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1

Time-dependent treatment of scattering - Integral equation approaches using the time-dependent amplitude density  

NASA Technical Reports Server (NTRS)

The time-dependent form of the Lippmann-Schwinger integral equation is used as the basis of several new wave packet propagation schemes. These can be formulated in terms of either the time-dependent wave function or a time-dependent amplitude density. The latter is nonzero only in the region of configuratiaon space for which the potential is nonzero, thereby in principle obviating the necessity of large grids or the use of complex absorbing potentials when resonances cause long collision times (leading, consequently, to long propagation times). Transition amplitudes are obtained in terms of Fourier transforms of the amplitude density from the time to the energy domain. The approach is illustrated by an application to a standard potential scattering model problem where, as in previous studies, the action of the kinetic energy operator is evaluated by fast Fourier transform (FFT) techniques.

Hoffman, David K.; Sharafeddin, Omar; Judson, Richard S.; Kouri, Donald J.

1990-01-01

2

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

3

Calculation of van der Waals coefficients in hydrodynamic approach to time-dependent density functional theory  

NASA Astrophysics Data System (ADS)

In this paper we employ hydrodynamic formulation of time-dependent density-functional theory to obtain coefficient C6 of the long-range part of the van der Waals interaction between alkali-metal clusters of large sizes. Such a calculation becomes computationally very demanding in the orbital-based Kohn-Sham formalism, but is quite simple in the hydrodynamic approach. This is because in hydrodynamic formulation, electron density and current density, rather than the orbitals, are employed as basic variables. We show that for intercations between the clusters of same sizes, C6 scales as the sixth power of the cluster radius and approaches the classically predicted value for large size clusters.

Banerjee, Arup; Harbola, Manoj K.

2002-11-01

4

Time-dependent localized Hartree-Fock density-functional linear response approach for photoionization of atomic excited states  

E-print Network

of atomic systems. This approach employs a spin-dependent localized Hartree- Fock exchange potential-shell 23 and open-shell 24 atomic systems. In this paper, we present a time-dependent localized HartreeTime-dependent localized Hartree-Fock density-functional linear response approach

Chu, Shih-I

5

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

6

A new temperature and humidity dependent surface site density approach for deposition ice nucleation  

NASA Astrophysics Data System (ADS)

Deposition nucleation experiments with Arizona Test Dust (ATD) as a surrogate for mineral dusts were conducted at the AIDA cloud chamber at temperatures between 220 and 250 K. The influence of the aerosol size distribution and the cooling rate on the ice nucleation efficiencies was investigated. Ice nucleation active surface site (INAS) densities were calculated to quantify the ice nucleation efficiency as a function of temperature, humidity and the aerosol surface area concentration. Additionally, a contact angle parameterization according to classical nucleation theory was fitted to the experimental data in order to relate the ice nucleation efficiencies to contact angle distributions. From this study it can be concluded that the INAS density formulation is a very useful tool to decribe the temperature and humidity dependent ice nucleation efficiency of ATD particles. Deposition nucleation on ATD particles can be described by a temperature and relative humidity dependent INAS density function ns(T, Sice) with ns(xtherm) = 1.88 × 105 \\centerdot exp(0.2659 \\centerdot xtherm) [m-2] (1) where the thermodynamic variable xtherm is defined as xtherm = -(T - 273.2) + (Sice-1) × 100 (2) with Sice>1 and within a temperature range between 226 and 250 K. For lower temperatures, xtherm deviates from a linear behavior with temperature and relative humidity over ice. Two different approaches for describing the time dependence of deposition nucleation initiated by ATD particles are proposed. Box model estimates suggest that the time dependent contribution is only relevant for small cooling rates and low number fractions of ice-active particles.

Steinke, I.; Hoose, C.; Möhler, O.; Connolly, P.; Leisner, T.

2014-07-01

7

A time-dependent momentum-space density functional theoretical approach for electron transport dynamics in molecular devices  

E-print Network

We propose a time-dependent density functional theoretical (TDDFT) approach in momentum (\\mathcal{P} ) space for the study of electron transport in molecular devices under arbitrary biases. The basic equation of motion, which is a time...

Chu, Shih-I; Zhou, Zhongyuan

2009-10-27

8

Theory of time-resolved photoelectron imaging. Comparison of a density functional with a time-dependent density functional approach  

Microsoft Academic Search

Time-resolved photoelectron differential cross sections are computed within a quantum dynamical theory that combines a formally exact solution of the nuclear dynamics with density functional theory (DFT)-based approximations of the electronic dynamics. Various observables of time-resolved photoelectron imaging techniques are computed at the Kohn-Sham and at the time-dependent DFT levels. Comparison of the results serves to assess the reliability of

Yoshi-Ichi Suzuki; Tamar Seideman; Mauro Stener

2004-01-01

9

A consumer-resource approach to the density-dependent population dynamics of mutualism  

USGS Publications Warehouse

Like predation and competition, mutualism is now recognized as a consumer resource (C-R) interaction, including, in particular, bi-directional (e.g., coral, plant- mycorrhizae) and uni-directional (e.g., ant-plant defense, plant-pollinator) C-R mutualisms. Here, we develop general theory for the density-dependent population dynamics of mutualism based on the C-R mechanism of interspecific interaction. To test the influence of C-R interactions on the dynamics and stability of bi- and uni-directional C-R mutualisms, we developed simple models that link consumer functional response of one mutualistic species with the resources supplied by another. Phase-plane analyses show that the ecological dynamics of C-R mutualisms are stable in general. Most transient behavior leads to an equilibrium of mutualistic coexistence, at which both species densities are greater than in the absence of interactions. However, due to the basic nature of C-R interactions, certain density-dependent conditions can lead to C-R dynamics characteristic of predator-prey interactions, in which one species overexploits and causes the other to go extinct. Consistent with empirical phenomena, these results suggest that the C-R interaction can provide a broad mechanism for understanding density-dependent population dynamics of mutualism. By unifying predation, competition, and mutualism under the common ecological framework of consumer-resource theory, we may also gain a better understanding of the universal features of interspecific interactions in general.

