneous fluids. PMID:24984592

Bollinger, Jonathan A; Jain, Avni; Truskett, Thomas M

2014-07-22

107

Cell density-dependent reduction of dihydroceramide desaturase activity in neuroblastoma cells.  

PubMed

We applied a metabolic approach to investigate the role of sphingolipids in cell density-induced growth arrest in neuroblastoma cells. Our data revealed that sphingolipid metabolism in neuroblastoma cells significantly differs depending on the cells' population context. At high cell density, cells exhibited G0/G1 cell-cycle arrest and reduced ceramide, monohexosylceramide, and sphingomyelin, whereas dihydroceramide was significantly increased. In addition, our metabolic-labeling experiments showed that neuroblastoma cells at high cell density preferentially synthesized very long chain (VLC) sphingolipids and dramatically decreased synthesis of sphingosine-1-phosphate (S1P). Moreover, densely populated neuroblastoma cells showed increased message levels of both anabolic and catabolic enzymes of the sphingolipid pathway. Notably, our metabolic-labeling experiments indicated reduced dihydroceramide desaturase activity at confluence, which was confirmed by direct measurement of dihydroceramide desaturase activity in situ and in vitro. Importantly, we could reduce dihydroceramide desaturase activity in low-density cells by applying conditional media from high-density cells, as well as by adding reducing agents, such as DTT and L-cysteine to the media. In conclusion, our data suggest a role of the sphingolipid pathway, dihydroceramides desaturase in particular, in confluence-induced growth arrest in neuroblastoma cells. PMID:22377532

Spassieva, Stefka D; Rahmaniyan, Mehrdad; Bielawski, Jacek; Clarke, Christopher J; Kraveka, Jacqueline M; Obeid, Lina M

2012-05-01

108

Density-dependent dispersal in the Glanville fritillary, Melitaea Karin Enfjall and Olof Leimar  

E-print Network

Density-dependent dispersal in the Glanville fritillary, Melitaea cinxia Karin Enfja¨ll and Olof Leimar Enfja¨ll, K. and Leimar, O. 2005. Density-dependent dispersal in the Glanville fritillary or willingness to leave a given patch. One such factor is conspecific density, which may affect the per capita

Leimar, Olof

109

Density-dependent influences on feeding and metabolism in a freshwater snail  

Microsoft Academic Search

We used laboratory experiments to assess the degree of, and the underlying mechanism for, density dependence in the grazing rate of the pulmonate gastropod Physella virgata. Both fecal pellet production and uptake and incorporation of 14C radioisotopes from labeled periphyton were used as indices of grazing rates. Pronounced density-dependent reductions in grazing rate were observed, especially at densities above 4

Kenneth M. Brown; Kevin R. Carman; Veronica Inchausty

1994-01-01

110

Density-dependent mother-yearling association in bighorn sheep  

Microsoft Academic Search

Post-weaning mother-daughter associations are typical of many ungulates, but their existence among sheep is controversial. In bighorn sheep, Ovis canadensis, at high population density, strong mother-yearling associations were found involving mostly ewes whose lamb-of-the-year died at or soon after birth. At low population density, there were no mother-yearling associations regardless of maternal reproductive status. Non-lactating ewes and most ewes caring

MAURO LUCHERINI; MARCO FESTA-BIANCHET; JON T. JORGENSON

111

Efficient time-dependent density functional theory approximations for hybrid density functionals: analytical gradients and parallelization.  

PubMed

In this paper, we present the implementation of efficient approximations to time-dependent density functional theory (TDDFT) within the Tamm-Dancoff approximation (TDA) for hybrid density functionals. For the calculation of the TDDFT/TDA excitation energies and analytical gradients, we combine the resolution of identity (RI-J) algorithm for the computation of the Coulomb terms and the recently introduced "chain of spheres exchange" (COSX) algorithm for the calculation of the exchange terms. It is shown that for extended basis sets, the RIJCOSX approximation leads to speedups of up to 2 orders of magnitude compared to traditional methods, as demonstrated for hydrocarbon chains. The accuracy of the adiabatic transition energies, excited state structures, and vibrational frequencies is assessed on a set of 27 excited states for 25 molecules with the configuration interaction singles and hybrid TDDFT/TDA methods using various basis sets. Compared to the canonical values, the typical error in transition energies is of the order of 0.01 eV. Similar to the ground-state results, excited state equilibrium geometries differ by less than 0.3 pm in the bond distances and 0.5° in the bond angles from the canonical values. The typical error in the calculated excited state normal coordinate displacements is of the order of 0.01, and relative error in the calculated excited state vibrational frequencies is less than 1%. The errors introduced by the RIJCOSX approximation are, thus, insignificant compared to the errors related to the approximate nature of the TDDFT methods and basis set truncation. For TDDFT/TDA energy and gradient calculations on Ag-TB2-helicate (156 atoms, 2732 basis functions), it is demonstrated that the COSX algorithm parallelizes almost perfectly (speedup ~26-29 for 30 processors). The exchange-correlation terms also parallelize well (speedup ~27-29 for 30 processors). The solution of the Z-vector equations shows a speedup of ~24 on 30 processors. The parallelization efficiency for the Coulomb terms can be somewhat smaller (speedup ~15-25 for 30 processors), but their contribution to the total calculation time is small. Thus, the parallel program completes a Becke3-Lee-Yang-Parr energy and gradient calculation on the Ag-TB2-helicate in less than 4 h on 30 processors. We also present the necessary extension of the Lagrangian formalism, which enables the calculation of the TDDFT excited state properties in the frozen-core approximation. The algorithms described in this work are implemented into the ORCA electronic structure system. PMID:21303101

Petrenko, Taras; Kossmann, Simone; Neese, Frank

2011-02-01

112

Density dependent stopping power and muon sticking in muon catalyzed DT fusion  

Microsoft Academic Search

The origin of the experimentally observed [1] density dependence of the muon alpha sticking fraction omegas in muon catalyzed deuterium- tritium fusion is investigated. We show that the reactivation probability depends sensitively on the target stopping power at low ion velocities. The density dependence of the stopping power for a singly charged projectile in liquid heavy hydrogen is parametrized to

H. E. Rafelski; B. Mueller

1988-01-01

113

Density dependent stopping power and muon sticking in muon catalyzed DT fusion  

Microsoft Academic Search

The origin of the experimentally observed [1] density dependence of the muon alpha sticking fraction ?s in muon catalyzed deuterium- tritium fusion is investigated. We show that the reactivation probability depends sensitively on the target stopping power at low ion velocities. The density dependence of the stopping power for a singly charged projectile in liquid heavy hydrogen is parametrized to

H. E. Rafelski

1988-01-01

114

Time-dependent density functional theory: Past, present, and future Kieron Burke  

E-print Network

Received 18 August 2004; accepted 17 March 2005; published online 17 August 2005 Time-dependent density-particle problem that can be solved much faster. Time-dependent density functional theory TDDFT applies the same electrons in a time-dependent field, wave function methods are prohibitive, and in the regime of not too

Burke, Kieron

115

Negative density-dependent dispersal in the American black bear (Ursus americanus) revealed by noninvasive sampling  

E-print Network

Negative density-dependent dispersal in the American black bear (Ursus americanus) revealed avoidance, philopatry, population density, Ursus americanus. Correspondence Louis Bernatchez; D on sex-specific dispersal behavior in the American black bear, Ursus americanus. We tested whether

Bernatchez, Louis

116

Density Dependent Functional Forms Drive Compensation in Populations Exposed to Stressors  

EPA Science Inventory

The interaction between density dependence (DD) and environmental stressors can result in a compensatory or synergistic response in population growth, and population models that use density-independent demographic rates or generic DD functions may be introducing bias into managem...

117

Frequency dependence of the plasma density for helicon plasmas  

SciTech Connect

Variations of plasma density are investigated as a function of frequency of rf power in a helicon plasma source. Abrupt, almost step-like changes in the plasma density are observed during the frequency scans under various conditions of the input rf power, the argon gas pressure, and the magnetic field. It is found that the transition frequencies shift to the lower value region as the input rf power and/or the argon gas pressure is increased, and to the higher value region as the magnetic field is increased. The observed density transitions are compared with semianalytical calculations based on the power balance relation and it has been shown that the results are in good agreement. {copyright} {ital 1997 American Institute of Physics.}

Kwak, J.G.; Choi, H.D.; Bak, H.I. [Department of Nuclear Engineering, Seoul National University, Seoul, 151-742 (Korea)] [Department of Nuclear Engineering, Seoul National University, Seoul, 151-742 (Korea); Cho, S. [Department of Physics, Kyonggi University, Suwon, Kyonggi-Do, 442-760 (Korea)] [Department of Physics, Kyonggi University, Suwon, Kyonggi-Do, 442-760 (Korea); Bak, J.G.; Kim, S.K. [Nuclear Fusion Laboratory, Korea Atomic Energy Research Institute, P.O. Box 105, Taejon, 305-600 (Korea)] [Nuclear Fusion Laboratory, Korea Atomic Energy Research Institute, P.O. Box 105, Taejon, 305-600 (Korea)

1997-05-01

118

Fibroblast PER2 Circadian Rhythmicity Depends on Cell Density  

PubMed Central

Like neurons in the suprachiasmatic nucleus (SCN), the master circadian pacemaker in the brain, single fibroblasts can function as independent oscillators. In the SCN, synaptic and paracrine signaling among cells creates a robust, synchronized circadian oscillation, whereas there is no evidence for such integration in fibroblast cultures. However, interactions among single-cell fibroblast oscillators cannot be completely excluded, because fibroblasts were not isolated in previous work. In this study, we tested the autonomy of fibroblasts as single-cell circadian oscillators in high and low density culture, by single-cell imaging of cells from PER2::LUC circadian reporter mice. We found greatly reduced PER2::LUC rhythmicity in low density cultures, which could result from lack of either constitutive or rhythmic paracrine signals from neighboring fibroblasts. To discriminate between these two possibilities, we mixed PER2::LUC wild type (WT) cells with non-luminescent, non-rhythmic Bmal1?/? cells, so that density of rhythmic cells was low but overall cell density remained high. In this condition, WT cells showed clear rhythmicity similar to high density cultures. We also mixed PER2::LUC WT cells with non-luminescent, long period Cry2?/? cells. In this condition, WT cells showed a period no different from cells cultured with rhythmic WT cells or non-rhythmic Bmal1?/? cells. In previous work, we found that low K+ suppresses fibroblast rhythmicity, and we and others have found that either low K+ or low Ca2+ suppresses SCN rhythmicity. Therefore, we attempted to rescue rhythmicity of low density fibroblasts with high K+ (21 mM), high Ca2+ (3.6 mM), or conditioned medium. Conditioned medium from high density fibroblast cultures rescued rhythmicity of low density cultures, whereas high K+ or Ca2+ medium did not consistently rescue rhythmicity. These data suggest that fibroblasts require paracrine signals from adjacent cells for normal expression of rhythmicity, but that these signals do not have to be rhythmic, and that rhythmic signals from other cells do not affect the intrinsic periods of fibroblasts. PMID:23735497

Noguchi, Takako; Wang, Lexie L.; Welsh, David K.

2013-01-01

119

Invasion rate of deer ked depends on spatiotemporal variation in host density.  

PubMed

Invasive parasites are of great global concern. Understanding the factors influencing the spread of invading pest species is a first step in developing effective countermeasures. Growing empirical evidence suggests that spread rates are essentially influenced by spatiotemporal dynamics of host-parasite interactions, yet approaches modelling spread rate have typically assumed static environmental conditions. We analysed invasion history of the deer ked (Lipoptena cervi) in Finland with a diffusion-reaction model, which assumed either the movement rate, the population growth rate, or both rates may depend on spatial and temporal distribution of moose (Alces alces), the main host of deer ked. We fitted the model to the data in a Bayesian framework, and used the Bayesian information criterion to show that accounting for the variation in local moose density improved the model's ability to describe the pattern of the invasion. The highest ranked model predicted higher movement rate and growth rate of deer ked with increasing moose density. Our results suggest that the historic increase in host density has facilitated the spread of the deer ked. Our approach illustrates how information about the ecology of an invasive species can be extracted from the spatial pattern of spread even with rather limited data. PMID:24521661

Meier, C M; Bonte, D; Kaitala, A; Ovaskainen, O

2014-06-01

120

How important is self-consistency for the dDsC density dependent dispersion correction?  

NASA Astrophysics Data System (ADS)

The treatment of dispersion interactions is ubiquitous but computationally demanding for seamless ab initio approaches. A highly popular and simple remedy consists in correcting for the missing interactions a posteriori by adding an attractive energy term summed over all atom pairs to standard density functional approximations. These corrections were originally based on atom pairwise parameters and, hence, had a strong touch of empiricism. To overcome such limitations, we recently proposed a robust system-dependent dispersion correction, dDsC, that is computed from the electron density and that provides a balanced description of both weak inter- and intramolecular interactions. From the theoretical point of view and for the sake of increasing reliability, we here verify if the self-consistent implementation of dDsC impacts ground-state properties such as interaction energies, electron density, dipole moments, geometries, and harmonic frequencies. In addition, we investigate the suitability of the a posteriori scheme for molecular dynamics simulations, for which the analysis of the energy conservation constitutes a challenging tests. Our study demonstrates that the post-SCF approach in an excellent approximation.

Brémond, Éric; Golubev, Nikolay; Steinmann, Stephan N.; Corminboeuf, Clémence

2014-05-01

121

Density-dependent liquid nitromethane decomposition: molecular dynamics simulations based on ReaxFF.  

PubMed

The decomposition mechanism of hot liquid nitromethane at various compressions was studied using reactive force field (ReaxFF) molecular dynamics simulations. A competition between two different initial thermal decomposition schemes is observed, depending on compression. At low densities, unimolecular C-N bond cleavage is the dominant route, producing CH(3) and NO(2) fragments. As density and pressure rise approaching the Chapman-Jouget detonation conditions (?30% compression, >2500 K) the dominant mechanism switches to the formation of the CH(3)NO fragment via H-transfer and/or N-O bond rupture. The change in the decomposition mechanism of hot liquid NM leads to a different kinetic and energetic behavior, as well as products distribution. The calculated density dependence of the enthalpy change correlates with the change in initial decomposition reaction mechanism. It can be used as a convenient and useful global parameter for the detection of reaction dynamics. Atomic averaged local diffusion coefficients are shown to be sensitive to the reactions dynamics, and can be used to distinguish between time periods where chemical reactions occur and diffusion-dominated, nonreactive time periods. PMID:21812413

Rom, Naomi; Zybin, Sergey V; van Duin, Adri C T; Goddard, William A; Zeiri, Yehuda; Katz, Gil; Kosloff, Ronnie

2011-09-15

122

Mapping Density Response in Maize: A Direct Approach for Testing Genotype and Treatment Interactions  

PubMed Central

Maize yield improvement has been strongly linked to improvements in stress tolerance, particularly to increased interplant competition. As a result, modern hybrids are able to produce kernels at high plant population densities. Identification of the genetic factors responsible for density response in maize requires direct testing of interactions between genetic effects and density and evaluation of that response in multiple traits. In this article we take a broad view of the problem and use a general approach based upon mixed models to analyze data from eight segmental inbred lines in a B73 background and their crosses to the unrelated parent Mo17 (hybrids). We directly test for the interaction between treatment effects and genetic effects instead of the commonly used overlaying of results on a common map. Additionally, we demonstrate one way to handle heteroscedasticity of variances common in stress responses. We find that some SILs are consistently different from the recurrent parent regardless of the density, while others differ from the recurrent parent in one density level but not in the other. Thus, we find positive evidence for both main effects and interaction between genetic loci and density in cases where the approach of overlapping results fails to find significant results. Furthermore, our study clearly identifies segments that respond differently to density depending upon the inbreeding level (inbred/hybrid). PMID:16489238

Gonzalo, Martin; Vyn, Tony J.; Holland, James B.; McIntyre, Lauren M.

2006-01-01

123

Regulation of nematode fecundity in the ring-necked pheasant (Phasianus colchicus): not just density dependence.  

PubMed

Patterns of nematode fecundity were investigated for infections of the caecal worm Heterakis gallinarum in the ring-necked pheasant (Phasianus colchicus). Worm length was a good predictor of parasite fecundity. After controlling for worm length no other factors, including parasite intensity, were related to worm fecundity. Density dependence in worm size was detected in natural infections at parasite intensities above a threshold of 96 worms (worm size decreased with increasing parasite intensity). However, below this threshold, worm size actually decreased with decreasing parasite intensity (inverse density dependence). The interaction between density dependence and inverse density dependence in regulating the development and subsequent fecundity of H. gallinarum worms in ring-necked pheasants was demonstrated in an infection experiment. Density dependence was observed in the stunted growth of worms in heavily infected hosts, relative to worms in lightly infected hosts. Inverse density dependence in worm size was the common pattern across hosts by the end of the experiment, when parasite intensities were below the density dependence threshold. This is the first study to document both density dependence and inverse density dependence in parasite fecundity in the same host-helminth system. PMID:10340333

Tompkins, D M; Hudson, P J

1999-04-01

124

Predation, habitat complexity, and variation in density-dependent mortality of temperate reef fishes.  

PubMed

Density dependence in demographic rates can strongly affect the dynamics of populations. However, the mechanisms generating density dependence (e.g., predation) are also dynamic processes and may be influenced by local conditions. Understanding the manner in which local habitat features affect the occurrence and/or strength of density dependence will increase our understanding of population dynamics in heterogeneous environments. In this study I conducted two separate field experiments to investigate how local predator density and habitat complexity affect the occurrence and form of density-dependent mortality of juvenile rockfishes (Sebastes spp.). I also used yearly censuses of rockfish populations on nearshore reefs throughout central California to evaluate mortality of juvenile rockfish at large spatial scales. Manipulations of predators (juvenile bocaccio, S. paucispinus) and prey (kelp, gopher, and black-and-yellow [KGB] rockfish, Sebastes spp.) demonstrated that increasing the density of predators altered their functional response and thus altered patterns of density dependence in mortality of their prey. At low densities of predators, the number of prey consumed per predator was a decelerating function, and mortality of prey was inversely density dependent. However, at high densities of predators, the number of prey killed per predator became an accelerating response, and prey mortality was directly density dependent. Results of field experiments and large-scale surveys both indicated that the strength of density-dependent mortality may also be affected by the structural complexity of the habitat. In small-scale field experiments, increased habitat complexity increased the strength of density-dependent mortality. However, at large scales, increasing complexity resulted in a decrease in the strength of density dependence. I suggest that these differences resulted from scale-dependent changes in the predatory response that generated mortality. Whether increased habitat complexity leads to an increase or a decrease in the strength of density-dependent mortality may depend on how specific predatory responses (e.g., functional or aggregative) are altered by habitat complexity. Overall, the findings of this study suggest that rates of demographic density dependence and the resulting dynamics of local populations may largely depend upon attributes of the local habitat. PMID:16761597

Johnson, Darren W

2006-05-01

125

2D collisionless magnetic reconnection: background density dependence.  

NASA Astrophysics Data System (ADS)

Even the simplest 2D configuration susceptible to magnetic reconnection (namely, Harris current sheet), possesses a number of "free parameters" that determine the dynamics and energetics of the process. Among such parameters are T_i/T_e ratio, guide field value, current sheet thickness, etc. In this report we systematically study the effect of changing the background density (from n_b/n_0=0.5 to n_b/n_0=0.003), which covers the range of lobe density values observed in the Earth's magnetotail. We performed two-dimensional Particle-in-Cell (PIC) simulations using implicit parallel code iPIC3D with double-periodic configuration. Increase of the jet front magnetic field (B_z) with n_b decrease is in agreement with 2D simulations reported previously. The elevated B_z region (magnetic field component normal to the current sheet) is several times larger than the initial current layer thickness, whereas large normal electric field (E_x) area is focused between the front and current sheet edge. Normal electric field has a bipolar profile (in the X direction), intensity scales roughly as (n_b/n_0)(-1/2) with changing n_b. In the low density case intense waves are generated near magnetic reconnection separatrices, what can be explained either by the separatrix electron flow disruption or by electron holes propagation.

Divin, Andrey; Lapenta, Giovanni; Markidis, Stefano; André, Mats; Khotyaintsev, Yuri; Olshevsky, Vyacheslav; Vaivads, Andris

126

Modeling density dependent flow in the sedimentary basin of Thuringia  

NASA Astrophysics Data System (ADS)

Salty groundwater reaching the surface or coming close to it is a phenomena that can be observed in many places of the Thuringian Basin. However, it is not obvious, why denser brine overlays lighter fresh water in this region. The hydrogeological processes which cause the rising of saltwater plumes from deeper geological layers to the surface are not yet fully understood. The goal of this modeling project is to investigate the mechanism of brine transport within the aquifers of sedimentary basins in general and of Thuringian Basin in particular. In this study we focus at investigating the fluid dynamics of the basin and how the fluid convection of the deep horizons interacts with groundwater flow near the surface. By gradually increasing the complexity of our model we analyze the major mechanism influencing the flow pattern: geology and hydraulic properties, fluid density differences caused by temperature and salt concentration gradients, fractures and faults as well as boundary conditions of the model, like inflow, outflow and groundwater recharge. For our numerical investigations we use a cross section of the Thuringian basin of approximately 80km length and maximal 800m height. The hydrogeological model is based on the major stratigraphical units from upper Perm (Zechstein) to upper Triassic (Keuper) with the lower Triassic (Bundsandstein) formations representing the main aquifer. The structural model as well as aquifer parameters are provided by geological partner groups of the University of Jena, Germany. To investigate hydrogeological scenarios we use the groundwater simulation program OpenGeoSys, which allows us to calculate thermally, hydrologically and chemically coupled processes. The challenge for us is to include density driven flow as a numerically very sensitive process on a grid that represents a large scale geologically realistic setting. With this work we contribute to the understanding of fluid convection processes influenced by density differences and local geological characteristics, especially for the local conditions in the sedimentary basin of Thuringia.

Zech, A.; Zehner, B.; Fischer, T.; Kolditz, O.; Attinger, S.

2012-04-01

127

Kinetic Density Functional Theory: A Microscopic Approach to Fluid Mechanics  

NASA Astrophysics Data System (ADS)

In the present paper we give a brief summary of some recent theoretical advances in the treatment of inhomogeneous fluids and methods which have applications in the study of dynamical properties of liquids in situations of extreme confinement, such as nanopores, nanodevices, etc. The approach obtained by combining kinetic and density functional methods is microscopic, fully self-consistent and allows to determine both configurational and flow properties of dense fluids. The theory predicts the correct hydrodynamic behavior and provides a practical and numerical tool to determine how the transport properties are modified when the length scales of the confining channels are comparable with the size of the molecules. The applications range from the dynamics of simple fluids under confinement, to that of neutral binary mixtures and electrolytes where the theory in the limit of slow gradients reproduces the known phenomenological equations such as the Planck—Nernst—Poisson and the Smolochowski equations. The approach here illustrated allows for fast numerical solution of the evolution equations for the one-particle phase-space distributions by means of the weighted density lattice Boltzmann method and is particularly useful when one considers flows in complex geometries.

Umberto Marini Bettolo, Marconi; Simone, Melchionna

2014-10-01

128

Broadcasting but not receiving: density dependence considerations for SETI signals  

E-print Network

This paper develops a detailed quantitative model which uses the Drake equation and an assumption of an average maximum radio broadcasting distance by an communicative civilization to derive a minimum civilization density for contact between two civilizations to be probable in a given volume of space under certain conditions, the amount of time it would take for a first contact, and whether reciprocal contact is possible. Results show that under certain assumptions, a galaxy can be teeming with civilizations yet not have a guarantee of communication between any of them given either short lifetimes or small maximum distances for communication.

Reginald D. Smith

2009-01-24

129

Application of a Spherical Harmonics Expansion Approach for Calculating Ligand Density Distributions around Proteins.  

PubMed

Protein-ligand interactions are central to many biological applications, including molecular recognition, protein formulations, and bioseparations. Complex, multisite ligands can have affinities for different locations on a protein's surface, depending on the chemical and topographical complementarity. We employ an approach based on the spherical harmonic expansion to calculate spatially resolved three-dimensional atomic density profiles of water and ligands in the vicinity of macromolecules. To illustrate the approach, we first study the hydration of model C180 buckyball solutes, with nonspherical patterns of hydrophobicity/-philicity on their surface. We extend the approach to calculate density profiles of increasingly complex ligands and their constituent groups around a protein (ubiquitin) in aqueous solution. Analysis of density profiles provides information about the binding face of the protein and the preferred orientations of ligands on the binding surface. Our results highlight that the spherical harmonic expansion based approach is easy to implement and efficient for calculation and visualization of three-dimensional density profiles around spherically nonsymmetric and topographically and chemically complex solutes. PMID:25198149

Parimal, Siddharth; Cramer, Steven M; Garde, Shekhar

2014-11-20

130

Density-dependent effects on group size are sex-specific in a gregarious ungulate.  

PubMed

Density dependence can have marked effects on social behaviors such as group size. We tested whether changes in population density of a large herbivore (elk, Cervus canadensis) affected sex-specific group size and whether the response was density- or frequency-dependent. We quantified the probability and strength of changes in group sizes and dispersion as population density changed for each sex. We used group size data from a population of elk in Manitoba, Canada, that was experimentally reduced from 1.20 to 0.67 elk/km(2) between 2002 and 2009. Our results indicated that functional responses of group size to population density are sex-specific. Females showed a positive density-dependent response in group size at population densities ?0.70 elk/km(2) and we found evidence for a minimum group size at population density ?0.70 elk/km(2). Changes in male group size were also density-dependent; however, the strength of the relationship was lower than for females. Density dependence in male group size was predominantly a result of fusion of solitary males into larger groups, rather than fusion among existing groups. Our study revealed that density affects group size of a large herbivore differently between males and females, which has important implications for the benefits e.g., alleviating predation risk, and costs of social behaviors e.g., competition for resources and mates, and intra-specific pathogen transmission. PMID:23326502

Vander Wal, Eric; van Beest, Floris M; Brook, Ryan K

2013-01-01

131

Charge density-dependent strength of hydration and biological structure.  

PubMed Central

Small ions of high charge density (kosmotropes) bind water molecules strongly, whereas large monovalent ions of low charge density (chaotropes) bind water molecules weakly relative to the strength of water-water interactions in bulk solution. The standard heat of solution of a crystalline alkali halide is shown here to be negative (exothermic) only when one ion is a kosmotrope and the ion of opposite charge is a chaotrope; this standard heat of solution is known to become proportionally more positive as the difference between the absolute heats of hydration of the corresponding gaseous anion and cation decreases. This suggests that inner sphere ion pairs are preferentially formed between oppositely charged ions with matching absolute enthalpies of hydration, and that biological organization arises from the noncovalent association of moieties with matching absolute free energies of solution, except where free energy is expended to keep them apart. The major intracellular anions (phosphates and carboxylates) are kosmotropes, whereas the major intracellular monovalent cations (K+; arg, his, and lys side chains) are chaotropes; together they form highly soluble, solvent-separated ion pairs that keep the contents of the cell in solution. PMID:8994593

Collins, K D

1997-01-01

132

Notes and Comments Plant Defense and Density Dependence in the Population Growth of Herbivores  

Microsoft Academic Search

Long-standing theory has predicted that plant defensive and nutritional traits contribute to the population dynamics of insect herbivores. To examine the role of plant variation in density depen- dence, I took a comparative approach by conducting density ma- nipulation experiments with the specialist aphid, Aphis nerii ,o n 18 species of milkweed (Asclepias spp.). The strength of density depen- dence

Anurag A. Agrawal

2004-01-01

133

Density-dependent fitness benefits in quorum-sensing bacterial populations  

E-print Network

Density-dependent fitness benefits in quorum-sensing bacterial populations Sophie E. Darcha beneficial at higher cell densities. However, this fundamental assumption has never been tested experimentally. Here, we directly test this by independently manipulating pop- ulation density and the induction

West, Stuart

134

Gaussian polynomial method for spin-dependent level density and new formula for spin distribution  

Microsoft Academic Search

Large-scale combinatorial calculations of level densities were performed for selected nuclei using Gaussian polynomial generating function method (GPM). Contrary to the results of previous combinatorial calculations, we find a good agreement of the combinatorial total level densities and Bethe formula. Combinatorial GPM calculations were performed also for spin-dependent level densities and comparison is made with various algebraic formulas. On the

V. Paar; D. K. Sunko; S. Brant; M. G. Mustafa; R. G. Lanier

1993-01-01

135

Progress towards Determining the Density Dependence of the Nuclear Symmetry Energy Using Heavy-Ion Reactions  

NASA Astrophysics Data System (ADS)

The latest development in determining the density dependence of the nuclear symmetry energy using heavy-ion collisions is reviewed. Within the IBUU04 version of an isospin- and momentum-dependent transport model using a modified Gogny effective interaction, recent experimental data from NSCL/MSU on isospin diffusion are found to be consistent with a nuclear symmetry energy of $E_{sym}(\\rho)\\approx 31.6(\\rho /\\rho_{0})^{1.05}$ at subnormal densities. Predictions on several observables sensitive to the density dependence of the symmetry energy at supranormal densities accessible at GSI and the planned Rare Isotope Accelerator (RIA) are also made.

Li, Bao-An; Chen, Lie-Wen; Ko, Che Ming; Yong, Gao-Chan; Zuo, Wei

2006-04-01

136

Thermodynamics of system with density- and/or temperature-dependent mass particles  

E-print Network

The thermodynamics with medium effects expressed by the dispersion relation of the temperature and density dependent particle mass is studied. Many previous treatments have been reviewed. A new thermodynamical treatment based on the equilibrium state is suggested. Employing the quark mass density- and temperature-dependent model, the discrepancies between our treatment and others are addressed.

Shaoyu Yin; Ru-Keng Su

2007-04-27

137

Density-dependent habitat selection by brown-headed cowbirds ( Molothrus ater ) in tallgrass prairie  

Microsoft Academic Search

Local distributions of avian brood parasites among their host habitats may depend upon conspecific parasite density. We used isodar analysis to test for density-dependent habitat selection in brown-headed cowbirds ( Molothrus ater) among tallgrass prairie adjacent to wooded edges, and prairie interior habitat (>100 m from wooded edges) with and without experimental perches. Eight study sites containing these three habitat treatments

William E. Jensen; Jack F. Cully

2005-01-01

138

Density-dependent carrier dynamics in a quantum dots-in-a-well heterostructure  

E-print Network

transmission spectroscopy to temporally and spectrally resolve density-dependent carrier dynamics in a quantumDensity-dependent carrier dynamics in a quantum dots-in-a-well heterostructure R. P. Prasankumar,1 The incorporation of semiconductor quantum dots into different heterostructures for applications in nanoscale lasing

Krishna, Sanjay

139

Age-specific patterns in density-dependent growth of white crappie, Pomoxis annularis  

E-print Network

Age-specific patterns in density-dependent growth of white crappie, Pomoxis annularis K . L . P O P University, Lubbock, TX, USA Abstract The relationship between growth in white crappie, Pomoxis annularis, Pomoxis annularis. Introduction Density-dependent growth is commonly observed in fishes (e.g. Le Cren 1958

Wilde, Gene

140

Quantification of trap state densities in GaAs heterostructures grown at varying rates using intensity-dependent time resolved photoluminescence  

NASA Astrophysics Data System (ADS)

Time-resolved photoluminescence is an established technique for characterizing carrier lifetimes in semiconductors, but the dependence of lifetime on excitation fluence has been only qualitatively investigated. We develop a quantitative approach for fitting fluence-dependent PL decay data to a Shockely-Read-Hall model of carrier recombination in order to extract the trap state density. We demonstrate this approach by investigating growth rate-dependent trap densities in gallium arsenide-indium gallium phosphide double heterostructures. The techniques developed here can be applied for rapid, non-destructive quantification of trap state densities in a variety of materials.

Haughn, C. R.; Schmieder, K. J.; Zide, J. M. O.; Barnett, A.; Ebert, C.; Opila, R.; Doty, M. F.

2013-05-01

141

Liquid water simulations with the density fragment interaction approach.  

PubMed

We reformulate the density fragment interaction (DFI) approach [Fujimoto and Yang, J. Chem. Phys., 2008, 129, 054102.] to achieve linear-scaling quantum mechanical calculations for large molecular systems. Two key approximations are developed to improve the efficiency of the DFI approach and thus enable the calculations for large molecules: the electrostatic interactions between fragments are computed efficiently by means of polarizable electrostatic-potential-fitted atomic charges; and frozen fragment pseudopotentials, similar to the effective fragment potentials that can be fitted from interactions between small molecules, are employed to take into account the Pauli repulsion effect among fragments. Our reformulated and parallelized DFI method demonstrates excellent parallel performance based on the benchmarks for the system of 256 water molecules. Molecular dynamics simulations for the structural properties of liquid water also show a qualitatively good agreement with experimental measurements including the heat capacity, binding energy per water molecule, and the radial distribution functions of atomic pairs of O-O, O-H, and H-H. With this approach, large-scale quantum mechanical simulations for water and other liquids become feasible. PMID:22466097

Hu, Xiangqian; Jin, Yingdi; Zeng, Xiancheng; Hu, Hao; Yang, Weitao

2012-06-01

142

Non-linear absorption and density dependent dephasing in Rydberg EIT-media  

E-print Network

Light propagation through an ensemble of ultra-cold Rydberg atoms in electromagnetically induced transparency (EIT) configuration is studied. In strongly interacting Rydberg EIT media, non-linear optical effects lead to a non-trivial dependence of the degree of probe beam attenuation on the medium density and on its initial intensity. We develop a Monte Carlo rate equation model that self-consistently includes the effect of the probe beam attenuation to investigate the steady state of the Rydberg medium driven by two laser fields. We compare our results to recent experimental data and to results of other state-of-the-art models for light propagation in Rydberg EIT-media. We find that for low probe field intensities, our results match the experimental data best if a density-dependent dephasing rate is included in the model. At higher probe intensities, our model deviates from other theoretical approaches, as it predicts a spectral asymmetry together with line broadening. These are likely due to off-resonant excitation channels, which however have not been observed in recent experiments. Atomic motion and coupling to additional Rydberg levels are discussed as possible origins for these deviations.

Martin Gärttner; Jörg Evers

2013-05-07

143

Density-dependent and frequency-dependent selection by bumblebees Bombus terrestris (L.) (Hymenoptera: Apidae)  

Microsoft Academic Search

The behaviour of bumblebee workers foraging on arrays of artificial flowers of two colour morphs was observed. Experiments were conducted on arrays of varying morph frequencies and at three different total flower densities. Bumblebees consistently showed a preference for the commonest colour morph, and this behaviour was not significantly affected by changing density. In contrast, frequency-independent preferences changed significantly with

ANN SMITHSON; MARK R. MACNAIR

1997-01-01

144

Effect of short-range correlation on the density dependence of nuclear symmetry energy at supra-saturation densities  

NASA Astrophysics Data System (ADS)

Short-range correlation (SRC) induces a significant high-momentum component in the nucleon momentum distribution n( k) in both finite nuclei and nuclear matter. Recent experiments suggest that the amount of correlation in pure neutron matter (PNM) may be much less than that in symmetric nuclear matter (SNM). In this paper we investigate on a qualitative level how this affects the density dependence of the symmetry energy, especially at supra-saturation densities, in the framework of the well-established MDI energy density functional. In the MDI formalism, SRC affects both the kinetic energy and the potential energy through its effect on n( k). For the n( k) of SNM, we use a formula proposed based on an interesting relation between n( k) at high momentum k ( i.e. k > k F ) and the average nucleon density of finite nuclei. For the n( k) of PNM, we use the prediction of a free Fermi gas model. The result is compared to the symmetry energy calculated assuming both the SNM and PNM to be Fermi-gas like. Our study shows that, to have a realistic estimation of the relative contributions to the symmetry energy from the kinetic part and the two-body and three-body potential part, one has to take the SRC effect into account. Furthermore, it is found that the density dependence of the symmetry energy at supra-saturation densities is sensitive to the competition between the SRC effect and the three-body force effect.

Zhang, Xin; Xu, Chang; Ren, Zhongzhou

2014-07-01

145

Physical Origin of Density Dependent Force of the Skyrme Type within the Quark Meson Coupling Model  

SciTech Connect

A density dependent, effective nucleon-nucleon force of the Skyrme type is derived from the quark-meson coupling model--a self-consistent, relativistic quark level description of nuclear matter. This new formulation requires no assumption that the mean scalar field is small and hence constitutes a significant advance over earlier work. The similarity of the effective interaction to the widely used SkM* force encourages us to apply it to a wide range of nuclear problems, beginning with the binding energies and charge distributions of doubly magic nuclei. Finding impressive results in this conventional arena, we apply the same effective interaction, within the Hartree-Fock-Bogoliubov approach, to the properties of nuclei far from stability. The resulting two neutron drip lines and shell quenching are quite satisfactory. Finally, we apply the relativistic formulation to the properties of dense nuclear matter in anticipation of future application to the properties of neutron stars.

Pierre Guichon; Hrayr Matevosyan; N. Sandulescu; Anthony Thomas

2006-03-17

146

Density-dependent nucleon-nucleon interaction from three-nucleon forces  

SciTech Connect

Microscopic calculations based on realistic nuclear Hamiltonians, while yielding accurate results for the energies of the ground and low-lying excited states of nuclei with A{<=}12, fail to reproduce the empirical equilibrium properties of nuclear matter, that are known to be significantly affected by three-nucleon forces. We discuss a scheme suitable to construct a density-dependent two-nucleon potential, in which the effects of n-particle interactions can be included by integrating out the degrees of freedom of (n-2) nucleons. Our approach, based on the formalism of correlated basis function and state-of-the-art models of the two- and three-nucleon potentials, leads to an effective interaction that can be easily employed in nuclear matter calculations, yielding results in good agreement with those obtained from the underlying three-body potential.

Lovato, Alessandro [SISSA and INFN, Sezione di Trieste, I-34014 Trieste (Italy); Benhar, Omar [INFN, Sezione di Roma, I-00185 Roma (Italy); Dipartimento di Fisica, Universita 'La Sapienza', I-00185 Roma (Italy); Fantoni, Stefano [SISSA and INFN, Sezione di Trieste, I-34014 Trieste (Italy); CNR-DEMOCRITOS National Supercomputing Center, I-34014 Trieste (Italy); Illarionov, Alexey Yu. [Dipartimento di Fisica, Universita di Trento, I-38123 Povo, Trento (Italy); Schmidt, Kevin E. [Department of Physics, Arizona State University, Tempe, Arizona 85287 (United States)

2011-05-15

147

Accurate Ground State Energies of Solids and Molecules from Time Dependent Density Functional Theory  

E-print Network

We demonstrate that ground state energies approaching chemical accuracy can be obtained by combining the adiabatic connection fluctuation-dissipation theorem (ACFDT) with time-dependent density functional theory (TDDFT). The key ingredient is a renormalization scheme, which eliminates the divergence of the correlation hole characteristic of any local kernel. This new class of renormalized kernels gives a significantly better description of the short-range correlations in covalent bonds compared to the random phase approximation (RPA) and yields a four fold improvement of RPA binding energies in both molecules and solids. We also consider examples of barrier heights in chemical reactions, molecular adsorption and graphene interacting with metal surfaces, which are three examples where RPA has been successful. In these cases, the renormalized kernel provides results that are of equal quality or even slightly better than RPA, with a similar computational cost.