Holland, J. Nathaniel; DeAngelis, Donald L.

2010-01-01

10

Correlated density-dependent chiral forces for infinite-matter calculations within the Green's function approach  

NASA Astrophysics Data System (ADS)

The properties of symmetric nuclear and pure neutron matter are investigated within an extended self-consistent Green's function method that includes the effects of three-body forces. We use the ladder approximation for the study of infinite nuclear matter and incorporate the three-body interaction by means of a density-dependent two-body force. This force is obtained via a correlated average over the third particle, with an in-medium propagator consistent with the many-body calculation we perform. We analyze different prescriptions in the construction of the average and conclude that correlations provide small modifications at the level of the density-dependent force. Microscopic as well as bulk properties are studied, focusing on the changes introduced by the density-dependent two-body force. The total energy of the system is obtained by means of a modified Galitskii-Migdal-Koltun sum rule. Our results validate previously used uncorrelated averages and extend the availability of chirally motivated forces to a larger density regime.

Carbone, Arianna; Rios, Arnau; Polls, Artur

2014-11-01

11

New truncation scheme for a time-dependent density-matrix approach applied to the ground state of 16O  

NASA Astrophysics Data System (ADS)

The ground state of 16O is calculated by using a time-dependent density-matrix approach derived from a new truncation scheme of the Bogoliubov-Born-Green-Kirkwood-Yvon hierarchy for reduced density matrices, where a three-body density matrix is approximated by an antisymmetrized product of two-body density matrices. The new scheme is compared with a simpler truncation scheme previously used for the calculation of the ground state of 16O where the three-body density matrix is neglected and only two-particle-two-hole elements of the two-body density matrix are considered. It is shown that the results obtained from the two truncation schemes agree well with the exact solution.

Tohyama, Mitsuru

2015-01-01

12

A consumer-resource approach to the density-dependent population dynamics of mutualism.  

PubMed

Like predation and competition, mutualism is now recognized as a consumer-resource (C-R) interaction, including, in particular, bi-directional (e.g., coral, plant-mycorrhizae) and uni-directional (e.g., ant-plant defense, plant-pollinator) C-R mutualisms. Here, we develop general theory for the density-dependent population dynamics of mutualism based on the C-R mechanism of interspecific interaction. To test the influence of C-R interactions on the dynamics and stability of bi- and uni-directional C-R mutualisms, we developed simple models that link consumer functional response of one mutualistic species with the resources supplied by another. Phase-plane analyses show that the ecological dynamics of C-R mutualisms are stable in general. Most transient behavior leads to an equilibrium of mutualistic coexistence, at which both species densities are greater than in the absence of interactions. However, due to the basic nature of C-R interactions, certain density-dependent conditions can lead to C-R dynamics characteristic of predator-prey interactions, in which one species overexploits and causes the other to go extinct. Consistent with empirical phenomena, these results suggest that the C-R interaction can provide a broad mechanism for understanding density-dependent population dynamics of mutualism. By unifying predation, competition, and mutualism under the common ecological framework of consumer-resource theory, we may also gain a better understanding of the universal features of interspecific interactions in general. PMID:20503862

Holland, J Nathaniel; DeAngelis, Donald L

2010-05-01

13

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

14

Photoionization of mercury: A relativistic time-dependent density-functional-theory approach  

NASA Astrophysics Data System (ADS)

The relativistic time-dependent density functional theory (RTDDFT) has been applied to the photoionization of mercury in the energy range from the threshold up to 300 eV, thus covering almost all the photon energy range for which experimental data are available. Partial cross sections and asymmetry parameter profiles for the 6s, 5d, 5p, and 4f subshells have been calculated and compared with earlier relativistic random-phase approximation and RTDDFT theoretical calculations and with the experimental results. A study of the spin polarization of photoelectrons from the outer subshells 6s and 5d at RTDDFT level is also presented. The use of the LB94 exchange-correlation potential together with an implementation of the RTDDFT equations in a B-spline basis set based on a noniterative procedure for the calculation of the induced response potential has permitted the study, at RTDDFT level, of the autoionization resonances converging to the 5d3/2,5/2 and 5p1/2,3/2 thresholds. Comparison of the RTDDFT results with the other theoretical and experimental data available confirms the effectiveness of the method in the description of correlation and relativistic effects in the photoionization of such a heavy system.

Toffoli, D.; Stener, M.; Decleva, P.

2002-07-01

15

Spin-dependent localized Hartree-Fock density-functional approach for the accurate treatment of inner-shell excitation of closed-shell atoms  

E-print Network

-shell excited states of closed-shell atoms by means of a spin- dependent localized Hartree-Fock SLHF densitySpin-dependent localized Hartree-Fock density-functional approach for the accurate treatment of inner-shell excitation of closed-shell atoms Zhongyuan Zhou1,2 and Shih-I Chu1 1 Department of Chemistry

Chu, Shih-I

16

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

17

Analytic derivative couplings in time-dependent density functional theory: Quadratic response theory versus pseudo-wavefunction approach.  