Olsen, Thomas

2014-01-01

148

Time-dependent density-functional and reduced density-matrix methods for few electrons: Exact versus adiabatic approximations  

E-print Network

f o Article history: Available online 28 June 2011 Keywords: Time-dependent density with an external resonant laser field shows Rabi oscillations which are detuned due to the lack of memory [4]. Other examples include the description of memory [5], charge- transfer excitations [6], Rabi

149

Time-dependent density-functional and reduced density-matrix methods for few electrons: Exact versus adiabatic approximations  

E-print Network

f o Article history: Available online xxxx Keywords: Time-dependent density-functional theory One with an external resonant laser field shows Rabi oscillations which are detuned due to the lack of memory [4]. Other examples include the description of memory [5], charge- transfer excitations [6], Rabi

Gross, E.K.U.

150

Time-dependent density-functional theory in massively parallel computer architectures: the OCTOPUS project.  

PubMed

Octopus is a general-purpose density-functional theory (DFT) code, with a particular emphasis on the time-dependent version of DFT (TDDFT). In this paper we present the ongoing efforts to achieve the parallelization of octopus. We focus on the real-time variant of TDDFT, where the time-dependent Kohn-Sham equations are directly propagated in time. This approach has great potential for execution in massively parallel systems such as modern supercomputers with thousands of processors and graphics processing units (GPUs). For harvesting the potential of conventional supercomputers, the main strategy is a multi-level parallelization scheme that combines the inherent scalability of real-time TDDFT with a real-space grid domain-partitioning approach. A scalable Poisson solver is critical for the efficiency of this scheme. For GPUs, we show how using blocks of Kohn-Sham states provides the required level of data parallelism and that this strategy is also applicable for code optimization on standard processors. Our results show that real-time TDDFT, as implemented in octopus, can be the method of choice for studying the excited states of large molecular systems in modern parallel architectures. PMID:22562950

Andrade, Xavier; Alberdi-Rodriguez, Joseba; Strubbe, David A; Oliveira, Micael J T; Nogueira, Fernando; Castro, Alberto; Muguerza, Javier; Arruabarrena, Agustin; Louie, Steven G; Aspuru-Guzik, Alán; Rubio, Angel; Marques, Miguel A L

2012-06-13

151

Negative density-dependent emigration of males in an increasing red deer population  

PubMed Central

In species with polygynous mating systems, females are regarded as food-limited, while males are limited by access to mates. When local density increases, forage availability declines, while mate access for males may increase due to an increasingly female-biased sex ratio. Density dependence in emigration rates may consequently differ between sexes. Here, we investigate emigration using mark-recovery data from 468 young red deer Cervus elaphus marked in Snillfjord, Norway over a 20-year period when the population size has increased sixfold. We demonstrate a strong negative density-dependent emigration rate in males, while female emigration rates were lower and independent of density. Emigrating males leaving the natal range settled in areas with lower density than expected by chance. Dispersing males moved 42 per cent longer at high density in 1997 (37?km) than at low density in 1977 (26?km), possibly caused by increasing saturation of deer in areas surrounding the marking sites. Our study highlights that pattern of density dependence in dispersal rates may differ markedly between sexes in highly polygynous species. Contrasting patterns reported in small-scale studies are suggestive that spatial scale of density variation may affect the pattern of temporal density dependence in emigration rates and distances. PMID:19364736

Loe, Leif Egil; Mysterud, Atle; Veiberg, Vebjørn; Langvatn, Rolf

2009-01-01

152

Equation of State for Nuclear Matter Based on Density Dependent Effective Interaction  

NASA Astrophysics Data System (ADS)

An interesting method of obtaining the equation of state for nuclear matter, from a density dependent M3Y interaction, by minimizing the energy per nucleon is described. The density dependence parameters of the interaction are obtained by reproducing the saturation energy per nucleon and the saturation density of spin and isospin symmetric cold infinite nuclear matter. The nuclear matter equation of state thus obtained is then used to calculate the pressure, energy density, nuclear incompressibility and velocity of sound in nuclear medium. The results obtained are in good agreement with experimental data and provide a unified description of radioactivity, scattering and nuclear matter.

Basu, D. N.

153

Density-dependent compensatory growth in brown trout (Salmo trutta) in nature.  

PubMed

Density-dependence is a major ecological mechanism that is known to limit individual growth. To examine if compensatory growth (unusually rapid growth following a period of imposed slow growth) in nature is density-dependent, one-year-old brown trout (Salmo trutta L.) were first starved in the laboratory, and then released back into their natural stream, either at natural or at experimentally increased population density. The experimental trout were captured three times over a one-year period. We found no differences in growth, within the first month after release (May-June), between the starved fish and the control group (i.e. no evidence of compensation). During the summer however (July-September), the starved fish grew more than the control group (i.e. compensation), and the starved fish released into the stream at a higher density, grew less than those released at a natural density, both in terms of weight and length (i.e. density-dependent compensation). Over the winter (October-April), there were no effects of either starvation or density on weight and length growth. After the winter, starved fish released at either density had caught up with control fish in body size, but recapture rates (proxy for survival) did not indicate any costs of compensation. Our results suggest that compensatory growth in nature can be density-dependent. Thus, this is the first study to demonstrate the presence of ecological restrictions on the compensatory growth response in free-ranging animals. PMID:23658820

Sundström, L Fredrik; Kaspersson, Rasmus; Näslund, Joacim; Johnsson, Jörgen I

2013-01-01

154

Density-Dependent Compensatory Growth in Brown Trout (Salmo trutta) in Nature  

PubMed Central

Density-dependence is a major ecological mechanism that is known to limit individual growth. To examine if compensatory growth (unusually rapid growth following a period of imposed slow growth) in nature is density-dependent, one-year-old brown trout (Salmo trutta L.) were first starved in the laboratory, and then released back into their natural stream, either at natural or at experimentally increased population density. The experimental trout were captured three times over a one-year period. We found no differences in growth, within the first month after release (May-June), between the starved fish and the control group (i.e. no evidence of compensation). During the summer however (July-September), the starved fish grew more than the control group (i.e. compensation), and the starved fish released into the stream at a higher density, grew less than those released at a natural density, both in terms of weight and length (i.e. density-dependent compensation). Over the winter (October-April), there were no effects of either starvation or density on weight and length growth. After the winter, starved fish released at either density had caught up with control fish in body size, but recapture rates (proxy for survival) did not indicate any costs of compensation. Our results suggest that compensatory growth in nature can be density-dependent. Thus, this is the first study to demonstrate the presence of ecological restrictions on the compensatory growth response in free-ranging animals. PMID:23658820

Sundström, L. Fredrik; Kaspersson, Rasmus; Näslund, Joacim; Johnsson, Jörgen I.

2013-01-01

155

Simultaneous positive and negative density-dependent dispersal in a colonial bird species.  

PubMed

Contradictory patterns of density-dependent animal dispersal can potentially be reconciled by integrating the conspecific attraction hypothesis with the traditional competition hypothesis. We propose a hypothesis that predicts a U-shaped relationship between density and both natal and breeding dispersal distance. Using 10 years of observations on a breeding colony of the Blue-footed Booby (Sula nebouxii), the hypothesis was confirmed by documenting simultaneous positive and negative density-dependent dispersal distances in natal and breeding dispersal of males and breeding dispersal of females within the colony. Point-pattern analyses demonstrated that the breeding sites of Blue-footed Boobies were highly aggregated in all years within a large study area, and aggregation presumably resulted in heterogeneity in patch density throughout the colony. As predicted, at moderate to high densities, dispersal distances showed positive density dependence, with individuals moving to lower density patches. In contrast, at low to moderate densities, dispersal distances showed negative density dependence, with individuals moving to higher density patches. In both sexes of the 1994 cohort, the higher the mean density in patches used by an individual over the long term (up to age 11 years), the fewer fledglings it produced. A positive effect of density on long-term reproductive success was not detected, possibly because birds that failed during pair formation or incubation were not sampled. Density of conspecifics may be an important influence on habitat selection of breeders, and dispersal may tend to carry individuals to patches where pair formation opportunities are better and negative effects of competition on reproductive success are reduced. PMID:19294928

Kim, Sin-Yeon; Torres, Roxana; Drummond, Hugh

2009-01-01

156

Nonlocal formulation of spin Coulomb drag in nanostructures: implications for time-dependent current-density-functional theory  

NASA Astrophysics Data System (ADS)

The spin Coulomb drag (SCD) effect occurs in materials and devices where charged carriers with different spins exchange momentum via Coulomb scattering. This causes frictional forces between spin-dependent currents that lead to dissipation and limit spin mobilities. We consider the role of the SCD in the damping of intersubband spin plasmons in semiconductor quantum wells, and show that a local density approximation leads to overdamping. A nonlocal formulation of the SCD is developed which agrees with experimental observations of spin plasmon linewidths. General consequences for using density-functional approaches to describe electronic many-body effects in nanostructures are discussed.

Ullrich, Carsten A.; D'Amico, Irene

2013-03-01

157

Photoabsorption spectra of small cationic xenon clusters from time-dependent density functional theory  

E-print Network

Photoabsorption spectra of small cationic xenon clusters from time-dependent density functional experimental evidence and by theoretical calculations of the photoabsorption of cationic xenon clusters.4­7 Up

158

Role of the density-dependent symmetry energy in multi-fragmentation  

E-print Network

The fragmentation of projectile and spectator matter is studied at different incident energies using an isospin-dependent QMD model with reduced isospin dependent cross-section. The sensitivity of the production of fragments is analyzed using different parametrizations of density dependence of symmetry energy. We shall also highlight the collective response of the momentum dependent interactions(MDI) and symmetry energy towards the fragmentation of colliding nuclei at intermediate energies.

Karan Singh Vinayak; Suneel Kumar

2011-05-11

159

Spin-dependent level density in interacting boson-fermion-fermion model  

Microsoft Academic Search

The level density of the odd-odd nucleus $^{132}Pr$ is investigated in the interacting boson-fermion-fermion model (IBFFM) which accounts for collectivity and complex interaction between quasiparticle and collective modes. The IBFFM total level density is fitted by a Gaussian and its tail is also fitted by Bethe formula and constant temperature Fermi gas model. The IBFFM spin-dependent level densities show high-spin

V. Paar; S. Brant; D. Paar

1996-01-01

160

Spin-dependent level density in interacting boson-fermion-fermion model  

Microsoft Academic Search

The level density of the odd-odd nucleus132\\u000a Pr is investigated in the interacting boson-fermion-fermion model (IBFFM) which accounts for collectivity and complex interaction\\u000a between quasiparticle and collective modes. The IBFFM total level density is fitted by a Gaussian and its tail is also fitted\\u000a by Bethe formula and constant temperature Fermi gas model. The IBFFM spin-dependent level densities show high-spin

V. Paar; S. Brant; D. Paar

1996-01-01

161

Nuclear matter and neutron matter for improved quark mass density- dependent model with $?$ mesons  

E-print Network

A new improved quark mass density-dependent model including u, d quarks, $\\sigma$ mesons, $\\omega$ mesons and $\\rho$ mesons is presented. Employing this model, the properties of nuclear matter, neutron matter and neutron star are studied. We find that it can describe above properties successfully. The results given by the new improved quark mass density- dependent model and by the quark meson coupling model are compared.

Chen Wu; Ru-Keng Su

2008-09-17

162

A metamaterial having a frequency dependent elasticity tensor and a zero effective mass density  

E-print Network

Within the context of linear elasticity we show that a two-terminal network of springs and masses, can respond exactly the same as a normal spring, but with a frequency dependent spring constant. As a consequence a network of such springs can have a frequency dependent effective elasticity tensor but zero effective mass density. The internal masses influence the elasticity tensor, but do not contribute to the effective mass density at any frequency.

Graeme Milton; Pierre Seppecher

2011-05-04

163

Experimental evidence for density-dependent survival in mallard ( Anas platyrhynchos ) ducklings  

Microsoft Academic Search

It is unresolved to what extent waterfowl populations are regulated by density-dependent processes. By doing a 2-year crossover perturbation experiment on ten oligotrophic boreal lakes we addressed the hypothesis that breeding output is density dependent. Wing-clipped mallard (Anas platyrhynchos) hens were introduced with their own brood and then monitored for 24 days. Predicted responses were that per capita duckling and hen

Gunnar Gunnarsson; Johan Elmberg; Kjell Sjöberg; Hannu Pöysä; Petri Nummi

2006-01-01

164

Life history correlates of density-dependent recruitment in marine fishes  

Microsoft Academic Search

Understanding the relationships among life history traits, density dependence, and population dynamics is a central goal in ecology. It is also vital if we are to predict how populations respond to and recover from exploitation. We used data for 54 stocks of commercially exploited fish species to examine relationships between maximum annual recruitment at low stock size and the density

Nicholas B. Goodwin; Alastair Grant; Allison L. Perry; Nicholas K. Dulvy; John D. Reynolds

2006-01-01

165

Density-dependent population dynamics in larvae of the dragonfly Pachydiplax longipennis : a field experiment  

Microsoft Academic Search

Several features of dragonfly population biology suggest that population regulation occurs in the larval stage. This study was designed to determine if density-dependent interactions among larval odonates can affect survival, growth and emergence. First-instar larvae of the dragonfly Pachydiplax longipennis were raised in outdoor experimental ponds at initial densities of 38, 152, and 608 larvae · m-2, under two levels

J. Van Buskirk

1987-01-01

166

Host-parasite population dynamics under combined frequency-and density-dependent transmission  

E-print Network

and M. Boots, Dept of Animal & Plant Sciences, Univ. of Sheffield, Sheffield, UK, S10 2TN. � R. J. Knell-driven extinction with frequency- but not density- dependence). Here, we consider the situation where transmission-driven extinction (and lowers the threshold for parasite persistence). Contrastingly, allowing some density

White, Andrew

167

Plasma density inside a femtosecond laser filament in air: Strong dependence on external focusing  

E-print Network

Plasma density inside a femtosecond laser filament in air: Strong dependence on external focusing strongly influences the plasma density and the diameter of femtosecond Ti-sapphire laser filaments spectroscopy, laser induced electrical discharge, and femtosecond laser material interactions. The measurements

Becker, Andreas

168

Size selectivity of predation by brown bears depends on the density of their sockeye salmon prey.  

PubMed

Can variation in prey density drive changes in the intensity or direction of selective predation in natural systems? Despite ample evidence of density-dependent selection, the influence of prey density on predatory selection patterns has seldom been investigated empirically. We used 20 years of field data on brown bears (Ursus arctos) foraging on sockeye salmon (Oncorhynchus nerka) in Alaska, to test the hypothesis that salmon density affects the strength of size-selective predation. Measurements from 41,240 individual salmon were used to calculate variance-standardized selection differentials describing the direction and magnitude of selection. Across the time series, the intensity of predatory selection was inversely correlated with salmon density; greater selection for smaller salmon occurred at low salmon densities as bears' tendency to kill larger-than-average salmon was magnified. This novel connection between density dependence and selective predation runs contrary to some aspects of optimal foraging theory and differs from many observations of density-dependent selection because (1) the direction of selection remains constant while its magnitude changes as a function of density and (2) stronger selection is observed at low abundance. These findings indicate that sockeye salmon may be subject to fishery-induced size selection from both direct mechanisms and latent effects of altered predatory selection patterns on the spawning grounds, resulting from reduced salmon abundance. PMID:23594549

Cunningham, Curry J; Ruggerone, Gregory T; Quinn, Thomas P

2013-05-01

169

Impacts of poor food availability on positive density dependence in a highly colonial seabird  

PubMed Central

For species with positive density dependence, costs and benefits of increasing density may depend on environmental conditions, but this has seldom been tested. By examining a colonial seabird (common guillemot) over a period of unprecedented poor food availability, we test two contrasting hypotheses suggesting that birds breeding at high density have: (i) greater leeway to increase foraging effort owing to more effective defence of unattended chicks against predators; and (ii) less leeway, owing to more attacks on unattended chicks by neighbouring adults. Supporting hypothesis 1, birds at high density increased provisioning rates and hence survival of chicks by foraging simultaneously with their partners, whereas at low density, unattended chicks were liable to be killed by predatory gulls and, unexpectedly, razorbills. Simultaneously, supporting hypothesis 2, heightened aggression towards unattended chicks at high density frequently resulted in infanticide, undermining benefits from collective defence against predators. Consequently, over 25 years, the magnitude of positive density dependence was independent of mean breeding success. These data indicate previously unsuspected trade-offs between costs and benefits of increasing density under changing environments. Previous generalizations about the importance of high density for reproductive success have so far remained robust, but such trade-offs could have unpredictable consequences for future population dynamics. PMID:20335206

Ashbrook, Kate; Wanless, Sarah; Harris, Mike P.; Hamer, Keith C.

2010-01-01

170

An evaluation of density-dependent and density-independent influences on population growth rates in Weddell seals  

USGS Publications Warehouse

Much of the existing literature that evaluates the roles of density-dependent and density-independent factors on population dynamics has been called into question in recent years because measurement errors were not properly dealt with in analyses. Using state-space models to account for measurement errors, we evaluated a set of competing models for a 22-year time series of mark-resight estimates of abundance for a breeding population of female Weddell seals (Leptonychotes weddellii) studied in Erebus Bay, Antarctica. We tested for evidence of direct density dependence in growth rates and evaluated whether equilibrium population size was related to seasonal sea-ice extent and the Southern Oscillation Index (SOI). We found strong evidence of negative density dependence in annual growth rates for a population whose estimated size ranged from 438 to 623 females during the study. Based on Bayes factors, a density-dependence-only model was favored over models that also included en! vironmental covariates. According to the favored model, the population had a stationary distribution with a mean of 497 females (SD = 60.5), an expected growth rate of 1.10 (95% credible interval 1.08-1.15) when population size was 441 females, and a rate of 0.90 (95% credible interval 0.87-0.93) for a population of 553 females. A model including effects of SOI did receive some support and indicated a positive relationship between SOI and population size. However, effects of SOI were not large, and including the effect did not greatly reduce our estimate of process variation. We speculate that direct density dependence occurred because rates of adult survival, breeding, and temporary emigration were affected by limitations on per capita food resources and space for parturition and pup-rearing. To improve understanding of the relative roles of various demographic components and their associated vital rates to population growth rate, mark-recapture methods can be applied that incorporate both environmental covariates and the seal abundance estimates that were developed here. An improved understanding of why vital rates change with changing population abundance will only come as we develop a better understanding of the processes affecting marine food resources in the Southern Ocean.

Rotella, J.J.; Link, W. A.; Nichols, J.D.; Hadley, G.L.; Garrott, R.A.; Proffitt, K.M.

2009-01-01

171

Role of prey and intraspecific density dependence on the population growth of an avian top predator  

NASA Astrophysics Data System (ADS)

Exploring predator-prey systems in diverse ecosystems increases our knowledge about ecological processes. Predator population growth may be positive when conspecific density is low but predators also need areas with prey availability, associated with competition, which increases the risk of suffering losses but stabilises populations. We studied relationships between European rabbits Oryctolagus cuniculus (prey) and adult eagle owls Bubo bubo (predators) in south-western Europe. We assessed models explaining the predator population growth and stability. We estimated the abundance of rabbits and adult eagle owls during three years in eight localities of central-southern Spain. We explored models including rabbit and adult eagle owl abundance, accounting for yearly variations and including the locality as a random variable. We found that population growth of adult eagle owls was positive in situations with low conspecific abundance and tended to be negative but approaching equilibrium in situations of higher conspecific abundance. Population growth was also positively related to previous summer rabbit density when taking into account eagle owl conspecific abundance, possibly indicating that rabbits may support recruitment. Furthermore, abundance stability of adult eagle owls was positively related to previous winter-spring rabbit density, which could suggest predator population stabilisation through quick territory occupation in high-quality areas. These results exemplify the trade-off between prey availability and abundance of adult predators related to population growth and abundance stability in the eagle owl-rabbit system in south-western Europe. Despite rabbits have greatly declined during the last decades and eagle owls locally specialise on them, eagle owls currently have a favourable conservation status. As eagle owls are the only nocturnal raptor with such dependence on rabbits, this could point out that predators may overcome prey decreases in areas with favourable climate and prey in the absence of superior competitors with similar foraging mode.

Fernandez-de-Simon, Javier; Díaz-Ruiz, Francisco; Cirilli, Francesca; Tortosa, Francisco S.; Villafuerte, Rafael; Ferreras, Pablo

2014-10-01

172

Determining excitation-energy transfer times and mechanisms from stochastic time-dependent density functional theory.  

PubMed

We developed an approach for calculating excitation-energy transfer times in supermolecular arrangements based on stochastic time-dependent density functional theory (STDDFT). The combination of real-time propagation and the stochastic Schrödinger equation with a Kohn-Sham Hamiltonian allows for simulating how an excitation spreads through an assembly of molecular systems. The influence that approximations, such as the dipole-dipole coupling approximation of Förster theory, have on energy-transfer times can be checked explicitly. As a first application of our approach we investigate a light-harvesting-inspired model ring system, calculating the time it takes for an excitation to travel from one side of the ring to the opposite side under ideal and perturbed conditions. Among other things we find that completely removing a molecule from the ring may inhibit energy transfer less than having an energetically detuned molecule in the ring. In addition, Förster's dipole coupling approximation may noticeably overestimate excitation-energy transfer efficiency. PMID:24147662

Hofmann-Mees, D; Appel, H; Di Ventra, M; Kümmel, S

2013-11-21

173

Density-dependent productivity depression in Pyrenean Bearded Vultures: implications for conservation.  

PubMed

The main objective of many conservation programs is to increase population size by improving a species' survival and reproduction. However, density dependence of demographic parameters may confound this approach. In this study we used a 25-year data set on Bearded Vultures (Gypaetus barbatus) in Spain to evaluate the consequences of population growth on reproductive performance. Unlike its coefficient of variation (cv), mean annual productivity decreased with increasing population size. After controlling for territorial heterogeneity, productivity also was negatively related to the distance to the nearest conspecific breeding pair and to supplementary feeding points where floaters congregate. These results suggest that vulture populations are regulated as posited by the site-dependency hypothesis: as the population increases, average productivity decreases because progressively poorer territories are used. The combined effects of the shrinkage of territories and the presence of floaters around supplementary feeding points seem to be the main causes of productivity decline and are therefore the main determinants of territory quality. This has conservation implications, especially concerning the role of supplementary feeding points. Supplementary feeding should be reviewed given that its usefulness in reducing preadult mortality has not yet been proved and its effect on productivity, as our results suggest, is negative. PMID:17069362

Carrete, Martina; Donázar, José A; Margalida, Antoni

2006-10-01

174

Finite Lifetime Effects on the Polarizability Within Time-dependent Density-functional Theory  

SciTech Connect

The research described in this product was performed in part in the Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the Department of Energy's Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory. We present an implementation for considering finite lifetime of the electronic excited states into linear-response theory within time-dependent density-functional theory. The lifetime of the excited states is introduced by a common phenomenological damping factor. The real and imaginary frequency-dependent polarizabilities can thus be calculated over a broad range of frequencies. This allows for the study of linear-response properties both in the resonance and nonresonance cases. The method is complementary to the standard approach of calculating the excitation energies from the poles of the polarizability. The real and imaginary polarizabilities can then be calculated in any specific energy range of interest, in contrast to the excitation energies which are usually solved only for the lowest electronic states. We have verified the method by investigating the photoabsorption properties of small alkali clusters. For these systems, we have calculated the real and imaginary polarizabilities in the energy range of 1–4 eV and compared these with excitation energy calculations. The results showed good agreement with both previous theoretical and experimental results.

Jensen, Lasse; Autschbach, Jochen; Schatz, George C.

2005-06-16

175

Density-dependent habitat selection by brown-headed cowbirds ( Molothrus ater) in tallgrass prairie.  

PubMed

Local distributions of avian brood parasites among their host habitats may depend upon conspecific parasite density. We used isodar analysis to test for density-dependent habitat selection in brown-headed cowbirds ( Molothrus ater) among tallgrass prairie adjacent to wooded edges, and prairie interior habitat (>100 m from wooded edges) with and without experimental perches. Eight study sites containing these three habitat treatments were established along a geographical gradient in cowbird abundance within the Flint Hills region of Eastern Kansas and Oklahoma, USA. The focal host species of our study, the dickcissel ( Spiza americana), is the most abundant and preferred cowbird host in the prairie of this region. Cowbird relative abundance and cowbird:host abundance ratios were used as estimates of female cowbird density, whereas cowbird egg density was measured as parasitism frequency (percent of dickcissel nests parasitized), and parasitism intensity (number of cowbird eggs per parasitized nest). Geographical variation in cowbird abundance was independent of host abundance. Within study sites, host abundance was highest in wooded edge plots, intermediate in the experimental perch plots, and lowest in prairie interior. Cowbirds exhibited a pattern of density-dependent selection of prairie edge versus experimental perch and interior habitats. On sites where measures of cowbird density were lowest, all cowbird density estimates (female cowbirds and their eggs) were highest near (< or =100 m) wooded edges, where host and perch availability are highest. However, as overall cowbird density increased geographically, these density estimates increased more rapidly in experimental perch plots and prairie interiors. Variation in cowbird abundance and cowbird:host ratios suggested density-dependent cowbird selection of experimental perch over prairie interior habitat, but parasitism levels on dickcissel nests were similar among these two habitats at all levels of local cowbird parasitism. The density-dependent pattern of cowbird distribution among prairie edge and interior suggested that density effects on perceived cowbird fitness are greatest at wooded edges. A positive relationship between daily nest mortality rates of parasitized nests during the nestling period with parasitism intensity levels per nest suggested a density-dependent effect on cowbird reproductive success. However, this relationship was similar among habitats, such that all habitats should have been perceived as being equally suitable to cowbirds at all densities. Other unmeasured effects on cowbird habitat suitability (e.g., reduced cowbird success in edge-dwelling host nests, cowbird despotism at edges) might have affected cowbird habitat selection. Managers attempting to minimize cowbird parasitism on sensitive cowbird hosts should consider that hosts in otherwise less-preferred cowbird habitats (e.g., habitat interiors) are at greater risk of being parasitized where cowbirds become particularly abundant. PMID:15375686

Jensen, William E; Cully, Jack F

2005-01-01

176

Density dependent growth in adult brown frogs Rana arvalis and Rana temporaria - A field experiment  

NASA Astrophysics Data System (ADS)

In species with complex life cycles, density regulation can operate on any of the stages. In frogs there are almost no studies of density effects on the performance of adult frogs in the terrestrial habitat. We therefore studied the effect of summer density on the growth rate of adult frogs during four years. Four 30 by 30 m plots in a moist meadow were used. In early summer, when settled after post-breeding migration, frogs ( Rana arvalis and Rana temporaria that have a very similar ecology and potentially compete) were enclosed by erecting a fence around the plots. Frogs were captured, measured, marked and partly relocated to create two high density and two low density plots. In early autumn the frogs were again captured and their individual summer growth determined. Growth effects were evaluated in relation to two density measures: density by design (high/low manipulation), and actual (numerical) density. R. arvalis in plots with low density by design grew faster than those in high density plots. No such effect was found for R. temporaria. For none of the species was growth related to actual summer density, determined by the Lincoln index and including the density manipulation. The result suggests that R. arvalis initially settled according to an ideal free distribution and that density had a regulatory effect (mediated through growth). The fact that there were no density effects on R. temporaria (and a significant difference in its response to that of R. arvalis) suggests it is a superior competitor to R. arvalis during the terrestrial phase. There were no density effects on frog condition index, suggesting that the growth rate modifications may actually be an adaptive trait of R. arvalis. The study demonstrates that density regulation may be dependent on resources in frogs' summer habitat.

Loman, Jon; Lardner, Björn

2009-11-01

177

Quantum fluid dynamics based current-density functional study of a helium atom in a strong time-dependent magnetic field  

NASA Astrophysics Data System (ADS)

Evolution of the helium atom in a strong time-dependent (TD) magnetic field ( B) of strength up to 1011 G is investigated through a quantum fluid dynamics (QFD) based current-density functional theory (CDFT). The TD-QFD-CDFT computations are performed through numerical solution of a single generalized nonlinear Schrödinger equation employing vector exchange-correlation potentials and scalar exchange-correlation density functionals that depend both on the electronic charge-density and the current-density. The results are compared with that obtained from a B-TD-QFD-DFT approach (based on conventional TD-DFT) under similar numerical constraints but employing only scalar exchange-correlation potential dependent on electronic charge-density only. The B-TD-QFD-DFT approach, at a particular TD magnetic field-strength, yields electronic charge- and current-densities as well as exchange-correlation potential resembling with that obtained from the time-independent studies involving static (time-independent) magnetic fields. However, TD-QFD-CDFT electronic charge- and current-densities along with the exchange-correlation potential and energy differ significantly from that obtained using B-TD-QFD-DFT approach, particularly at field-strengths >109 G, representing dynamical effects of a TD field. The work concludes that when a helium atom is subjected to a strong TD magnetic field of order >109 G, the conventional TD-DFT based approach differs "dynamically" from the CDFT based approach under similar computational constraints.

Vikas, Hash(0x125f4490)

2011-02-01

178

A seasonal, density-dependent model for the management of an invasive weed.  

PubMed

The population effects of harvest depend on complex interactions between density dependence, seasonality, stage structure, and management timing. Here we present a periodic nonlinear matrix population model that incorporates seasonal density dependence with stage-selective and seasonally selective harvest. To this model, we apply newly developed perturbation analyses to determine how population densities respond to changes in harvest and demographic parameters. We use the model to examine the effects of popular control strategies and demographic perturbations on the invasive weed garlic mustard (Alliaria petiolata). We find that seasonality is a major factor in harvest outcomes, because population dynamics may depend significantly on both the season of management and the season of observation. Strategies that reduce densities in one season can drive increases in another, with strategies giving positive sensitivities of density in the target seasons leading to compensatory effects that invasive species managers should avoid. Conversely, demographic parameters to which density is very elastic (e.g., seeding survival, second-year rosette spring survival, and the flowering to fruiting adult transition for maximum summer densities) may indicate promising management targets. PMID:24555315

Shyu, Esther; Pardini, Eleanor A; Knight, Tiffany M; Caswell, Hal

2013-12-01

179

Density-dependent regulation of the sex ratio in an annual plant.  

PubMed

Sex ratios are subject to strong frequency-dependent selection regulated by the mating system and the relative male versus female investment. In androdioecious plant populations, where males co-occur with hermaphrodites, the sex ratio depends on the rate of self-fertilization by hermaphrodites and on the relative pollen production of males versus hermaphrodites. Here, we report evolutionary changes in the sex ratio from experimental mating arrays of the androdioecious plant Mercurialis annua. We found that the progeny sex ratio depended strongly on density, with fewer males in the progeny of plants grown under low density. This occurred in part because of a plastic adjustment in pollen production by hermaphrodites, which produced more pollen when grown at low density than at high density. Our results provide support for the prediction that environmental conditions govern sex ratios through their effects on the relative fertility of unisexual versus hermaphrodite individuals. PMID:18429672

Dorken, Marcel E; Pannell, John R

2008-06-01

180

Magnetized strange quark matter in a mass-density-dependent model  

E-print Network

We investigate the properties of strange quark matter in a strong magnetic field with quark confinement by the density dependence of quark mass considering the total baryon number conservation, charge neutrality and chemical equilibrium. The strength of the magnetic field considered in this article is $10^{16} \\sim 10^{20}$ G. It is found that an additional term should appear in the pressure expression to maintain thermodynamic consistency. At fixed density, the energy density of magnetized strange quark matter varies with the magnetic field strength. The exists a minimum with increasing the field strength, depending on the density. It is about $6\\times10^{19}$ Gauss at two times the normal nuclear saturation density.

J. X. Hou; G. X. Peng; C. J. Xia; J. F. Xu

2014-03-05

181

Carrier-Density-Dependent Lattice Stability in InSb P. B. Hillyard,1,2  

E-print Network

Carrier-Density-Dependent Lattice Stability in InSb P. B. Hillyard,1,2 K. J. Gaffney,2,3,* A. M of an InSb crystal has been utilized to observe carrier dependent changes in the potential energy surface

von der Linde, D.

182

Density dependence in a recovering osprey population: demographic and behavioural processes.  

PubMed

1. Understanding how density-dependent and independent processes influence demographic parameters, and hence regulate population size, is fundamental within population ecology. We investigated density dependence in growth rate and fecundity in a recovering population of a semicolonial raptor, the osprey Pandion haliaetus [Linnaeus, 1758], using 31 years of count and demographic data in Corsica. 2. The study population increased from three pairs in 1974 to an average of 22 pairs in the late 1990s, with two distinct phases during the recovery (increase followed by stability) and contrasted trends in breeding parameters in each phase. 3. We show density dependence in population growth rate in the second phase, indicating that the stabilized population was regulated. We also show density dependence in productivity (fledging success between years and hatching success within years). 4. Using long-term data on behavioural interactions at nest sites, and on diet and fish provisioning rate, we evaluated two possible mechanisms of density dependence in productivity, food depletion and behavioural interference. 5. As density increased, both provisioning rate and the size of prey increased, contrary to predictions of a food-depletion mechanism. In the time series, a reduction in fledging success coincided with an increase in the number of non-breeders. Hatching success decreased with increasing local density and frequency of interactions with conspecifics, suggesting that behavioural interference was influencing hatching success. 6. Our study shows that, taking into account the role of non-breeders, in particular in species or populations where there are many floaters and where competition for nest sites is intense, can improve our understanding of density-dependent processes and help conservation actions. PMID:18564291

Bretagnolle, V; Mougeot, F; Thibault, J-C

2008-09-01

183

A New Approach of Designing Superalloys for Low Density  

NASA Technical Reports Server (NTRS)

New low-density single-crystal (LDS) alloy, have bee. developed for turbine blade applications, which have the potential for significant improvements in the thrust-to-weight ratio over current production superalloys. An innovative alloying strategy was wed to achieve alloy density reductions, high-temperature creep resistance, microstructural stability, and cyclic oxidation resistance. The alloy design relies on molybdenum as a potent. lower-density solid-solution strengthener in the nickel-based superalloy. Low alloy density was also achieved with modest rhenium levels tmd the absence of tungsten. Microstructural, physical mechanical, and environmental testing demonstrated the feasibility of this new LDS superalloy design.

MacKay, Rebecca A.; Gabb, Timothy P.; Smialek, James L.; Nathal, Michael V.

2010-01-01

184

Density Dependence, Prey Dependence, and Population Dynamics of Martens in Ontario  

Microsoft Academic Search

Abstract. Ecological factors influencing demographic,parameters,of mammalian,car- nivores are poorly understood, due to the difficulty of simultaneously measuring predator and,prey populations,over an extended,period. We used,cohort,analysis based,on age- specific harvest,data to estimate population,densities over 20 yr for martens,(Martes amer- icana). Marten abundance increased threefold over the study period, probably due to re- laxation,in harvest,intensity at the beginning,of the study,interval. Changes,in rates of

John M. Fryxell; J. Bruce Falls; E. Ann Falls; Ronald J. Brooks; Linda Dix; Marjorie A. Strickland

1999-01-01

185

Density dependence of the "symmetry energy" in the lattice gas model  

E-print Network

Isoscaling behavior of the statistical emission fragments from the equilibrated sources with $Z$ = 30 and $N$ = 30, 33, 36 and 39, resepectively, is investigated in the framework of isospin dependent lattice gas model. The dependences of isoscaling parameters $\\alpha$ on source isospin asymmetry, temperature and freeze-out density are studied and the "symmetry energy" is deduced from isoscaling parameters. Results show that "symmetry energy" $C_{sym}$ is insensitive to the change of temperature but follows the power-law dependence on the freeze-out density $\\rho$. The later gives $C_{sym}$ = 30$(\\rho/\\rho_0)^{0.62}$ if the suitable asymmetric nucleon-nucleon potential is taken. The effect of strength of asymmetry of nucleon-nucleon interaction potential on the density dependence of the "symmetry energy" is dicussed.

Q. M. Su; Y. G. Ma; W. D. Tian; D. Q. Fang; X. Z. Cai; K. Wang

2007-11-01

186

Structural Implications of Reciprocal Exchange: A Power-Dependence Approach  

ERIC Educational Resources Information Center

This paper presents and tests a general model to predict emergent exchange patterns and power differences in reciprocal exchange networks when individual actors follow the norm of reciprocity. With an interesting qualification, the experimental results reported here support the power-dependence approach (Emerson 1972a, b): those who acquire the…

Bonacich, Phillip; Bienenstock, Elisa Jayne

2009-01-01

187

Dependence of various SOL widths on plasma current and density in NSTX H-mode plasmas  

SciTech Connect

The dependence of various SOL widths on the line-averaged density ({ovr n}{sub e}) and plasma current (l{sub p}) for the quiescent H-mode plasmas with Type-V ELMs in the National Spherical Torus Experiment (NSTX) was investigated. It is found that the heat flux SOL width ({lambda}{sub q}), measured by the IR camera, is virtually insensitive to {ovr n}{sub e} and has a strong negative dependence on l{sub p}. This insensitivity of {lambda}{sub q} to {ovr n}{sub e} is consistent with the scaling law from JET H-mode plasmas that shows a very weak dependence on the upstream density. The electron temperature, ion saturation current density, electron density, and electron pressure decay lengths ({lambda}{sub Te}, {lambda}{sub jsat}, {lambda}{sub ne}, and {lambda}{sub pe}, respectively) measured by the probe showed that {lambda}{sub Te} and {lambda}{sub jsat} have strong negative dependence on l{sub p}, whereas {lambda}{sub ne} and {lambda}{sub pe} revealed only a little or no dependence. The dependence of {lambda}{sub Te} on l{sub p} is consistent with the scaling law in the literature while {lambda}{sub ne} and {lambda}{sub pe} dependence shows a different trend.

Ahn, J; Maingi, R; Boedo, J; Soukhanovskii, V A

2009-02-12

188

Population dynamics of Norwegian red deer: density-dependence and climatic variation.  