PubMed

We revisit the formalism for analytic derivative couplings between excited states in time-dependent density functional theory (TDDFT). We derive and implement these couplings using quadratic response theory, then numerically compare this response-theory formulation to couplings implemented previously based on a pseudo-wavefunction formalism and direct differentiation of the Kohn-Sham determinant. Numerical results, including comparison to full configuration interaction calculations, suggest that the two approaches perform equally well for many molecular systems, provided that the underlying DFT method affords accurate potential energy surfaces. The response contributions are found to be important for certain systems with high symmetry, but can be calculated with only a moderate increase in computational cost beyond what is required for the pseudo-wavefunction approach. In the case of spin-flip TDDFT, we provide a formal proof that the derivative couplings obtained using response theory are identical to those obtained from the pseudo-wavefunction formulation, which validates our previous implementation based on the latter formalism. PMID:25681889

Zhang, Xing; Herbert, John M

2015-02-14

18

Solvent effects on optical properties of molecules: A combined time-dependent density functional theory/effective fragment potential approach  

NASA Astrophysics Data System (ADS)

A quantum mechanics/molecular mechanics (QM/MM) type of scheme is employed to calculate the solvent-induced shifts of molecular electronic excitations. The effective fragment potential (EFP) method was used for the classical potential. Since EFP has a density dependent functional form, in contrast with most other MM potentials, time-dependent density functional theory (TDDFT) has been modified to combine TDDFT with EFP. This new method is then used to perform a hybrid QM/MM molecular dynamics simulation to generate a simulated spectrum of the n ??? vertical excitation energy of acetone in vacuum and with 100 water molecules. The calculated water solvent effect on the vertical excitation energy exhibits a blueshift of the n ??? vertical excitation energy in acetone (??1=0.211 eV), which is in good agreement with the experimental blueshift.

Yoo, Soohaeng; Zahariev, Federico; Sok, Sarom; Gordon, Mark S.

2008-10-01

19

Time Dependent Density Functional Theory An introduction  

E-print Network

Time Dependent Density Functional Theory An introduction Francesco Sottile LSI, Ecole Polytechnique (ETSF) Time Dependent Density Functional Theory Palaiseau, 7 February 2012 1 / 32 #12;Outline 1 Frontiers 4 Perspectives and Resources Francesco Sottile (ETSF) Time Dependent Density Functional Theory

Botti, Silvana

20

First principles calculation of field emission from nanostructures using time-dependent density functional theory: A simplified approach  

NASA Astrophysics Data System (ADS)

We introduce a new simplified method for computing the electron field emission current in short carbon nanotubes and graphene sheets using ab-initio computation in slab-periodic simulation cells. The evolution of the wave functions using Time-Dependent Density Functional Theory is computed by utilizing the Crank-Nicholson propagator and using the Octopus code (Castro et al., 2006 [1]), where we skip the wave function relaxation step elaborated by Han et al. (2002) [2], and apply a norm-conserving wave propagation method instead of the norm-nonconserving seventh-order Taylor Expansion method used by Araidai et al. (2004) [3]. Our method is mainly geared towards reducing the time it takes to compute the wave function propagation and enhancing the calculation precision. We found that in pristine carbon nanotubes, the emitted charge tends to emerge mostly from electrons that are concentrated at the nanotube tip region. The charge beam concentrates into specific channel structures, showing the utility of carbon nanotubes in precision emission applications.

Tawfik, Sherif A.; El-Sheikh, S. M.; Salem, N. M.

2011-05-01

21

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

22

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

23

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

24

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

25

Time Dependent Density Functional Theory An Introduction  

E-print Network

Time Dependent Density Functional Theory An Introduction Francesco Sottile Laboratoire des Solides) Belfast, 29 Jun 2007 Time Dependent Density Functional Theory Francesco Sottile #12;Intro Formalism Linear Response Formalism 3 TDDFT in practice: The ALDA: Achievements and Shortcomings 4 Resources Time

Botti, Silvana

26

Toward effective and reliable fluorescence energies in solution by a new state specific polarizable continuum model time dependent density functional theory approach  

Microsoft Academic Search

A state specific (SS) model for the inclusion of solvent effects in time dependent density functional theory (TD-DFT) computations of emission energies has been developed and coded in the framework of the so called polarizable continuum model (PCM). The new model allows for a rigorous and effective treatment of dynamical solvent effects in the computation of fluorescence and phosphorescence spectra

Roberto Improta; Giovanni Scalmani; Michael J. Frisch; Vincenzo Barone

2007-01-01

27

Orbital-dependent density functionals: Theory and applications Stephan Kmmel*  

E-print Network

Orbital-dependent density functionals: Theory and applications Stephan Kümmel* Physikalisches-generalized-gradient approximation 36 2. Perturbation theory 38 3. Range-separated functionals and other approaches 40 IV. Orbital provides a perspective on the use of orbital-dependent functionals, which is currently considered one

Wu, Zhigang

28

Excited-state properties of organic photovoltaic materials from a many-body Lanczos-GW approach and time-dependent density functional theory  

Microsoft Academic Search

Many-body GW and time-dependent DFT (TDDFT) are two approaches that in principle allow us to access electronic and optical excited-state properties, which are critical to characterize and engineer organic photovoltaic materials. Recently we have implemented a novel approach to GW that constructs an optimal polarizability basis without using conduction states and avoids plasmon-pole approximation and sum-over-states bottlenecks using a modified

Xiaofeng Qian; Paolo Umari; Davide Ceresoli; Nicola Marzari

2010-01-01

29

Effects of electron structure and multielectron dynamical response on strong-field multiphoton ionization of diatomic molecules with arbitrary orientation: An all-electron time-dependent density-functional-theory approach  

E-print Network

We present a time-dependent density-functional-theory approach for the ab initio study of the effect of correlated multielectron responses on the multiphoton ionization (MPI) of diatomic molecules N2, O2, and F2 in intense short laser pulse fields...

Chu, Shih-I; Telnov, Dmitry A.

2009-04-03

30

Density functional approaches to atomic nuclei  

E-print Network

Nuclear mean-field models are briefly reviewed to illustrate its foundation and necessity of state dependence in effective interactions. This state dependence is successfully taken into account by the density dependence, leading to the energy density functional. Recent results for photoabsorption cross sections in spherical and deformed Nd isotopes are shown.

Takashi Nakatsukasa

2012-09-22

31

The cosmological dependence of cluster density profiles  

NASA Technical Reports Server (NTRS)

We use N-body simulations to study the shape of mean cluster density and velocity profiles in the nonlinear regime 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(k) varies as k(exp n). Einstein-de Sitter, open (Omega(sub 0) = 0.2 and 0.1) and flat, low density (Omega(sub 0) = 0.2 lambda(sub 0) = 0.8) models are examined, with initial spectral indices n = -2, -1 and 0. For each model, we stack clusters in an appropriately scaled manner to define an average density profile in the nonlinear regime. The profiles are well fit by a power law rho(r) varies as r(exp -alpha) for radii whereat the local density contrast is between 100 and 3000. This covers 99% of the cluster volume. We find a clear trend toward steeper slopes (larger alphas) with both increasing n and decreasing Omega(sub 0). The Omega(sub 0) dependence is partially masked by the n dependence; there is degeneracy in the values of alpha between the Einstein-de Sitter and flat, low-density cosmologies. However, the profile slopes in the open models are consistently higher than the Omega = 1 values for the range of n examined. 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. An attempt is made to recover the unsoftened profiles assuming angular momentum invariance. The recovered profiles in Einstein-de Sitter cosmologies are consistent with a pure power law up to the highest density contrasts (10(exp 6)) accessible with our resolution. The low-density models show significant deviation from a power law above density contrasts approximately 10(exp 5). We interpret this curvature as reflecting the non-scale-invariant nature of the background cosmology in these models. These results are at the limit of our resolution and so should be tested in the future using simulations with larger numbers of particles. Such simulations will also provide insight on the broader problem of understanding, in a statistical sense, the full phase space structure of collapsed, cosmological halos.

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

1994-01-01

32

Mechanisms of density dependence in fluctuating vole populations: deducing annual density dependence from seasonal processes  

Microsoft Academic Search

Based on recent advances in time-series analyses of ecological dynamics using statistical and mathematical models, we summarise our recent results on the seasonal processes in the annual population dynamics of the grey-sided vole Clethrionomys rufocanus (Sundevall, 1846) in Hokkaido, Japan, and report additional analyses on annual and seasonal density dependence. Annual direct density dependence was strong in almost all populations.

Takashi Saitoh; NilsChr. Stenseth; Hildegunn Viljugrein; Marte O. Kittilsen

2003-01-01

33

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

34

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.  

PubMed

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. PMID:24682050

Kityk, A V

2014-07-15

35

Size-dependent density of zirconia nanoparticles.  

PubMed

The correlation between density and specific surface area of ZrO2 nanoparticles (NPs) was studied. The NPs were produced using a hydrothermal process involving microwave heating. The material was annealed at 1100 °C which resulted in an increase in the average grain size of the ZrO2 NPs from 11 to 78 nm and a decrease in the specific surface area from 97 to 15 m(2)/g. At the same time, the density increased from 5.22 g/m(3) to 5.87 g/m(3). This effect was interpreted to be the result of the presence of a hydroxide monolayer on the NP surface. A smaller ZrO2 grain size was correlated with a larger contribution of the low density surface layer to the average density. To prove the existence of such a layer, the material was synthesized using 50% heavy water. Fourier transform infrared spectroscopy (FTIR) permitted the identification of the -OD groups created during synthesis. It was found that the -OD groups persisted on the ZrO2 surface even after annealing at 1100 °C. This hydroxide layer is responsible for the decrease in the average density of the NPs as their size decreases. This study of the correlation between particle size and density may be used to assess the quality of the NPs. In most cases, the technological aim is to avoid an amorphous layer and to obtain fully crystalline nanoparticles with the highest density possible. However, due to the effect of the surface layers, there is a maximum density which can be achieved for a given average NP diameter. The effect of the surface layer on the NP density becomes particularly evident for NPs smaller than 50 nm, and thus, the density of nanoparticles is size dependent. PMID:25671149

Opalinska, Agnieszka; Malka, Iwona; Dzwolak, Wojciech; Chudoba, Tadeusz; Presz, Adam; Lojkowski, Witold

2015-01-01

36

Size-dependent density of zirconia nanoparticles  

PubMed Central

Summary The correlation between density and specific surface area of ZrO2 nanoparticles (NPs) was studied. The NPs were produced using a hydrothermal process involving microwave heating. The material was annealed at 1100 °C which resulted in an increase in the average grain size of the ZrO2 NPs from 11 to 78 nm and a decrease in the specific surface area from 97 to 15 m2/g. At the same time, the density increased from 5.22 g/m3 to 5.87 g/m3. This effect was interpreted to be the result of the presence of a hydroxide monolayer on the NP surface. A smaller ZrO2 grain size was correlated with a larger contribution of the low density surface layer to the average density. To prove the existence of such a layer, the material was synthesized using 50% heavy water. Fourier transform infrared spectroscopy (FTIR) permitted the identification of the –OD groups created during synthesis. It was found that the –OD groups persisted on the ZrO2 surface even after annealing at 1100 °C. This hydroxide layer is responsible for the decrease in the average density of the NPs as their size decreases. This study of the correlation between particle size and density may be used to assess the quality of the NPs. In most cases, the technological aim is to avoid an amorphous layer and to obtain fully crystalline nanoparticles with the highest density possible. However, due to the effect of the surface layers, there is a maximum density which can be achieved for a given average NP diameter. The effect of the surface layer on the NP density becomes particularly evident for NPs smaller than 50 nm, and thus, the density of nanoparticles is size dependent. PMID:25671149

Opalinska, Agnieszka; Dzwolak, Wojciech; Chudoba, Tadeusz; Presz, Adam; Lojkowski, Witold

2015-01-01

37

Multireference density functional theory with orbital-dependent correlation corrections  

Microsoft Academic Search

The features of ab initio wave function-driven multireference (MR)-density functional theory (DFT) approach based on the partially interacting reference system for ground and excited states are discussed from the viewpoint of a simple classification of electron correlation effects. Our previous MR-DFT approach covers (i) structure-free, and (ii) resonating-type correlations, but does not include (iii) orbital-dependent correlation effects. Indeed, the computational

Shusuke Yamanaka; Kazuto Nakata; Takeshi Ukai; Toshikazu Takada; Kizashi Yamaguchi

2006-01-01

38

Sublinear scaling for time-dependent stochastic density functional theory.  