PubMed Central

We present a model on plant-deer climate interactions developed for improving our understanding of the temporal dynamics of deer abundance and, in particular, how intrinsic (density-dependent) and extrinsic (plants, climate) factors influence these dynamics. The model was tested statistically by analysing the dynamics of five Norwegian red deer populations between 1964 and 1993. Direct and delayed density-dependence significantly influenced the development of the populations: delayed density-dependence primarily operated through female density, whereas direct density-dependence acted through both female and male densities. Furthermore, population dynamics of Norwegian red deer were significantly affected by climate (as measured by the global weather phenomenon, the North Atlantic Oscillation: NAO). Warm, snowy winters (high NAO) were associated with decreased deer abundance, whereas the delayed (two-year) effect of warm, snowy winters had a positive effect on deer abundance. Our analyses are argued to have profound implications for the general understanding of climate change and terrestrial ecosystem functioning. PMID:9523435

Forchhammer, M C; Stenseth, N C; Post, E; Langvatn, R

1998-01-01

189

Adaptive nest clustering and density-dependent nest survival in dabbling ducks  

USGS Publications Warehouse

Density-dependent population regulation is observed in many taxa, and understanding the mechanisms that generate density dependence is especially important for the conservation of heavily-managed species. In one such system, North American waterfowl, density dependence is often observed at continental scales, and nest predation has long been implicated as a key factor driving this pattern. However, despite extensive research on this topic, it remains unclear if and how nest density influences predation rates. Part of this confusion may have arisen because previous studies have studied density-dependent predation at relatively large spatial and temporal scales. Because the spatial distribution of nests changes throughout the season, which potentially influences predator behavior, nest survival may vary through time at relatively small spatial scales. As such, density-dependent nest predation might be more detectable at a spatially- and temporally-refined scale and this may provide new insights into nest site selection and predator foraging behavior. Here, we used three years of data on nest survival of two species of waterfowl, mallards and gadwall, to more fully explore the relationship between local nest clustering and nest survival. Throughout the season, we found that the distribution of nests was consistently clustered at small spatial scales (˜50–400 m), especially for mallard nests, and that this pattern was robust to yearly variation in nest density and the intensity of predation. We demonstrated further that local nest clustering had positive fitness consequences – nests with closer nearest neighbors were more likely to be successful, a result that is counter to the general assumption that nest predation rates increase with nest density.

Ringelman, Kevin M.; Eadie, John M.; Ackerman, Joshua T.

2014-01-01

190

The effects of density-dependent dispersal on the spatiotemporal dynamics of cyclic populations.  

PubMed

Density-dependent dispersal occurs throughout the animal kingdom, and has been shown to occur in some taxa whose populations exhibit multi-year population cycles. However, the importance of density-dependent dispersal for the spatiotemporal dynamics of cyclic populations is unknown. We investigated the potential effects of density-dependent dispersal on the properties of periodic travelling waves predicted by two coupled reaction-diffusion models: a commonly used predator-prey model, and a general model of cyclic trophic interactions. We compared the effects of varying the gradient of both positive and negative density-dependent dispersal rates, to varying the ratio of the (constant) dispersal rates of the two interacting populations. Our comparison focussed on the possible range of wave properties, and on the waves generated by landscape obstacles and invasions. In all scenarios that we studied, varying the gradient of density-dependent dispersal has small quantitative effects on the travelling wave properties, relative to the effects of varying the ratio of the diffusion coefficients. PMID:18640692

Smith, Matthew J; Sherratt, Jonathan A; Lambin, Xavier

2008-09-21

191

Restricted dispersal reduces the strength of spatial density dependence in a tropical bird population  

PubMed Central

Spatial processes could play an important role in density-dependent population regulation because the disproportionate use of poor quality habitats as population size increases is widespread in animal populations—the so-called buffer effect. While the buffer effect patterns and their demographic consequences have been described in a number of wild populations, much less is known about how dispersal affects distribution patterns and ultimately density dependence. Here, we investigated the role of dispersal in spatial density dependence using an extraordinarily detailed dataset from a reintroduced Mauritius kestrel (Falco punctatus) population with a territorial (despotic) breeding system. We show that recruitment rates varied significantly between territories, and that territory occupancy was related to its recruitment rate, both of which are consistent with the buffer effect theory. However, we also show that restricted dispersal affects the patterns of territory occupancy with the territories close to release sites being occupied sooner and for longer as the population has grown than the territories further away. As a result of these dispersal patterns, the strength of spatial density dependence is significantly reduced. We conclude that restricted dispersal can modify spatial density dependence in the wild, which has implications for the way population dynamics are likely to be impacted by environmental change. PMID:18285284

Burgess, Malcolm D; Nicoll, Malcolm A.C; Jones, Carl G; Norris, Ken

2008-01-01

192

Density dependence and population differentiation of genetic architecture in Impatiens capensis in natural environments.  

PubMed

We identified environment-dependent constraints on the evolution of plasticity to density under natural conditions in two natural populations of Impatiens capensis. We also examined the expression of population divergence in genetic variance-covariance matrices in these natural environments. Inbred lines, originally collected from a sunny site with high seedling densities and a woodland site with low seedling densities, were planted in both original sites at natural high densities and at low density. Morphological and life-history characters were measured. More genetic variation for plastic responses to density was expressed in the sun site than in the woodland site, so the evolutionary potential of plasticity was greater in the sun site. Strong genetic correlations between the same character expressed at different densities and correlations among different characters could constrain the evolution of plasticity in both sites. Genetically based trade-offs in meristem allocation to vegetative growth and reproduction were apparent only in the high-resource environment with no overhead canopy and no intraspecific competition. Therefore, genetic constraints on the evolution of plasticity depended on the site and density in which plants were grown, and correlated responses to selection on plastic characters are also expected to differ between sites and densities. Population differentiation in genetic variance-covariance matrices was detected, but matrix structural differences, as opposed to proportional differences, were detected between populations only in the sun site at natural high density. Thus, population divergence in genetic architecture can occur rapidly and on a fine spatial scale, but the expression of such divergence may depend on the environment. PMID:11209774

Donohue, K; Pyle, E H; Messiqua, D; Heschel, M S; Schmitt, J

2000-12-01

193

Density-dependent habitat selection and partitioning between two sympatric ungulates.  

PubMed

Theory on density-dependent habitat selection predicts that as population density of a species increases, use of higher quality (primary) habitat by individuals declines while use of lower quality (secondary) habitat rises. Habitat partitioning is often considered the primary mechanism for coexistence between similar species, but how this process evolves with changes in population density remains to be empirically tested for free-ranging ungulates. We used resource-selection functions to quantify density effects on landscape-scale habitat selection of two sympatric species of ungulates [moose (Alces alces) and elk (Cervus canadensis manitobensis)] in Riding Mountain National Park, Manitoba, Canada (2000-2011). The density of elk was actively reduced from 1.2 to 0.4 elk km(-2) through increased hunting effort during the period of study, while moose density decreased without additional human influence from 1.6-0.7 moose km(-2). Patterns of habitat selection during winter by both species changed in accordance to expectations from density-dependent habitat-selection theory. At low intraspecific density, moose and elk did not partition habitat, as both species selected strongly for mixed forest (primary habitat providing both food and cover), but did so in different areas segregated across an elevational gradient. As intraspecific density increased, selection for primary habitat by both species decreased, while selection for secondary, lower quality habitat such as agricultural fields (for elk) and built-up areas (for moose) increased. We show that habitat-selection strategies during winter for moose and elk, and subsequent effects on habitat partitioning, depend heavily on the position in state space (density) of both species. PMID:24913777

van Beest, Floris M; McLoughlin, Philip D; Vander Wal, Eric; Brook, Ryan K

2014-08-01

194

Small-scale field experiments accurately scale up to predict density dependence in reef fish populations at large scales  

PubMed Central

Field experiments provide rigorous tests of ecological hypotheses but are usually limited to small spatial scales. It is thus unclear whether these findings extrapolate to larger scales relevant to conservation and management. We show that the results of experiments detecting density-dependent mortality of reef fish on small habitat patches scale up to have similar effects on much larger entire reefs that are the size of small marine reserves and approach the scale at which some reef fisheries operate. We suggest that accurate scaling is due to the type of species interaction causing local density dependence and the fact that localized events can be aggregated to describe larger-scale interactions with minimal distortion. Careful extrapolation from small-scale experiments identifying species interactions and their effects should improve our ability to predict the outcomes of alternative management strategies for coral reef fishes and their habitats. PMID:16150721

Steele, Mark A.; Forrester, Graham E.

2005-01-01

195

Exact-exchange kernel of time-dependent density functional theory: Frequency dependence and photoabsorption spectra of atoms  

SciTech Connect

In this work we have calculated excitation energies and photoionization cross sections of Be and Ne in the exact-exchange (EXX) approximation of time-dependent density functional theory (TDDFT). The main focus has been on the frequency dependence of the EXX kernel and on how it affects the spectrum as compared to the corresponding adiabatic approximation. We show that for some discrete excitation energies the frequency dependence is essential to reproduce the results of time-dependent Hartree-Fock theory. Unfortunately, we have found that the EXX approximation breaks down completely at higher energies, producing a response function with the wrong analytic structure and making inner-shell excitations disappear from the calculated spectra. We have traced this failure to the existence of vanishing eigenvalues of the Kohn-Sham non-interacting response function. Based on the adiabatic TDDFT formalism we propose a new way of deriving the Fano parameters of autoionizing resonances.

Hellgren, Maria; Barth, Ulf von [Department of Mathematical Physics, Institute of Physics, Lund University, Soelvegatan 14A, S-22362 Lund (Sweden)

2009-07-28

196

Exact-exchange kernel of time-dependent density functional theory: Frequency dependence and photoabsorption spectra of atoms  

E-print Network

In this work we have calculated excitation energies and photoionization cross sections of Be and Ne in the exact-exchange (EXX) approximation of time-dependent density functional theory (TDDFT). The main focus has been on the frequency dependence of the EXX kernel and on how it affects the spectrum as compared to the corresponding adiabatic approximation. We show that for some discrete excitation energies the frequency dependence is essential to reproduce the results of time-dependent Hartree-Fock theory. Unfortunately, we have found that the EXX approximation breaks down completely at higher energies, producing a response function with the wrong analytic structure and making inner-shell excitations disappear from the calculated spectra. We have traced this failure to the existence of vanishing eigenvalues of the Kohn-Sham non-interacting response function. Based on the adiabatic TDDFT formalism we propose a new way of deriving the Fano parameters of autoionizing resonances.

Hellgren, Maria

2009-01-01

197

Sampling-Based Approaches to Calculating Marginal Densities  

Microsoft Academic Search

Stochastic substitution, the Gibbs sampler, and the sampling-importance-resampling algorithm can be viewed as three alternative sampling- (or Monte Carlo-) based approaches to the calculation of numerical estimates of marginal probability distributions. The three approaches will be reviewed, compared, and contrasted in relation to various joint probability structures frequently encountered in applications. In particular, the relevance of the approaches to calculating

Alan E. Gelfand; Adrian F. M. Smith

1990-01-01

198

Polarization-dependent density-functional theory and quasiparticle theory: Optical response beyond local-density approximations  

Microsoft Academic Search

The polarization ({ital P}) dependence of the exchange-correlation energy ({ital E}{sub {ital xc}}) of semiconductors results in an effective field (â²{ital E}{sub {ital xc}}\\/â{ital P}²){ital P}â¡Î³â{ital P} in the Kohn-Sham equations [Gonze {ital et} {ital al}., Phys. Rev. Lett {bold 74}, 4035 (1995)]. This effective field is absent in local-density approximations such as LDA and GGA. We show that in

Wilfried Aulbur; L. Joensson; John Wilkins

1996-01-01

199

Polarization-dependent density-functional theory and quasiparticle theory: Optical response beyond local-density approximations  

Microsoft Academic Search

The polarization (P) dependence of the exchange-correlation energy (Exc) of semiconductors results in an effective field (?2Exc\\/?P2)P?gamma1P in the Kohn-Sham equations [Gonze et al., Phys. Rev. Lett 74, 4035 (1995)]. This effective field is absent in local-density approximations such as LDA and GGA. We show that in the long-wavelength limit gamma1~=chi-1LDA-chi-1expt where chi is the linear susceptibility. We find that

Wilfried G. Aulbur; Lars Jönsson; John W. Wilkins

1996-01-01

200

Density-dependent growth and survival in a natural population of the facultative biennial Digitalis purpurea  

Microsoft Academic Search

Summary 1 Density-dependent effects on vital rates may vary in both magnitude and direction at different stages of the life cycle. In monocarpic perennials, however, it is often assumed that recruitment is the stage most affected by density. 2 The spatial pattern of newly emerged individuals of the facultative biennial Digitalis purpurea was recorded and followed in five 0.5-m 2

NINA SLETVOLD

2005-01-01

201

Inversely density-dependent natal dispersal in brown bears Ursus arctos  

Microsoft Academic Search

There is considerable controversy in the literature about the presence of density dependence in dispersal. In this study, we exploit a data series from a long-term study (>18 years) on radio-marked brown bears (Ursus arctos L.) in two study areas in Scandinavia to investigate how individual-based densities influence the probability of natal dispersal and natal dispersal distances. Cumulatively, 32% and 46%

Ole-Gunnar Støen; Andreas Zedrosser; Solve Sæbø; Jon E. Swenson

2006-01-01

202

Low-magnetic-field dependence and anisotropy of the critical current density in coated conductors  

Microsoft Academic Search

Many applications of ReBCO-coated conductors operate at low magnetic fields in the superconductor (below 200 mT). In order to predict the critical current and AC loss in these applications, it is necessary to know the anisotropy and field dependence of the critical current density at low magnetic fields. In this paper, we obtain a formula for the critical current density

E. Pardo; M. Vojenciak; F. Gömöry; J. Souc

2011-01-01

203

Density dependence in the terrestrial life history stage of two anurans  

Microsoft Academic Search

Populations of species with complex life cycles have the potential to be regulated at multiple life history stages. However,\\u000a research tends to focus on single stage density-dependence, which can lead to inaccurate conclusions about population regulation\\u000a and subsequently hinder conservation efforts. In amphibians, many studies have demonstrated strong effects of larval density\\u000a and have often assumed that populations are regulated

Elizabeth B. Harper; Raymond D. Semlitsch

2007-01-01

204

Dependencies of Ultrasonic Properties on Apparent Bone Density in Trabecular Bone  

Microsoft Academic Search

The present study aims to provide insight into the dependencies of ultrasonic properties, such as speed of sound (SOS), broadband ultrasonic attenuation (BUA), and integrated reflection coefficient (IRC), on apparent bone density in trabecular bone. SOS, BUA, and IRC were measured in 19 bovine trabecular bone specimens with apparent densities from 0.411 to 0.928 g\\/cm3, using a matched pair of

Kang Il Lee

2008-01-01

205

Time-Dependent Density Functional Theory Calculations of Photoabsorption Spectra in the Vacuum Ultraviolet Region  

Microsoft Academic Search

Time-dependent density functional theory (TD-DFT) calculations of transition energy and oscillator strength of formaldehyde, benzene, ethylene and methane molecules are performed. It was found that the LDFT transition energies tend to be smaller than experimental values by 0.1 - 1.3 eV. Inclusion of nonlocal effects made the calculated energies to be larger than the LDFT (local density functional theory) values

Nobuyuki N. Matsuzawa; Ahihiko Ishitani; David A. Dixon; Tsuyoshi Uda

2001-01-01

206

Estimates of Leaf Vein Density Are Scale Dependent1[C][W][OPEN  

PubMed Central

Leaf vein density (LVD) has garnered considerable attention of late, with numerous studies linking it to the physiology, ecology, and evolution of land plants. Despite this increased attention, little consideration has been given to the effects of measurement methods on estimation of LVD. Here, we focus on the relationship between measurement methods and estimates of LVD. We examine the dependence of LVD on magnification, field of view (FOV), and image resolution. We first show that estimates of LVD increase with increasing image magnification and resolution. We then demonstrate that estimates of LVD are higher with higher variance at small FOV, approaching asymptotic values as the FOV increases. We demonstrate that these effects arise due to three primary factors: (1) the tradeoff between FOV and magnification; (2) geometric effects of lattices at small scales; and; (3) the hierarchical nature of leaf vein networks. Our results help to explain differences in previously published studies and highlight the importance of using consistent magnification and scale, when possible, when comparing LVD and other quantitative measures of venation structure across leaves. PMID:24259686

Price, Charles A.; Munro, Peter R.T.; Weitz, Joshua S.

2014-01-01

207

An efficient multiresolution algorithm for compensating density-dependent media blurring  

NASA Astrophysics Data System (ADS)

The sharpness of a printed image may suffer due to the presence of material layers above and below the dye layers. These layers contribute to scattering and surface reflections that make the degradation in sharpness density-dependent. We present data that illustrate this effect, and model the phenomenon numerically. A digital non-linear sharpening filter is proposed to compensate for this density-dependent blurring. The support and shape of this filter is constrained to lie in a space spanned by a set of basis filters that can be computed efficiently. Burt and Adelson's Laplacian pyramid is used to develop an efficient scale-recursive algorithm in which the image is decomposed into the high-pass basis images in a fine-to-coarse scale sweep, and the sharpened image along with a local density image is subsequently synthesized by a coarse-to-fine scale sweep using these basis images. The local density image is employed, in combination with a scale dependent gain function, to modulate the high-pass basis images in a space-varying fashion. A robust method is proposed for the estimation of the gain functions directly from measured data. Experimental results demonstrate that the proposed algorithm successfully compensates for media-related density dependent blurring.

Saquib, Suhail S.; Vetterling, William T.

2005-03-01

208

Modeling the Effect of Density-Dependent Chemical Interference upon Seed Germination  

PubMed Central

A mathematical model is presented to estimate the effects of phytochemicals on seed germination. According to the model, phytochemicals tend to prevent germination at low seed densities. The model predicts that at high seed densities they may increase the probability of seed germination and the number of germinating seeds. Hence, the effects are reminiscent of the density-dependent effects of allelochemicals on plant growth, but the involved variables are germination probability and seedling number. The results imply that it should be possible to bypass inhibitory effects of allelopathy in certain agricultural practices and to increase the efficiency of nature conservation in several plant communities. PMID:18648596

Sinkkonen, Aki

2006-01-01

209

Modeling the Effect of Density-Dependent Chemical Interference Upon Seed Germination  

PubMed Central

A mathematical model is presented to estimate the effects of phytochemicals on seed germination. According to the model, phytochemicals tend to prevent germination at low seed densities. The model predicts that at high seed densities they may increase the probability of seed germination and the number of germinating seeds. Hence, the effects are reminiscent of the density-dependent effects of allelochemicals on plant growth, but the involved variables are germination probability and seedling number. The results imply that it should be possible to bypass inhibitory effects of allelopathy in certain agricultural practices and to increase the efficiency of nature conservation in several plant communities. PMID:19330163

Sinkkonen, Aki

2005-01-01

210

Effects of the density dependence of the nuclear symmetry energy on the properties of superheavy nuclei  

SciTech Connect

Effects of the density dependence of the nuclear symmetry energy on ground-state properties of superheavy nuclei are studied in the relativistic mean-field theory. It is found that the softening of the symmetry energy plays an important role in the empirical shift [Phys. Rev. C 67, 024309 (2003)] of spherical orbitals in superheavy nuclei. The calculation based on the relativistic mean-field models NL3 and FSUGold supports the double shell closure in {sup 292}120 with the softening of the symmetry energy. In addition, the significant effect of the density dependence of the symmetry energy on the neutron skin thickness in superheavy nuclei is investigated.

Jiang Weizhou [Department of Physics, Southeast University, Nanjing 211189 (China) and National Laboratory of Heavy Ion Accelerator, Lanzhou 730000 (China)

2010-04-15

211

An effective method for state population within time-dependent density functional theory  

NASA Astrophysics Data System (ADS)

The determination of state population probability within the framework of time-dependent density functional theory (TDDFT) has remained a widely open question. The aim of this study is to find out whether and how this probability can be extracted from time-dependent density, which has been used as the basic variable within TDDFT. We propose an effective method to calculate state population probabilities, which has been well validated in benchmark case studies on nonresonant (detuned) Rabi oscillations of a Na atom, Na2 dimer, and Na4 cluster irradiated by a monochromatic laser.

Wang, Feng; Jiang, Lan; Hong, Xuhai; Jiao, Yalong; Wang, Jianguo; Zhang, Fengshou

2013-09-01

212

Temperature dependence of the electron density of states and dc electrical resistivity of disordered binary alloys.  

NASA Technical Reports Server (NTRS)

A model calculation of the temperature dependence of the electronic density of states and the electrical conductivity of disordered binary alloys, based on the coherent-potential approximation is made by introducing thermal disorder in the single-band model (Velicky and others). Thermal disorder is found to broaden and smear the static-alloy density of states. The electrical resistivity in weak-scattering alloys always increases with temperature. However, in the strong-scattering case, the temperature coefficient of resistivity can be positive, zero, or negative, depending on the location of the Fermi energy.-

Chen, A.-B.; Weisz, G.; Sher, A.

1972-01-01

213

Optical response of extended systems using time-dependent density functional theory.  

PubMed

In this chapter, time-dependent density functional theory is introduced and an outline of the Runge-Gross theorem is presented. An equation for linear response within time-dependent density functional theory is derived. A key ingredient of this equation is the exchange-correlation kernel for which several modern-day approximation exist. These approximations are investigated for their ability to capture the excitonic physics in absorption and electron loss spectra. To this end, results for medium (Si and diamond) to large (LiF and Ar) band-gap insulators are presented, which exhibit excitonic physics to varying degrees. PMID:24647840

Sharma, S; Dewhurst, J K; Gross, E K U

2014-01-01

214

Density-dependent survival and fecundity of hemlock woolly adelgid (hemiptera: adelgidae).  

PubMed

The hemlock woolly adelgid (Adelges tsugae Annand) has decimated eastern hemlocks (Tsuga canadensis Carrière) in forests throughout the eastern United Sates, but its densities in central New England appear to have stabilized. To find out why, we infested 64 eastern hemlocks with varying densities of adelgid ovisacs in a typical eastern hemlock forest in western Massachusetts. We subsequently documented adelgid density, fecundity, and the amount of new growth on experimental trees over two consecutive years. We used a 2 by 2 randomized block design using previously and newly infested hemlocks that were either 1-m tall saplings or branches of mature trees. There was a density-dependent decline in the survival and fecundity of adelgid in both the spring and winter generations. This response was a function of both previous infestation by adelgid and current year's crawler density in the spring generation. Additionally, the production of sexuparae in the spring generation played a key role in the overall density-dependent survival of adelgid, suggesting that sexuparae production is strongly linked to developing crawler density. PMID:25203223

Sussky, Elizabeth M; Elkinton, Joseph S

2014-10-01

215

Density!  

NSDL National Science Digital Library

What is Density? Density is the amount of "stuff" in a given "space". In science terms that means the amount of "mass" per unit "volume". Using units that means the amount of "grams" per "centimeters cubed". Check out the following links and learn about density through song! Density Beatles Style Density Chipmunk Style Density Rap Enjoy! ...

Witcher, Miss

2011-10-06

216

Estimating density dependence in time-series of age-structured populations.  

PubMed Central

For a life history with age at maturity alpha, and stochasticity and density dependence in adult recruitment and mortality, we derive a linearized autoregressive equation with time-lags of from 1 to alpha years. Contrary to current interpretations, the coefficients for different time-lags in the autoregressive dynamics do not simply measure delayed density dependence, but also depend on life-history parameters. We define a new measure of total density dependence in a life history, D, as the negative elasticity of population growth rate per generation with respect to change in population size, D = - partial differential lnlambda(T)/partial differential lnN, where lambda is the asymptotic multiplicative growth rate per year, T is the generation time and N is adult population size. We show that D can be estimated from the sum of the autoregression coefficients. We estimated D in populations of six avian species for which life-history data and unusually long time-series of complete population censuses were available. Estimates of D were in the order of 1 or higher, indicating strong, statistically significant density dependence in four of the six species. PMID:12396510

Lande, R; Engen, S; Saether, B-E

2002-01-01

217

Multiple density-dependence mechanisms regulate a migratory bird population during the breeding season.  

PubMed Central

The mechanisms regulating bird populations are poorly understood and controversial. We provide evidence that a migratory songbird, the black-throated blue warbler (Dendroica caerulescens), is regulated by multiple density-dependence mechanisms in its breeding quarters. Evidence of regulation includes: stability in population density during 1969-2002, strong density dependence in time-series analyses of this period, an inverse relationship between warbler density and annual fecundity, and a positive relationship between annual fecundity and recruitment of yearlings in the subsequent breeding season. Tests of the mechanisms causing regulation were carried out within the Hubbard Brook Experimental Forest, New Hampshire, during 1997-1999. When individuals from abutting territories were experimentally removed in a homogeneous patch of high-quality habitat, the fecundity of focal pairs nearly doubled, revealing a locally operating crowding mechanism. A site-dependence mechanism was indicated by an inverse relationship between population size and mean territory quality, as well as by greater annual fecundity on the sites that were most frequently occupied and of highest quality. These site-dependence relationships were revealed by intensive monitoring of territory quality and demography at the landscape spatial scale. Crowding and site-dependence mechanisms, therefore, acted simultaneously but at different spatial scales to regulate local abundance of this migratory bird population. PMID:14561272

Rodenhouse, Nicholas L; Sillett, T Scott; Doran, Patrick J; Holmes, Richard T

2003-01-01

218

Compositional or charge density modification of the endothelial glycocalyx accelerates flow-dependent concentration polarization of low-density lipoproteins.  

PubMed

We hypothesized that diminished endothelial glycocalyx (GCX) at atherosclerotic lesion-prone sites accelerates flow-dependent concentration polarization of low-density lipoproteins (LDLs) at the luminal surface, and in turn contributes to vulnerability of these sites to atherosclerosis. A parallel plate flow chamber was applied to expose cultured endothelial monolayers to three different levels of shear stress (3, 12, 20 dyn/cm(2)). Heparinase III (Hep.III) was employed to degrade heparan sulfate proteoglycans selectively and 3-(N-morpholino) propanesulfonic acid-buffered physiological salt solutions (MOPS-PSS) were used at either normal ionic strength (Normal-MOPS), low ionic strength (LO-MOPS) or high ionic strength (HI-MOPS) to modify the effective charge density of the endothelial GCX. Water filtration velocity (V(w)) across the endothelial monolayer, the luminal concentration of LDLs (C(w)) and the uptake of LDLs by endothelial cells were measured and compared among the following five groups of cells: (1) Control; (2) Hep.III treatment; (3) LO-MOPS; (4) Normal-MOPS; and (5) HI-MOPS. The results obtained substantiated the aforementioned hypothesis and demonstrated that compositional or charge density modification of the endothelial GCX facilitated water filtration across the endothelium, enhanced the accumulation of LDLs on the luminal surface and increased the uptake of LDLs by endothelial cells, therefore contributing to atherogenesis. PMID:21659384

Kang, Hongyan; Fan, Yubo; Sun, Anqiang; Deng, Xiaoyan

2011-07-01

219

The Environmental Dependence of Galaxy Properties in the Local Universe: Dependence on Luminosity, Local Density, and System Richness  

E-print Network

We investigate the environmental dependence of star formation and the morphology of galaxies in the local universe based on a volume-limited sample (0.030galaxy properties change with local density. The star formation and morphology show a 'break' at the critical local density of log Sigma_crit~0.4 galaxies h_75^2 Mpc^-2, which is in agreement with previous studies. However, the break can be seen only for faint galaxies (M^*_r+1 galaxies (M_r galaxies of different luminosities are found to show different environmental dependencies. Next, we examine dependencies on richness of galaxy systems. Star formation activities of galaxies are different from those of field galaxies even in systems as poor as sigma~100 km/s. This result suggests that environmental mechanisms that are effective only in rich systems, such as ram-pressure stripping of cold gas and harassment, have not played a major role in transforming galaxies into red early-type galaxies. Strangulation and interactions between galaxies, however, remain candidates of the driver of the environmental dependence. In the dense environments in the local universe, the slow transformation of faint galaxies occurs to some extent, but the transformation of bright galaxies is not clearly visible. We suggest that the evolution of bright galaxies is not strongly related to galaxy system, such as groups and clusters, while the evolution of faint galaxies is likely to be closely connected to galaxy system.

Masayuki Tanaka; Tomotsugu Goto; Sadanori Okamura; Kazuhiro Shimasaku; Jon Brinkman

2004-11-05

220

Exploration of Plasma Jets Approach to High Energy Density Physics  

SciTech Connect

High-energy-density laboratory plasma (HEDLP) physics is an emerging, important area of research in plasma physics, nuclear physics, astrophysics, and particle acceleration. While the HEDLP regime occurs at extreme conditions which are often found naturally in space but not on the earth, it may be accessible by colliding high intensity plasmas such as high-energy-density plasma jets, plasmoids or compact toroids from plasma guns. The physics of plasma jets is investigated in the context of high energy density laboratory plasma research. This report summarizes results of theoretical and computational investigation of a plasma jet undergoing adiabatic compression and adiabatic expansion. A root-mean-squared (rms) envelope theory of plasma jets is developed. Comparison between theory and experiment is made. Good agreement between theory and experiment is found.

Chen, Chiping [Massachusetts Institute of Technology

2013-08-26

221

Hydrodynamic perspective on memory in time-dependent density-functional theory  

Microsoft Academic Search

The adiabatic approximation of time-dependent density-functional theory is studied in the context of nonlinear excitations of two-electron singlet systems. We compare the exact time evolution of these systems to the adiabatically exact one obtained from time-dependent Kohn-Sham calculations relying on the exact ground-state exchange-correlation potential. Thus, we can show under which conditions the adiabatic approximation breaks down and memory effects

M. Thiele; S. Kümmel

2009-01-01

222

Dipicolinic acid prevents the copper-dependent oxidation of low density lipoprotein  

Microsoft Academic Search

Effect of dipicolinic acid (pyridine 2,6-dicarboxylic acid) and pyridine compounds on the copper-dependent oxidation of human low density lipoprotein was analyzed in relation to the inhibition of copper reduction. Dipicolinic acid inhibited copper-dependent LDL oxidation completely, but the LDL oxidation was slightly inhibited by pyridine compounds with one carboxyl group at 2 or 6-position. Reduction of copper by LDL itself

Keiko Murakami; Yasuko Tanemura; Masataka Yoshino

2003-01-01

223

Equation of state of dense matter from a density dependent relativistic mean field model  

SciTech Connect

We calculate the equation of state (EOS) of dense matter using a relativistic mean field (RMF) model with a density dependent coupling that is a slightly modified form of the original NL3 interaction. For nonuniform nuclear matter we approximate the unit lattice as a spherical Wigner-Seitz cell, wherein the meson mean fields and nucleon Dirac wave functions are solved fully self-consistently. We also calculate uniform nuclear matter for a wide range of temperatures, densities, and proton fractions, and match them to nonuniform matter as the density decreases. The calculations took over 6000 CPU days in Indiana University's supercomputer clusters. We tabulate the resulting EOS at over 107,000 grid points in the proton fraction range Y{sub P}=0 to 0.56. For the temperature range T=0.16 to 15.8 MeV, we cover the density range n{sub B}=10{sup -4} to 1.6 fm{sup -3}; and for the higher temperature range T=15.8 to 80 MeV, we cover the larger density range n{sub B}=10{sup -8} to 1.6 fm{sup -3}. In the future we plan to study low density, low temperature (T<15.8 MeV), nuclear matter using a Virial expansion, and we will match the low-density and high-density results to generate a complete EOS table for use in astrophysical simulations of supernova and neutron star mergers.

Shen, G.; Horowitz, C. J. [Nuclear Theory Center and Department of Physics, Indiana University Bloomington, Indiana 47405 (United States); Teige, S. [University Information Technology Services, Indiana University, Bloomington, Indiana 47408 (United States)

2010-07-15

224

A Concept for Airborne Precision Spacing for Dependent Parallel Approaches  

NASA Technical Reports Server (NTRS)

The Airborne Precision Spacing concept of operations has been previously developed to support the precise delivery of aircraft landing successively on the same runway. The high-precision and consistent delivery of inter-aircraft spacing allows for increased runway throughput and the use of energy-efficient arrivals routes such as Continuous Descent Arrivals and Optimized Profile Descents. This paper describes an extension to the Airborne Precision Spacing concept to enable dependent parallel approach operations where the spacing aircraft must manage their in-trail spacing from a leading aircraft on approach to the same runway and spacing from an aircraft on approach to a parallel runway. Functionality for supporting automation is discussed as well as procedures for pilots and controllers. An analysis is performed to identify the required information and a new ADS-B report is proposed to support these information needs. Finally, several scenarios are described in detail.

Barmore, Bryan E.; Baxley, Brian T.; Abbott, Terence S.; Capron, William R.; Smith, Colin L.; Shay, Richard F.; Hubbs, Clay

2012-01-01

225

Time-dependent approach to the continuum shell model  

SciTech Connect

The continuum shell model represents a merger of the traditional shell model, the tool for understanding nuclear structure, with the physics of reactions. In this work a new time-dependent approach to the continuum shell model is presented, where construction and application of the time-dependent evolution operator culminate in an effective and successful strategy for tackling the nonstationary many-body dynamics. Details behind the technique and methods to overcome general problems associated with quantum many-body physics on the verge of stability are discussed. Topics presented include the construction of the time-dependent Green's function, the full propagator from the exact solution of Dyson's equation, a treatment of decays and virtual self-energy terms, the explicit time dependence and survival probability of states, the strength function and collective features of unstable systems, the center-of-mass problem, computation of the cross section and its Blatt-Biedenharn angular decomposition, Coulomb amplitudes, and interference. An extensive comparison with the R-matrix approach is offered. Realistic examples are used to demonstrate the techniques.

Volya, Alexander [Department of Physics, Florida State University, Tallahassee, Florida 32306-4350 (United States)

2009-04-15

226

Strong density-dependent competition and acquired immunity constrain parasite establishment: implications for parasite aggregation.  

PubMed

The vast majority of parasites exhibit an aggregated frequency distribution within their host population, such that most hosts have few or no parasites while only a minority of hosts are heavily infected. One exception to this rule is the trophically transmitted parasite Pterygodermatites peromysci of the white-footed mouse (Peromyscus leucopus), which is randomly distributed within its host population. Here, we ask: what are the factors generating the random distribution of parasites in this system when the majority of macroparasites exhibit non-random patterns? We hypothesise that tight density-dependent processes constrain parasite establishment and survival, preventing the build-up of parasites within individual hosts, and preclude aggregation within the host population. We first conducted primary infections in a laboratory experiment using white-footed mice to test for density-dependent parasite establishment and survival of adult worms. Secondary or challenge infection experiments were then conducted to investigate underlying mechanisms, including intra-specific competition and host-mediated restrictions (i.e. acquired immunity). The results of our experimental infections show a dose-dependent constraint on within-host-parasite establishment, such that the proportion of mice infected rose initially with exposure, and then dropped off at the highest dose. Additional evidence of density-dependent competition comes from the decrease in worm length with increasing levels of exposure. In the challenge infection experiment, previous exposure to parasites resulted in a lower prevalence and intensity of infection compared with primary infection of naïve mice; the magnitude of this effect was also density-dependent. Host immune response (IgG levels) increased with the level of exposure, but decreased with the number of worms established. Our results suggest that strong intra-specific competition and acquired host immunity operate in a density-dependent manner to constrain parasite establishment, driving down aggregation and ultimately accounting for the observed random distribution of parasites. PMID:21215747

Luong, Lien T; Vigliotti, Beth A; Hudson, Peter J

2011-04-01

227

Solving inverse problems using an EM approach to density estimation  

E-print Network

, the acoustics of a four-tube articulatory model, and the localization of multiple objects from sensor data then be used to form any input/output map. Thus, to estimate the vector function y = f(x) the joint density P

Ghahramani, Zoubin

228

Solving inverse problems using an EM approach to density estimation  

E-print Network

­joint planar arm, the acoustics of a four­tube articulatory model, and the localization of multiple objects from sensor data. The learning algorithm presented differs from regression­based algorithms, to estimate the vector function y = f(x) the joint density P (x; y) is estimated and, given a particular input

Ghahramani, Zoubin

229

Native Birds and Alien Insects: Spatial Density Dependence in Songbird Predation of Invading Oak Gallwasps  

PubMed Central

Revealing the interactions between alien species and native communities is central to understanding the ecological consequences of range expansion. Much has been learned through study of the communities developing around invading herbivorous insects. Much less, however, is known about the significance of such aliens for native vertebrate predators for which invaders may represent a novel food source. We quantified spatial patterns in native bird predation of invading gall-inducing Andricus wasps associated with introduced Turkey oak (Quercus cerris) at eight sites across the UK. These gallwasps are available at high density before the emergence of caterpillars that are the principle spring food of native insectivorous birds. Native birds showed positive spatial density dependence in gall attack rates at two sites in southern England, foraging most extensively on trees with highest gall densities. In a subsequent study at one of these sites, positive spatial density dependence persisted through four of five sequential week-long periods of data collection. Both patterns imply that invading galls are a significant resource for at least some native bird populations. Density dependence was strongest in southern UK bird populations that have had longest exposure to the invading gallwasps. We hypothesise that this pattern results from the time taken for native bird populations to learn how to exploit this novel resource. PMID:23342048

Schönrogge, Karsten; Begg, Tracey; Stone, Graham N.

2013-01-01

230

Controlled suppression of the photoluminescence superlinear dependence on excitation density in quantum dots  

PubMed Central

We have shown that it is possible to tune, up to complete suppression, the photoluminescence superlinear dependence on the excitation density in quantum dot samples at high temperatures by annealing treatments. The effect has been attributed to the reduction of the defectivity of the material induced by annealing. PMID:23033918

2012-01-01

231

Modeling density dependence and climatic disturbances in caribou: a case study from the Bathurst Island complex,  

E-print Network

doi:10.1111/j.1469-7998.2006.00257.x Abstract Peary caribou Rangifer tarandus pearyi important roles in the population dynamics of caribou and reindeer Rangifer tarandus (Solberg et al., 2001Modeling density dependence and climatic disturbances in caribou: a case study from the Bathurst

232

A test of statistical techniques for detecting density dependence in sequential censuses of animal populations  

Microsoft Academic Search

Principal and reduced major axes, and Bulmer's (1975) tests have been suggested as methods for detecting the presence of density dependence in a series of population censuses that are unsuitable for analysis by alternative means e.g. by k-factor analysis. These alternative methods are tested using census data, some of which are previously unpublished, from natural populations known from independent evidence

K. J. Gaston; J. H. Lawton

1987-01-01

233

Carrier-Density-Dependent Lattice Stability in InSb P. B. Hillyard,1,2  

E-print Network

Carrier-Density-Dependent Lattice Stability in InSb P. B. Hillyard,1,2 K. J. Gaffney,2,3,* A. M April 2007) The ultrafast decay of the x-ray diffraction intensity following laser excitation of an InSb

234

Dependence of Camomile Flower Solubility on Carbon Dioxide Density at Supercritical Extraction  

Microsoft Academic Search

The solubility of camomile flowers in supercritical carbon dioxide as it depends on pressure (80–160 bar) and temperature (313–353 K) changes has been determined by using a dynamic procedure. The solubilities obtained were correlated with the density of carbon dioxide by using Chrastil's equation. This equation allows the calculation of the solubility of a complex system.

Aleksandar Toli?; Zoran Zekovi?; Branislav Peki?