PubMed

A stochastic approach to time-dependent density functional theory 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 first 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 infinite, 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 using real-space grids and find that the overall scaling of the algorithm is sublinear with computational time and memory. PMID:25612688

Gao, Yi; Neuhauser, Daniel; Baer, Roi; Rabani, Eran

2015-01-21

39

Density and isospin asymmetry dependence of high-momentum components  

E-print Network

We study the one-body momentum distribution at different densities in nuclear matter, with special emphasis on its components at high momentum. Explicit calculations for finite neutron-proton asymmetry, based on the ladder self-consistent Green's function approach, allow us to access the isospin dependence of momentum distributions and elucidate their role in neutron-rich systems. Comparisons with the deuteron momentum distribution indicate that a substantial proportion of high-momentum components are dominated by tensor correlations. We identify the density dependence of these tensor correlations in the momentum distributions. Further, we find that high-momentum components are determined by the density of each sub-species and we provide a new isospin asymmetry scaling of these components. We use different realistic nucleon-nucleon interactions to quantify the model dependence of our results.

A. Rios; A. Polls; W. H. Dickhoff

2014-04-02

40

Evolution of density-dependent cooperation.  

PubMed

Cooperation is surprisingly common in life despite of its vulnerability to selfish cheating, i.e. defecting. Defectors do not contribute to common resources but take the advantage of cooperators' investments. Therefore, the emergence and maintenance of cooperation have been considered irrational phenomena. In this study, we focus on plastic, quantitative cooperation behaviour, especially on its evolution. We assume that individuals are capable to sense the population density in their neighbourhood and adjust their real-valued investments on public goods based on that information. The ecological setting is described with stochastic demographic events, e.g. birth and death, occurring at individual level. Individuals form small populations, which further constitute a structured metapopulation. For evolutionary investigations, we apply the adaptive dynamics framework. The cost of cooperative investment is incorporated into the model in two ways, by decreasing the birth rate or by increasing the death rate. In the first case, density-dependent cooperation evolves to be a decreasing function of population size as expected. In the latter case, however, the density-dependent cooperative investment can have a qualitatively different form as it may evolve to be highest in intermediate-sized populations. Indeed, we emphasize that some details in modelling may have a significant impact on the results obtained. PMID:25213153

Seppänen, Anne; Parvinen, Kalle

2014-12-01

41

Selective fishing induces density-dependent growth  

PubMed Central

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

42

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 York 14627-0216 Received 14 October 2002; published 15 April 2003 The time-dependent density-electron density matrix in Liouville space. A collective-oscillator, quasi- particle, representation of the density

Mukamel, Shaul

43

Temperature and density dependent solute vibrational relaxation in supercritical fluoroform  

E-print Network

Temperature and density dependent solute vibrational relaxation in supercritical fluoroform D. J Received 29 November 2000; accepted 13 June 2001 Temperature- and density-dependent vibrational relaxation in reproducing the temperature- and density-dependent trends of the experimental data with a minimum

Fayer, Michael D.

44

Time Dependent Density Functional Theory Applications, limitations and ... new frontiers  

E-print Network

Time Dependent Density Functional Theory Applications, limitations and ... new frontiers Francesco Spectroscopy Facility (ETSF) Vienna, 19 January 2007 1/55 Time Dependent Density Functional Theory Francesco Sottile #12;Time-Dependent Density Functional Theory Applications and results: The ETSF Outline 1 Time

Botti, Silvana

45

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.

46

Density-dependent mortality and the latitudinal gradient in species  

E-print Network

.............................................................. Density-dependent mortality.B.) ............................................................................................................................................................................. Ecologists have long postulated that density-dependent mortality maintains high tree diversity in the tropics,2,7­9 . Agents of density-dependent mortality (such as host-specific predators, and pathogens) may be more

Beckage, Brian

47

Density dependence of the optical Kerr effect of linear molecules  

Microsoft Academic Search

We investigated the density dependence of the optical Kerr effect of two linear molecules, CO2 and N2O, by the optical Kerr effect experiment for a wide range of densities higher than the critical density. We also performed molecular dynamics simulation for CO2 in order to analyze the mechanism of the optical response. The density was controlled from the critical density

Kenji Kiyohara; Yoshifumi Kimura; Yoshihiro Takebayashi; Noboru Hirota; Koji Ohta

2002-01-01

48

High-Order Harmonic Generation of H2 in Intense Laser Fields: Self-Interaction-Free Time-Dependent Density Functional Theoretical Approach  

Microsoft Academic Search

We present a self-interaction-free TDDFT approach for nonperturbative treatment of multiphoton processes of many-electron molecular systems in intense laser fields. The theory is applied to the study of multiple high-order harmonic generation (HHG) of H2 and N2 in intense laser pulsed fields (X. Chu and S. I. Chu, Phys. Rev. A63) (2001) 023411.^,(X. Chu and S. I. Chu, to be

Xi Chu; Shih-I. Chu

2001-01-01

49

Multiphoton Processes and Dynamical Responses of Individual Valence Electrons of N2 in Intense Laser Fields: Self-Interaction-Free Time-Dependent Density Functional Theoretical Approach  

Microsoft Academic Search

We present a self-interaction-free TDDFT approach for nonperturbative treatment of multiphoton processes of many-electron molecular systems in intense laser fields. The theory is applied to the all-electron study of multiple photon ionization (MPI) and multiple high-order harmonic generation (HHG) of H2 (X. Chu and S. I. Chu, Phys. Rev. A63) (2001) 023411. and N2 (X. Chu and S. I. Chu,