1996-01-01

235

Evolutionary Ecology, 1994, 8, 328-329 On the difficulty of detecting density-dependent  

E-print Network

:polyrnorphism; density-dependent selection; Odonata Female colour polymorphism is a prevalent, but perplexing likely than heteromorphs to be ignored by males as potential mates. He suggested that the benefit of avoiding time-consuming supernumerary matings balanced the cost of relatively greater predation

Fincke, Ola M.

236

How to test different density-dependent fecundity hypotheses in an increasing or stable population  

Microsoft Academic Search

Summary 1. We report on a simulation study of increasing and stable populations working under two different hypotheses of density dependence of fecundity: the habitat heterogeneity hypothesis (HHH) and the individual adjustment hypothesis (IAH). Our aim is to find critical differences between the two regulatory hypotheses in natural populations. 2. Populations under HHH show a strong negative relationship between fecundity

MIGUEL FERRER; IAN NEWTON; EVA CASADO

2006-01-01

237

Cannibalism and density-dependent mortality in the wolf spider Pardosa milvina (Araneae: Lycosidae)  

E-print Network

Cannibalism and density-dependent mortality in the wolf spider Pardosa milvina (Araneae: Lycosidae) Christopher M. Buddle, Sean E. Walker, and Ann L. Rypstra Abstract: Cannibalism is an important regulating that cannibalism with wolf spiders (Araneae: Lycosidae) in the genera Schizocosa and Pardosa is common and can act

Walker, Sean E.

238

Host immune responses are necessary for density dependence in nematode infections  

E-print Network

283 Host immune responses are necessary for density dependence in nematode infections S. PATERSON (Received 23 November 2001; revised 23 January and 15 April 2002; accepted 18 April 2002) SUMMARY Nematode a host. These effects act to regulate and stabilize the size of nematode populations. Understanding how

Paterson, Steve

239

Boca-dependent maturation of b-propeller/EGF modules in low-density lipoprotein receptor  

E-print Network

Boca-dependent maturation of b-propeller/EGF modules in low-density lipoprotein receptor proteins ortholog, Mesoderm development, in the mouse. All LDLRs have at least one six-bladed b-propeller domain is specifically required for the maturation of these b-propeller/EGF modules through the secretory pathway

Springer, Timothy A.

240

Density-dependent predation by blue crabs Callinectes sapidus on natural prey populations of infaunal bivalves  

Microsoft Academic Search

We used field and laboratory mesocosm experiments to examine the effects of the func- tional response of the blue crab Callinectes sapidus foraging on Balthic clams Macoma balthica in the upper Chesapeake Bay. Field experiments measured the density-dependent effect of blue crabs on clam patches in both mud and sand substrates at multiple sites spanning the natural range of clam

Mark L. Kuhlmann; Anson H. Hines

2005-01-01

241

TEMPERATURE DEPENDENT CREEP EXPANSION OF Ti-6Al-4V LOW DENSITY CORE SANDWICH STRUCTURES  

E-print Network

TEMPERATURE DEPENDENT CREEP EXPANSION OF Ti-6Al-4V LOW DENSITY CORE SANDWICH STRUCTURES Douglas T The application of lightweight, structurally efficient metal based honeycomb structures has been limited for the low cost manufacture of porous metal sandwich structures. These porous cored sandwich structures

Wadley, Haydn

242

DENSITY-DEPENDENT SEARCHING TIME: IMPLICATIONS IN SURPLUS-PRODUCTION MODELSl  

E-print Network

be incorporated into sUrplus-production models of fisheries. A simple simulation is used to demonstrate the management implications associated with failure to account for this parameter in fisheries where handling and Dickie (1964) model. The failure to measure density-dependent search- ing time in assessing fishing

243

The interplay between climate variability and density dependence in the population viability of Chinook salmon.  

PubMed

The viability of populations is influenced by driving forces such as density dependence and climate variability, but most population viability analyses (PVAs) ignore these factors because of data limitations. Additionally, simplified PVAs produce limited measures of population viability such as annual population growth rate (lamda) or extinction risk. Here we developed a "mechanistic" PVA of threatened Chinook salmon (Oncorhynchus tshawytscha) in which, based on 40 years of detailed data, we related freshwater recruitment of juveniles to density of spawners, and third-year survival in the ocean to monthly indices of broad-scale ocean and climate conditions. Including climate variability in the model produced important effects: estimated population viability was very sensitive to assumptions of future climate conditions and the autocorrelation contained in the climate signal increased mean population abundance while increasing probability of quasi extinction. Because of the presence of density dependence in the model, however we could not distinguish among alternative climate scenarios through mean lamda values, emphasizing the importance of considering multiple measures to elucidate population viability. Our sensitivity analyses demonstrated that the importance of particular parameters varied across models and depended on which viability measure was the response variable. The density-dependent parameter associated with freshwater recruitment was consistently the most important, regardless of viability measure, suggesting that increasing juvenile carrying capacity is important for recovery. PMID:16909672

Zabel, Richard W; Scheuerell, Mark D; McClure, Michelle M; Williams, John G

2006-02-01

244

DENSITY-DEPENDENT FLOW IN ONE-DIMENSIONAL VARIABLY-SATURATED MEDIA  

EPA Science Inventory

A one-dimensional finite element is developed to simulate density-dependent flow of saltwater in variably saturated media. The flow and solute equations were solved in a coupled mode (iterative), in a partially coupled mode (non-iterative), and in a completely decoupled mode. P...

245

Behavioral signature of intraspecific competition and density dependence in colony-breeding marine predators  

E-print Network

Behavioral signature of intraspecific competition and density dependence in colony-breeding marine was supported by the Future of Marine Animal Populations program, Fisheries and Oceans Canada, Dalhousie In populations of colony-breeding marine animals, foraging around colonies can lead to intraspecific competition

Breed, Greg A.

246

Consistent patterns of maturity and density-dependent growth among populations of walleye  

E-print Network

of walleye (Sander vitreus) from Ontario and Quebec, Canada (mean annual GDD = 1200 to 2300 8CÁdays´ provenant de 416 populations de dore´s jaunes (Sander vitreus) de l'Ontario et du Que´bec, Canada (moyenneConsistent patterns of maturity and density- dependent growth among populations of walleye (Sander

Venturelli, Paul

247

Measuring point defect density in individual carbon nanotubes using polarization-dependent X-ray microscopy.  

PubMed

The presence of defects in carbon nanotubes strongly modifies their electrical, mechanical, and chemical properties. It was long thought undesirable, but recent experiments have shown that introduction of structural defects using ion or electron irradiation can lead to novel nanodevices. We demonstrate a method for detecting and quantifying point defect density in individual carbon nanotubes (CNTs) based on measuring the polarization dependence (linear dichroism) of the C 1s --> pi* transition at specific locations along individual CNTs with a scanning transmission X-ray microscope (STXM). We show that STXM can be used to probe defect density in individual CNTs with high spatial resolution. The quantitative relationship between ion dose, nanotube diameter, and defect density was explored by purposely irradiating selected sections of nanotubes with kiloelectronvolt (keV) Ga(+) ions. Our results establish polarization-dependent X-ray microscopy as a new and very powerful characterization technique for carbon nanotubes and other anisotropic nanostructures. PMID:20608685

Felten, Alexandre; Gillon, Xavier; Gulas, Michal; Pireaux, Jean-Jacques; Ke, Xiaoxing; Van Tendeloo, Gustaaf; Bittencourt, Carla; Najafi, Ebrahim; Hitchcock, Adam P

2010-08-24

248

Cosmological density perturbations with a scale-dependent Newton's constant G  

SciTech Connect

We explore possible cosmological consequences of a running Newton's constant G({open_square}), as suggested by the nontrivial ultraviolet fixed point scenario in the quantum field-theoretic treatment of Einstein gravity with a cosmological constant term. In particular, we focus here on what possible effects the scale-dependent coupling might have on large scale cosmological density perturbations. Starting from a set of manifestly covariant effective field equations derived earlier, we systematically develop the linear theory of density perturbations for a nonrelativistic, pressureless fluid. The result is a modified equation for the matter density contrast, which can be solved and thus provides an estimate for the growth index parameter {gamma} in the presence of a running G. We complete our analysis by comparing the fully relativistic treatment with the corresponding results for the nonrelativistic (Newtonian) case, the latter also with a weakly scale-dependent G.

Hamber, Herbert W.; Toriumi, Reiko [Max Planck Institute for Gravitational Physics (Albert Einstein Institute), D-14476 Potsdam (Germany); Department of Physics and Astronomy, University of California, Irvine, California 92697-4575 (United States)

2010-08-15

249

Density dependence of microscopic nucleon optical potential in first order Brueckner theory  

NASA Astrophysics Data System (ADS)

In the present work we apply the lowest order Brueckner theory of infinite nuclear matter to obtain nucleon-nucleus optical potential for p-40Ca elastic scattering at 200 MeV using Urbana V14 soft core internucleon potential. We have investigated the effect of target density on the calculated nucleon-nucleus optical potential. We find that the calculated optical potentials depend quite sensitively on the density distribution of the target nucleus. The important feature is that the real part of calculated central optical potential for all densities shows a wine-bottle-bottom type behaviour at this energy. We also discuss the effect of our new radial dependent effective mass correction. Finally, we compare the prediction of our calculated nucleon optical potential using V14 with the prediction using older hard core Hamada-Johnston internucleon potential for p-40Ca elastic scattering at 200 MeV.

Saliem, S. M.; Haider, W.

2002-06-01

250

turboTDDFT 2.0-Hybrid functionals and new algorithms within time-dependent density-functional perturbation theory  

NASA Astrophysics Data System (ADS)

We present a new release of the turboTDDFT code featuring an implementation of hybrid functionals, a recently introduced pseudo-Hermitian variant of the Liouville-Lanczos approach to time-dependent density-functional perturbation theory, and a newly developed Davidson-like algorithm to compute selected interior eigenvalues/vectors of the Liouvillian super-operator. Our implementation is thoroughly validated against benchmark calculations performed on the cyanin (C21O11H21) molecule using the Gaussian 09 and turboTDDFT 1.0 codes.

Ge, Xiaochuan; Binnie, Simon J.; Rocca, Dario; Gebauer, Ralph; Baroni, Stefano

2014-07-01

251

Coverage Dependence of the Local Density of States at the Fermi Energy: Li Adsorbed on Ru(001)  

NASA Astrophysics Data System (ADS)

The local density of states (LDOS) of Li on a Ru surface has been determined between very low and 0.15 monolayer coverage. ?-decay detected nuclear magnetic resonance has been employed to measure the spin lattice relaxation rate between 200 and 1250 K. The LDOS\\(EF,0\\) = 0.13\\(1\\) eV-1 Å-3, which is 0.6 of the bulk metallic value supporting a view of an at most weak ionic bond. No coverage dependence of the LDOS is seen in the regime investigated. Thus the adsorbate wave functions do not overlap. The LDOS provides a benchmark against which theoretical approaches must be measured.

Jänsch, H. J.; Arnolds, H.; Ebinger, H. D.; Polenz, C.; Polivka, B.; Pietsch, G. J.; Preyss, W.; Saier, V.; Veith, R.; Fick, D.

1995-07-01

252

Long-term demographic analysis in goshawk Accipiter gentilis: the role of density dependence and stochasticity.  

PubMed

Density dependence and environmental stochasticity are both potentially important processes influencing population demography and long-term population growth. Quantifying the importance of these two processes for population growth requires both long-term population as well as individual-based data. I use a 30-year data set on a goshawk Accipiter gentilis population from Eastern Westphalia, Germany, to describe the key vital rate elements to which the growth rate is most sensitive and test how environmental stochasticity and density dependence affect long-term population growth. The asymptotic growth rate of the fully age-structured mean matrix model was very similar to the observed one (0.7% vs. 0.3% per annum), and population growth was most elastic to changes in survival rate at age classes 1-3. Environmental stochasticity led only to a small change in the projected population growth rate (between -0.16% and 0.67%) and did not change the elasticities qualitatively, suggesting that the goshawk life history of early reproduction coupled with high annual fertility buffers against a variable environment. Age classes most crucial to population growth were those in which density dependence seemed to act most strongly. This emphasises the importance of density dependence as a regulatory mechanism in this goshawk population. It also provides a mechanism that might enable the population to recover from population lows, because a mean matrix model incorporating observed functional responses of both vital rates to population density coupled with environmental stochasticity reduced long-term extinction risk of 30% under density-independent environmental stochasticity and 60% under demographic stochasticity to zero. PMID:17356810

Krüger, Oliver

2007-06-01

253

Nesting success of the great-tailed grackle (Cassidix mexicanus prosopidicola) in relation to certain density dependent and density independent factors  

E-print Network

NESTING SUCCESS OF THE GREAT-TAILED GRACKLE (CASSIDIX MEXICANUS PROSOPIDICOLA) IN RELATION TO CERTAIN DENSITY DEPENDENT AND DENSITY INDEPENDENT FACTORS A Thesis by ROBERT FRANCIS GOTIE Submitted to the Graduate College of Texas A&M University... in partial fulfillment of the requirement for the degree of MASTER OF SCIENCE December 1972 Major Subject: Wildlife Science NESTING SUCCESS OF THE GREAT-TAILED GRACKLE (CASSIDIX MEXICANUS PROSOPIDICOLA) IN RELATION TO CERTAIN DENSITY DEPENDENT...

Gotie, Robert Francis

2012-06-07

254

Determination of the surface charge density and temperature dependence of purple membrane by electric force microscopy.  

PubMed

We report here the measurement of the temperature-dependent surface charge density of purple membrane (PM) by using electrostatic force microscopy (EFM). The surface charge density was measured to be 3.4 × 10(5) e/cm(2) at room temperature and reaches the minimum at around 52 °C. The initial decrease of the surface charge density could be attributed to the reduced dipole alignment because of the thermally induced protein mobility in PM. The increase of charge density at higher temperature could be ascribed to the weakened interaction between proteins and the lipids, which leads to the exposure of the charged amino acids. This work could be a benefit to the direct assessment of the structural stability and electric properties of biological membranes at the nanoscale. PMID:23909786

Du, Huiwen; Li, Denghua; Wang, Yibing; Wang, Chenxuan; Zhang, Dongdong; Yang, Yan-lian; Wang, Chen

2013-08-29

255

UC/MALDI-MS analysis of HDL; evidence for density-dependent post-translational modifications  

PubMed Central

The purpose of this study is to determine whether the nature of the post-translational modifications of the major apolipoproteins of HDL is different for density-distinct subclasses. These subclasses were separated by ultracentrifugation using a novel density-forming solute to yield a high-resolution separation. The serum of two subjects, a control with a normolipidemic profile and a subject with diagnosed cardiovascular disease, was studied. Aliquots of three HDL subclasses were analyzed by MALDI and considerable differences were seen when comparing density-distinct subclasses and also when comparing the two subjects. A detailed analysis of the post-translational modification pattern of apoA-1 shows evidence of considerable protease activity, particularly in the more dense fractions. We conclude that part of the heterogeneity of the population of HDL particles is due to density-dependent protease activity. PMID:19050741

Johnson, Jeffery D.; Henriquez, Ronald R.; Tichy, Shane E.; Russell, David H.; McNeal, Catherine J.; Macfarlane, Ronald D.

2007-01-01

256

The nutrient density approach to healthy eating: challenges and opportunities.  

PubMed

The term 'nutrient density' for foods/beverages has been used loosely to promote the Dietary Guidelines for Americans. The 2010 Dietary Guidelines for Americans defined 'all vegetables, fruits, whole grains, fat-free or low-fat milk and milk products, seafood, lean meats and poultry, eggs, beans and peas (legumes), and nuts and seeds that are prepared without added solid fats, added sugars, and sodium' as nutrient dense. The 2010 Dietary Guidelines for Americans further states that nutrient-dense foods and beverages provide vitamins, minerals and other substances that may have positive health effects with relatively few (kilo)calories or kilojoules. Finally, the definition states nutrients and other beneficial substances have not been 'diluted' by the addition of energy from added solid fats, added sugars or by the solid fats naturally present in the food. However, the Dietary Guidelines Advisory Committee and other scientists have failed to clearly define 'nutrient density' or to provide criteria or indices that specify cut-offs for foods that are nutrient dense. Today, 'nutrient density' is a ubiquitous term used in the scientific literature, policy documents, marketing strategies and consumer messaging. However, the term remains ambiguous without a definitive or universal definition. Classifying or ranking foods according to their nutritional content is known as nutrient profiling. The goal of the present commentary is to address the research gaps that still exist before there can be a consensus on how best to define nutrient density, highlight the situation in the USA and relate this to wider, international efforts in nutrient profiling. PMID:25166614

Nicklas, Theresa A; Drewnowski, Adam; O'Neil, Carol E

2014-12-01

257

Unified approach to nuclear densities from exotic atoms  

E-print Network

Parameters of nuclear density distributions are derived from least-squares fits to strong interaction observables in exotic atoms. Global analyses of antiprotonic and pionic atoms show reasonably good agreement between the two types of probes regarding the average behaviour of root-mean-square radii of the neutron distributions. Apparent conflict regarding the shape of the neutron distribution is attributed to different radial sensitivities of these two probes.

E. Friedman

2008-10-10

258

Plot the Dot: A Graphical Approach to Density  

NSDL National Science Digital Library

In this activity, learners work in groups to determine the mass and volume of four samples: glass marbles, steel washers or nuts, pieces of pine wood, and pieces of PVC pipe. Learners then plot the data points on a large class graph of mass vs. volume to discover that data points for a particular material form a straight line, the slope of which gives the density of the material.

Rathjen, Don

2009-01-01

259

Latitudinal Density Dependence of Magnetic Field Lines Inferred from Polar Plasma Wave Data  

NASA Technical Reports Server (NTRS)

Using observations of the electron density, n(sub e), based on measurement of the upper hybrid resonance frequency by the Polar spacecraft Plasma Wave Instrument, we have examined the radial density dependence along field lines in the outer plasmasphere and the near plasmatrough. Sampled L values range from 2.5 to 6.6. Our technique depends on the fact that Polar crosses particular L values at two different points with different radial distance R. In our plasmaspheric data set (n(sub e) > 100/cm3), we find that on average n(sub e) is flat along field lines from the equator up to the latitudes sampled by Polar (R approximately equal to or > 2.0). In the plasmatrough data set (n(sub e) < 100/cm-3), there is on average a mild radial dependence n(sub e) varies as R(exp -1.7).

Goldstein, J.; Denton, R. E.; Hudson, M. K.; Miftakhova, E. G.; Menietti, J. D.; Gallagher, D. L.

2000-01-01

260

Lianas escape self-thinning: Experimental evidence of positive density dependence in temperate lianas Celastrus orbiculatus and C. scandens  

Microsoft Academic Search

The neighborhood density of plants strongly affects their growth, reproduction, and survival. In most cases, high density increases competition and negatively affects a focal plant in predictable ways, leading to the self-thinning law. There are, however, situations in which high densities of plants facilitate focal plant performance, resulting in positive density dependence. Despite their importance in forest gap dynamics and

Stacey A. Leicht-Young; Andrew M. Latimer; John A. Silander

2011-01-01

261

Density  

NSDL National Science Digital Library

What is density? Density is a relationship between mass (usually in grams or kilograms) and volume (usually in L, mL or cm 3 ). Below are several sights to help you further understand the concept of density. Click the following link to review the concept of density. Be sure to read each slide and watch each video: Chemistry Review: Density Watch the following video: Pop density video The following is a fun interactive sight you can use to review density. Your job is #1, to play and #2 to calculate the density of the ...

Hansen, Mr.

2010-10-26

262

Joint effects of density dependence and toxicant exposure on Drosophila melanogaster populations.  

PubMed

Risk assessment of environmental contaminants is usually based on experiments on well-fed individuals held at low population densities. However, field populations are often subject to resource limitation. Individuals who are already stressed by crowding or food limitation may show greater susceptibility to toxicants. But density dependence could also reduce population-level impacts as toxicant-related mortalities may reduce competition for resources. This study examines the joint effects of toxicants and food availability on populations of Drosophila melanogaster. The interactions between the effects of food limitation and toxicant stress were dose dependent and strongly influenced by toxicity mechanisms. In food-limited conditions, a compensatory effect often occurred, with toxicant exposure having a lower proportional impact than at higher food levels. This provides further evidence that density-dependent population processes can produce an assimilative capacity for the effects of toxicants. But synergistic food-toxicant effects were also common and the interaction often switched between synergistic and compensatory at different toxicant concentrations and food supplies. There is no simple "less-than-additive", "additive" or "more-than-additive" relationship between density and toxicant effects, even for a single toxicant. PMID:17658601

Gui, Yue; Grant, Alastair

2008-06-01

263

Thermodynamics and phase behavior of a triangle-well model and density-dependent variety  

NASA Astrophysics Data System (ADS)

A hard sphere+triangle-well potential is employed to test a recently proposed thermodynamic perturbation theory (TPT) based on a coupling parameter expansion. It is found that the second-order term of the coupling parameter expansion surpasses by far that of a high temperature series expansion under a macroscopic compressibility approximation and several varieties. It is also found that the fifth-order version displays best among all of the numerically accessible versions with dissimilar truncation orders. Particularly, the superiority of the fifth-order TPT from other available liquid state theories is exhibited the most incisively when the temperature of interest obviously falls. We investigate the modification of the phase behavior of the hard sphere+triangle-well fluid resulting from a density dependence imposed on the original potential function. It is shown that (1) the density dependence induces polymorphism of fluid phase, particularly liquid-liquid transition in metastable supercooled region, and (2) along with enhanced decaying of the potential function as a function of bulk density, both the liquid-liquid transition and vapor-liquid transition tend to be situated at the domain of lower temperature, somewhat similar to a previously disclosed thumb rule that the fluid phase transition tends to metastable with respect to the fluid-solid transition as the range of the attraction part of a density-independence potential is sufficiently short compared to the range of the repulsion part of the same density-independence potential.

Zhou, Shiqi

2009-01-01

264

Time-Dependent Density Wave Theory for Co-orbital Satellites  

NASA Astrophysics Data System (ADS)

The standard theory of density waves in planetary rings assumes that the orbit of the perturbing satellite is on a fixed orbit. However, the co-orbital satellites, Janus and Epimetheus, trade orbits every four years. Cassini images of Saturn's rings record the time evolution of the density waves excited by these satellites. Ticareno et al. (Ap. J. 651: L65-L68, 2006) attempted to model the observations by assuming the steady-state density waves shift locations in the rings instantaneously when the co-orbital satellites trade orbits. They found that certain wave-like features are observed in the evanescent region of the wave that were not predicted by the steady state theory. We will present the results of a new time-dependent density wave theory that can model the perturbations by the co-orbital satellites during their orbital exchange. Similar to our previous result for temporally modulated density waves excited by the satellite, Pandora (Stewart and Sremcevic, DDA 2008), we find that the effective sound velocity in the wave plays an important role in propagating the time-dependent signal in the wave train. Comparisons with Cassini observations as well as predictions for the next co-orbital exchange event in January 2010 will be presented. This research was supported by NASA's Outer Planets Research Program.

Stewart, Glen R.

2009-05-01

265

Benzodiazepines: Dependence and a Therapeutic Approach to Gradual Withdrawal  

PubMed Central

The extensive use of benzodiazepines in the treatment of anxiety and insomnia has led clinicians to wonder about the dependence potential of these drugs. It is estimated that, in Canada, the United States, and western Europe, between 10% and 20% of the population take tranquilizers or hypnotics (mainly benzodiazepines). Of these, 70% are 50 years of age or older; 15% have been taking these drugs for more than 12 months. One researcher reported that 50% of patients who received a prescription for benzodiazepines renewed it without consulting their physician. In this article, the authors briefly review the concept of dependence, the risk factors, and the clinical signs of a benzodiazepine withdrawal syndrome. They then develop a rational therapeutic approach to make it easier to discontinue these drugs. PMID:21234000

Blais, Diane; Petit, Louise

1990-01-01

266

Ground state of heavy closed shell nuclei: An effective interaction and local density approximation approach  

SciTech Connect

We study the ground-state properties of heavy closed-shell nuclei such as {sup 48}Ca, {sup 90}Zr, {sup 120}Sn, and {sup 208}Pb as well as {sup 4}He, {sup 16}O, and {sup 40}Ca. Similar to our recent work, the local density approximation in the harmonic oscillator basis and different channel-dependent effective two-body interactions that are generated through the lowest-order constrained variational calculation for asymmetric nuclear matter with the Reid68Day, Reid68, and {delta}-Reid68 potentials are used. Unlike nuclear matter, it is shown that Reid68 potential gives ground-state binding energies closer to the experimental data with respect to the {delta}-Reid68 potential and there is not much difference between Reid68 and Reid68Day potentials, which have been define up to J=5. The different channel-dependent effective interactions (J>2) and one- and two-body density distribution functions are discussed and they are compared with the results of other approaches such as the Brueckner local density approximation, correlated basis function, variational fermion hypernetted chain, variational cluster Monte Carlo, Brueckner-Hartree-Fock, fermionic molecular dynamics, and coupled cluster. Finally it is concluded that the three-body force (isobar degrees of freedom) is very important for light (heavy) nuclei because in the most of recent many-body calculations, it is observed that the available two-body nuclear forces usually underbind light nuclei and overbind heavy nuclei and nuclear matter.

Modarres, M.; Rasekhinejad, N. [Physics Department, University of Tehran, 1439955961 Tehran (Iran, Islamic Republic of)

2005-12-15

267

Ignitor and the High Density Approach for Fusion*  

NASA Astrophysics Data System (ADS)

The high plasma density regimes discovered by high magnetic field toroidal experiments have both outstanding confinement characteristics and degree of purity, and are at the basis of the Ignitor design. The main purpose of the Ignitor experiment is, in fact, that of establishing the reactor physics in regimes close to ignition, where the thermonuclear instability can set in with all its associated non linear effects. ``Extended limiter'' and double X-point configurations have been analyzed and relevant transport simulations show that similar burning plasma conditions can be attained with both, by Ohmic heating only or with modest amounts of ICRH auxiliary heating. The driving factor for the machine design (R01.32 m, a xb0.47x0.83 m^2, BT<=13 T, Ip<=11 MA) is the poloidal field pressure that can contain, under macroscopically stable conditions, the peak plasma pressures corresponding to ignition. Objectives other than ignition can be envisioned for the relatively near term, for example that of high flux neutron sources for material testing involving compact, high density fusion machines. This has been one of the incentives that have led the Ignitor Project to adopt magnesium diboride (MgB2) superconducting cables in the machine design, a first in fusion research. Accordingly, the largest coils (about 5 m diameter) of the machine will be made entirely of MgB2 cables. *Sponsored in part by ENEA of Italy and by the U.S. D.O.E.

Bombarda, F.; Coppi, B.

2010-11-01

268

Time-dependent, lattice approach to atomic collisions  

SciTech Connect

Recent progress in developing and applying methods of direct numerical solution of atomic collision problems is described. Various forms of the three-body problem are used to illustrate these techniques. Specifically, the process of ionization in proton-, antiproton-, and electron-impact of atomic hydrogen is considered in applications ranging in computational intensity from collisions simulated in two spatial dimensions to treatment of the three-dimensional, fully correlated two-electron Schroedinger equation. These examples demonstrate the utility and feasibility of treating strongly interacting atomic systems through time-dependent, lattice approaches.

Schultz, D.R. [Oak Ridge National Lab., TN (United States). Physics Div.

1995-12-31

269

Density-dependent self-fertilization and male versus hermaphrodite siring success in an androdioecious plant.  

PubMed

Models of mating-system evolution emphasize the importance of frequency-dependent interactions among mating partners. It is also known that outcross siring success and the selfing rate in self-compatible hermaphrodites can be density dependent. Here, we use array experiments to show that the mating system (i.e., the outcrossing rate) and the siring success of morphs with divergent sex allocation strategies are both density dependent and frequency dependent in androdioecious populations of the wind-pollinated, annual plant Mercurialis annua. In particular, the outcrossing rate is a decreasing function of the mean interplant distance, regulated by a negative exponential pollen fall-off curve. Our results indicate that pollen dispersed from a male inflorescence are over 60% more likely to sire outcrossed progeny than equivalent pollen dispersed from hermaphrodites, likely due to the fact that males, but not hermaphrodites, disperse their pollen from erect inflorescence stalks. Because of this difference, and because males of M. annua produce much more pollen than hermaphrodites, the presence of males in the experimental arrays reduced both the selfing rate and the outcross siring success of hermaphrodites. We use our results to infer a density threshold below which males are unable to persist with hermaphrodites but above which they can invade hermaphroditic populations. We discuss our findings in the context of a metapopulation model, in which males can only persist in well-established populations but are excluded from small, sparse populations, for example, in the early stages of colonization. PMID:17711472

Eppley, Sarah M; Pannell, John R

2007-10-01

270

Simulating Nanofluids via the Weighted Density Lattice Boltzmann Approach  

NASA Astrophysics Data System (ADS)

The structure and transport of neutral and charged fluids under nanoscopic confinement are derived from the kinetic and microscopic perspective. As compared to lumped parameter approaches, the strategy is to resolve the collision between particles for hard-core forces and to use a mean field treatment for soft- and long-ranged forces. The numerical strategy adapts the Lattice Boltzmann (LB) scheme to handle interatomic and electrostatic interactions.

Melchionna, Simone; Marconi, Umberto Marini Bettolo

2013-12-01

271

Quantum-dot density dependence of power conversion efficiency of intermediate-band solar cells  

NASA Astrophysics Data System (ADS)

For intermediate-band solar cells containing GaAs/InAs quantum dots (QDs), the QD density dependence of the power conversion efficiency (PCE) was theoretically calculated for various sun concentrations under AM1.5 conditions based on detailed balance principles. A QD density of over 5 × 1013 cm-2 was required to achieve a PCE of more than 50% under 10 000 suns. However, under the photo-filled state and 1 sun, the PCE decreased over a wide total QD density range from about 3 × 1010 to 1 × 1013 cm-2. This reduction was attributed to the negative net carrier generation rate through the intermediate band, which was due to insufficient two-step optical absorption. The short-circuit current density increased as the QD density increased up to about 1 × 1011 cm-2 and it then saturated. In contrast, the open-circuit voltage decreased with increasing QD density. This reduction in the open-circuit voltage was suppressed at high sun concentrations.

Sakamoto, Katsuyoshi; Kondo, Yasunori; Uchida, Keisuke; Yamaguchi, Koichi

2012-12-01

272

Density  

NSDL National Science Digital Library

Students will explain the concept of and be able to calculate density based on given volumes and masses. Throughout today's assignment, you will need to calculate density. You can find a density calculator at this site. Make sure that you enter the correct units. For most of the problems, grams and cubic centimeters will lead you to the correct answer: Density Calculator What is Density? Visit the following website to answer questions ...

Petersen, Mrs.

2013-10-28

273

Density  

NSDL National Science Digital Library

This page introduces students to the concept of density by presenting its definition, formula, and two blocks representing materials of different densities. Students are given the mass and volume of each block and asked to calculate the density. Their answers are then compared against a table of densities of common objects (air, wood, gold, etc.) and students must determine, using the density of the blocks, which substance makes up each block.

Carpi, Anthony

2003-01-01

274

Density-dependent prophylaxis in the coral-eating crown-of-thorns sea star, Acanthaster planci  

NASA Astrophysics Data System (ADS)

The density-dependent prophylaxis hypothesis predicts that individuals at high density will invest more resources into immune defence than individuals at lower densities as a counter-measure to density-dependent pathogen transmission rates. Evidence has been found for this hypothesis in insects, but not in a non-arthropod taxon. To investigate this hypothesis in the coral-eating crown-of-thorns sea star, Acanthaster planci, density treatments were set up over 21 days, and pathogen infection was simulated with bacterial injection. Five immune responses: amoebocyte count, amoebocyte viability, lysosomal membrane integrity, respiratory burst and peroxidase activity were all upregulated at high density. These results demonstrate that immune investment shows phenotypic plasticity with adult population density in agreement with the density-dependent prophylaxis hypothesis. Here I show that the density-dependent prophylaxis hypothesis is neither dependent on larval density nor restricted to insects, and hence may potentially have important consequences on disease dynamics in any species with widely fluctuating population densities. This is the first demonstration of the density-dependent prophylaxis hypothesis outside arthropods.

Mills, S. C.

2012-06-01

275

Protein Adsorption on Poly(N-isopropylacrylamide) Brushes: dependence on grafting density and chain collapse  

PubMed Central

The protein resistance of poly(N-isopropylacrylamide) brushes grafted from silicon wafers was investigated as a function of the chain molecular weight, grafting density, and temperature. Above the lower critical solution temperature (LCST) of 32°C, the collapse of the water swollen chains, determined by ellipsometry, depends on the grafting density and molecular weight. Ellipsometry, radio assay, and fluorescence imaging demonstrated that, below the lower critical solution temperature, the brushes repel protein as effectively as oligoethylene oxide terminated monolayers. Above 32°C, very low levels of protein adsorb on densely grafted brushes, and the amounts of adsorbed protein increase with decreasing brush grafting densities. Brushes that do not exhibit a collapse transition also bind protein, even though the chains remain extended above the LCST. These findings suggest possible mechanisms underlying protein interactions with end-grafted PNIPAM brushes. PMID:21662243

Xue, Changying; Yonet-Tanyeri, Nihan; Brouette, Nicolas; Sferrazza, Michele; Braun, Paul V.; Leckband, Deborah E.

2012-01-01

276

Optoelectronic Properties of Carbon Nanorings: Excitonic Effects from Time-Dependent Density Functional Theory  

PubMed Central

The electronic structure and size-scaling of optoelectronic properties in cycloparaphenylene carbon nanorings are investigated using time-dependent density functional theory (TDDFT). The TDDFT calculations on these molecular nanostructures indicate that the lowest excitation energy surprisingly becomes larger as the carbon nanoring size is increased, in contradiction with typical quantum confinement effects. In order to understand their unusual electronic properties, I performed an extensive investigation of excitonic effects by analyzing electron-hole transition density matrices and exciton binding energies as a function of size in these nanoring systems. The transition density matrices allow a global view of electronic coherence during an electronic excitation, and the exciton binding energies give a quantitative measure of electron-hole interaction energies in the nanorings. Based on overall trends in exciton binding energies and their spatial delocalization, I find that excitonic effects play a vital role in understanding the unique photoinduced dynamics in these carbon nanoring systems. PMID:22481999

2009-01-01

277

PTPN14 is required for the density-dependent control of YAP1  

PubMed Central

Through an shRNA-mediated loss-of-function screen, we identified PTPN14 as a potential tumor suppressor. PTPN14 interacts with yes-associated protein 1 (YAP1), a member of the hippo signaling pathway. We showed that PTPN14 promotes the nucleus-to-cytoplasm translocation of YAP1 during contact inhibition and thus inhibits YAP1 transactivation activity. Interestingly, PTPN14 protein stability was positively controlled by cell density. We identified the CRL2LRR1 (cullin2 RING ubiquitin ligase complex/leucine-rich repeat protein 1) complex as the E3 ligase that targets PTPN14 for degradation at low cell density. Collectively, these data suggest that PTPN14 acts to suppress cell proliferation by promoting cell density-dependent cytoplasmic translocation of YAP1. PMID:22948661

Wang, Wenqi; Huang, Jun; Wang, Xin; Yuan, Jingsong; Li, Xu; Feng, Lin; Park, Jae-Il; Chen, Junjie

2012-01-01

278

Time-dependent density-functional theory for molecules and molecular solids  

Microsoft Academic Search

Time-dependent density-functional theory (TDDFT) has become a well-established part of the modern theoretical chemist’s toolbox for treating electronic excited states. Yet, though applications of TDDFT abound in quantum chemistry, review articles specifically focusing on TDDFT for chemical applications are relatively rare. This article helps to fill the void by first giving a historical review of TDDFT, with emphasis on molecular

Mark E. Casida

2009-01-01

279

Large-scale noninvasive genetic monitoring of wolverines using scats reveals density dependent adult survival  

Microsoft Academic Search

Noninvasive genetic monitoring has the potential to estimate vital rates essential for conservation and management of many species. In a long-term genetic capture-mark-recapture study using scats we evaluated temporal variation in adult survival in a wolverine (Gulo gulo) population in southern Norway. In contrast to most previous studies of large mammals we found evidence for negative density dependence in adult

Henrik Brøseth; Øystein Flagstad; Cecilia Wärdig; Malin Johansson; Hans Ellegren

2010-01-01

280

Resveratrol inhibits metal ion-dependent and independent peroxidation of porcine low-density lipoproteins  

Microsoft Academic Search

Resveratrol, a phytoalexin (3, 4?, 5, trihydroxystilbene) present in some red wines, has been reported to inhibit copper-mediated low-density lipoprotein (LDL) oxidation. In this study, we examined the efficiency of this compound in inhibiting metal ion-dependent and independent peroxidation of porcine LDL. At 0.5, 1, or 1.5 ?M, transresveratrol prolonged the lag time preceding the onset of conjugated diene formation

Leila Belguendouz; Lucie Fremont; Alain Linard

1997-01-01

281

Temperature and density dependence of the electron Landé g factor in semiconductors  

Microsoft Academic Search

The temperature and density dependence of spin quantum beats of electrons is measured by time-resolved photoluminescence spectroscopy and yields the electron Landé g factor in bulk GaAs, InP, and CdTe. In GaAs the g factor increases linearly from -0.44 at 4 K to -0.30 at 280 K; in InP the g factor is 1.20 at 4 K, exhibiting a very

M. Oestreich; S. Hallstein; A. P. Heberle; K. Eberl; E. Bauser; W. W. Rühle

1996-01-01

282

Effects of acorn abundance on density dependence in a Japanese wood mouse ( Apodemus speciosus ) population  

Microsoft Academic Search

We analysed the effects of Quercus crispula acorn abundance on the density dependence of the large Japanese wood mouse Apodemus speciosus using time series data (1992–2007). The data were obtained in a forest in northern Hokkaido, Japan, by live-trapping rodents\\u000a and directly counting acorns on the ground. Acorn abundance in one year clearly influenced the abundance of wood mice in the

Takashi Saitoh; Jon Olav Vik; Nils Chr. Stenseth; Toshikazu Takanishi; Shintaro Hayakashi; Nobuo Ishida; Masaaki Ohmori; Toshio Morita; Shigeru Uemura; Masahiko Kadomatsu; Jun Osawa; Koji Maekawa

2008-01-01

283

Diminished gradient dependence of density functionals: Constraint satisfaction and self-interaction correction  

Microsoft Academic Search

The Perdew-Burke-Ernzerhof (PBE) generalized gradient approximation for the exchange-correlation energy functional has two nonempirical constructions, based on satisfaction of universal exact constraints on the hole density or on the energy. We show here that, by identifying one possible free parameter in exchange and a second in correlation, we can continue to satisfy these constraints while diminishing the gradient dependence almost

Gábor I. Csonka; Oleg A. Vydrov; Gustavo E. Scuseria; Adrienn Ruzsinszky; John P. Perdew

2007-01-01

284

Parametric dependence of turbulent particle transport in high density electron heated FTU plasmas  

Microsoft Academic Search

In this paper we present the result of a study carried out at the Frascati Tokamak Upgrade (FTU), on a set of full non-inductive current driven, electron heated, L-mode discharges aimed at investigating the parametric dependence of the electron density profile on the electron temperature and safety factor gradients as predicted by quasi-linear drift-turbulence transport theory. Experiments in FTU allow

M. Romanelli; G. T. Hoang; C. Bourdelle; C. Gormezano; E. Giovannozzi; M. Leigheb; M. Marinucci; D. Marocco; C. Mazzotta; L. Panaccione; V. Pericoli; G. Regnoli; O. Tudisco

2007-01-01

285

Density dependence of resonance broadening and shadowing effects in nuclear photoabsorption  

E-print Network

Medium effects as a function of the mass number $A$ are studied in the total photonuclear cross section from the $\\Delta$-resonance region up to the region where shadowing effects are known to exist. A consistent picture is obtained by simply assuming a density dependence of the different mechanisms of resonance broadening and shadowing. The $\\Delta$-mass shift is found to increase with $A$.

S. Boffi; Ye. Golubeva; L. A. Kondratyuk; M. I. Krivoruchenko; E. Perazzi

1996-06-24

286

Overwinter survival in subadult European rabbits: weather effects, density dependence, and the impact of individual characteristics  

Microsoft Academic Search

The survival probability of an individual may be limited by density-dependent mechanisms and by environmental stochasticity, but can also be modified by individual characteristics. In our study, we investigated over-winter survival of subadults of an enclosed European rabbit Oryctolagus cuniculus population in a temperate zone habitat over the period 1992–2002. We: (1) selected for appropriate models to explain individual variation

H. G. Rödel; A. Bora; P. Kaetzke; M. Khaschei; H. Hutzelmeyer; D. von Holst

2004-01-01

287

Long-term demographic analysis in goshawk Accipiter gentilis : the role of density dependence and stochasticity  

Microsoft Academic Search

Density dependence and environmental stochasticity are both potentially important processes influencing population demography\\u000a and long-term population growth. Quantifying the importance of these two processes for population growth requires both long-term\\u000a population as well as individual-based data. I use a 30-year data set on a goshawk Accipiter gentilis population from Eastern Westphalia, Germany, to describe the key vital rate elements to

Oliver Krüger

2007-01-01

288

FOXO3 Growth Inhibition of Colonic Cells Is Dependent on Intraepithelial Lipid Droplet Density*  

PubMed Central

Forkhead transcription factor FOXO3 plays a critical role in suppressing tumor growth, in part, by increasing the cell cycle inhibitor p27kip1, and Foxo3 deficiency in mice results in marked colonic epithelial proliferation. Here, we show in Foxo3-deficient colonic epithelial cells a striking increase in intracytoplasmic lipid droplets (LDs), a dynamic organelle recently observed in human tumor tissue. Although the regulation and function of LDs in non-adipocytes is unclear, we hypothesize that the anti-proliferative effect of FOXO3 was dependent on lowering LD density, thus decreasing fuel energy in both normal and colon cancer cells. In mouse colonic tumors, we found an increased expression of LD coat protein PLIN2 compared with normal colonic epithelial cells. Stimulation of LD density in human colon cancer cells led to a PI3K-dependent loss of FOXO3 and a decrease in the negative regulator of lipid metabolism in Sirtuin6 (SIRT6). Foxo3 deficiency also led to a decrease in SIRT6, revealing the existence of LD and FOXO3 feedback regulation in colonic cells. In parallel, LD-dependent loss of FOXO3 led to its dissociation from the promoter and decreased expression of the cell cycle inhibitor p27kip1. Stimulation of LD density promoted proliferation in colon cancer cells, whereas silencing PLIN2 or overexpression of FOXO3 inhibited proliferation. Taken together, FOXO3 and LDs might serve as new targets for therapeutic intervention of colon cancer. PMID:23603907

Qi, Wentao; Fitchev, Philip S.; Cornwell, Mona L.; Greenberg, Jordan; Cabe, Maleen; Weber, Christopher R.; Roy, Hemant K.; Crawford, Susan E.; Savkovic, Suzana D.

2013-01-01

289

Optical response of C60 fullerene from a Time Dependent Thomas Fermi approach  

E-print Network

We study the collective electron dynamics in C60 clusters within the Time Dependent Thomas Fermi method in the frame of jellium model. The results regarding the optical spectrum are in good agreement with the experimental data, our simulations being able to reproduce both resonances from 20eV and 40eV . We compare also, the results with those from other theoretical approaches and investigate the implications of quantum effects including exchange-correlation corrections, or gradient corrections from a Weizsacker term. The nature of the second resonance is studied using transition densities and phase analysis and interpreted as being a collective surface plasmon

Palade, D I

2014-01-01

290

Variational approach to the time-dependent Schrödinger-Newton equations  

E-print Network

Using a variational approach based on a Lagrangian formulation and Gaussian trial functions, we derive a simple dynamical system that captures the main features of the time-dependent Schr\\"odinger-Newton equations. With little analytical or numerical effort, the model furnishes information on the ground state density and energy eigenvalue, the linear frequencies, as well as the nonlinear long-time behaviour. Our results are in good agreement with those obtained through analytical estimates or numerical simulations of the full Schr\\"odinger-Newton equations.

Giovanni Manfredi; Paul-Antoine Hervieux; Fernando Haas

2012-10-29

291

Polymer density functional approach to efficient evaluation of path integrals.  

PubMed

A polymer density functional theory (P-DFT) has been extended to the case of quantum statistics within the framework of Feynman path integrals. We start with the exact P-DFT formalism for an ideal open chain and adapt its efficient numerical solution to the case of a ring. We show that, similarly, the path integral problem can, in principle, be solved exactly by making use of the two-particle pair correlation function (2p-PCF) for the ends of an open polymer, half of the original. This way the exact data for one-dimensional quantum harmonic oscillator are reproduced in a wide range of temperatures. The exact solution is not, though, reachable in three dimensions (3D) because of a vast amount of storage required for 2p-PCF. In order to treat closed paths in 3D, we introduce a so-called "open ring" approximation which proves to be rather accurate in the limit of long chains. We also employ a simple self-consistent iteration so as to correctly account for the interparticle interactions. The algorithm is speeded up by taking convolutions with the aid of fast Fourier transforms. We apply this approximate path integral DFT (PI-DFT) method to systems within spherical symmetry: 3D harmonic oscillator, atoms of hydrogen and helium, and ions of He and Li. Our results compare rather well to the known data, while the computational effort (some seconds or minutes) is about 100 times less than that with Monte Carlo simulations. Moreover, the well-known "sign problem" is expected to be considerably reduced within the reported PI-DFT, since it allows for a direct estimate of the corresponding partition functions. PMID:16383563

Broukhno, Andrei; Vorontsov-Velyaminov, Pavel N; Bohr, Henrik

2005-10-01

292

Microwave DENSITY—INDEPENDENT Permittivity Functions as Spring Oats Kernels' Moisture Calibrators: a New Approach  

NASA Astrophysics Data System (ADS)

The present work makes use of experimental data for real part of microwave complex permittivity of spring oats (Avena sativa L.) at 2.45 GHz and 24 °C as a function of moisture content, as extracted from the literature. These permittivity data were individually converted to those for solid materials using seven independent mixture equations for effective permittivity of random media. Moisture dependent quadratic models for complex permittivity of spring oats (Avena sativa L.), as developed by the present group, were used to evaluate the dielectric loss factor of spring oats kernels. Using these data, seven density—independent permittivity functions were evaluated and plotted as a function of moisture content of the samples. Second and third order polynomial regression equations were used for curve fittings with these data and their performances are reported. Coefficients of determination (r2) approaching unity (˜ 0.95-0.9999) and very small Standard Deviation (SD) ˜0.001-8.87 show good acceptability for these models. The regularity in the nature of these variations revealed the usefulness of the density—independent permittivity functions as indicators/calibrators of moisture content of spring oats kernels. Keeping in view the fact that moisture content of grains and seeds is an important factor determining quality and affecting the storage, transportation, and milling of grains and seeds, the work has the potentiality of its practical applications.

Das, Priyanka; Ahmad, Zeeshan; Singh, P. N.; Prasad, Ashutosh

2011-11-01

293

Parametric dependence of turbulent particle transport in high density electron heated FTU plasmas  

NASA Astrophysics Data System (ADS)

In this paper we present the result of a study carried out at the Frascati Tokamak Upgrade (FTU), on a set of full non-inductive current driven, electron heated, L-mode discharges aimed at investigating the parametric dependence of the electron density profile on the electron temperature and safety factor gradients as predicted by quasi-linear drift-turbulence transport theory. Experiments in FTU allow the extension of similar studies carried out on other tokamaks to plasmas with higher density and higher magnetic field. Magnetic shear and electron temperature gradients are found to drive opposite turbulent particle flows in the gradient region (0.3 < r/a <= 0.5), while inward thermo-diffusion alone is found in the plasma core (r/a <= 0.3). Density profiles at midradius appear to be controlled by a convective term proportional to the density and independent of the gradient of temperature and magnetic shear. A linear increase in density peaking versus effective collisionality is found, differing from the scaling observed in other FTU plasma regimes.

Romanelli, M.; Hoang, G. T.; Bourdelle, C.; Gormezano, C.; Giovannozzi, E.; Leigheb, M.; Marinucci, M.; Marocco, D.; Mazzotta, C.; Panaccione, L.; Pericoli, V.; Regnoli, G.; Tudisco, O.; FTU Team

2007-06-01

294

The ecological-evolutionary interplay: density-dependent sexual selection in a migratory songbird  

PubMed Central

Little is understood about how environmental heterogeneity influences the spatial dynamics of sexual selection. Within human-dominated systems, habitat modification creates environmental heterogeneity that could influence the adaptive value of individual phenotypes. Here, we used the gray catbird to examine if the ecological conditions experienced in the suburban matrix (SM) and embedded suburban parks (SP) influence reproductive strategies and the strength of sexual selection. Our results show that these habitats varied in a key ecological factor, breeding density. Moreover, this ecological factor was closely tied to reproductive strategies such that local breeding density predicted the probability that a nest would contain extra-pair offspring. Partitioning reproductive variance showed that while within-pair success was more important in both habitats, extra-pair success increased the opportunity for sexual selection by 39% at higher breeding densities. Body size was a strong predictor of relative reproductive success and was under directional selection in both habitats. Importantly, our results show that the strength of sexual selection did not differ among habitats at the landscape scale but rather that fine-scale variation in an ecological factor, breeding density, influenced sexual selection on male phenotypes. Here, we document density-dependent sexual selection in a migratory bird and hypothesize that coarse-scale environmental heterogeneity, in this case generated by anthropogenic habitat modification, changed the fine-scale ecological conditions that drove the spatial dynamics of sexual selection. PMID:22837842

Ryder, Thomas B; Fleischer, Robert C; Shriver, W Greg; Marra, Peter P

2012-01-01

295

Density-dependent fitness benefits in quorum-sensing bacterial populations  

PubMed Central

It has been argued that bacteria communicate using small diffusible signal molecules to coordinate, among other things, the production of factors that are secreted outside of the cells in a process known as quorum sensing (QS). The underlying assumption made to explain QS is that the secretion of these extracellular factors is more beneficial at higher cell densities. However, this fundamental assumption has never been tested experimentally. Here, we directly test this by independently manipulating population density and the induction and response to the QS signal, using the opportunistic pathogen Pseudomonas aeruginosa as a model organism. We found that the benefit of QS was relatively greater at higher population densities, and that this was because of more efficient use of QS-dependent extracellular “public goods.” In contrast, the benefit of producing “private goods,” which are retained within the cell, does not vary with cell density. Overall, these results support the idea that QS is used to coordinate the switching on of social behaviors at high densities when such behaviors are more efficient and will provide the greatest benefit. PMID:22566647

Darch, Sophie E.; West, Stuart A.; Winzer, Klaus; Diggle, Stephen P.

2012-01-01

296

Non-swarming grasshoppers exhibit density-dependent phenotypic plasticity reminiscent of swarming locusts.  

PubMed

Locusts are well known for exhibiting an extreme form of density-dependent phenotypic plasticity known as locust phase polyphenism. At low density, locust nymphs are cryptically colored and shy, but at high density they transform into conspicuously colored and gregarious individuals. Most of what we know about locust phase polyphenism come from the study of the desert locust Schistocerca gregaria (Forskål), which is a devastating pest species affecting many countries in North Africa and the Middle East. The desert locust belongs to the grasshopper genus Schistocerca Stål, which includes mostly non-swarming, sedentary species. Recent phylogenetic studies suggest that the desert locust is the earliest branching lineage within Schistocerca, which raises a possibility that the presence of density-dependent phenotypic plasticity may be a plesiomorphic trait for the whole genus. In order to test this idea, we have quantified the effect of rearing density in terms of the resulting behavior, color, and morphology in two non-swarming Schistocerca species native to Florida. When reared in both isolated and crowded conditions, the two non-swarming species, Schistocerca americana (Drury) and Schistocerca serialis cubense (Saussure) clearly exhibited plastic reaction norms in all traits measured, which were reminiscent of the desert locust. Specifically, we found that both species were more active and more attracted to each other when reared in a crowded condition than in isolation. They were mainly bright green in color when isolated, but developed strong black patterns and conspicuous background colors when crowded. We found a strong effect of rearing density in terms of size. There were also more mechanoreceptor hairs on the outer face of the hind femora in the crowded nymphs in both species. Although both species responded similarly, there were some clear species-specific differences in terms of color and behavior. Furthermore, we compare and contrast our findings with those on the desert locust and other relevant studies. We attribute the presence of density-dependent phenotypic plasticity in the non-swarming Schistocerca species to phylogenetic conservatism, but there may be a possible role of local adaptation in further shaping the ultimate expressions of plasticity. PMID:24035748

Gotham, Steven; Song, Hojun

2013-11-01

297

A density functional approach to many-body effects in the optical response of atoms  

Microsoft Academic Search

A method for calculating the optical response of finite electronic system which is accurate, computationally simple and lends itself to a ready physical interpretation of the results is presented. A time dependent local density approximation (TDLDA) represents a natural generalization of the usual local density approximation to the ground state properties of a many electron system. Using standard first order

A. Zangwill

1981-01-01

298

Particle-number restoration within the energy density functional formalism: Nonviability of terms depending on noninteger powers of the density matrices  

Microsoft Academic Search

We discuss the origin of pathological behaviors that have been recently identified in particle-number-restoration calculations performed within the nuclear energy density functional framework. A regularization method that removes the problematic terms from the multi reference energy density functional and that applies (i) to any symmetry-restoration- and\\/or generator-coordinate-method-based configuration mixing calculation and (ii) to energy density functionals depending only on integer

T. Duguet; M. Bender; K. Bennaceur; D. Lacroix; T. Lesinski

2009-01-01

299

Sex modulates approach systems and impulsivity in substance dependence  

PubMed Central

Background Personality traits such as pathological engagement in approach behaviors, high levels of impulsivity and heightened negative affect are consistently observed in substance dependent individuals (SDI). The clinical course of addiction has been shown to differ between sexes. For example, women increase their rates of consumption of some drugs of abuse more quickly than men. Despite the potential influence of personality and sex on features of addiction, few studies have investigated the interaction of these factors in substance dependence. Methods Fifty-one SDI (26 male, 25 female) and 66 controls (41 male, 25 female) completed the Behavioral Inhibition/Behavioral Activation System (BIS/BAS) Scales, the Barratt Impulsiveness Scale, and the Positive and Negative Affect Schedule (PANAS-X). Data were analyzed with 2×2 ANCOVAs testing for main effects of group, sex and group by sex interactions, adjusting for education level. Results Significant group by sex interactions were observed for BAS scores [F(1,116)=7.03, p<.01] and Barratt Motor Impulsiveness [F(1,116)=6.11, p<.02] with female SDI showing the highest approach tendencies and impulsivity followed by male SDI, male controls, and finally female controls. SDI scored higher on negative affect [F(1,116)=25.23, p<.001] than controls. Behavioral Inhibition System scores were higher in women than men [F(1,116)=14.03, p< .001]. Conclusion Higher BAS and motor impulsivity in SDI women relative to SDI men and control women suggest that personality traits that have been previously associated with drug use may be modulated by sex. These factors may contribute to differences in the disease course observed in male compared to female drug users. PMID:23725607

Perry, Robert I.; Krmpotich, Theodore; Thompson, Laetitia L.; Mikulich-Gilbertson, Susan K.; Banich, Marie T.; Tanabe, Jody

2013-01-01

300

Stocking density affects the growth performance of broilers in a sex-dependent fashion.  

PubMed

The effects of stocking density, sex, and dietary ME concentration on live performance, footpad burns, and leg weakness of broilers were investigated. A total of 876 male and 1,020 female 1-d-old chicks were placed in 24 pens to simulate final stocking density treatments of 26 kg (LSD; 10 males or 12 females/m(2)) and 42 kg (HSD; 16 males or 18 females/m(2)) of BW/m(2) floor space. Two series of experimental diets with a 150 kcal/kg difference in ME concentration (2,800, 2,900, and 3,000 or 2,950, 3,050, and 3,150 kcal of ME/kg) were compared in a 3-phase feeding program. The HSD treatment significantly decreased BW gain and feed conversion ratio (FCR). The HSD chickens consumed less feed by 35 d of age; thereafter, the reverse was true. Male chickens had significantly higher feed intake (FI), BW gain, and FCR compared with females. A significant interaction was found of stocking density and age for FI, BW gain, and FCR. Compared with LSD treatment, HSD broilers had a higher FI and a lower FCR from 36 to 42 d of age. Stocking density, sex, and age had a significant interaction for BW gain and FCR. Female broilers had worse BW gain and FCR when stocked at high density from 36 to 42 d of age. Stocking density had no significant influence on breast, thigh, or abdominal fat yield. Female broilers had significantly higher breast yield and abdominal fat. Male broilers and HSD treatment had high footpad burn and gait scores. A low ME diet increased footpad burn score but had no effect on gait score. The result indicated that stocking density had a more severe effect on the growth of male broilers before 35 d of age. Female broilers need more space than males at similar BW per square meter near marketing age. The incidence and severity of leg weakness are associated with sex, diet, and stocking density. This result suggests that the deteriorated effect of high stocking density is sex and age dependent. PMID:21673155

Zuowei, S; Yan, L; Yuan, L; Jiao, H; Song, Z; Guo, Y; Lin, H

2011-07-01

301

Nalmefene: a new approach to the treatment of alcohol dependence.  

PubMed

Reduction of alcohol consumption is not yet a widely accepted treatment objective for alcohol-dependent patients, as abstinence is often considered to be the only possible objective in this situation. However, various studies have demonstrated the value of proposing these two options to such patients. Firstly, reduction of alcohol consumption very significantly reduces the risk of alcohol-related damage, and also modifies the patient's and the doctor's perception of the disease, resulting in improved access to care and better patient adherence with the proposed treatment objective and consequently better clinical results. Recent studies have shown that some medicinal products can help patients reduce their alcohol consumption. One such product, nalmefene, has been granted European marketing authorization and is now being released onto the market in various countries. The ESENSE 1 and 2 studies in alcohol-dependent patients showed that, in combination with BRENDA, a psychosocial intervention focusing on reinforcement of motivation and treatment adherence, nalmefene significantly reduced the number of heavy drinking days and mean daily total alcohol consumption versus placebo. This reduction was more marked in the marketing authorization target population, ie, patients with a high or very high drinking risk level according to World Health Organization criteria. Another original feature of this molecule is that it can be used as needed if the patient perceives a risk of drinking, which is a more flexible approach and more likely to ensure the patient's active involvement in the treatment of his/her disease. This molecule opens up interesting and original therapeutic prospects in the treatment of alcohol dependence. PMID:25187751

Paille, François; Martini, Hervé

2014-01-01

302

Evaluating Systematic Dependencies of Type Ia Supernovae: The Influence of Deflagration to Detonation Density  

NASA Astrophysics Data System (ADS)

We explore the effects of the deflagration to detonation transition density on the production of 56Ni in thermonuclear supernova explosions (type Ia supernovae). The transition density indirectly sets the amount of expansion during the deflagration phase of the explosion and therefore the amount of Fe-group material produced. A fraction of the Fe-group material will be radioactive 56Ni that powers the supernova lightcurve. We employ a theoretical framework for a well-controlled statistical study of two-dimensional simulations of thermonuclear supernovae with randomized initial conditions that produce 56Ni masses with a similar average and range to those inferred from observations (Townsley et al. 2009) for a fiducial transition density of 107 g cm-3. Within this framework, we utilize a realistic 'simmered' white dwarf progenitor model and a detailed flame model and energetics scheme to calculate the amount of Fe-group material synthesized for a suite of simulated explosions in which the transition density is varied. Understanding the effect of transition density on the amount of Fe-group material produced in combination with the effect of 22Ne (and metallicity) on transition density allows us to construct the functional dependence of the amount of Fe-group material and 56Ni mass synthesized in the explosion on metallicity through the 22Ne content. This work was supported by NASA under grant No. NNX09AD19G and utilized resources at the New York Center for Computational Sciences at Stony Brook University/Brookhaven National Laboratory, which is supported by the U.S. Department of Energy under Contract No. DE-AC02-98CH10886 and by the State of New York.

Jackson, Aaron P.; Calder, A. C.; Townsley, D. M.; Chamulak, D. A.; Brown, E. F.; Timmes, F. X.

2010-01-01

303

Differential tolerance to direct and indirect density-dependent costs of viral infection in Arabidopsis thaliana.  

PubMed

Population density and costs of parasite infection may condition the capacity of organisms to grow, survive and reproduce, i.e. their competitive ability. In host-parasite systems there are different competitive interactions: among uninfected hosts, among infected hosts, and between uninfected and infected hosts. Consequently, parasite infection results in a direct cost, due to parasitism itself, and in an indirect cost, due to modification of the competitive ability of the infected host. Theory predicts that host fitness reduction will be higher under the combined effects of costs of parasitism and competition than under each factor separately. However, experimental support for this prediction is scarce, and derives mostly from animal-parasite systems. We have analysed the interaction between parasite infection and plant density using the plant-parasite system of Arabidopsis thaliana and the generalist virus Cucumber mosaic virus (CMV). Plants of three wild genotypes grown at different densities were infected by CMV at various prevalences, and the effects of infection on plant growth and reproduction were quantified. Results demonstrate that the combined effects of host density and parasite infection may result either in a reduction or in an increase of the competitive ability of the host. The two genotypes investing a higher proportion of resources to reproduction showed tolerance to the direct cost of infection, while the genotype investing a higher proportion of resources to growth showed tolerance to the indirect cost of infection. Our findings show that the outcome of the interaction between host density and parasitism depends on the host genotype, which determines the plasticity of life-history traits and consequently, the host capacity to develop different tolerance mechanisms to the direct or indirect costs of parasitism. These results indicate the high relevance of host density and parasitism in determining the competitive ability of a plant, and stress the need to simultaneously consider both factors to understand the selective pressures that drive host-parasite co-evolution. PMID:19649316

Pagán, Israel; Alonso-Blanco, Carlos; García-Arenal, Fernando

2009-07-01

304

Density-dependent reduction and induction of milkweed cardenolides by a sucking insect herbivore.  

PubMed

The effect of aphid population size on host-plant chemical defense expression and the effect of plant defense on aphid population dynamics were investigated in a milkweed-specialist herbivore system. Density effects of the aposematic oleander aphid, Aphis nerii, on cardenolide expression were measured in two milkweed species, Asclepias curassavica and A. incarnata. These plants vary in constitutive chemical investment with high mean cardenolide concentration in A. curassavica and low to zero in A. incarnata. The second objective was to determine whether cardenolide expression in these two host plants impacts mean A. nerii colony biomass (mg) and density. Cardenolide concentration (microgram/g) of A. curassavica in both aphid-treated leaves and opposite, herbivore-free leaves decreased initially in comparison with aphid-free controls, and then increased significantly with A. nerii density. Thus, A. curassavica responds to aphid herbivory initially with density-dependent phytochemical reduction, followed by induction of cardenolides to concentrations above aphid-free controls. In addition, mean cardenolide concentration of aphid-treated leaves was significantly higher than that of opposite, herbivore-free leaves. Therefore, A. curassavica induction is strongest in herbivore-damage tissue. Conversely, A. incarnata exhibited no such chemical response to aphid herbivory. Furthermore, neither host plant responded chemically to herbivore feeding duration time (days) or to the interaction between herbivore initial density and feeding duration time. There were also no significant differences in mean colony biomass or population density of A. nerii reared on high cardenolide (A. curassavica) and low cardenolide (A. incarnata) hosts. PMID:15139307

Martel, John W; Malcolm, Stephen B

2004-03-01

305

Invasive leaf resources alleviate density dependence in the invasive mosquito, Aedes albopictus  

PubMed Central

Interactions between invasive species can have important consequences for the speed and impact of biological invasions. Containers occupied by the invasive mosquito, Aedes albopictus Skuse, may be sensitive to invasive plants whose leaves fall into this larval habitat. To examine the potential for interactions between invasive leaf species and larval A. albopictus, we conducted a field survey of leaf material found with A. albopictus in containers in Palm Beach County, Florida and measured density dependent responses of A. albopictus larvae to two invasive and one native leaf species in laboratory experiments. We found increased diversity of leaf species, particularly invasive species, in areas further from the urbanized coast, and a significant positive association between the presence of Schinus terebinthifolious (Brazilian pepper) and the abundance of A. albopictus. In laboratory experiments, we determined that larval growth and survivorship were significantly affected by both larval density and leaf species which, in turn, resulted in higher population performance on the most abundant invasive species (Brazilian pepper) relative to the most abundant native species, Quercus virginiana (live oak). These results suggest invasive leaf species can alleviate density dependent reductions in population performance in A. albopictus, and may contribute to its invasion success and potential to spread infectious disease. PMID:22523473

Zarrabi, Ali A.; Lounibos, L. Philip

2012-01-01

306

Low-magnetic-field dependence and anisotropy of the critical current density in coated conductors  

NASA Astrophysics Data System (ADS)

Many applications of ReBCO-coated conductors operate at low magnetic fields in the superconductor (below 200 mT). In order to predict the critical current and AC loss in these applications, it is necessary to know the anisotropy and field dependence of the critical current density at low magnetic fields. In this paper, we obtain a formula for the critical current density in a coated conductor as a function of the local magnetic field and its orientation. Afterwards, we apply this formula to predict the critical current of a pancake coil that we constructed. We extract the critical current density of the tape from measurements of the in-field critical current at several orientations. Numerical simulations correct the effect of the self-field in the measurements and successfully predict the critical current in the pancake coil. We found that a simple elliptical model is not enough to describe the anisotropy of the critical current density. In conclusion, the analytical fit that we present is useful to predict the critical current of actual coils. Therefore, it may also be useful for other structures made of coated conductor, like power-transmission cables, Roebel cables and resistive fault current limiters.

Pardo, E.; Vojen?iak, M.; Gömöry, F.; Šouc, J.

2011-06-01

307

Is the prevalence of Taenia taeniaeformis in Microtus arvalis dependent on population density?  

PubMed

Populations of common voles Microtus arvalis were studied as hosts of the tapeworm Taenia taeniaeformis during a 14-yr survey. They were monitored in spring, summer, and autumn in a pastoral ecosystem in eastern France. A total of 7,574 voles were sampled during 2 multiannual population fluctuations. A third fluctuation was sampled during the increase phase only. Overall prevalence was lowest in summer (0.6%), increased by 3 times in autumn (1.5%) and a further 5 times in spring (7.8%). Analysis of prevalence, based on 7,384 voles, by multiple logistic regression revealed that extrinsic factors such as season and intrinsic factors such as host age and host density have a combined effect. In the longer term, host age and host density were positively associated with prevalence in summer. Host density was strongly associated with autumn prevalence. There was no association between the fluctuation phase and prevalence. The study shows that a long timescale (here a multiannual survey) is necessary to demonstrate the positive effect of host density on the prevalence of this indirectly transmitted parasite. The demonstration of this relationship depends on the rodents being intermediate rather than definitive hosts. PMID:14740902

Fichet-Calvet, Elisabeth; Giraudoux, Patrick; Quéré, Jean-Pierre; Ashford, Richard William; Delattre, Pierre

2003-12-01

308

Dependence of light attenuation and backscattering on collagen concentration and chondrocyte density in agarose scaffolds  

NASA Astrophysics Data System (ADS)

Optical coherence tomography (OCT) has been applied for high resolution imaging of articular cartilage. However, the contribution of individual structural elements of cartilage on OCT signal has not been thoroughly studied. We hypothesize that both collagen and chondrocytes, essential structural components of cartilage, act as important light scatterers and that variation in their concentrations can be detected by OCT through changes in backscattering and attenuation. To evaluate this hypothesis, we established a controlled model system using agarose scaffolds embedded with variable collagen concentrations and chondrocyte densities. Using OCT, we measured the backscattering coefficient (µb) and total attenuation coefficient (µt) in these scaffolds. Along our hypothesis, light backscattering and attenuation in agarose were dependent on collagen concentration and chondrocyte density. Significant correlations were found between µt and chondrocyte density (? = 0.853, p < 0.001) and between µt and collagen concentration (? = 0.694, p < 0.001). µb correlated significantly with chondrocyte density (? = 0.504, p < 0.001) but not with collagen concentration (? = 0.103, p = 0.422) of the scaffold. Thus, quantitation of light backscattering and, especially, attenuation could be valuable when evaluating the integrity of soft tissues, such as articular cartilage with OCT.

Puhakka, P. H.; Ylärinne, J. H.; Lammi, M. J.; Saarakkala, S.; Tiitu, V.; Kröger, H.; Virén, T.; Jurvelin, J. S.; Töyräs, J.

2014-11-01

309

Temperature-Dependent Maximum Density of 1D Excitons in Carbon Nanotubes  

NASA Astrophysics Data System (ADS)

Previous studies have shown that an upper limit exists on the density of 1D excitons in single-walled carbon nanotubes (SWNTs) due to very efficient exciton-exciton annihilation (EEA). A recent theoretical study based on a dark-bright two-band exciton model predicts that there is a temperature at which the achievable exciton density will be maximized, surpassing the room-temperature upper limit. Therefore, we performed temperature-dependent (300 K to 11 K) photoluminescence (PL) on HiPco SWNTs embedded in an i-carrageenan matrix under high resonant excitation. To achieve high densities, we used pump fluences up to ˜ 10^14 photons/cm^2, utilizing intense fs pulses from a wavelength-tunable optical parametric amplifier. We found that for each temperature the PL intensity saturates as a function of pump fluence and the saturation intensity increases from 300 K to a moderate temperature around 100-150 K. Below that critical temperature, the PL intensity decreases with decreasing temperature. Within the framework of diffusion-limited EEA, we successfully estimated the upper limit of the density of 1D excitons in SWNTs as a function of temperature and chirality

Searles, Thomas; Walsh, Ian; Nosaka, Takayuki; Rice, William; Kono, Junichiro

2011-03-01

310

Dependence of light attenuation and backscattering on collagen concentration and chondrocyte density in agarose scaffolds.  

PubMed

Optical coherence tomography (OCT) has been applied for high resolution imaging of articular cartilage. However, the contribution of individual structural elements of cartilage on OCT signal has not been thoroughly studied. We hypothesize that both collagen and chondrocytes, essential structural components of cartilage, act as important light scatterers and that variation in their concentrations can be detected by OCT through changes in backscattering and attenuation. To evaluate this hypothesis, we established a controlled model system using agarose scaffolds embedded with variable collagen concentrations and chondrocyte densities. Using OCT, we measured the backscattering coefficient (µb) and total attenuation coefficient (µt) in these scaffolds. Along our hypothesis, light backscattering and attenuation in agarose were dependent on collagen concentration and chondrocyte density. Significant correlations were found between µt and chondrocyte density (? = 0.853, p < 0.001) and between µt and collagen concentration (? = 0.694, p < 0.001). µb correlated significantly with chondrocyte density (? = 0.504, p < 0.001) but not with collagen concentration (? = 0.103, p = 0.422) of the scaffold. Thus, quantitation of light backscattering and, especially, attenuation could be valuable when evaluating the integrity of soft tissues, such as articular cartilage with OCT. PMID:25310088

Puhakka, P H; Ylärinne, J H; Lammi, M J; Saarakkala, S; Tiitu, V; Kröger, H; Virén, T; Jurvelin, J S; Töyräs, J

2014-11-01

311

Equilibrium Sampling Approach to the Interpretation of Electron Density Maps Andreas Vitalis,1,  

E-print Network

microscopy is an active field of research. Here, we introduce and evaluate an approach relying1 Equilibrium Sampling Approach to the Interpretation of Electron Density Maps Andreas Vitalis,1 rigid components. For the most difficult system studied, we highlight the importance of advanced data

Caflisch, Amedeo

312

A quadrature-based LES/transported probability density function approach for  

E-print Network

unstable in highly compressible shock-containing flows. In this work, a novel Eulerian approach called. The DQMOM approach is then used to simulate two practical flow configurations: a supersonic reacting jet is the transported probability density function (PDF) method [8]. Here, the joint-PDF of the gas phase thermochemical

Raman, Venkat

313

Approach for control of high-density plasma reactors through optimal pulse shaping*  

E-print Network

Approach for control of high-density plasma reactors through optimal pulse shaping* Tyrone L and operation of glow-discharge plasma process reactors. Several approaches have been proposed recently, one of which is the operation of plasma reactors in the pulsed mode where the power input to the reactor

Raja, Laxminarayan L.

314

Testing density-dependent groundwater models: two-dimensional steady state unstable convection in infinite, finite and inclined porous layers  

Microsoft Academic Search

This study proposes the use of several problems of unstable steady state convection with variable fluid density in a porous layer of infinite horizontal extent as two-dimensional (2-D) test cases for density-dependent groundwater flow and solute transport simulators. Unlike existing density-dependent model benchmarks, these problems have well-defined stability criteria that are determined analytically. These analytical stability indicators can be compared

Douglas Weatherill; Craig T Simmons; Clifford I Voss; Neville I Robinson

2004-01-01

315

Monte Carlo study of voxel S factor dependence on tissue density and atomic composition  

NASA Astrophysics Data System (ADS)

Voxel dosimetry is a common approach to the internal dosimetry of non-uniform activity distributions in nuclear medicine therapies with radiopharmaceuticals and in the estimation of the radiation hazard due to internal contamination of radionuclides. Aim of the present work is to extend our analytical approach for the calculation of voxel S factors to materials different from the soft tissue. We used a Monte Carlo simulation in GEANT4 of a voxelized region of each material in which the source of monoenergetic electrons or photons was uniformly distributed within the central voxel, and the energy deposition was scored over the surrounding 11×11×11 voxels. Voxel S factors were obtained for the following standard ICRP materials: Adipose tissue, Bone cortical, Brain, Lung, Muscle skeletal and Tissue soft with 1 g cm-3 density. Moreover, we considered the standard ICRU materials: Bone compact and Muscle striated. Voxel S factors were represented as a function of the “normalized radius”, defined as the ratio between the source-target voxel distance and the voxel side. We found that voxel S factors and related analytical fit functions are mainly affected by the tissue density, while the material composition gives only a slight contribution to the difference between data series, which is negligible for practical purposes. Our results can help in broadening the dosimetric three-dimensional approach based on voxel S factors to other tissues where diagnostic and therapeutic radionuclides can be taken up and radiation can propagate.

Amato, Ernesto; Italiano, Antonio; Baldari, Sergio

2013-11-01

316

Dynamical response function in sodium and aluminum from time-dependent density-functional theory  

NASA Astrophysics Data System (ADS)

We present a detailed study of the dynamical electronic response in bulk sodium and aluminum within time-dependent density-functional theory (TDDFT). The poor results of the random-phase approximation (RPA) and the time-dependent local-density approximation (TDLDA) in sodium are greatly improved by the approximate inclusion of the finite lifetimes of electrons and holes via a modified independent-particle polarizability, which brings the calculated spectra into good agreement with experiment. For aluminum the changes are less visible, but at some values of momentum-transfer lifetime effects are necessary to obtain qualitatively correct spectra. The double-peak structure in aluminum, induced by band-structure effects, is partially washed out by the inclusion of the finite lifetimes. The latter do not, however, create a double peak by themselves as they do in the case of the homogeneous electron gas. Studying the performance of different time-dependent and nonlocal TDDFT kernels, we conclude that the Gross-Kohn, Corradini , and the Hubbard local-field factors improve the spectra compared to the RPA results. However, the results agree less well with experiment than those obtained using TDLDA with added lifetime effects. These results apply to both the loss spectra and the plasmon dispersion.

Cazzaniga, Marco; Weissker, Hans-Christian; Huotari, Simo; Pylkkänen, Tuomas; Salvestrini, Paolo; Monaco, Giulio; Onida, Giovanni; Reining, Lucia

2011-08-01

317

Photoabsorption spectra from adiabatically exact time-dependent density-functional theory in real time.  

PubMed

Photoabsorption spectra for 2-electron singlet systems are obtained from the real-time propagation of the time-dependent Kohn-Sham equations in the adiabatically exact approximation. The latter is provided by the exact ground state exchange-correlation potential corresponding to the instantaneous density. The results are compared to exact data obtained from the solution of the interacting Schrödinger equation. We find that the adiabatically exact approximation provides very good results for transitions of genuinely single excitation character but yields incorrect results if double excitations contribute substantially. However, the extent of the error can vary: some double excitations are just shifted in energy whereas others are missed completely. These situations are analyzed with the help of transition densities. PMID:19475184

Thiele, Mark; Kümmel, Stephan

2009-06-14

318

Dependence of the cosmic microwave background lensing power spectrum on the matter density  

NASA Astrophysics Data System (ADS)

The anisotropies in the cosmic microwave background (CMB) provide our best laboratory for testing models of the formation and evolution of large-scale structure. The rich features in the CMB anisotropy spectrum, in combination with highly precise observations and theoretical predictions, also allow us to simultaneously constrain a number of cosmological parameters. As observations have progressed, measurements at smaller angular scales have provided increasing leverage. These smaller angular scales provide sensitive measures of the matter density through the effect of gravitational lensing. In this work, we provide an analytic explanation of the manner in which the lensing of CMB anisotropies depends on the matter density, finding that the dominant effect comes from the shape of the matter power spectrum set by the decay of small-scale potentials between horizon crossing and matter-radiation equality.

Pan, Z.; Knox, L.; White, M.

2014-12-01

319

High-performance algorithm to calculate spin- and parity-dependent nuclear level densities  

SciTech Connect

A new algorithm for calculating the spin- and parity-dependent shell-model nuclear level densities using the moments method in the proton-neutron formalism is presented. A new, parallelized code based on this algorithm was developed and tested using up to 4000 cores for a set of nuclei from the sd-, pf-, and pf+g{sub 9/2}-model spaces. By comparing the nuclear level densities at low excitation energy for a given nucleus calculated in two model spaces, such as pf and pf+g{sub 9/2}, one can estimate the ground-state energy in the larger model space, which is not accessible to direct shell-model calculations due to the unmanageable dimension. Examples for the ground-state energies of for {sup 64}Ge and {sup 68}Se in the pf+g{sub 9/2} model space are presented.