Xi Chu; Shih-I. Chu

2002-01-01

50

Excitation energies from range-separated time-dependent density and density matrix functional theory  

NASA Astrophysics Data System (ADS)

Time-dependent density functional theory (TD-DFT) in the adiabatic formulation exhibits known failures when applied to predicting excitation energies. One of them is the lack of the doubly excited configurations. On the other hand, the time-dependent theory based on a one-electron reduced density matrix functional (time-dependent density matrix functional theory, TD-DMFT) has proven accurate in determining single and double excitations of H2 molecule if the exact functional is employed in the adiabatic approximation. We propose a new approach for computing excited state energies that relies on functionals of electron density and one-electron reduced density matrix, where the latter is applied in the long-range region of electron-electron interactions. A similar approach has been recently successfully employed in predicting ground state potential energy curves of diatomic molecules even in the dissociation limit, where static correlation effects are dominating. In the paper, a time-dependent functional theory based on the range-separation of electronic interaction operator is rigorously formulated. To turn the approach into a practical scheme the adiabatic approximation is proposed for the short- and long-range components of the coupling matrix present in the linear response equations. In the end, the problem of finding excitation energies is turned into an eigenproblem for a symmetric matrix. Assignment of obtained excitations is discussed and it is shown how to identify double excitations from the analysis of approximate transition density matrix elements. The proposed method used with the short-range local density approximation (srLDA) and the long-range Buijse-Baerends density matrix functional (lrBB) is applied to H2 molecule (at equilibrium geometry and in the dissociation limit) and to Be atom. The method accounts for double excitations in the investigated systems but, unfortunately, the accuracy of some of them is poor. The quality of the other excitations is in general much better than that offered by TD-DFT-LDA or TD-DMFT-BB approximations if the range-separation parameter is properly chosen. The latter remains an open problem.

Pernal, Katarzyna

2012-05-01

51

Simulating Density-Dependent Flows Using the Lattice Boltzmann Method  

NASA Astrophysics Data System (ADS)

Seawater intrusion is a classic density-dependent problem in hydrogeology. It must be fully understood in order to be able to predict and prevent groundwater deterioration in coastal areas. Although there is extensive research on this topic, more tools are required to fully understand and predict the location and behavior of the freshwater/seawater boundary. Due to the difficulty of sampling, one widely used method to obtain this information is numerical modeling. Various software programs have been developed and are being used to model coupled fluid flow and solute transport for density-dependent applications. All of the current programs are either finite difference or finite element methods. Density-dependent flow problems are exceptionally challenging for conventional numerical methods due to inherent non-linearity; definitive solutions are often elusive and a completely different modeling approach may be advantageous. The lattice Boltzmann method represents such a radically different numerical tool because it is not based on discretization of a series of differential equations. Instead, its foundation lies in the kinetic theory of gasses as proposed by Boltzmann. Recent advances in lattice Boltzmann modeling permit simulation of large-scale density-dependent ground water flow and heat/solute transport. A key advantage of lattice Boltzmann method is that it has the ability to solve the Navier-Stokes equations in larger conduits and pores. Hence it allows for eddy diffusion brought on by inertial components of flow at higher Reynolds numbers, which may occur in some coastal aquifers. Simulation of these phenomena is not possible with traditional Darcy's law-based groundwater models. Some geologists and engineers have been able to successfully apply lattice Boltzmann methods to fluid flow and contaminant transport problems. There are only a handful of scientists attempting to apply lattice Boltzmann methods to density-dependent flows in general; even fewer have considered seawater intrusion. Almost all previous simulations were conducted in closed or periodic domains. We simulate temperature- or concentration-induced density-dependent flows in domains with boundary conditions such as constant flow and hydrostatic pressure that are relevant to real-world systems with lattice Boltzmann models and compare with other solutions.

Bardsley, K. J.; Sukop, M. C.

2008-05-01

52

Density-matrix representation of nonadiabatic couplings in time-dependent density functional ,,TDDFT... theories  

E-print Network

Density-matrix representation of nonadiabatic couplings in time-dependent density functional of a molecule are derived by representing the time-dependent density functional TDDFT equations in a form of classical dynamics for the Kohn-Sham KS single-electron density matrix. Applicability of Krylov

Mukamel, Shaul

53

Evolution of Behavior by Density-Dependent Natural Selection  

Microsoft Academic Search

Theories of density-dependent natural selection predict that evolution should favor those genotypes with the highest per capita rates of population growth under the current density conditions. These theories are silent about the mechanisms that may give rise to these increases in density-dependent growth rates. We have observed the evolution of six populations of Drosophila melanogaster recently placed in crowded environments

Pingzhong Guo; Laurence D. Mueller; Francisco J. Ayala

1991-01-01

54

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.

Stetcu, I; Bulgac, A; Magierski, P; Roche, K J

2014-01-01

55

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

56

THE DEPENDENCE OF STAR FORMATION EFFICIENCY ON GAS SURFACE DENSITY  

SciTech Connect

Studies by Lada et al. and Heiderman et al. have suggested that star formation mostly occurs above a threshold in gas surface density {Sigma} of {Sigma}{sub c} {approx} 120 M{sub Sun} pc{sup -2} (A{sub K} {approx} 0.8). Heiderman et al. infer a threshold by combining low-mass star-forming regions, which show a steep increase in the star formation rate per unit area {Sigma}{sub SFR} with increasing {Sigma}, and massive cores forming luminous stars which show a linear relation. We argue that these observations do not require a particular density threshold. The steep dependence of {Sigma}{sub SFR}, approaching unity at protostellar core densities, is a natural result of the increasing importance of self-gravity at high densities along with the corresponding decrease in evolutionary timescales. The linear behavior of {Sigma}{sub SFR} versus {Sigma} in massive cores is consistent with probing dense gas in gravitational collapse, forming stars at a characteristic free-fall timescale given by the use of a particular molecular tracer. The low-mass and high-mass regions show different correlations between gas surface density and the area A spanned at that density, with A {approx} {Sigma}{sup -3} for low-mass regions and A {approx} {Sigma}{sup -1} for the massive cores; this difference, along with the use of differing techniques to measure gas surface density and star formation, suggests that connecting the low-mass regions with massive cores is problematic. We show that the approximately linear relationship between dense gas mass and stellar mass used by Lada et al. similarly does not demand a particular threshold for star formation and requires continuing formation of dense gas. Our results are consistent with molecular clouds forming by galactic hydrodynamic flows with subsequent gravitational collapse.

Burkert, Andreas [University Observatory Munich, Scheinerstrasse 1, D-81679 Munich (Germany); Hartmann, Lee, E-mail: burkert@usm.lmu.de, E-mail: lhartm@umich.edu [Department of Astronomy, University of Michigan, 830 Dennison, 500 Church St., Ann Arbor, MI 48109-1042 (United States)

2013-08-10

57

Application of time-dependent density functional response theory to Raman scattering  

Microsoft Academic Search

Recently, the first density functional theory (DFT) calculations on Raman intensities and depolarisation ratios were published. Those calculations were done in the static approximation. Here, we use time-dependent DFT in order to include the dependence of those properties on the frequency of the exciting light wave. By analytically calculating the frequency-dependent polarisability at different nuclear positions, our approach is closer

S. J. A. van Gisbergen; J. G. Snijders; E. J. Baerends

1996-01-01

58

Elevational variation in density dependence in a subtropical forest  

PubMed Central

Density-dependent mortality has been recognized as an important mechanism that underpins tree species diversity, especially in tropical forests. However, few studies have attempted to explore how density dependence varies with spatial scale and even fewer have attempted to identify why there is scale-dependent differentiation. In this study, we explore the elevational variation in density dependence. Three 1-ha permanent plots were established at low and high elevations in the Heishiding subtropical forest, southern China. Using data from 1200 1 m2 seedling quadrats, comprising of 200 1 m2 quadrats located in each 1-ha plot, we examined the variation in density dependence between elevations using a generalized linear mixed model with crossed random effects. A greenhouse experiment also investigated the potential effects of the soil biota on density-dependent differentiation. Our results demonstrated that density-dependent seedling mortality can vary between elevations in subtropical forests. Species found at a lower elevation suffered stronger negative density dependence than those found at a higher elevation. The greenhouse experiment indicated that two species that commonly occur at both elevations suffered more from soilborne pathogens during seed germination and seedling growth when they grew at the lower elevation, which implied that soil pathogens may play a crucial role in density-dependent spatial variation. PMID:25165522

Xu, Meng; Yu, Shixiao

2014-01-01

59

Simulation of salt migrations in density dependent groundwater flow  

E-print Network

Simulation of salt migrations in density dependent groundwater flow E.S. van Baaren Master's Thesis for the salt migration in the groundwater underneath the polders near the coast. The problem description of this thesis is to investigate the possibilities of modelling salt migrations in density dependent groundwater

Vuik, Kees

60

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

61

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

62

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

63

Density dependence of the nuclear symmetry energy: A microscopic perspective  

SciTech Connect

We perform a systematic analysis of the density dependence of nuclear symmetry energy within the microscopic Brueckner-Hartree-Fock (BHF) approach using the realistic Argonne V18 nucleon-nucleon potential plus a phenomenological three-body force of Urbana type. Our results are compared thoroughly with those arising from several Skyrme and relativistic effective models. The values of the parameters characterizing the BHF equation of state of isospin asymmetric nuclear matter fall within the trends predicted by those models and are compatible with recent constraints coming from heavy ion collisions, giant monopole resonances, or isobaric analog states. In particular we find a value of the slope parameter L=66.5 MeV, compatible with recent experimental constraints from isospin diffusion, L=88{+-}25 MeV. The correlation between the neutron skin thickness of neutron-rich isotopes and the slope L and curvature K{sub sym} parameters of the symmetry energy is studied. Our BHF results are in very good agreement with the correlations already predicted by other authors using nonrelativistic and relativistic effective models. The correlations of these two parameters and the neutron skin thickness with the transition density from nonuniform to {beta}-stable matter in neutron stars are also analyzed. Our results confirm that there is an inverse correlation between the neutron skin thickness and the transition density.

Vidana, Isaac; Providencia, Constanca; Polls, Artur; Rios, Arnau [Centro de Fisica Computacional, Department of Physics, University of Coimbra, PT-3004-516 Coimbra (Portugal); Departament d'Estructura i Constituents de la Materia and Institut de Ciencies del Cosmos, Universitat de Barcelona, Avda. Diagonal 647, E-08028 Barcelona (Spain); Faculty of Engineering and Physical Sciences, Department of Physics, University of Surrey, Guildford, Surrey GU2 7XH (United Kingdom) and Kavli Institute for Theoretical Physics China (CAS), 100190 Beijing (China)

2009-10-15

64

A time-dependent semiempirical approach to determining excited states  

Microsoft Academic Search

We study a time-dependent semiempirical method to determine excitation energies, TD-PM3. This semiempirical method allows large molecules to be treated. A Linear-response Chebyshev approach yields the TD-PM3 spectrum very efficiently. Spectra and excitation energies were tested by comparing it with the results obtained using TD-DFT (Time Dependent-Density Functional Theory), using both small and large basis sets. They were also compared

Lizette A. Bartell; Michael R. Wall; Daniel Neuhauser

2010-01-01

65

Phase space explorations in time dependent density functional theory  

NASA Astrophysics Data System (ADS)