Sen'kov, R. A.; Horoi, M. [Department of Physics, Central Michigan University, Mount Pleasant, Michigan 48859 (United States)

2010-08-15

320

Density-dependent Lectin-Glycan Interactions as a Paradigm for Conditional Regulation by Posttranslational Modifications*  

PubMed Central

Mice with null mutations in specific Golgi glycosyltransferases show evidence of glycan compensation where missing carbohydrate epitopes are found on biosynthetically related structures. Repetitive saccharide sequences within the larger glycan structures are functional epitopes recognized by animal lectins. These studies provide the first in vivo support for the existence of a feedback system that maintains and regulates glycan epitope density in cells. Receptor regulation by lectin–glycan interactions and the Golgi provides a mechanism for the adaptation of cell surface receptors and solute transporters in response to environmental cues and intracellular signaling. We suggest that other posttranslational modification systems might have similar conditional features regulated by density-dependent ligand–epitope interactions. PMID:23378517

Dennis, James W.; Brewer, C. Fred

2013-01-01

321

Photoabsorption and photoionization dynamics study of silicon tetrafluoride in the framework of time-dependent density-functional theory  

SciTech Connect

Photoionization cross sections and angular distributions of silicon tetrafluoride have been calculated in the framework of the time-dependent density-functional theory approach. Both valence and inner shell ionizations have been considered in an extended photon energy range. Calculations have been carried out by using two different exchange-correlation (xc) potentials characterized by the correct asymptotic behavior. Theoretical results obtained with both the van Leeuwen-Baerends and statistical average of orbital potentials (SAOP) xc potentials are compared with photoabsorption, photoionization, and electron-scattering experiments as well as with previous theoretical calculations. It is suggested that even if both xc potentials provide a reasonably good description of the photoionization dynamics, correlation effects are phenomenologically better accounted for by the SAOP xc potential. Overall, the good accuracy attained with the linear combination of atomic orbitals-time dependent density-functional theory method in reproducing the experimental findings for SiF{sub 4} makes it a promising and powerful method for the characterization of the photoionization dynamics from medium and large-size molecules.

Toffoli, D. [Dipartimento di Scienze Chimiche, Universita degli Studi di Trieste, Via L. Giorgieri 1, I-34127, Trieste (Italy); INFM DEMOCRITOS, National Simulation Center, Trieste (Italy); Stener, M.; Decleva, P. [Dipartimento di Scienze Chimiche, Universita degli Studi di Trieste, Via L. Giorgieri 1, I-34127, Trieste (Italy); INFM DEMOCRITOS, National Simulation Center, Trieste (Italy); Consorzio Interuniversitario Nazionale per la Scienza e Tecnologia dei Materiali, INSTM, Unita di Trieste, Trieste (Italy)

2006-04-15

322

Demographic, mechanistic and density-dependent determinants of population growth rate: a case study in an avian predator.  

PubMed Central

Identifying the determinants of population growth rate is a central topic in population ecology. Three approaches (demographic, mechanistic and density-dependent) used historically to describe the determinants of population growth rate are here compared and combined for an avian predator, the barn owl (Tyto alba). The owl population remained approximately stable (r approximately 0) throughout the period from 1979 to 1991. There was no evidence of density dependence as assessed by goodness of fit to logistic population growth. The finite (lambda) and instantaneous (r) population growth rates were significantly positively related to food (field vole) availability. The demographic rates, annual adult mortality, juvenile mortality and annual fecundity were reported to be correlated with vole abundance. The best fit (R(2) = 0.82) numerical response of the owl population described a positive effect of food (field voles) and a negative additive effect of owl abundance on r. The numerical response of the barn owl population to food availability was estimated from both census and demographic data, with very similar results. Our analysis shows how the demographic and mechanistic determinants of population growth rate are linked; food availability determines demographic rates, and demographic rates determine population growth rate. The effects of food availability on population growth rate are modified by predator abundance. PMID:12396509

Hone, Jim; Sibly, Richard M

2002-01-01

323

Density dependence and risk of extinction in a small population of sea otters  

USGS Publications Warehouse

Sea otters (Enhydra lutris (L.)) were hunted to extinction off the coast of Washington State early in the 20th century. A new population was established by translocations from Alaska in 1969 and 1970. The population, currently numbering at least 550 animals, A major threat to the population is the ongoing risk of majour oil spills in sea otter habitat. We apply population models to census and demographic data in order to evaluate the status of the population. We fit several density dependent models to test for density dependence and determine plausible values for the carrying capacity (K) by comparing model goodness of fit to an exponential model. Model fits were compared using Akaike Information Criterion (AIC). A significant negative relationship was found between the population growth rate and population size (r2=0.27, F=5.57, df=16, p<0.05), suggesting density dependence in Washington state sea otters. Information criterion statistics suggest that the model is the most parsimonious, followed closely by the logistic Beverton-Holt model. Values of K ranged from 612 to 759 with best-fit parameter estimates for the Beverton-Holt model including 0.26 for r and 612 for K. The latest (2001) population index count (555) puts the population at 87-92% of the estimated carrying capacity, above the suggested range for optimum sustainable population (OSP). Elasticity analysis was conducted to examine the effects of proportional changes in vital rates on the population growth rate (??). The elasticity values indicate the population is most sensitive to changes in survival rates (particularly adult survival).

Gerber, L.R.; Buenau, K.E.; VanBlaricom, G.

2004-01-01

324

Quark mean field model with density dependent couplings for finite nuclei  

E-print Network

The quark mean field model, which describes the nucleon using the constituent quark model, is applied to investigate the properties of finite nuclei. The couplings of the scalar and vector mesons with quarks are made density dependent through direct coupling to the scalar field so as to reproduce the relativistic Brueckner-Hartree-Fock results of nuclear matter. The present model provides satisfactory results on the properties of spherical nuclei, and predicts an increasing size of the nucleon as well as a reduction of the nucleon mass in the nuclear environment

Y. H. Tan; H. Shen; P. Z. Ning

2001-03-04

325

Defect-concentration dependence of the charge-density-wave transport in tetrathiafulvalene tetracyanoquinodimethane  

NASA Astrophysics Data System (ADS)

The effect of electron-irradiation-induced defects on the non-Ohmic dc conductivity of tetrathiafulvalene tetracyanoquinodimethane (TTF-TCNQ) has been studied. The threshold field (ET) for non-Ohmic transport increases linearly with the defect concentration. This impurity study lends strong support to the explanation of the nonlinear conductivity in TTF-TCNQ by the depinning of the charge-density-wave condensate in strong electric fields. Both the nonlinear current and the Ohmic conductivity present a similar temperature dependence in the Peierls state.

Forro, L.; Lacoe, R.; Bouffard, S.; Jérome, D.

1987-04-01

326

Observation of plasma density dependence of electromagnetic soliton excitation by an intense laser pulse  

SciTech Connect

The experimental evidence of the correlation between the initial electron density of the plasma and electromagnetic soliton excitation at the wake of an intense (10{sup 19} W/cm{sup 2}) and short (1 ps) laser pulse is presented. The spatial distribution of the solitons, together with their late time evolution into post-solitons, is found to be dependent upon the background plasma parameters, in agreement with published analytical and numerical findings. The measured temporal evolution and electrostatic field distribution of the structures are consistent with their late time evolution and the occurrence of multiple merging of neighboring post-solitons.

Sarri, G.; Kar, S.; Kourakis, I.; Borghesi, M. [Centre for Plasma Physics, The Queen's University of Belfast, Belfast BT7 1NN (United Kingdom); Romagnani, L. [LULI, Ecole Polytechnique, CNRS, route de Saclay, Palaiseau 91128 (France); Bulanov, S. V. [Kansai Photon Science Institute, JAEA, Kizugawa, Kyoto 619-0215 (Japan); Cecchetti, C. A.; Gizzi, L. A. [IPCF, Consiglio Nazionale delle Ricerche, CNR campus, Pisa (Italy); Galimberti, M.; Heathcote, R. [Rutherford Appleton Laboratory, Central Laser Facility, Chilton OX11 0QX (United Kingdom); Jung, R.; Osterholz, J.; Willi, O. [Institute for Laser and Plasma Physics, Heinrich Heine University, Dusseldorf (Germany); Schiavi, A. [Dipartimento di Energetica, Universita di Roma 1 'La Sapienza', Roma (Italy)

2011-08-15

327

Depth dependency of neutron density produced by cosmic rays in the lunar subsurface  

NASA Astrophysics Data System (ADS)

Depth dependency of neutrons produced by cosmic rays (CRs) in the lunar subsurface was estimated using the three-dimensional Monte Carlo particle and heavy ion transport simulation code, PHITS, incorporating the latest high energy nuclear data, JENDL/HE-2007. The PHITS simulations of equilibrium neutron density profiles in the lunar subsurface were compared with the measurement by Apollo 17 Lunar Neutron Probe Experiment (LNPE). Our calculations reproduced the LNPE data except for the 350-400 mg/cm2 region under the improved condition using the CR spectra model based on the latest observations, well-tested nuclear interaction models with systematic cross section data, and JENDL/HE-2007.

Ota, S.; Sihver, L.; Kobayashi, S.; Hasebe, N.

2014-11-01

328

Pressure dependence of the charge-density-wave gap in rare-earth tritellurides.  

PubMed

We investigate the pressure dependence of the optical properties of CeTe3, which exhibits an incommensurate charge-density-wave (CDW) state already at 300 K. Our data are collected in the midinfrared spectral range at room temperature and at pressures between 0 and 9 GPa. The energy for the single particle excitation across the CDW gap decreases upon increasing the applied pressure, similarly to the chemical pressure by rare-earth substitution. The broadening of the bands upon lattice compression removes the perfect nesting condition of the Fermi surface and therefore diminishes the impact of the CDW transition on the electronic properties of RTe3. PMID:17358625

Sacchetti, A; Arcangeletti, E; Perucchi, A; Baldassarre, L; Postorino, P; Lupi, S; Ru, N; Fisher, I R; Degiorgi, L

2007-01-12

329

Non-monotonic density dependence of the diffusion of DNA fragments in low-salt suspensions  

E-print Network

The high linear charge density of 20-base-pair oligomers of DNA is shown to lead to a striking non-monotonic dependence of the long-time self-diffusion on the concentration of the DNA in low-salt conditions. This generic non-monotonic behavior results from both the strong coupling between the electrostatic and solvent-mediated hydrodynamic interactions, and from the renormalization of these electrostatic interactions at large separations, and specifically from the dominance of the far-field hydrodynamic interactions caused by the strong repulsion between the DNA fragments.

M. G. McPhie; G. Naegele

2008-11-26

330

Density determination of nano-layers depending to the thickness by non-destructive method  

SciTech Connect

Non-destructive tests used to characterize and observe the state of the solids near the surface or at depth, without damaging them or damaging them. Density is frequently used to follow the variations of the physical structure of the samples, as well as in the calculation of quantity of material required to fill a given volume, and it is also used to determine the homogeneity of a sample. However, the measurement of the acoustic properties (density, elastic constants,…) of a thin film whose thickness is smaller than several atomic layers is not easy to perform. For that reason, we expose in this work the effects of the thicknesses of thin films on the evolution of the density, where several samples are analyzed. The samples selected structures are thin films deposited on substrates, these coatings have thicknesses varying from a few atomic layers to ten or so micrometers and can change the properties of the substrate on which they are deposited. To do so, we considered a great number of layers (Cr, Al, SiO{sub 2}, ZnO, Cu, AlN, Si{sub 3}N{sub 4}, SiC) deposited on different substrates (Al{sub 2}O{sub 3}, Cu and Quartz). It is first shown that the density exhibits a dispersive behaviour. Such a behaviour is characterized by an initial increase (or decrease) followed by a saturated region. Further investigations of these dependences led to the determination of a semi-empirical universal relations, ?=f(h/?{sub T}), for all the investigated layer/substrate combination. Such expression could be of great importance in the density prediction of even layers thicknesses.

Gacem, A. [Département des Sciences Fondamentales, Faculté des Sciences et Sciences de l'Ingénieur, Université 20 Aout.1955, Skikda, BP 26, DZ-21000 Algérie and Laboratoire des Semi-Conducteurs, Département de Physique (Algeria); Doghmane, A.; Hadjoub, Z. [Laboratoire des Semi-Conducteurs, Département de Physique, Faculté des Sciences, Université Badji-Mokhtar, BP 12, Annaba, DZ-23000 (Algeria)

2013-12-16

331

Survival, growth and food conversion of cultured larvae of Pangasianodon hypophthalmus, depending on feeding level, prey density and fish density  

Microsoft Academic Search

In young fish larvae feeding efficiency is generally proportional to prey density, so feeding in excess is needed to maximise growth and survival. Increasing fish density might contribute to improve food conversion, but it can also impact negatively on fish growth or survival. Larvae of Pangasianodon hypophthalmus were raised until 192 h after hatching (hah) in 30-L tanks in a recirculating

J. Slembrouck; E. Baras; J. Subagja; L. T. Hung; M. Legendre

2009-01-01

332

LETTER: Dependence of the density shape on the heat flux profile in ASDEX Upgrade high density H modes  

NASA Astrophysics Data System (ADS)

For high density H modes with constant gas puff on ASDEX Upgrade a slow evolution of the density profile is observed with a time constant much larger than ?E. While the edge density remains constant the central density increases as does the stored energy until sawteeth are lost and impurity influx sets in. This peaking process is only observed with NBI heating. Substituting half of the heating by central ICRH leads to completely flat profiles even though half of the original NBI particle fuelling remains. This behaviour, as well as the slow timescale of the peaking process with pure NBI heating, is successfully modelled with a pinch of the order of the neoclassical Ware pinch and a proportionality between the particle diffusion coefficient D and the heat conductivity ?. Such an assumption links D to the heat flux profile qheat, since the temperature profile is observed to be stiff. Such a model implies flat density profiles for a centrally heated fusion reactor.

Stober, J.; Fuchs, C.; Gruber, O.; Kaufmann, M.; Kurzan, B.; Meo, F.; Müller, H. W.; Ryter, F.; ASDEX Upgrade Team

2001-11-01

333

Undoing static correlation: Long-range charge transfer in time-dependent density-functional theory  

NASA Astrophysics Data System (ADS)

Long-range charge-transfer excited states are notoriously badly underestimated in time-dependent density-functional theory (TDDFT). We discuss how exact TDDFT captures charge transfer between open-shell species: in particular, the role of the step in the ground-state potential, and the severe frequency dependence of the exchange-correlation kernel. An expression for the latter is derived, that becomes exact in the limit that the charge-transfer excitations are well separated from other excitations. The exchange-correlation kernel has the task of undoing the static correlation in the ground state introduced by the step, in order to accurately recover the physical charge-transfer states.

Maitra, Neepa T.

2005-06-01

334

Rotochemical heating with a density-dependent superfluid energy gap in neutron stars  

SciTech Connect

When a rotating neutron star loses angular momentum, the reduction of the centrifugal force makes it contract. This perturbs each fluid element, raising the local pressure and originating deviations from beta equilibrium, inducing reactions that release heat (rotochemical heating). This effect has previously been studied by Fernandez and Reisenegger for neutron stars of non-superfluid matter and by Petrovich and Reisenegger for superfluid matter, finding that the system in both cases reaches a quasi-steady state, corresponding to a partial equilibration between compression, due to the loss of angular momentum, and reactions that try to restore the equilibrium. However, Petrovich and Reisenegger assumes a constant value of the superfluid energy gap, whereas theoretical models predict density-dependent gap amplitudes, and therefore gaps that depend on the location in the star. In this work, we try to discriminate between several proposed gap models, comparing predicted surface temperatures to the value measured for the nearest millisecond pulsar, J0437-4715.

Gonzalez-Jimenez, Nicolas; Petrovich, Cristobal; Reisenegger, Andreas [Departamento de Astronomia y Astrofisica, Pontificia Universidad Catolica de Chile, Casilla 306, Santiago 22 (Chile)

2010-08-04

335

Time-Dependent Density Functional Theory Calculations of Photoabsorption of Fluorinated Cyclic Molecules in the Vacuum Ultraviolet Region  

SciTech Connect

Time-dependent density functional theory (TD-DFT) calculations of the photoabsorption of molecules in the vacuum region have been performed in order to aid in the design of transparent materials for use as photoresists for F2 lithography (157 nm). The method including an empirical equation for correcting the calculated transition energy is described. We have used the TD-DFT approach to predict the photoabsorption of substituted benzenes, and cycloalkane including norbornane. The calculations show that norbornane is the best cycloalkane on which to start substitution studies. We report results on the mono-, di-, and tera-fluorinated derivatives. The results show that tetrafluoro-norbornanes have the lowest absorption in the 157 nm range for the molecules that we studied.

Dixon, David A.(BATTELLE (PACIFIC NW LAB)) [BATTELLE (PACIFIC NW LAB); Matsuzawa, Nobuyuki N.(SONY) [SONY; Ishitani, Ahihiko (Atsugi Research Center) [Atsugi Research Center; Uda, Tsuyoshi (Joint Research Center for ATom Technology) [Joint Research Center for ATom Technology

2001-05-30

336

Landscape-scale density-dependent recruitment of oaks in planted forests: more is not always better.  

PubMed

Plant colonization studies usually address density-dependent processes in the narrow sense of recruitment constraints due to negative density-dependent seed and seedling mortality. However, complex density-dependent effects may be involved in additional stages of the recruitment process. We hypothesized that seed arrival and seedling establishment are influenced by density dependence acting at small scales at the site of colonization, and at larger scales as a function of the colonizing species' landscape abundance. These hypotheses were tested in a study of colonization of pine forests by oaks in a heterogeneous Mediterranean landscape. Maximum-likelihood models show that density effects switch from positive to negative along the range of landscape-scale oak seed source abundance. Contrary to expectations, high seed source densities limited oak recruitment, suggesting a landscape-scale Janzen-Connell effect. We propose a range of mechanisms that generate positive or negative density dependence during colonization, resulting in nonlinear density-dependent feedbacks that can generate unexpected colonization patterns. PMID:24015516

Sheffer, Efrat; Canham, Charles D; Kigel, Jaime; Perevolotsky, Avi

2013-08-01

337

Lifetime reproductive success and density-dependent, multi-variable resource selection.  

PubMed

Individuals are predicted to maximize lifetime reproductive success (LRS) through selective use of resources; however, a wide range of ecological and social processes may prevent individuals from always using the highest-quality resources available. Resource selection functions (RSFs) estimate the relative amount of time an individual spends using a resource as a function of the proportional availability of that resource. We quantified the association between LRS and coefficients of individual-based RSFs describing lifetime resource selection for 267 female red deer (Cervus elaphus) of the Isle of Rum, Scotland, from 1970 to 2001. LRS was significantly related to first- and second-order effects of selection for Agrostis/Festuca grassland and proximity to the sea coast (quality of forage within Agrostis/Festuca grassland was highest nearest the coast (ratio of short:long grassland)). The benefits of selecting for quality in Agrostis/Festuca grassland, however, traded-off with increases in LRS gained by avoiding conspecific density. LRS was inversely associated with local density, which was highest along the coast, and reproductive benefits of selecting Agrostis/Festuca grassland diminished with increasing density. We discuss the relevance of these results to our understanding of the spatial distribution of red deer abundance, and potential applications of our approach to evolutionary and applied ecology. PMID:16777736

McLoughlin, Philip D; Boyce, Mark S; Coulson, Tim; Clutton-Brock, Tim

2006-06-22

338

Individual differences, density dependence and offspring birth traits in a population of red deer.  

PubMed

Variation between individuals is an essential component of natural selection and evolutionary change, but it is only recently that the consequences of persistent differences between individuals on population dynamics have been considered. In particular, few authors have addressed whether interactions exist between individual quality and environmental variation. In part, this is due to the difficulties of collecting sufficient data, but also the challenge of defining individual quality. Using a long-established study population of red deer, Cervus elaphus, inhabiting the North Block of the Isle of Rum, and three quality measures, this paper investigates how differences in maternal quality affect variation in birth body mass and date, as population density varies, and how this differs depending on the sex of the offspring and the maternal quality measure used. Significant interactions between maternal quality, measured as a hind's total contribution to population growth, and population density are reported for birth mass, but only for male calves. Analyses using dominance or age at primiparity to define maternal quality showed no significant interactions with population density, highlighting the difficulties of defining a consistent measure of individual quality. PMID:18522909

Stopher, Katie V; Pemberton, Josephine M; Clutton-Brock, Tim H; Coulson, Tim

2008-09-22

339

A High-Performance Fortran Code to Calculate Spin- and Parity-Dependent Nuclear Level Densities  

E-print Network

A high-performance Fortran code is developed to calculate the spin- and parity-dependent shell model nuclear level densities.The algorithm is based on the extension of methods of statistical spectroscopy and implies exact calculation of the first and second Hamiltonian moments for different configurations at fixed spin and parity. The proton-neutron formalism is used. We have applied the method for calculating the level densities for a set of nuclei in the sd-, pf-, and pf+g9/2 - model spaces. Examples of the calculations for 28Si (in the sd-model space) and 64Ge (in the pf+g9/2-model space) are presented. To illustrate the power of the method we estimate the ground state energy of 64Ge in the larger model space pf+g9/2, which is not accessible to direct shell model diagonalization due to the prohibitively large dimension, by comparing with the nuclear level densities at low excitation energy calculated in the smaller model space pf.

Sen'kov, R; Zelevinsky, V G

2012-01-01

340

A High-Performance Fortran Code to Calculate Spin- and Parity-Dependent Nuclear Level Densities  

E-print Network

A high-performance Fortran code is developed to calculate the spin- and parity-dependent shell model nuclear level densities.The algorithm is based on the extension of methods of statistical spectroscopy and implies exact calculation of the first and second Hamiltonian moments for different configurations at fixed spin and parity. The proton-neutron formalism is used. We have applied the method for calculating the level densities for a set of nuclei in the sd-, pf-, and pf+g9/2 - model spaces. Examples of the calculations for 28Si (in the sd-model space) and 64Ge (in the pf+g9/2-model space) are presented. To illustrate the power of the method we estimate the ground state energy of 64Ge in the larger model space pf+g9/2, which is not accessible to direct shell model diagonalization due to the prohibitively large dimension, by comparing with the nuclear level densities at low excitation energy calculated in the smaller model space pf.

R. Sen'kov; M. Horoi; V. G. Zelevinsky

2012-06-20

341

Hippo signaling regulates microprocessor and links cell-density-dependent miRNA biogenesis to cancer.  

PubMed

Global downregulation of microRNAs (miRNAs) is commonly observed in human cancers and can have a causative role in tumorigenesis. The mechanisms responsible for this phenomenon remain poorly understood. Here, we show that YAP, the downstream target of the tumor-suppressive Hippo-signaling pathway regulates miRNA biogenesis in a cell-density-dependent manner. At low cell density, nuclear YAP binds and sequesters p72 (DDX17), a regulatory component of the miRNA-processing machinery. At high cell density, Hippo-mediated cytoplasmic retention of YAP facilitates p72 association with Microprocessor and binding to a specific sequence motif in pri-miRNAs. Inactivation of the Hippo pathway or expression of constitutively active YAP causes widespread miRNA suppression in cells and tumors and a corresponding posttranscriptional induction of MYC expression. Thus, the Hippo pathway links contact-inhibition regulation to miRNA biogenesis and may be responsible for the widespread miRNA repression observed in cancer. PMID:24581491

Mori, Masaki; Triboulet, Robinson; Mohseni, Morvarid; Schlegelmilch, Karin; Shrestha, Kriti; Camargo, Fernando D; Gregory, Richard I

2014-02-27

342

A Density Dependence for Protostellar Luminosity in Class I Sources: Collaborative Accretion  

NASA Astrophysics Data System (ADS)

Class I protostars in three high-mass star-forming regions are found to have correlations among the local projected density of other Class I protostars, the summed flux from these other protostars, and the protostellar luminosity in the WISE 22 ?m band. Brighter Class I sources form in higher-density and higher-flux regions, while low luminosity sources form anywhere. These correlations depend slightly on the number of neighbors considered (from 2 to 20) and could include a size-of-sample effect from the initial mass function (i.e., larger numbers include rarer and more massive stars). Luminosities seem to vary by neighborhood with nearby protostars having values proportional to each other and higher density regions having higher values. If Class I luminosity is partially related to the accretion rate, then this luminosity correlation is consistent with the competitive accretion model, although it is more collaborative than competitive. The correlation is also consistent with primordial mass segregation and could explain why the stellar initial mass function resembles the dense core mass function even when cores form multiple stars.

Elmegreen, Bruce G.; Hurst, Rachel; Koenig, Xavier

2014-02-01

343

Wavelet-Based Linear-Response Time-Dependent Density-Functional Theory  

E-print Network

Linear-response time-dependent (TD) density-functional theory (DFT) has been implemented in the pseudopotential wavelet-based electronic structure program BigDFT and results are compared against those obtained with the all-electron Gaussian-type orbital program deMon2k for the calculation of electronic absorption spectra of N2 using the TD local density approximation (LDA). The two programs give comparable excitation energies and absorption spectra once suitably extensive basis sets are used. Convergence of LDA density orbitals and orbital energies to the basis-set limit is significantly faster for BigDFT than for deMon2k. However the number of virtual orbitals used in TD-DFT calculations is a parameter in BigDFT, while all virtual orbitals are included in TD-DFT calculations in deMon2k. As a reality check, we report the x-ray crystal structure and the measured and calculated absorption spectrum (excitation energies and oscillator strengths) of the small organic molecule N-cyclohexyl-2-(4-methoxyphenyl)imidaz...

Natarajan, Bhaarathi; Casida, Mark E; Deutsch, Thierry; Burchak, Olga N; Philouze, Christian; Balakirev, Maxim Y

2011-01-01

344

Tobacco Retailer Proximity and Density and Nicotine Dependence Among Smokers With Serious Mental Illness  

PubMed Central

Objectives We examined the density and proximity of tobacco retailers and associations with smoking behavior and mental health in a diverse sample of 1061 smokers with serious mental illness (SMI) residing in the San Francisco Bay Area of California. Methods Participants’ addresses were geocoded and linked with retailer licensing data to determine the distance between participants’ residence and the nearest retailer (proximity) and the number of retailers within 500-meter and 1-kilometer service areas (density). Results More than half of the sample lived within 250 meters of a tobacco retailer. A median of 3 retailers were within 500 meters of participants’ residences, and a median of 12 were within 1 kilometer. Among smokers with SMI, tobacco retailer densities were 2-fold greater than for the general population and were associated with poorer mental health, greater nicotine dependence, and lower self-efficacy for quitting. Conclusions Our findings provide further evidence of the tobacco retail environment as a potential vector contributing to tobacco-related disparities among individuals with SMI and suggest that this group may benefit from progressive environmental protections that restrict tobacco retail licenses and reduce aggressive point-of-sale marketing. PMID:24922145

Young-Wolff, Kelly C.; Henriksen, Lisa; Delucchi, Kevin; Prochaska, Judith J.

2014-01-01

345

Individual differences, density dependence and offspring birth traits in a population of red deer  

PubMed Central

Variation between individuals is an essential component of natural selection and evolutionary change, but it is only recently that the consequences of persistent differences between individuals on population dynamics have been considered. In particular, few authors have addressed whether interactions exist between individual quality and environmental variation. In part, this is due to the difficulties of collecting sufficient data, but also the challenge of defining individual quality. Using a long-established study population of red deer, Cervus elaphus, inhabiting the North Block of the Isle of Rum, and three quality measures, this paper investigates how differences in maternal quality affect variation in birth body mass and date, as population density varies, and how this differs depending on the sex of the offspring and the maternal quality measure used. Significant interactions between maternal quality, measured as a hind's total contribution to population growth, and population density are reported for birth mass, but only for male calves. Analyses using dominance or age at primiparity to define maternal quality showed no significant interactions with population density, highlighting the difficulties of defining a consistent measure of individual quality. PMID:18522909

Stopher, Katie V; Pemberton, Josephine M; Clutton-Brock, Tim H; Coulson, Tim

2008-01-01

346

Cosmological Constraints from the Redshift Dependence of the Alcock-Paczynski Test: Galaxy Density Gradient Field  

NASA Astrophysics Data System (ADS)

We propose a method based on the redshift dependence of the Alcock-Paczynski (AP) test to measure the expansion history of the universe. It uses the isotropy of the galaxy density gradient field to constrain cosmological parameters. If the density parameter ? m or the dark energy equation of state w are incorrectly chosen, the gradient field appears to be anisotropic with the degree of anisotropy varying with redshift. We use this effect to constrain the cosmological parameters governing the expansion history of the universe. Although redshift-space distortions (RSD) induced by galaxy peculiar velocities also produce anisotropies in the gradient field, these effects are close to uniform in magnitude over a large range of redshift. This makes the redshift variation of the gradient field anisotropy relatively insensitive to the RSD. By testing the method on mock surveys drawn from the Horizon Run 3 cosmological N-body simulations, we demonstrate that the cosmological parameters can be estimated without bias. Our method is complementary to the baryon acoustic oscillation or topology methods as it depends on DAH , the product of the angular diameter distance and the Hubble parameter.

Li, Xiao-Dong; Park, Changbom; Forero-Romero, J. E.; Kim, Juhan

2014-12-01

347

Photoabsorption spectra of small cationic xenon clusters from time-dependent density functional theory  

SciTech Connect

Upon ionization, rare-gas (like Ar and Xe) clusters shift their absorption spectrum from the ultraviolet to the visible. This happens as bonding becomes much stronger due to the removal of an electron from a strongly antibonding orbital. In this article, we study the absorption spectrum of small cationic xenon clusters (Xe{sub n}{sup +}, with n=3,...,35) by means of time-dependent density functional theory. These calculations include relativistic effects through the use of relativistic j-dependent pseudopotentials in a two-spinor formulation of the Kohn-Sham equations. The peak positions in our calculated spectra are in fairly good agreement with experiment and confirm that absorption is mainly due to a charged linear core composed of 3, 4, or 5 Xe atoms where the positive charge is localized. However, we find large deviations concerning the oscillator strengths, which can be partially explained by the unsatisfactory treatment of exchange in common density functionals. Furthermore, we find that adequate ground-state geometries are necessary for the correct prediction of the qualitative features of the spectra.

Oliveira, Micael J. T. [Center for Computational Physics, University of Coimbra, Rua Larga, 3004-516 Coimbra (Portugal) and Laboratoire de Physique de la Matiere Condense et Nanostructures, Universite Lyon I, CNRS, UMR 5586, Domaine Scientifique de la Doua, F-69622 Villeurbanne Cedex (France); European Theoretical Spectroscopy Facility (ETSF) (Country Unknown); Nogueira, Fernando [Center for Computational Physics, University of Coimbra, Rua Larga, 3004-516 Coimbra (Portugal); Marques, Miguel A. L. [Laboratoire de Physique de la Matiere Condense et Nanostructures, Universite Lyon I, CNRS, UMR 5586, Domaine Scientifique de la Doua, F-69622 Villeurbanne Cedex (France); European Theoretical Spectroscopy Facility (ETSF) (Country Unknown); Rubio, Angel [Dpto. Fisica de Materiales, Nano-Bio Spectroscopy group and ETSF Scientific Development Centre, Universidad del Pais Vasco, Centro de Fisica de Materiales CSIC-UPV/EHU-MPC and DIPC, Av. Tolosa 72, E-20018 San Sebastian (Spain); Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6 D-14 195 Berlin-Dahlem (Germany); European Theoretical Spectroscopy Facility (ETSF) (Country Unknown)

2009-12-07

348

Anion height dependence of T c and the density of states in iron-based superconductors  

NASA Astrophysics Data System (ADS)

Systematic ab initio LDA calculations were performed for all the typical representatives of recently discovered class of iron-based high-temperature superconductors: REOFe(As,P) (RE = La, Ce, Nd, Sm, Tb), Ba2Fe2As, (Sr,Ca)FFeAs, Sr4Sc2O6Fe2P2, LiFeAs and Fe(Se,Te). Non-monotonic behavior of total density of states at the Fermi level is observed as a function of anion height relative to Fe layer with maximum at about ? z a ˜ 1.37 Å, attributed to changing Fe-As (P, Se, Te) hybridization. This leads to a similar dependence of superconducting transition temperature T c as observed in the experiments. The fit of this dependence to elementary BCS theory produces semiquantitative agreement with experimental data for T c for the whole class of iron-based superconductors. The similar fit to Allen-Dynes formula underestimates T c in the vicinity of the maximum, signifying the possible importance of non-phonon pairing in this region. These results unambiguously demonstrate that the main effect of T c variation between different types of iron-based superconductors is due to the corresponding variation of the density of states at the Fermi level.

Kuchinskii, E. Z.; Nekrasov, I. A.; Sadovskii, M. V.

2010-05-01

349

Herbivore-Specific, Density-Dependent Induction of Plant Volatiles: Honest or "Cry Wolf" Signals?  

PubMed Central

Plants release volatile chemicals upon attack by herbivorous arthropods. They do so commonly in a dose-dependent manner: the more herbivores, the more volatiles released. The volatiles attract predatory arthropods and the amount determines the probability of predator response. We show that seedlings of a cabbage variety (Brassica oleracea var. capitata, cv Shikidori) also show such a response to the density of cabbage white (Pieris rapae) larvae and attract more (naive) parasitoids (Cotesia glomerata) when there are more herbivores on the plant. However, when attacked by diamondback moth (Plutella xylostella) larvae, seedlings of the same variety (cv Shikidori) release volatiles, the total amount of which is high and constant and thus independent of caterpillar density, and naive parasitoids (Cotesia vestalis) of diamondback moth larvae fail to discriminate herbivore-rich from herbivore-poor plants. In contrast, seedlings of another cabbage variety of B. oleracea (var. acephala: kale) respond in a dose-dependent manner to the density of diamondback moth larvae and attract more parasitoids when there are more herbivores. Assuming these responses of the cabbage cultivars reflect behaviour of at least some genotypes of wild plants, we provide arguments why the behaviour of kale (B. oleracea var acephala) is best interpreted as an honest signaling strategy and that of cabbage cv Shikidori (B. oleracea var capitata) as a “cry wolf” signaling strategy, implying a conflict of interest between the plant and the enemies of its herbivores: the plant profits from being visited by the herbivore's enemies, but the latter would be better off by visiting other plants with more herbivores. If so, evolutionary theory on alarm signaling predicts consequences of major interest to students of plant protection, tritrophic systems and communication alike. PMID:20808961

Shiojiri, Kaori; Ozawa, Rika; Kugimiya, Soichi; Uefune, Masayoshi; van Wijk, Michiel; Sabelis, Maurice W.; Takabayashi, Junji

2010-01-01

350

Density matrix renormalization group approach for many-body open quantum systems  

E-print Network

The density matrix renormalization group (DMRG) approach is extended to complex-symmetric density matrices characteristic of many-body open quantum systems. Within the continuum shell model, we investigate the interplay between many-body configuration interaction and coupling to open channels. It is shown that the DMRG technique applied to broad resonances in the unbound neutron-rich nucleus 7He provides a highly accurate treatment of the coupling to the non-resonant scattering continuum.

J. Rotureau; N. Michel; W. Nazarewicz; M. Ploszajczak; J. Dukelsky

2006-03-07

351

Structure of fluid mixtures near a solute: A density functional approach  

NASA Astrophysics Data System (ADS)

The structure of fluid mixtures near a spherical solute is studied using a density functional approach and computer simulation. The input direct correlation function is obtained from integral equation theory with an accurate closure relation. The density and concentration profiles of binary as well as ternary hard-sphere mixtures near a large hard-spherical solute compare quite well with the computer simulation results over a wide range of parametric conditions.

Patra, Chandra N.

2014-09-01

352

Theoretical and numerical assessments of spin-flip time-dependent density functional theory.  

PubMed

Spin-flip time-dependent density functional theory (SF-TD-DFT) with the full noncollinear hybrid exchange-correlation kernel and its approximate variants are critically assessed, both formally and numerically. As demonstrated by the ethylene torsion and the C(2v) ring-opening of oxirane, SF-TD-DFT is very useful for describing nearly degenerate situations. However, it may occasionally yield unphysical results. This stems from the noncollinear form of the generalized gradient approximation, which becomes numerically instable in the presence of spin-flip excitations from the closed- to vacant-shell orbitals of an open-shell reference. To cure this defect, a simple modification, dubbed as ALDA0, is proposed in the spirit of adiabatic local density approximation (ALDA). It is applicable to all kinds of density functionals and yields stable results without too much loss of accuracy. In particular, the combination of ALDA0 with the Tamm-Dancoff approximation is a promising tool for studying global potential energy surfaces. In addition to the kernel problem, SF-TD-DFT is also rather sensitive to the choice of reference states, as demonstrated by the spin multiplet states of closed-shell molecules of H(2)O, CH(2)O, and C(2)H(4). Surprisingly, SF-TD-DFT with pure density functionals may also fail for valance excitations with large orbital overlaps, at variance with the spin-conserving counterpart (SC-TD-DFT). In this case, the inclusion of a large amount of Hartree-Fock exchange is mandatory for quantitative results. Nonetheless, for spatially degenerate cases such as CF, CH, and NH(+), SF-TD-DFT is more advantageous than SC-TD-DFT, unless the latter is also space adapted. These findings are very instructive for future development and applications of TD-DFT. PMID:22260564

Li, Zhendong; Liu, Wenjian

2012-01-14

353

Dependency of irradiation damage density on tritium migration behaviors in Li2TiO3  

NASA Astrophysics Data System (ADS)

Tritium migration behaviors in Li2TiO3 with the increase of irradiation damage density were investigated by means of electron spin resonance and thermal desorption spectroscopy. The irradiation damages of F+-centers and O--centers were formed by neutron irradiation, and their damage densities were increased with increasing neutron fluence. Tritium release temperature was clearly shifted toward higher temperature side with increasing neutron fluence, i.e. increasing damage density. The rate determining process for tritium release was also clearly changed depending on the damage density. Tritium release was mainly controlled by tritium diffusion process in crystalline grain of Li2TiO3 at lower neutron fluence. The apparent tritium diffusivity was reduced as the damage density in Li2TiO3 increased due to the introduction of tritium trapping/detrapping sites for diffusing tritium. Then, tritium trapping/detrapping processes began to control the overall tritium release with further damage introductions as the amount of tritium trapping sites increased enough to trap most of tritium in Li2TiO3. The effects of water vapor in purge gas on tritium release behaviors were also investigated. It was considered that hydrogen isotopes in purge gas would be dissociated and adsorbed on the surface of Li2TiO3. Then, hydrogen isotopes diffused inward Li2TiO3 would occupy the tritium trapping sites before diffusing tritium reaches to these sites, promoting apparent tritium diffusion consequently. Kinetics analysis of tritium release for highly damaged Li2TiO3 showed that the rate determining process of tritium release was the detrapping process of tritium formed as hydroxyl groups. The rate of tritium detrapping as hydroxyl groups was determined by the kinetic analysis, and was comparable to tritium release kinetics for Li2O, LiOH and Li4TiO4. The dangling oxygen atoms (O--centers) formed by neutron irradiation would contribute strongly on the formation of hydroxyl groups. The efficiency of tritium trapping/detrapping by the dangling oxygen atoms was clearly increased with increasing damage density due to the stabilization of damages by neighboring irradiation damages and/or the lithium burn-up which produces lithium vacancy acting as a pass way of tritium to the dangling oxygen atoms.

Kobayashi, Makoto; Toda, Kensuke; Oya, Yasuhisa; Okuno, Kenji

2014-04-01

354

A unifying probabilistic Bayesian approach to derive electron density from MRI for radiation therapy treatment planning  

NASA Astrophysics Data System (ADS)

MRI significantly improves the accuracy and reliability of target delineation in radiation therapy for certain tumors due to its superior soft tissue contrast compared to CT. A treatment planning process with MRI as the sole imaging modality will eliminate systematic CT/MRI co-registration errors, reduce cost and radiation exposure, and simplify clinical workflow. However, MRI lacks the key electron density information necessary for accurate dose calculation and generating reference images for patient setup. The purpose of this work is to develop a unifying method to derive electron density from standard T1-weighted MRI. We propose to combine both intensity and geometry information into a unifying probabilistic Bayesian framework for electron density mapping. For each voxel, we compute two conditional probability density functions (PDFs) of electron density given its: (1) T1-weighted MRI intensity, and (2) geometry in a reference anatomy, obtained by deformable image registration between the MRI of the atlas and test patient. The two conditional PDFs containing intensity and geometry information are combined into a unifying posterior PDF, whose mean value corresponds to the optimal electron density value under the mean-square error criterion. We evaluated the algorithm’s accuracy of electron density mapping and its ability to detect bone in the head for eight patients, using an additional patient as the atlas or template. Mean absolute HU error between the estimated and true CT, as well as receiver operating characteristics for bone detection (HU > 200) were calculated. The performance was compared with a global intensity approach based on T1 and no density correction (set whole head to water). The proposed technique significantly reduced the errors in electron density estimation, with a mean absolute HU error of 126, compared with 139 for deformable registration (p = 2? × ?10?4), 283 for the intensity approach (p = 2? × ?10?6) and 282 without density correction (p = 5? × ?10?6). For 90% sensitivity in bone detection, the proposed method achieved a specificity of 86%, compared with 80, 11 and 10% using deformable registration, intensity and without density correction, respectively. Notably, the Bayesian approach was more robust against anatomical differences between patients, with a specificity of 62% in the worst case (patient), compared to 30% specificity in registration-based approach. In conclusion, the proposed unifying Bayesian method provides accurate electron density estimation and bone detection from MRI of the head with highly heterogeneous anatomy.

Sudhan Reddy Gudur, Madhu; Hara, Wendy; Le, Quynh-Thu; Wang, Lei; Xing, Lei; Li, Ruijiang

2014-11-01

355

A unifying probabilistic Bayesian approach to derive electron density from MRI for radiation therapy treatment planning.  

PubMed

MRI significantly improves the accuracy and reliability of target delineation in radiation therapy for certain tumors due to its superior soft tissue contrast compared to CT. A treatment planning process with MRI as the sole imaging modality will eliminate systematic CT/MRI co-registration errors, reduce cost and radiation exposure, and simplify clinical workflow. However, MRI lacks the key electron density information necessary for accurate dose calculation and generating reference images for patient setup. The purpose of this work is to develop a unifying method to derive electron density from standard T1-weighted MRI. We propose to combine both intensity and geometry information into a unifying probabilistic Bayesian framework for electron density mapping. For each voxel, we compute two conditional probability density functions (PDFs) of electron density given its: (1) T1-weighted MRI intensity, and (2) geometry in a reference anatomy, obtained by deformable image registration between the MRI of the atlas and test patient. The two conditional PDFs containing intensity and geometry information are combined into a unifying posterior PDF, whose mean value corresponds to the optimal electron density value under the mean-square error criterion. We evaluated the algorithm's accuracy of electron density mapping and its ability to detect bone in the head for eight patients, using an additional patient as the atlas or template. Mean absolute HU error between the estimated and true CT, as well as receiver operating characteristics for bone detection (HU > 200) were calculated. The performance was compared with a global intensity approach based on T1 and no density correction (set whole head to water). The proposed technique significantly reduced the errors in electron density estimation, with a mean absolute HU error of 126, compared with 139 for deformable registration (p = 2? × ?10(-4)), 283 for the intensity approach (p = 2? × ?10(-6)) and 282 without density correction (p = 5? × ?10(-6)). For 90% sensitivity in bone detection, the proposed method achieved a specificity of 86%, compared with 80, 11 and 10% using deformable registration, intensity and without density correction, respectively. Notably, the Bayesian approach was more robust against anatomical differences between patients, with a specificity of 62% in the worst case (patient), compared to 30% specificity in registration-based approach. In conclusion, the proposed unifying Bayesian method provides accurate electron density estimation and bone detection from MRI of the head with highly heterogeneous anatomy. PMID:25321341

Gudur, Madhu Sudhan Reddy; Hara, Wendy; Le, Quynh-Thu; Wang, Lei; Xing, Lei; Li, Ruijiang

2014-11-01

356

Partitioning the sources of demographic variation reveals density-dependent nest predation in an island bird population.  

PubMed

Ecological factors often shape demography through multiple mechanisms, making it difficult to identify the sources of demographic variation. In particular, conspecific density can influence both the strength of competition and the predation rate, but density-dependent competition has received more attention, particularly among terrestrial vertebrates and in island populations. A better understanding of how both competition and predation contribute to density-dependent variation in fecundity can be gained by partitioning the effects of density on offspring number from its effects on reproductive failure, while also evaluating how biotic and abiotic factors jointly shape demography. We examined the effects of population density and precipitation on fecundity, nest survival, and adult survival in an insular population of orange-crowned warblers (Oreothlypis celata) that breeds at high densities and exhibits a suite of traits suggesting strong intraspecific competition. Breeding density had a negative influence on fecundity, but it acted by increasing the probability of reproductive failure through nest predation, rather than through competition, which was predicted to reduce the number of offspring produced by successful individuals. Our results demonstrate that density-dependent nest predation can underlie the relationship between population density and fecundity even in a high-density, insular population where intraspecific competition should be strong. PMID:25077023

Sofaer, Helen R; Sillett, T Scott; Langin, Kathryn M; Morrison, Scott A; Ghalambor, Cameron K

2014-07-01

357

Partitioning the sources of demographic variation reveals density-dependent nest predation in an island bird population  

PubMed Central

Ecological factors often shape demography through multiple mechanisms, making it difficult to identify the sources of demographic variation. In particular, conspecific density can influence both the strength of competition and the predation rate, but density-dependent competition has received more attention, particularly among terrestrial vertebrates and in island populations. A better understanding of how both competition and predation contribute to density-dependent variation in fecundity can be gained by partitioning the effects of density on offspring number from its effects on reproductive failure, while also evaluating how biotic and abiotic factors jointly shape demography. We examined the effects of population density and precipitation on fecundity, nest survival, and adult survival in an insular population of orange-crowned warblers (Oreothlypis celata) that breeds at high densities and exhibits a suite of traits suggesting strong intraspecific competition. Breeding density had a negative influence on fecundity, but it acted by increasing the probability of reproductive failure through nest predation, rather than through competition, which was predicted to reduce the number of offspring produced by successful individuals. Our results demonstrate that density-dependent nest predation can underlie the relationship between population density and fecundity even in a high-density, insular population where intraspecific competition should be strong. PMID:25077023

Sofaer, Helen R; Sillett, T Scott; Langin, Kathryn M; Morrison, Scott A; Ghalambor, Cameron K

2014-01-01

358

Statistical characteristics of the density-independent population fluctuation and the evaluation of density-dependence and regulation in animal populations  

Microsoft Academic Search

Summary  In this study it was proved that the “density-independent population” shows the following characteristics if the analysis\\u000a is made using time-series samples:\\u000a \\u000a \\u000a (1) \\u000a \\u000a The slopeb in the density-on-density regression between successive developmental stages has a general tendency to become smaller than\\u000a 1, though the bias approaches zero if one or more of the following three conditions are satisfied: (a) the

Eizi Kuno

1973-01-01

359

Field dependence of the zero energy density of states of an anisotropic s± superconductor  

NASA Astrophysics Data System (ADS)

The pairing symmetry in iron-based superconductors (SC) is generally believed to be an s±-wave state. Although ARPES suggests a mainly isotropic gap on all Fermi surface sheets, different thermodynamic and transport measurements are still inconclusive about the existence and orientation of gap nodes. Specific heat measurements in a magnetic field showing a square root like dependence of the Sommerfeld coefficient ?(B) have been reported, contradicting the linear behavior expected for a fully gapped system. For a d-wave SC, ?(B)?B is well-known as Volovik effect. For a fully gapped s±-wave SC with ?+?-, a similar concave field dependence is expected. To distinguish these two effects we apply a two-band model using the Riccati parametrization of the Eilenberger equation to study the density of states around a single vortex and compare it with self-consistent calculations in the vortex lattice. Different models for the momentum dependence of the gap on each band relevant to the iron-based SC, ranging from isotropic to strongly anisotropic and nodal gaps are investigated. Partial support was provided by DOE DE-FG02-05ER46236 (PJH).

Wang, Yan; Hirschfeld, Peter; Graser, Siegfried

2011-03-01

360

Mixed convection and density-dependent seawater circulation in coastal aquifers  

NASA Astrophysics Data System (ADS)

Density-dependent circulation of seawater in coastal aquifers results in submarine groundwater discharge (SGD) across the seabed that is a mixture of terrestrial groundwater and former marine water. In this study, the controls of the relative amount of seawater to freshwater in SGD were investigated numerically using the FEFLOW and SUTRA codes. It was found that the key controls could be expressed in the form of a single nondimensional recirculation number that incorporates the combined effects of free convection, forced convection, and hydrodynamic dispersion on convective overturn within the coastal salt wedge. Anisotropy effects were incorporated into the recirculation number with limited success based on the principle of equivalent isotropic hydraulic conductivity. The type of boundary condition employed along the seabed was shown to be important. Convective overturn was substantially increased if backward dispersion of salt into the aquifer from along the outflowing portion of the seabed boundary was prevented. Overall, the results demonstrated a strong dependence of convective overturn on the aquifer dispersivities, suggesting that results from numerical simulations are problematic to apply to real aquifer systems that typically exhibit uncertain, scale-dependent dispersion properties.

Smith, Anthony J.

2004-08-01

361

Density-dependent life-history compensation of an iteroparous salmonid.  

PubMed

Over the course of a decade, the bull trout (Salvelinus confluentus) population in Lower Kananaskis Lake, Alberta, Canada, recovered from a heavily overexploited state, experiencing a 28-fold increase in adult abundance after the implementation of zero-harvest regulations. This system provided a unique opportunity to monitor the changes in life-history characteristics in a natural population throughout the recovery process. The purpose of this study was to examine the degree to which life-history traits were able to compensate for harvest-induced changes and the implications of this for management. Density-dependent changes in growth, survival, and reproductive life-history characteristics were observed. As density increased, maturation was delayed, and the frequency of skipped reproductive events, primarily by individuals of poor condition, increased. However, size at maturation and the proportion of fish skipping reproduction differed between the sexes, suggesting that life-history trade-offs differ between the sexes. The rapid response of these life-history traits to changes in density suggests that these changes were primarily due to phenotypic plasticity, although the importance of natural and artificial selection should not be discounted. The magnitude of the variation in the traits represents the degree to which the population was able to compensate for overharvest, although the overexploited state of the population at the beginning of the study demonstrates it was not able to fully compensate for this mortality. However, no evidence of depensatory processes was found. This, in combination with the plasticity of the life-history traits, has important implications for the resilience of the population to overharvest. Furthermore, density-dependent growth may have the unintended result of making size-based regulations less conservative at low levels of population abundance, as younger fish, perhaps even immature fish, become vulnerable to harvest. Finally, the variation in life-history traits in relation to evolutionary change is discussed. Results from this study demonstrate the importance of considering not only survival, but also changes in life-history characteristics for management and conservation. PMID:19323202

Johnston, Fiona D; Post, John R

2009-03-01

362

Electronic spectroscopy of UO2(2+), NUO(+) and NUN: an evaluation of time-dependent density functional theory for actinides.  

PubMed

The performance of the time-dependent density functional theory (TDDFT) approach has been evaluated for the electronic spectrum of the UO(2)(2+), NUO(+) and NUN molecules. Different exchange-correlation functionals (LDA, PBE, BLYP, B3LYP, PBE0, M06, M06-L, M06-2X, CAM-B3LYP) and the SAOP model potential have been investigated, as has the relative importance of the adiabatic local density approximation (ALDA) to the exchange-correlation kernel. The vertical excitation energies have been compared with reference data obtained using accurate wave-function theory (WFT) methods. PMID:21359301

Tecmer, Pawe?; Gomes, André Severo Pereira; Ekström, Ulf; Visscher, Lucas

2011-04-01

363

Topside ionosphere plasma bubbles, seen in He+ density: longitudinal dependence and thermosphere meridional wind influence  

NASA Astrophysics Data System (ADS)

He+ density depletions, considered as originating from equatorial plasma bubbles, or as fossil bubble signatures, were involved in this study. He+ density depletions, obtained from ISS-b spacecraft data, were observed during a high solar activity (1978-80, F10.7=200) in the topside ionosphere (900-1100 km) deeply inside the plasmasphere (L=1.3-3) (Sidorova, 2004, 2007). (1) He+ density depletion statistics with respect to longitude is considered for the post-sunset hours under winter, summer and equinoctial conditions within of 35° invariant latitudes. The map of He+ density depletion distribution as function of latitude- and longitude was also derived. The statistics and the map were compared with Equatorial Spread-F statistics, plasma bubble distribution and Range Spread-F statistics, obtained by Maruyama and Matuura (1984, 1980) from ISS-b spacecraft data for the same period (1978-80). The longitudinal variations of the Equatorial F-region Irregularities probability, obtained from the AE-E spacecraft data (McClure et al., 1998) for the same period, were also taken. Comparison shows good conformity in statistics/spatial distributions of all mentioned irregularities. Their predominant occurrence area for all seasons and both hemispheres covers the region of Brasilia, Atlantic Ocean and Africa (270°-0°-30°), where the range of magnetic field declination angle varies from 0° to 20°. (2) It is also suggested, that the plasma bubbles, produced by Rayleigh-Taylor (R-T) instability at the bottomside of ionosphere and transported up to the topside ionosphere/plasmasphere, could be strong affected by meridional wind during a generation due to inhibiting the growth of R-T instability and flux tube integrated conductivity. For better understanding competing/complementary roles of thermosphere winds in the development of plasma bubbles, observed in He+ density, the evaluation of the possible influence of the thermosphere meridional winds was done. The diurnal He+ density depletion statistics, averaged for solstices and equinoxes, were compared with the model velocity variations of the thermosphere meridional wind, taken from (Maruyama, 1996). It was revealed that the meridional wind influence shows itself as modulation effect. The modulation has seasonal dependence and the best correlation in equinoxes (R=0.87). The best amplitude correlation was found for the longitudes of 270°-360° (Brasilia, Atlantic regions), where the declination angle of the magnetic meridional wind component is near 20°. It was concluded that the topside plasma bubbles, seen as He+ density depletions, are strong enough affected by thermosphere meridional wind. REFERENCE Maryama, T. and N. Matuura, Global distribution of occurrence probability of spread echoes based on ISS-b observation, RRL, vol.27, N 124, 201-216, 1980. Maryama, T. and N. Matuura, Longitudinal variability of annual changes in activity of equatorial spread F and plasma bubbles, J. Geophys. Res., 89(A12), 10,903-10,912, 1984. Maruyama, T., Modeling study of equatorial ionospheric height and spread F occurrence, J. Geophys. Res., vol. 101, A3, pp. 5157-5163, 1996. McClure, J. P., Singh S., Bamgboye D.K., Johnson F.S., Hyosub Kil. Occurrence of equatorial F region irregularities: Evidence for tropospheric seeding, J. Geophys. Res., 103(A12), 29,119-29,135, 1998. Sidorova, L.N., He+ density topside modeling based on ISS-b satellite data, Adv. Space Res., vol. 33, pp. 850-854, 2004. Sidorova, L.N., Plasma bubble phenomenon in the topside ionosphere, Adv. Space Res., Special issue (COSPAR), doi: 10.1016/j.asr.2007.03.067, 2007.

Sidorova, L.

2009-04-01

364

Integration of Density Dependence and Concentration Response Models Provides an Ecologically Relevant Assessment of Populations Exposed to Toxicants  

EPA Science Inventory

The assessment of toxic exposure on wildlife populations involves the integration of organism level effects measured in toxicity tests (e.g., chronic life cycle) and population models. These modeling exercises typically ignore density dependence, primarily because information on ...

365

Impact of numerical artifact of the forward model in the inverse solution of density-dependent flow problem  

NASA Astrophysics Data System (ADS)

investigate the effect of computational error on the inversion of a density-dependent flow and transport model, using SEAWAT and UCODE-2005 in an inverse identification of hydraulic conductivity and dispersivity using head and concentration data from a 2-D laboratory experiment. We investigated inversions using three different solution schemes including variation of number of particles and time step length, in terms of the three aspects: the shape and smoothness of the objective function surface, the consequent impacts to the optimization, and the resulting Pareto analyses. This study demonstrates that the inversion is very sensitive to the choice of the forward model solution scheme. In particular, standard finite difference methods provide the smoothest objective function surface; however, this is obtained at the cost of numerical artifacts that can lead to erroneous warping of the objective function surface. Total variation diminishing (TVD) schemes limit these impacts at the cost of more computation time, while the hybrid method of characteristics (HMOC) approach with increased particle numbers and/or reduced time step gives both smoothed and accurate objective function surface. Use of the most accurate methods (TVD and HMOC) did lead to successful inversion of the two parameters; however, with distinct results for Pareto analyses. These results illuminate the sensitivity of the inversion to a number of aspects of the forward solution of the density-driven flow problem and reveal that parameter values may result that are erroneous but that counteract numerical errors in the solution.

Nassar, Mohamed K.; Ginn, Timothy R.

2014-08-01

366

Density-dependent resistance of the gypsy moth Lymantria dispar to its nucleopolyhedrovirus, and the consequences for population dynamics  

Microsoft Academic Search

The processes controlling disease resistance can strongly influence the population dynamics of insect outbreaks. Evidence\\u000a that disease resistance is density-dependent is accumulating, but the exact form of this relationship is highly variable from\\u000a species to species. It has been hypothesized that insects experiencing high population densities might allocate more energy\\u000a to disease resistance than those at lower densities, because they

James R. Reilly; Ann E. Hajek

2008-01-01

367

Detection of the density of fine particulate matter employing laser beam divergence and inertia-dependent particle motion  

NASA Astrophysics Data System (ADS)

We present a miniaturized sensor setup capable of determining the density of airborne particles employing size information provided by an enhanced light-scattering intensity ratio technique and inertia-dependent particle motion. The method is based on the particle density-dependent spatial particle spreading, measured as the time of flight using a divergent laser beam. Measurement results using polystyrene latex and silica particles in a size range of 500-1,600 nm show good agreement with theoretical estimations.

Schrobenhauser, R.; Strzoda, R.; Hartmann, A.; Fleischer, M.; Amann, M.-C.

2014-10-01

368

Density Functional and Basis Set Dependence of Hydrated Ln(III) Properties  

SciTech Connect

Benchmark studies of Ln(H?O)?‚???³? (Ln ) La, Lu) have been performed to assess the calculated properties obtained with local density approximation, generalized gradient approximation (GGA), meta-GGA, and hybrid functionals, when used with small- and largecore relativistic effective core potentials and their associated bases. Basis set dependence and the importance of specific functions to adequately describe the Ln atomic orbitals have been determined. The lanthanide contraction has been found to be an insufficient metric for characterizing the quality of a method/basis set combination due to cancellation of the errors. The electrostatic description obtained by natural population analysis has been examined, and an alternative partitioning of the valence space, which includes the 6s6p5d4f natural atomic orbitals, has been proposed.

Clark, Aurora E.

2008-04-05

369

Time-Dependent Density-Functional Calculations of Photoabsorption Spectra of Carbon Nanostructures  

NASA Astrophysics Data System (ADS)

Optical properties of nanoscale structures have attracted much attention experimentally and theoretically. It is not appropriate to apply the conventional density-functional theory (DFT) to investigation of the optical properties, because the excited states, which are not adequately represented by the DFT, play essential roles in these phenomena. To go beyond the DFT, we adopt time-dependent DFT (TDDFT) calculations with the linear- response theory, which is a powerful computational tool for calculating the excited states of nanostructures properly. In this study, we report the results of excitation energies and photo-absorption spectra in aromatic molecules, such as naphthalene and anthracene. A high spectral intensity in a low frequency region occurs in the spectra for these molecules. This characteristic spectrum is shifted to lower energy with increasing molecular size. We also discuss the details of the electronic excitations utilizing the TDDFT calculations in a real-time scheme.

Noguchi, Tomoyuki; Araidai, Masaaki; Watanabe, Kazuyuki

2005-03-01

370

Relativistic Hartree-Fock-Bogoliubov theory with density dependent meson-nucleon couplings  

SciTech Connect

Relativistic Hartree-Fock-Bogoliubov (RHFB) theory with density-dependent meson-nucleon couplings is presented. The integrodifferential RHFB equations are solved by expanding the different components of the quasiparticle spinors in the complete set of eigensolutions of the Dirac equations with Woods-Saxon potentials. Using the finite-range Gogny force D1S as an effective interaction in the pairing channel, systematic RHFB calculations are performed for Sn isotopes and N=82 isotones. It is demonstrated that an appropriate description of both mean field and pairing effects can be obtained within RHFB theory with finite range Gogny pairing forces. Better systematics are also found in the regions from the stable to the neutron-rich side with the inclusion of Fock terms, especially in the presence of rho-tensor couplings.

Long Wenhui [School of Nuclear Science and Technology, Lanzhou University, 730000 Lanzhou (China); Physik-Department der Technischen Universitaet Muenchen, D-85748 Garching (Germany); School of Physics and State Key Laboratory of Nuclear Physics and Technology, Peking University, 100871 Beijing (China); Ring, Peter [Physik-Department der Technischen Universitaet Muenchen, D-85748 Garching (Germany); Giai, Nguyen Van [CNRS-IN2P3, UMR 8608, F-91406 Orsay Cedex (France); Universite Paris-Sud, F-91405 Orsay (France); Meng Jie [School of Physics and State Key Laboratory of Nuclear Physics and Technology, Peking University, 100871 Beijing (China)

2010-02-15

371

{alpha}-cluster structure above doubly closed shells in a generalized density-dependent cluster model  

SciTech Connect

An extension of the generalized density-dependent cluster model (GDDCM) is presented to study {alpha}-cluster structure above doubly closed shells. In all cases, the microscopic {alpha}-core potential is numerically constructed in the double-folding model with CDM3Y6 nucleon-nucleon interactions plus proton-proton Coulomb interactions. The properties of intraband E2 transitions and {alpha} decays are calculated by the exact solution of the Schroedinger equation with appropriate boundary conditions. It is found that the enhanced B(E2) transition strengths are well reproduced without any effective charge and the calculations of {alpha}-decay properties show good agreement with the available experimental data. This indicates that the GDDCM has universal applicability and equal validity regardless of whether the {alpha}-cluster states are in light or heavy nuclei.

Ni Dongdong [Department of Physics, Nanjing University, Nanjing 210093 (China); Kavli Institute for Theoretical Physics China, Beijing 100190 (China); Ren Zhongzhou [Department of Physics, Nanjing University, Nanjing 210093 (China); Kavli Institute for Theoretical Physics China, Beijing 100190 (China); Center of Theoretical Nuclear Physics, National Laboratory of Heavy-Ion Accelerator, Lanzhou 730000 (China)

2011-01-15

372

Gradient-dependent density functionals of the PBE type for atoms, molecules and solids  

E-print Network

One of the standard generalized-gradient approximations (GGAs) in use in modern electronic-structure theory, PBE, and a recently proposed modification designed specifically for solids, PBEsol, are identified as particular members of a family of functionals taking their parameters from different properties of homogeneous or inhomogeneous electron liquids. Three further members of this family are constructed and tested, together with the original PBE and PBEsol, for atoms, molecules and solids. We find that PBE, in spite of its popularity in solid-state physics and quantum chemistry, is not always the best performing member of the family, and that PBEsol, in spite of having been constructed specifically for solids, is not the best for solids. The performance of GGAs for finite systems is found to sensitively depend on the choice of constraints steaming from infinite systems. Guidelines both for users and for developers of density functionals emerge from this work.

Pedroza, Luana S; Capelle, Klaus

2009-01-01

373

Density dependence of fixed-node errors in diffusion quantum Monte Carlo: Triplet pair correlations  

NASA Astrophysics Data System (ADS)

We analyze trial wave function fixed-node errors for a three-electron fully spin-polarized atomic systems with varying atomic number Z. The fully spin-polarized state S(1s2s3s) is the lowest quadruplet with S-symmetry and even parity and it is nearly-degenerate with the S(1s2p3p) state. We find significant fixed-node errors proportional to Z at the Hartree-Fock level while the two-configuration wave function leads to nearly exact energies. This broadly agrees with our analogous study on Be-like atoms so that the dependence of fixed-node errors on density appears to be similar in both spin-polarized and unpolarized atomic systems.

Kulahlioglu, Adem H.; Rasch, Kevin; Hu, Shuming; Mitas, Lubos

2014-01-01

374

Thickness dependence of the charge-density-wave transition temperature in VSe2  

NASA Astrophysics Data System (ADS)

A set of three-dimensional charge-density-wave (3D CDW) VSe2 nano-flakes with different thicknesses were obtained by the scotch tape-based micro-mechanical exfoliation method. Resistivity measurements showed that the 3D CDW transition temperature Tp decreases systematically from 105 K in bulk to 81.8 K in the 11.6 nm thick flake. The Hall resistivity ?xy of all the flakes showed a linear dependent behavior against the magnetic field with a residual electron concentration of the order of ˜1021 cm-3 at 5 K. The electron concentration n increases slightly as the thickness d decreases, possibly due to the CDW gap is reduced with the decrease of the thickness.

Yang, Jiyong; Wang, Weike; Liu, Yan; Du, Haifeng; Ning, Wei; Zheng, Guolin; Jin, Chiming; Han, Yuyan; Wang, Ning; Yang, Zhaorong; Tian, Mingliang; Zhang, Yuheng

2014-08-01

375

Rotochemical heating of millisecond and classical pulsars with anisotropic and density-dependent superfluid gap models  

E-print Network

When a rotating neutron star loses angular momentum, the progressive reduction of the centrifugal force makes it contract. This perturbs each fluid element, raising the local pressure and originating deviations from beta equilibrium, inducing reactions that release heat (rotochemical heating). This effect has previously been studied by Fern\\'andez & Reisenegger (2005) for non-superfluid neutron stars and by Petrovich & Reisenegger (2010) for superfluid millisecond pulsars. Both studies found that pulsars reach a quasi-steady state in which the compression driving the matter out of beta equilibrium is balanced by the reactions trying to restore the equilibrium. We extend previous studies by considering the effect of density-dependence and anisotropy of the superfluid energy gaps, for the case in which the dominant reactions are the modified Urca processes, the protons are non-superconducting, and the neutron superfluidity is parametrized by models proposed in the literature. By comparing our prediction...

González-Jiménez, Nicolás; Reisenegger, Andreas

2014-01-01

376

Charge transfer in time-dependent density-functional theory: Insights from the asymmetric Hubbard dimer  

NASA Astrophysics Data System (ADS)

We show that propagation with the best possible adiabatic approximation in time-dependent density-functional theory fails to properly transfer charge in an asymmetric two-site Hubbard model when beginning in the ground state. The approximation is adiabatic but exact otherwise, constructed from the exact ground-state exchange-correlation functional that we compute via constrained search. The model shares the essential features of charge-transfer dynamics in a real-space long-range molecule, so the results imply that the best possible adiabatic approximation, despite being able to capture nonlocal ground-state step features relevant to dissociation and charge-transfer excitations, cannot capture fully time-resolved charge-transfer dynamics out of the ground state.

Fuks, J. I.; Maitra, N. T.

2014-06-01

377

A robust and efficient numerical model for analyses of density-dependent flow in porous media  

NASA Astrophysics Data System (ADS)

A new computer code (MITSU3D) has been developed to simulate variable density-dependent flow in porous media. The techniques involve a control-volume discretization, Newton-Raphson linearization, and a flux limiter scheme. The assembled Jacobian matrices are solved by using preconditioned conjugate gradient-type methods employing a block-incomplete LU factorization and reduced system preconditioning. Numerical simulations involving contaminant migration in a two-dimensional cross-section show MITSU3D to be much faster and more robust than SUTRA. MITSU3D successfully avoids an "error-driven" contaminant plume instability in a coarse grid discretization and shows increased performance by a factor of five. The enhanced speed and robustness will enable us to examine much larger coupled problems than heretofore had been considered computationally practical.

Ibaraki, Motomu

1998-10-01

378

Risk-disturbance overrides density dependence in a hunted colonial rodent, the black-tailed prairie dog Cynomys ludovicianus  

Microsoft Academic Search

Summary 1. Traditional understanding of how hunting affects vertebrate populations empha- sizes competitive release and density dependence of vital rates, but more recent thinking has proposed complex non-lethal responses to hunting disturbance and predation risk. Colonial species have been proposed to be more vulnerable than dispersed, solitary species to disturbance and perceived risk from hunting. However, empirical comparisons of density

JONATHAN N. PAULI; STEVEN W. BUSKIRK

2007-01-01

379

Density-dependent processes in cohorts of Tubifex tubifex, with special emphasis on the control of fecundity  

Microsoft Academic Search

Laboratory cohort cultures of the tubificid Tubifex tubifex with different initial densities were carried out at 20° C with the condition of unlimited food. The main results were: 1) Intracocoon mortality was 37% of the laid eggs (observation of 689 eggs); 2) The principal bionomic parameters (generation time, r, R0) appeared to be density dependent; 3) Recruitment was regulated through

Carla Bonacinal; Giuliano Bonomi; Carlo Monti

1989-01-01

380

Reconstruction of Layer Densities in a Multilayer Snowpack using a Bayesian Approach to Inverse Modeling  

NASA Astrophysics Data System (ADS)

Estimation of snowpack parameters such as depth, density and grain structure is a central focus of hydrology in seasonally snow-covered lands. These parameters are directly estimated by field observations, indirectly estimated from other parameters using statistical correlations, or simulated with a model. Difficulty in sampling thin layers and uncertainty in the transition between layers can cause significant uncertainty in measurements of these parameters. Snow density is one of the most important parameters to measure because it is strictly related with snow water content, an important component of the global water balance. We develop a mathematical framework to estimate snow density from measurements of temperature and thickness of snowpack layers over a particular time period, in conjunction with a physics-based model of snowpack evolution. We formulate a Bayesian approach to estimate the snowpack density profile, using a full range of possible simulations that incorporate key sources of uncertainty to build in prior snowpack knowledge. The posterior probability density function of the snow density, conditioned on snowpack temperature measurements, is computed by multiplying the likelihoods and assumed prior distribution function. Random sampling is used to generate a range of densities with same probability when prior uniform probability function is assumed. A posterior probability density function calculated directly via Bayes' theorem is used to calculate the probability of every sample generated. The forward model is a 1D, multilayer snow energy and mass balance model, which solves for snow temperature, density, and liquid water content on a finite element mesh. The surface and ground temperature data of snowpack (boundary conditions), are provided by the Center for Snow and Avalanche Studies (CSAS), Silverton CO, from snow pits made at Swamp Angel and Senator Beck study plot sites. Standard errors between field observations and results computed denote the quality of the estimations and facilitate further arrangements of this approach.

Aguayo, M.; Marshall, H.; McNamara, J. P.; Mead, J.; Flores, A. N.

2013-12-01

381

A Multiple Motives Approach to Tobacco Dependence: The Wisconsin Inventory of Smoking Dependence Motives (WISDM-68)  

ERIC Educational Resources Information Center

The dependence construct fills an important explanatory role in motivational accounts of smoking and relapse. Frequently used measures of dependence are either atheoretical or grounded in a unidimensional model of physical dependence. This research creates a multidimensional measure of dependence that is based on theoretically grounded motives for…

Piper, Megan E.; Piasecki, Thomas M.; Federman, E. Belle; Bolt, Daniel M.; Smith, Stevens S.; Fiore, Michael C.; Baker, Timothy C.

2004-01-01

382

Functionalization density dependent toxicity of oxidized multiwalled carbon nanotubes in a murine macrophage cell line.  

PubMed

The present study investigates the effect of functionalization density on the toxicity and cellular uptake of oxidized multiwalled carbon nanotubes (f-MWCNTs) in vitro. The toxicity of f-MWCNTs at varying degrees of carboxylation was assessed in a murine macrophage RAW 264.7 cell line, a model for liver Kupffer cells. In vitro cytotoxicity of oxidized MWCNTs was directly proportional to their functionalization density. The increased cytotoxicity was associated with a concurrent increase in the number of apoptotic cells and production of reactive nitrogen species (RNS). In contrast, reactive oxygen species (ROS) generation was the highest in the case of pristine MWCNTs and decreased with increased functionalization density. Quantitative cellular uptake studies indicated that endogenous ROS production was independent of the concentration of CNTs internalized by a specific cell population and was directly proportional to their surface hydrophobicity. Mechanistic studies suggested that cellular uptake of CNTs was critically charge-dependent and mediated through scavenger receptors, albeit the involvement of nonscavenger receptor mechanisms at low CNT concentrations and their saturation at the experimental concentration cannot be ruled out. A mathematical model was established to correlate between the cellular uptake of CNTs with their length and zeta potential. In an attempt to correlate the results of in vitro toxicity experiments with those of the in vivo toxicity in the mouse model, we found that the toxicity trends in vitro and in vivo are rather opposing. The apparent anomaly was explained on the basis of different experimental conditions and doses associated with cells under in vivo and in vitro culture conditions. PMID:22994501

Singh, Raman Preet; Das, Manasmita; Thakare, Vivek; Jain, Sanyog

2012-10-15

383

Density-dependent cooperative non-specific binding in solid-phase SELEX affinity selection  

PubMed Central

The non-specific binding of undesired ligands to a target is the primary factor limiting the enrichment of tight-binding ligands in affinity selection. Solution-phase non-specific affinity is determined by the free-energy of ligand binding to a single target. However, the solid-phase affinity might be higher if a ligand bound concurrently to multiple adjacent immobilized targets in a cooperative manner. Cooperativity could emerge in this case as a simple consequence of the relationship between the free energy of binding, localization entropy and the spatial distribution of the immobilized targets. We tested this hypothesis using a SELEX experimental design and found that non-specific RNA aptamer ligands can concurrently bind up to four bead-immobilized peptide targets, and that this can increase their effective binding affinity by two orders-of-magnitude. Binding curves were quantitatively explained by a new statistical mechanical model of density-dependent cooperative binding, which relates cooperative binding to both the target concentration and the target surface density on the immobilizing substrate. Target immobilization plays a key role in SELEX and other ligand enrichment methods, particularly in new multiplexed microfluidic purification devices, and these results have strong implications for optimizing their performance. PMID:23737446

Ozer, Abdullah; White, Brian S.; Lis, John T.; Shalloway, David

2013-01-01

384

Embedded density functional approach for calculations of adsorption on ionic crystals  

E-print Network

Embedded density functional approach for calculations of adsorption on ionic crystals Eugene V is assumed to be the same as in the ideal crystal. In this case the Kohn�Sham Hamiltonian for electrons molecular complexes X Li , Na , K , F or Cl and water adsorption on the 001 surface of the NaCl crystal

Truong, Thanh N.

385

Nucleation in n-alkanes: A density-functional approach Chaok Seok and David W. Oxtobya)  

E-print Network

Nucleation in n-alkanes: A density-functional approach Chaok Seok and David W. Oxtobya) Department and nucleation are discussed. It is shown that the n-alkane systems cannot be well approximated by a system char- acter the alcohols , or elongated and flexible chains the n-alkanes . In order to confront theory

Seok, Chaok

386

Affinity Density: a novel genomic approach to the identification of transcription factor regulatory targets  

Microsoft Academic Search

Methods: A new method was developed for identifying novel transcription factor regulatory targets based on calculating Local Affinity Density. Techniques from the signal-processing field were used, in particular the Hann digital filter, to calculate the relative binding affinity of different regions based on previously published in vitro binding data. To illustrate this approach, the genomes of Drosophila melanogaster and D.

Dennis J. Hazelett; Daniel L. Lakeland; Joseph B. Weiss

2009-01-01

387

LARGE EDDY SIMULATION/EULERIAN PROBABILITY DENSITY FUNCTION APPROACH FOR SIMULATING HYDROGEN-ENRICHED GAS TURBINE  

E-print Network

LARGE EDDY SIMULATION/EULERIAN PROBABILITY DENSITY FUNCTION APPROACH FOR SIMULATING HYDROGEN-ENRICHED GAS TURBINE COMBUSTORS Christopher Lietz Dept. of Aerospace Engg. & Engg. Mechanics The University:scott.m.martin@siemens.com ABSTRACT To describe partially-premixed combustion inside hydrogen- rich combustors, a novel quadrature

Raman, Venkat

388

Associating domain-dependent knowledge and Monte Carlo approaches within a Go program  

Microsoft Academic Search

This paper underlines the association of two computer go approaches, a domain-dependent knowledge approach and Monte Carlo. First, the strengthes and weaknesses of the two existing approaches are related. Then, the association is described in two steps. A rst step consists in using domain-dependent knowledge within the random games enabling the program to compute evaluations that are more signican t

Bruno Bouzy

2005-01-01

389

Nonlinear Feedback Controllers and Compensators: A StateDependent Riccati Equation Approach  

E-print Network

Nonlinear Feedback Controllers and Compensators: A State­Dependent Riccati Equation Approach H. T Computation North Carolina State University Raleigh, NC 27695 Abstract State­dependent Riccati equation (SDRE­dependent Riccati equation (SDRE) approach in the context of the nonlinear regulator problem. This method, which

390

Nonlinear Feedback Controllers and Compensators: A State-Dependent Riccati Equation Approach  

E-print Network

Nonlinear Feedback Controllers and Compensators: A State-Dependent Riccati Equation Approach H. T North Carolina State University Raleigh, NC 27695 Abstract State-dependent Riccati equation (SDRE-dependent Riccati equation (SDRE) approach in the context of the nonlinear regulator problem. This method, which

391

Context Dependency Management in Ontology Engineering: A Formal Approach  

Microsoft Academic Search

A viable ontology engineering methodology requires supporting do- main experts in gradually building and managing increasingly complex versions of ontological elements and their converging and diverging interrelationships. Contexts are necessary to formalise and reason about such a dynamic wealth of knowledge. However, context dependencies introduce many complexities. In this article, we introduce a formal framework for supporting context dependency management

Pieter De Leenheer; Aldo De Moor; Robert Meersman

2007-01-01

392

Density-dependent effects of multiple predators sharing a common prey in an endophytic habitat.  

PubMed

Multiple predator species feeding on a common prey can lead to higher or lower predation than would be expected by simply combining their individual effects. Such emergent multiple predator effects may be especially prevalent if predators share feeding habitat. Despite the prevalence of endophagous insects, no studies have examined how multiple predators sharing an endophytic habitat affect prey or predator reproduction. We investigated density-dependent predation of Thanasimus dubius (Coleoptera: Cleridae) and Platysoma cylindrica (Coleoptera: Histeridae) on a bark beetle prey, Ips pini (Coleoptera: Scolytidae), in a laboratory assay. I. pini utilize aggregation pheromones to group-colonize and reproduce within the stems of conifers. T. dubius and P. cylindrica exploit these aggregation pheromones to arrive simultaneously with the herbivore. Adult T. dubius prey exophytically, while P. cylindrica adults enter and prey within the bark beetle galleries. Larvae of both predators prey endophytically. We used a multiple regression analysis, which avoids confounding predator composition with density, to examine the effects of varying predator densities alone and in combination on herbivore establishment, herbivore reproduction, and predator reproduction. Predators reduced colonization success by both sexes, and decreased I. pini reproduction on a per male and per female basis. The combined effects of these predators did not enhance or reduce prey establishment or reproduction in unexpected manners, and these predators were entirely substitutable. The herbivore's net replacement rate was never reduced significantly below one at prey and predator densities emulating field conditions. Similar numbers of each predator species emerged from the logs, but predator reproduction suffered from high intraspecific interference. The net replacement rate of P. cylindrica was not affected by conspecifics or T. dubius. In contrast, the net replacement rate of T. dubius decreased with the presence of conspecifics or P. cylindrica. Combinations of both predators led to an emergent effect, a slightly increased net replacement rate of T. dubius. This may have been due to predation by larval T. dubius on pupal P. cylindrica, as P. cylindrica develops more rapidly than T. dubius within this shared habitat. PMID:14968356

Aukema, Brian H; Clayton, Murray K; Raffa, Kenneth F

2004-05-01

393

Emergent multicellular life cycles in filamentous bacteria owing to density-dependent population dynamics  

PubMed Central

Filamentous bacteria are the oldest and simplest known multicellular life forms. By using computer simulations and experiments that address cell division in a filamentous context, we investigate some of the ecological factors that can lead to the emergence of a multicellular life cycle in filamentous life forms. The model predicts that if cell division and death rates are dependent on the density of cells in a population, a predictable cycle between short and long filament lengths is produced. During exponential growth, there will be a predominance of multicellular filaments, while at carrying capacity, the population converges to a predominance of short filaments and single cells. Model predictions are experimentally tested and confirmed in cultures of heterotrophic and phototrophic bacterial species. Furthermore, by developing a formulation of generation time in bacterial populations, it is shown that changes in generation time can alter length distributions. The theory predicts that given the same population growth curve and fitness, species with longer generation times have longer filaments during comparable population growth phases. Characterization of the environmental dependence of morphological properties such as length, and the number of cells per filament, helps in understanding the pre-existing conditions for the evolution of developmental cycles in simple multicellular organisms. Moreover, the theoretical prediction that strains with the same fitness can exhibit different lengths at comparable growth phases has important implications. It demonstrates that differences in fitness attributed to morphology are not the sole explanation for the evolution of life cycles dominated by multicellularity. PMID:21593029

Rossetti, Valentina; Filippini, Manuela; Svercel, Miroslav; Barbour, A. D.; Bagheri, Homayoun C.

2011-01-01

394

An exponential scaling law for the strain dependence of the Nb3Sn critical current density  

NASA Astrophysics Data System (ADS)

The critical current density of the Nb3Sn superconductor is strongly dependent on the strain applied to the material. In order to investigate this dependence, it is a common practice to measure the critical current of Nb3Sn strands for different values of applied axial strain. In the literature, several models have been proposed to describe these experimental data in the reversible strain region. All these models are capable of fitting the measurement results in the strain region where data are collected, but tend to predict unphysical trends outside the range of data, and especially for large strain values. In this paper we present a model of a new strain function, together with the results obtained by applying the new scaling law on relevant datasets. The data analyzed consisted of the critical current measurements at 4.2 K that were carried out under applied axial strain at Durham University and the University of Geneva on different strand types. With respect to the previous models proposed, the new scaling function does not present problems at large strain values, has a lower number of fitting parameters (only two instead of three or four), and is very stable, so that, starting from few experimental points, it can estimate quite accurately the strand behavior in a strain region where there are no data. A relationship is shown between the proposed strain function and the elastic strain energy, and an analogy is drawn with the exponential form of the McMillan equation for the critical temperature.

Bordini, B.; Alknes, P.; Bottura, L.; Rossi, L.; Valentinis, D.

2013-07-01

395

Synaptic state-dependent functional interplay between postsynaptic density-95 and synapse-associated protein 102.  

PubMed

Activity-dependent regulation of AMPA receptor (AMPAR)-mediated synaptic transmission is the basis for establishing differences in synaptic weights among individual synapses during developmental and experience-dependent synaptic plasticity. Synaptic signaling scaffolds of the Discs large (DLG)-membrane-associated guanylate kinase (MAGUK) protein family regulate these processes by tethering signaling proteins to receptor complexes. Using a molecular replacement strategy with RNAi-mediated knockdown in rat and mouse hippocampal organotypic slice cultures, a postsynaptic density-95 (PSD-95) knock-out mouse line and electrophysiological analysis, our current study identified a functional interplay between two paralogs, PSD-95 and synapse-associated protein 102 (SAP102) to regulate synaptic AMPARs. During synaptic development, the SAP102 protein levels normally plateau but double if PSD-95 expression is prevented during synaptogenesis. For an autonomous function of PSD-95 in regulating synaptic AMPARs, in addition to the previously demonstrated N-terminal multimerization and the first two PDZ (PSD-95, Dlg1, zona occludens-1) domains, the PDZ3 and guanylate kinase domains were required. The Src homology 3 domain was dispensable for the PSD-95-autonomous regulation of basal synaptic transmission. However, it mediated the functional interaction with SAP102 of PSD-95 mutants to enhance AMPARs. These results depict a protein domain-based multifunctional aspect of PSD-95 in regulating excitatory synaptic transmission and unveil a novel form of domain-based interplay between signaling scaffolds of the DLG-MAGUK family. PMID:23946397

Bonnet, Stéphanie A D; Akad, Derya S; Samaddar, Tanmoy; Liu, Yanling; Huang, Xiaojie; Dong, Yan; Schlüter, Oliver M

2013-08-14

396

Using an Epidemiological Model for Phylogenetic Inference Reveals Density Dependence in HIV Transmission  

PubMed Central

The control, prediction, and understanding of epidemiological processes require insight into how infectious pathogens transmit in a population. The chain of transmission can in principle be reconstructed with phylogenetic methods which analyze the evolutionary history using pathogen sequence data. The quality of the reconstruction, however, crucially depends on the underlying epidemiological model used in phylogenetic inference. Until now, only simple epidemiological models have been used, which make limiting assumptions such as constant rate parameters, infinite total population size, or deterministically changing population size of infected individuals. Here, we present a novel phylogenetic method to infer parameters based on a classical stochastic epidemiological model. Specifically, we use the susceptible-infected-susceptible model, which accounts for density-dependent transmission rates and finite total population size, leading to a stochastically changing infected population size. We first validate our method by estimating epidemic parameters for simulated data and then apply it to transmission clusters from the Swiss HIV epidemic. Our estimates of the basic reproductive number R0 for the considered Swiss HIV transmission clusters are significantly higher than previous estimates, which were derived assuming infinite population size. This difference in key parameter estimates highlights the importance of careful model choice when doing phylogenetic inference. In summary, this article presents the first fully stochastic implementation of a classical epidemiological model for phylogenetic inference and thereby addresses a key aspect in ongoing efforts to merge phylogenetics and epidemiology. PMID:24085839

Leventhal, Gabriel E.; Gunthard, Huldrych F.; Bonhoeffer, Sebastian; Stadler, Tanja

2014-01-01

397

2007 Time_Dependent Density-Functional Therory (July 15-20, 2007 Colby College, Maine)  

SciTech Connect

Time-dependent density-functional theory (TDDFT) provides an efficient, elegant, and formally exact way of describing the dynamics of interacting many-body quantum systems, circumventing the need for solving the full time-dependent Schroedinger equation. In the 20 years since it was first rigorously established in 1984, the field of TDDFT has made rapid and significant advances both formally as well as in terms of successful applications in chemistry, physics and materials science. Today, TDDFT has become the method of choice for calculating excitation energies of complex molecules, and is becoming increasingly popular for describing optical and spectroscopic properties of a variety of materials such as bulk solids, clusters and nanostructures. Other growing areas of applications of TDDFT are nonlinear dynamics of strongly excited electronic systems and molecular electronics. The purpose and scope of this Gordon Research Conference is to provide a platform for discussing the current state of the art of the rapidly progressing, highly interdisciplinary field of TDDFT, to identify and debate open questions, and to point out new promising research directions. The conference will bring together experts with a diverse background in chemistry, physics, and materials science.

Ullrich Carsten

2008-09-19

398

THE COLUMN DENSITY VARIANCE IN TURBULENT INTERSTELLAR MEDIA: A FRACTAL MODEL APPROACH  

SciTech Connect

Fractional Brownian motion structures are used to investigate the dependency of column density variance ({sigma}{sup 2}{sub lnN}) in the turbulent interstellar medium on the variance of three-dimensional density ({sigma}{sup 2}{sub ln{rho}}) and the power-law slope of the density power spectrum. We provide quantitative expressions to infer the three-dimensional density variance, which is not directly observable, from the observable column density variance and spectral slope. We also investigate the relationship between the column density variance and sonic Mach number (M{sub s}) in the hydrodynamic (HD) regime by assuming the spectral slope and density variance to be functions of sonic Mach number, as obtained from the HD turbulence simulations. They are related by the expression {sigma}{sup 2}{sub lnN} = A{sigma}{sub ln{rho}} {sup 2} = Aln (1 + b {sup 2} M{sup 2}{sub s}), suggested by Burkhart and Lazarian for the magnetohydrodynamic case. The proportional constant A varies from Almost-Equal-To 0.2 to Almost-Equal-To 0.4 in the HD regime as the turbulence forcing parameter b increases from 1/3 (purely solenoidal forcing) to 1 (purely compressive forcing). It is also discussed that the parameter A is lowered in the presence of a magnetic field.

Seon, Kwang-Il, E-mail: kiseon@kasi.re.kr [Korea Astronomy and Space Science Institute, Daejeon 305-348 (Korea, Republic of); Astronomy and Space Science Major, University of Science and Technology, Daejeon 305-350 (Korea, Republic of)

2012-12-20

399

Density-dependent flowering phenology, outcrossing, and reproduction in Impatiens capensis  

Microsoft Academic Search

We investigated the effects of plant density on cleistogamous (CL) and chasmogamous (CH) flowering phenology and seed production in a natural Impatiens capensis population, by censusing individually marked plants at experimentally reduced and natural densities. CL flowering was earlier at natural density. This plastic density response may have resulted from a stress-related threshold for CL flowering; slower growing plants at

J. Schmitt; J. Eccleston; D. W. Ehrhardt

1987-01-01

400

Density-dependent response to mycorrhizal infection in Abutilon theophrasti Medic  

Microsoft Academic Search

One purpose of this study was to determine whether an increase in plant density would result in a decrease in response to mycorrhizal infection (particularly as measured by phosphorus content). Increases in plant density generally result in increases in root density in the volume of soil occupied by the plants. Root density, in turn, largely determines phosphorus uptake. If mycorrhizal

Roger T. Koide

1991-01-01

401

An Unusual Approach to Dependability for Space SW-Applications  

NASA Astrophysics Data System (ADS)

Much more than fault tolerance, space applications require dependability, which is the combination of availability, reliability and safety. Dependability is a major challenge when designing space computing systems, both at software and hardware level. After 30 years of contentious research on how to achieve high dependability, not an universal solution has been found. A huge effort has been invested to improve reliability, using reliable radiation hardened components. However, failures cannot be eliminated totally. The best solution one can achieve is just to postpone failures. Even when significantly postponed, failures still may occur.

Montenegro, S.; Vitulli, R.

2007-08-01

402

Density-Dependent Regulation of Brook Trout Population Dynamics along a Core-Periphery Distribution Gradient in a Central Appalachian Watershed  

PubMed Central

Spatial population models predict strong density-dependence and relatively stable population dynamics near the core of a species' distribution with increasing variance and importance of density-independent processes operating towards the population periphery. Using a 10-year data set and an information-theoretic approach, we tested a series of candidate models considering density-dependent and density-independent controls on brook trout population dynamics across a core-periphery distribution gradient within a central Appalachian watershed. We sampled seven sub-populations with study sites ranging in drainage area from 1.3–60 km2 and long-term average densities ranging from 0.335–0.006 trout/m. Modeled response variables included per capita population growth rate of young-of-the-year, adult, and total brook trout. We also quantified a stock-recruitment relationship for the headwater population and coefficients of variability in mean trout density for all sub-populations over time. Density-dependent regulation was prevalent throughout the study area regardless of stream size. However, density-independent temperature models carried substantial weight and likely reflect the effect of year-to-year variability in water temperature on trout dispersal between cold tributaries and warm main stems. Estimated adult carrying capacities decreased exponentially with increasing stream size from 0.24 trout/m in headwaters to 0.005 trout/m in the main stem. Finally, temporal variance in brook trout population size was lowest in the high-density headwater population, tended to peak in mid-sized streams and declined slightly in the largest streams with the lowest densities. Our results provide support for the hypothesis that local density-dependent processes have a strong control on brook trout dynamics across the entire distribution gradient. However, the mechanisms of regulation likely shift from competition for limited food and space in headwater streams to competition for thermal refugia in larger main stems. It also is likely that source-sink dynamics and dispersal from small headwater habitats may partially influence brook trout population dynamics in the main stem. PMID:24618602

Huntsman, Brock M.; Petty, J. Todd

2014-01-01

403

Frequency-and electric-field-dependent conductivity of single-walled carbon nanotube networks of varying density  

E-print Network

Frequency- and electric-field-dependent conductivity of single-walled carbon nanotube networks October 2007; published 20 February 2008 We present measurements of the frequency- and electric-field-dependent conductivity of single-walled car- bon nanotube SWCNT networks of various densities. The ac conductivity

Gruner, George

404

Temperature dependence of density, thermal expansion coefficient and shear viscosity of supercooled glycerol as a reflection of its structure  

Microsoft Academic Search

The relationship of the microstructure of supercooled, highly viscous glycerol to the temperature dependence of its density, thermal expansion coefficient, and shear viscosity are discussed. The character of this temperature dependence at the transition from low viscosity state to the solid amorphous state (solidified state without nuclei) is described with help of function psi, which can be interpreted as the

Ivan V. Blazhnov; Nikolay P. Malomuzh; Sergey V. Lishchuk

2004-01-01

405

The Effects of Text Density Levels and the Cognitive Style of Field Dependence on Learning from a CBI Tutorial  

ERIC Educational Resources Information Center

The purpose of this study was to investigate the effects of variations in text density levels and the cognitive style of field dependence on learning from a CBI tutorial, based on the dependent measures of achievement, reading comprehension, and reading rate, and of lesson completion time. Eighty college undergraduate students were randomly…

Ipek, Ismail

2011-01-01

406

Conditional Dependencies: A Principled Approach to Improving Data Quality  

E-print Network

by supporting similarity and matching operators across relations. In con- trast to traditional dependencies, loading), as well as research prototype systems, e.g., Ajax, Potter's Wheel, Artkos and Telcordia (see [2

Antwerpen, Universiteit

407

Conditional Dependencies: A Principled Approach to Improving Data Quality  

E-print Network

by supporting similarity and matching operators across relations. In con- trast to traditional dependencies prototype systems, e.g., Ajax, Potter's Wheel, Artkos and Telcordia (see [2,23] for a survey). Most data

Fan, Wenfei

408

Pharmacologic approaches to the treatment of cocaine dependence.  

PubMed Central

When pharmacologic agents are considered in the treatment of cocaine addiction, the objective of such treatment--sustained abstinence--must be considered. Medication and medical approaches have been disappointing in the treatment of cocaine overdose. The central neurobiologic mechanism(s) involved in cocaine toxicity are poorly understood. Without a cocaine antagonist, pharmacologic approaches have been less than promising in preventing relapse. Various psychoactive medications have been tried in early cocaine abstinence, with some success. PMID:1971975

Taylor, W A; Gold, M S

1990-01-01

409

Many-Body Approaches to Time-Dependent Resonance Fluorescence  

Microsoft Academic Search

The problem of methods to find time-dependent spectra of resonance fluorescence for a medium excited by a short laser pulse is considered. Using the generalized second Born approximation for BBGKY-hierarchy, along with the method of coherent states, we have derived the equations to describe time-dependent scattering of a laser pulse in a two-level medium omitting the rescattering of spontaneous emission.

Vl. C. Roerich; A. N. Starostin; A. A. Panteleev

2003-01-01

410

Current and emerging treatment approaches for tobacco dependence  

Microsoft Academic Search

Nicotine in tobacco is the primary reason why most people find it hard to stop using tobacco. Nicotine creates dependence\\u000a by activating the dopaminergic reward system in the brain. Physiologic withdrawal symptoms that occur when nicotine is no\\u000a longer administered reinforce continued nicotine administration to avoid withdrawal. Extrapolating from this evidence has\\u000a led to the development of tobacco dependence pharmacotherapy

K. Michael Cummings; Martin Mahoney

2006-01-01

411

Assessment of density functional theory for thermochemical approaches based on bond separation reactions.  

PubMed

The recently proposed ATOMIC protocol is a fully ab initio thermochemical protocol that rests upon the concept of bond separation reactions (BSRs) to correct for systematic errors of composite wave function approaches. It achieves high accuracy for atomization energies and derived heats of formation if basis set requirements for all contributing components are balanced carefully. The present work explores the potential of density functionals as possible replacements of composite wave function approaches in the ATOMIC protocol. Twenty density functionals are examined for their accuracy in thermochemical predictions based on calculated bond-separation energies and precomputed high-level data for the small parent molecules entering BSRs. The best density functionals outperform CCSD (coupled cluster with singles and doubles excitations), but none reaches the accuracy of well-balanced composite wave function approaches that consider quasiperturbational connected triples excitations at least with small basis sets. Some functionals show unexpected problems with bond separation reactions and are analyzed further with a model of empirically calibrated bond additivity corrections. Finally, the benefit of adding empirical dispersion terms to common density functionals is analyzed in the context of BSR-corrected thermochemistry. PMID:23214917

Bakowies, Dirk

2013-01-10

412

Polarization corrections to single-particle energies studied within the energy-density-functional and QRPA approaches  

E-print Network

Background: Models based on using perturbative polarization corrections and mean-field blocking approximation give conflicting results for masses of odd nuclei. Purpose: Systematically investigate the polarization and mean-field models, implemented within self-consistent approaches that use identical interactions and model spaces, so as to find reasons for the conflicts between them. Methods: For density-dependent interactions and with pairing correlations included, we derive and study links between the mean-field and polarization results obtained for energies of odd nuclei. We also identify and discuss differences between the polarization-correction and full particle-vibration-coupling (PVC) models. Numerical calculations are performed for the mean-field ground-state properties of deformed odd nuclei and then compared to the polarization corrections determined by using the approach that conserves spherical symmetry. Results: We have identified and numerically evaluated self-interaction (SI) energies that are...

Tarpanov, D; Dobaczewski, J; Carlsson, B G

2014-01-01

413

Factors Influencing Density-Dependent Groundwater Flow in the Michigan Basin  

NASA Astrophysics Data System (ADS)

Regional-scale density-dependent groundwater flow is analyzed in an approximately 18000 sq km domain of the Michigan basin centered on a site at Tiverton Ontario near the shore of Lake Huron for a proposed deep geologic repository (DGR) for low and intermediate level nuclear waste. Flow was also analyzed in an approximately 600 km west-to-east cross-section through the center of the basin. Both domains extend from the Precambrian basement to the surface and include minimal upscaling of the complex stratigraphy in the basin. The model FRAC3DVS-OPG was used for all analyses. The hydraulic gradients across the basin are small as both Lake Huron and Lake Michigan have the same water surface elevation. As a result, groundwater flow in the basin is expected to be stagnant. Hydrogeologic parameters for the models were developed from borehole and petrophysics data from the DGR site for units from the Cambrian sandstone to the Devonian. Literature data were used for the shallower units in Michigan. Excluding the surficial drift, the hydraulic conductivity in the basin ranges from 3x10e-6 m/s in the Cambrian to less than 10e-14 m/s in the Ordovician sediments. Groundwater flow is sensitive to the distribution of total dissolved solids concentration with concentrations ranging up to 384 g/L in the Guelph formation in the Silurian. Both TDS data from porewater and groundwater at the DGR site and literature data for TDS versus depth were assigned to the sedimentary rock. The TDS distribution with depth for the Precambrian rock was assigned using both data for the Canadian Shield and a literature based model. Data at the DGR site indicates that the Cambrian is overpressured with respect to the surface while the Ordovician sediments are underpressured. It is hypothesized that the underpressures are the result of the presence of a gas phase in the units. The steps in determining a converged solution for saturated density-dependent flow were as follows: (1) solve steady state density-independent flow, (2) assign a TDS concentration distribution and allow the initial equivalent freshwater heads to equilibrate to the fixed TDS distribution, (3) allow the resulting heads to equilibrate to the TDS concentration with solute transport enabled. Convergence of a solution for the regional-scale model was not achieved without the middle step. For the Michigan Basin cross-section, the 3rd step would not yield a converged solution using FRAC3DVS-OPG. Flow in both analyses is sensitive to surface topography and the TDS distribution. The overpressure in the Cambrian could be described by density differences across the basin and surface topography differences. The underpressures can be described with a gas water analysis using TOUGH2-MP. Paleoclimate analyses that included mechanical loading could not describe the underpressures. Flow in the intracratonic Michigan Basin is complex and dynamic as a result of glaciation. Converged solutions are difficult to achieve. Flow in the low permeability units such as those of the Ordovician and Silurian is negligible with solute transport being diffusion dominant. The analyses provide a bench mark for evaluating the upscaling of stratigraphic units required in continental-scale simulations.

Sykes, J. F.; Normani, S. D.; Yin, Y.

2010-12-01

414

Dynamic Inversion via State Dependent Riccati Equation Approach: Application to Flight Vehicles  

E-print Network

Dynamic Inversion via State Dependent Riccati Equation Approach: Application to Flight Vehicles Algebraic Riccati Equation (SDRE) technique is rapidly emerging as a design method, which provides Riccati equations (SDRE) and Dynamic In- version control law. The State Dependent Riccati Equation

Yedavalli, Rama K.

415

Treatment for Tobacco Dependence: Effect on Brain Nicotinic Acetylcholine Receptor Density  

PubMed Central

Cigarette smoking leads to upregulation of brain nicotinic acetylcholine receptors (nAChRs), including the common ?4?2* nAChR subtype. Although a substantial percentage of smokers receive treatment for tobacco dependence with counseling and/or medication, the effect of a standard course of these treatments on nAChR upregulation has not yet been reported. In the present study, 48 otherwise healthy smokers underwent positron emission tomography (PET) scanning with the radiotracer 2-FA (for labeling ?4?2* nAChRs) before and after treatment with either cognitive-behavioral therapy, bupropion HCl, or pill placebo. Specific binding volume of distribution (VS/fP), a measure proportional to ?4?2* nAChR density, was determined for regions known to have nAChR upregulation with smoking (prefrontal cortex, brainstem, and cerebellum). In the overall study sample, significant decreases in VS/fP were found for the prefrontal cortex, brainstem, and cerebellum of ?20 (±35), ?25 (±36), and ?25 (±31)%, respectively, which represented movement of VS/fP values toward values found in non-smokers (mean 58.2% normalization of receptor levels). Participants who quit smoking had significantly greater reductions in VS/fP across regions than non-quitters, and correlations were found between reductions in cigarettes per day and decreases in VS/fP for brainstem and cerebellum, but there was no between-group effect of treatment type. Thus, smoking reduction and cessation with commonly used treatments (and pill placebo) lead to decreased ?4?2* nAChR densities across brain regions. Study findings could prove useful in the treatment of smokers by providing encouragement with the knowledge that decreased smoking leads to normalization of specific brain receptors. PMID:23429692

Brody, Arthur L; Mukhin, Alexey G; Stephanie Shulenberger; Mamoun, Michael S; Kozman, Maggie; Phuong, Jonathan; Neary, Meaghan; Luu, Trinh; Mandelkern, Mark A

2013-01-01

416

Neutron star properties in density-dependent relativistic Hartree-Fock theory  

E-print Network

With the equations of state provided by the newly developed density dependent relativistic Hartree-Fock (DDRHF) theory for hadronic matter, the properties of the static and $\\beta$-equilibrium neutron stars without hyperons are studied for the first time, and compared to the predictions of the relativistic mean field (RMF) models and recent observational data. The influences of Fock terms on properties of asymmetric nuclear matter at high densities are discussed in details. Because of the significant contributions from the $\\sigma$- and $\\omega$-exchange terms to the symmetry energy, large proton fractions in neutron stars are predicted by the DDRHF calculations, which strongly affect the cooling process of the star. The critical mass about 1.45 $M_\\odot$, close to the limit 1.5 $M_\\odot$ determined by the modern soft X-ray data analysis, is obtained by DDRHF with the effective interactions PKO2 and PKO3 for the occurrence of direct Urca process in neutron stars. The maximum masses of neutron stars given by the DDRHF calculations lie between 2.45 M$_\\odot$ and 2.49 M$_\\odot$, which are in reasonable agreement with high pulsar mass $2.08 \\pm 0.19 M_\\odot$ from PSR B1516+02B. It is also found that the mass-radius relations of neutron stars determined by DDRHF are consistent with the observational data from thermal radiation measurement in the isolated neutron star RX J1856, QPOs frequency limits in LMXBs 4U 0614+09 and 4U 1636-536, and redshift determined in LMXBs EXO 0748-676.

Bao Yuan Sun; Wen Hui Long; Jie Meng; U. Lombardo

2009-10-22

417

Generalized Floquet theoretical formulation of time-dependent density functional theory for many-electron systems in multicolor laser fields  

Microsoft Academic Search

We generalize the Floquet formulation of the time-dependent density . w . x functional theory TDDFT Telnov and Chu, Chem. Phys. Lett. 264, 466 1997 to the case of many-electron systems in multicolor or polychromatic time-dependent fields. It is shown that the time-dependent Kohn)Sham equations can be transformed into an equivalent time-independent infinite-dimensional Floquet Hamiltonian eigenvalue problem. For the case

Dmitry A. Telnov; Shih-I Chu

1998-01-01

418

Plasmonlike resonances in atomic chains: A time-dependent density-functional theory study  

NASA Astrophysics Data System (ADS)

We studied plasmonlike resonances in one-dimensional (1D) atomic chain systems by using time-dependent density-functional theory (TDDFT) and local density functional theory. Recent TDDFT studies have shown the coexistence of longitudinal and transverses collective plasmonlike resonances in the atomic chains of simple and noble metals. Such atomic chains contain only a few atoms. The induced polarization occurs along the atomic chain in longitudinal mode and perpendicular to the atomic chain in transverse mode. To understand the emergence of plasmonlike resonance in 1D atomic chains better, we studied carbon chains in which plasmonic resonances are not expected to occur. We used TDDFT to study the emergence of collective resonances in various forms of carbon chains, cumulenes CnH4, polyynes CnH2, and alkenes CnHn+2. The excitation energy and dipole oscillation strengths of these systems were determined through TDDFT by using the turbomole package. We determined how collective plasmonlike resonances arise from single-electron excitations when the number of electrons increases as the carbon chain lengthens. The collective excitation behavior is then compared with that of metallic atomic chains. Our TDDFT results showed longitudinal collective modes for cumulenes and polyynes, as well as for finite-length chains. These collective excitations exhibit the same behavior as that of longitudinal "plasmon" previously identified in sodium and silver chains, although polyynes are gapped in the long chain limit. Such longitudinal excitations are absent in alkenes. However, unlike metal atomic chains, carbon chains exhibited no transverse collective mode. The band structure of periodic atomic chains was calculated by using the standard local density functional method. These structures were used to interpret the results and to relate the single-electron excitation to the collective plasmonlike response. Within the one-particle quantum-well picture, the longitudinal mode in the linear atomic chain arises from intraband transition with ?q =1, where q is the quantum number of quantum wells. ?q =1 intraband transitions can be found in metallic (e.g., Na) chains and in carbon chains (cumulenes and polyynes), such longitudinal collective mode is rather "generic". Meanwhile, the transverse modes of the sodium chains are attributed to interband transitions with an even ?q (dominated by ?q =0), and such transverse collective excitations only form if the allowed ?q =0 transitions occur between bands that are parallel to each other. Such bands can be found in simple metals, but not in carbon chains.

Huang, Yu-Hui; Lin, Ken-Ming; Leung, T. C.; Chan, C. T.

2014-08-01

419

Flowing afterglow measurements of the density dependence of gas-phase ion-ion mutual neutralization reactions  

SciTech Connect

We have studied the dependence of several ion-ion mutual neutralization (MN) reactions on helium density in the range from 1.6 Multiplication-Sign 10{sup 16} to 1.5 Multiplication-Sign 10{sup 17} cm{sup -3} at 300 K, using the Variable Electron and Neutral Density Attachment Mass Spectrometry method. The rate coefficients of the reactions Ar{sup +}+ Br{sub 2}{sup -}, Ar{sup +}+ SF{sub 6}{sup -}, and Ar{sup +}+ C{sub 7}F{sub 14}{sup -} were found to be independent of gas density over the range studied, in disagreement with earlier observations that similar MN reactions are strongly enhanced at the same gas densities. The cause of the previous enhancement with density is traced to the use of 'orbital-motion-limit' theory to infer ion densities from the currents collected by ion-attracting Langmuir probes in a region where it is not applicable.

Shuman, Nicholas S.; Viggiano, Albert A. [Air Force Research Laboratory, Space Vehicles Directorate, Kirtland Air Force Base, New Mexico 87117 (United States); Johnsen, Rainer [Department of Physics and Astronomy, University of Pittsburgh, Pittsburgh, Pennsylvania 15260 (United States)

2013-05-28

420

Time-dependent density functional theory study of cobalt corrinoids: Electronically excited states of methylcobalamin  

NASA Astrophysics Data System (ADS)

Time-dependent density functional theory (TDDFT) has been applied to the analysis of the electronic spectra of methylcobalamin (MeCbl) and its derivative in which the trans axial base was replaced by a water molecule (MeCbi-H2O). The latter corresponds to the situation encountered in strongly acidic solutions. The study primarily focuses on the accuracy of two functionals, the hybrid B3LYP and the gradient corrected BP86, in dealing with the electronic excitations. The high resolution crystal structure of MeCbl was the source of the initial coordinates. To generate the initial structures, the full MeCbl was simplified by replacing the corrin side chains by H atoms. The vertical excitation energies, together with the corresponding oscillator strengths, were calculated at the optimized BP86 and B3LYP structures of the ground electronic state of the complexes. The NBO analysis shows that the B3LYP functional gives a bonding description of the ground state as a more polarized covalent bond compared to that given by BP86. The latter functional has more covalent bonding and is thus more appropriate for modeling the axial bonding properties. To validate the accuracy of the present TDDFT analysis, the computed excitations were directly compared to the absorption spectra of MeCbl. In order to obtain a reliable agreement between experiment and theory, the two-parameter scaling technique was introduced, which compensates differently the low-energy and high-energy excitations. Electronic excitations strongly depend on the choice of the functional. Transitions involving corrin ?-->?* excitations are better described by the B3LYP functional while transitions associated with metal-to-ligand (d/?-->?*/d) excitations are better described by BP86. These differences can be associated with the different bonding descriptions obtained by B3LYP and BP86.

Andruniów, Tadeusz; Jaworska, Maria; Lodowski, Piotr; Zgierski, Marek Z.; Dreos, Renata; Randaccio, Lucio; Kozlowski, Pawel M.

2008-08-01

421

Two-electron Rabi oscillations in real-time time-dependent density-functional theory.  

PubMed

We investigate the Rabi oscillations of electrons excited by an applied electric field in several simple molecular systems using time-dependent configuration interaction (TDCI) and real-time time-dependent density-functional theory (RT-TDDFT) dynamics. While the TDCI simulations exhibit the expected single-electron Rabi oscillations at a single resonant electric field frequency, Rabi oscillations in the RT-TDDFT simulations are a two-electron process. The existence of two-electron Rabi oscillations is determined both by full population inversion between field-free molecular orbitals and the behavior of the instantaneous dipole moment during the simulations. Furthermore, the Rabi oscillations in RT-TDDFT are subject to an intensity threshold of the electric field, below which Rabi oscillations do not occur and above which the two-electron Rabi oscillations occur at a broad range of frequencies. It is also shown that at field intensities near the threshold intensity, the field frequency predicted to induce Rabi oscillations by linear response TDDFT only produces detuned Rabi oscillations. Instead, the field frequency that yields the full two-electron population inversion and Rabi oscillation behavior is shown to be the average of single-electron transition frequencies from the ground S0 state and the doubly-excited S2 state. The behavior of the two-electron Rabi oscillations is rationalized via two possible models. The first model is a multi-photon process that results from the electric field interacting with the three level system such that three level Rabi oscillations may occur. The second model suggests that the mean-field nature of RT-TDDFT induces paired electron propagation. PMID:25399137

Habenicht, Bradley F; Tani, Noriyuki P; Provorse, Makenzie R; Isborn, Christine M

2014-11-14

422

Modes of competition: adding and removing brown trout in the wild to understand the mechanisms of density-dependence.  

PubMed

While the prevalence of density-dependence is well-established in population ecology, few field studies have investigated its underlying mechanisms and their relative population-level importance. Here, we address these issues, and more specifically, how differences in body-size influence population regulation. For this purpose, two experiments were performed in a small coastal stream on the Swedish west coast, using juvenile brown trout (Salmo trutta) as a study species. We manipulated densities of large and small individuals, and observed effects on survival, migration, condition and individual growth rate in a target group of intermediate-sized individuals. The generality of the response was investigated by reducing population densities below and increasing above the natural levels (removing and adding large and small individuals). Reducing the density (relaxing the intensity of competition) had no influence on the response variables, suggesting that stream productivity was not a limiting factor at natural population density. Addition of large individuals resulted in a negative density-dependent response, while no effect was detected when adding small individuals or when maintaining the natural population structure. We found that the density-dependent response was revealed as reduced growth rate rather than increased mortality and movement, an effect that may arise from exclusion to suboptimal habitats or increased stress levels among inferior individuals. Our findings confirm the notion of interference competition as the primary mode of competition in juvenile salmonids, and also show that the feedback-mechanisms of density-dependence are primarily acting when increasing densities above their natural levels. PMID:23658736

Kaspersson, Rasmus; Sundström, Fredrik; Bohlin, Torgny; Johnsson, Jörgen I

2013-01-01

423

Modes of Competition: Adding and Removing Brown Trout in the Wild to Understand the Mechanisms of Density-Dependence  

PubMed Central

While the prevalence of density-dependence is well-established in population ecology, few field studies have investigated its underlying mechanisms and their relative population-level importance. Here, we address these issues, and more specifically, how differences in body-size influence population regulation. For this purpose, two experiments were performed in a small coastal stream on the Swedish west coast, using juvenile brown trout (Salmo trutta) as a study species. We manipulated densities of large and small individuals, and observed effects on survival, migration, condition and individual growth rate in a target group of intermediate-sized individuals. The generality of the response was investigated by reducing population densities below and increasing above the natural levels (removing and adding large and small individuals). Reducing the density (relaxing the intensity of competition) had no influence on the response variables, suggesting that stream productivity was not a limiting factor at natural population density. Addition of large individuals resulted in a negative density-dependent response, while no effect was detected when adding small individuals or when maintaining the natural population structure. We found that the density-dependent response was revealed as reduced growth rate rather than increased mortality and movement, an effect that may arise from exclusion to suboptimal habitats or increased stress levels among inferior individuals. Our findings confirm the notion of interference competition as the primary mode of competition in juvenile salmonids, and also show that the feedback-mechanisms of density-dependence are primarily acting when increasing densities above their natural levels. PMID:23658736

Kaspersson, Rasmus; Sundström, Fredrik; Bohlin, Torgny; Johnsson, Jörgen I.

2013-01-01

424

Lianas escape self-thinning: Experimental evidence of positive density dependence in temperate lianas Celastrus orbiculatus and C. scandens  

USGS Publications Warehouse

The neighborhood density of plants strongly affects their growth, reproduction, and survival. In most cases, high density increases competition and negatively affects a focal plant in predictable ways, leading to the self-thinning law. There are, however, situations in which high densities of plants facilitate focal plant performance, resulting in positive density dependence. Despite their importance in forest gap dynamics and distinctive growth form, there have been very few studies of the effect of density on lianas or vines. We grew an invasive (Celastrus orbiculatus) and a native (Celastrus scandens) liana species together in three different density treatments, while also manipulating the light and support availability. We found that across treatment conditions, C. orbiculatus always out-performed C. scandens, showing greater relative growth rate in height and diameter, greater biomass and higher survival. Both species responded similarly to the density treatments: with plants in high density not showing a decrease in relative height growth rate compared to medium density. Aboveground biomass for C. scandens was not affected by density, while for C. orbiculatus, the most massive plants were growing in medium density without support. More surprisingly, survival analysis indicated that the two species both had significantly lower mortality rates in the highest density treatment; this trend held true across the other treatments of light and supports. More generally, this study demonstrates that these lianas can escape the consequences of high density and thus the self-thinning law that affects self-supporting plants. This suggests a broader hypothesis about lianas in general: their greater flexibility in allocating growth resources allows them to grow taller and thinner without collapsing and thereby potentially escape shading and mortality even at high densities.

Leicht-Young, S. A.; Latimer, A. M.; Silander, J. A.

2011-01-01

425

Case management of technology- dependent children: A family- centered approach  

Microsoft Academic Search

The number of technology-dependent children receiving home care has grown significantly in re cent years. This article focuses on the hardships encountered by the families of these children as well as the direct and indirect costs that occur. These families face a bureaucratic maze that can be overwhelming and frustrating. In spite of the many drawbacks, these families always say

Laura V. Hill; Mary K. Thompson