Time dependent density functional theory (TDDFT) is one of the useful tools for the study of the dynamic behavior of correlated electronic systems under the influence of external potentials. The success of this formally exact theory practically relies on approximations for the exchange-correlation potential which is a complicated functional of the co-ordinate density, non-local in space and time. Adiabatic approximations (such as ALDA), which are local in time, are most commonly used in the increasing applications of the field. Going beyond ALDA, has been proved difficult leading to mathematical inconsistencies. We explore the regions where the theory faces challenges, and try to answer some of them via the insights from two electron model systems. In this thesis work we propose a phase-space extension of the TDDFT. We want to answer the challenges the theory is facing currently by exploring the one-body phase-space. We give a general introduction to this theory and its mathematical background in the first chapter. In second chapter, we carryout a detailed study of instantaneous phase-space densities and argue that the functionals of distributions can be a better alternative to the nonlocality issue of the exchange-correlation potentials. For this we study in detail the interacting and the non-interacting phase-space distributions for Hookes atom model. The applicability of ALDA-based TDDFT for the dynamics in strongfields can become severely problematic due to the failure of single-Slater determinant picture.. In the third chapter, we analyze how the phase-space distributions can shine some light into this problem. We do a comparative study of Kohn-Sham and interacting phase-space and momentum distributions for single ionization and double ionization systems. Using a simple model of two-electron systems, we have showed that the momentum distribution computed directly from the exact KS system contains spurious oscillations: a non-classical description of the essentially classical two-electron dynamics. In Time dependent density matrix functional theory (TDDMFT), the evolution scheme of the 1RDM (first order reduced density matrix) contains second-order reduced density matrix (2RDM), which has to be expressed in terms of 1RDMs. Any non-correlated approximations (Hartree-Fock) for 2RDM would fail to capture the natural occupations of the system. In our fourth chapter, we show that by applying the quasi-classical and semi-classical approximations one can capture the natural occupations of the excited systems. We study a time-dependent Moshinsky atom model for this. The fifth chapter contains a comparative work on the existing non-local exchange-correlation kernels that are based on current density response frame work and the co-moving frame work. We show that the two approaches though coinciding with each other in linear response regime, actually turn out to be different in non-linear regime.

Rajam, Aruna K.

66

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.; Bølviken, Erik; Glöckner, Fredrik

2003-01-01

67

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

2008-09-04

68

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

69

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

70

A Wigner Monte Carlo approach to density functional theory  

NASA Astrophysics Data System (ADS)

In order to simulate quantum N-body systems, stationary and time-dependent density functional theories rely on the capacity of calculating the single-electron wave-functions of a system from which one obtains the total electron density (Kohn-Sham systems). In this paper, we introduce the use of the Wigner Monte Carlo method in ab-initio calculations. This approach allows time-dependent simulations of chemical systems in the presence of reflective and absorbing boundary conditions. It also enables an intuitive comprehension of chemical systems in terms of the Wigner formalism based on the concept of phase-space. Finally, being based on a Monte Carlo method, it scales very well on parallel machines paving the way towards the time-dependent simulation of very complex molecules. A validation is performed by studying the electron distribution of three different systems, a Lithium atom, a Boron atom and a hydrogenic molecule. For the sake of simplicity, we start from initial conditions not too far from equilibrium and show that the systems reach a stationary regime, as expected (despite no restriction is imposed in the choice of the initial conditions). We also show a good agreement with the standard density functional theory for the hydrogenic molecule. These results demonstrate that the combination of the Wigner Monte Carlo method and Kohn-Sham systems provides a reliable computational tool which could, eventually, be applied to more sophisticated problems.

Sellier, J. M.; Dimov, I.

2014-08-01

71

Density-dependent synthetic gauge fields using periodically modulated interactions.  

PubMed

We show that density-dependent synthetic gauge fields may be engineered by combining periodically modulated interactions and Raman-assisted hopping in spin-dependent optical lattices. These fields lead to a density-dependent shift of the momentum distribution, may induce superfluid-to-Mott insulator transitions, and strongly modify correlations in the superfluid regime. We show that the interplay between the created gauge field and the broken sublattice symmetry results, as well, in an intriguing behavior at vanishing interactions, characterized by the appearance of a fractional Mott insulator. PMID:25479501

Greschner, S; Sun, G; Poletti, D; Santos, L

2014-11-21

72

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

73

Computational complexity of time-dependent density functional theory  

E-print Network

Time-dependent density functional theory (TDDFT) is rapidly emerging as a premier method for solving dynamical many-body problems in physics and chemistry. The mathematical foundations of TDDFT are established through the formal existence of a fictitious non-interacting system (known as the Kohn-Sham system), which can reproduce the one-electron reduced probability density of the actual system. We build upon these works and show that on the interior of the domain of existence, the Kohn-Sham system can be efficiently obtained given the time-dependent density. Since a quantum computer can efficiently produce such time-dependent densities, we present a polynomial time quantum algorithm to generate the time-dependent Kohn-Sham potential with controllable error bounds. As a consequence, in contrast to the known intractability result for ground state density functional theory (DFT), the computation of the necessary time-dependent potentials given the initial state is in the complexity class described by bounded error quantum computation in polynomial time (BQP).

J. D. Whitfield; M. -H. Yung; D. G. Tempel; S. Boixo; A. Aspuru-Guzik

2014-08-21

74

Analyzing density dependent symmetry energy and dynamics for mass-asymmetric heavy-ion reactions  

NASA Astrophysics Data System (ADS)

We attempt to study the interplay of density-dependent symmetry energy and its impact on the dynamics evolved in intermediate energy mass-asymmetric reactions. The investigation includes the theoretical analysis of various mass-asymmetric heavy-ion reactions (keeping total mass the same) based on the microscopic isospin-dependent quantum molecular dynamical (IQMD) approach. We present the time evolution, colliding geometry (impact parameter), and excitation energy dependence (100 MeV nucleon?1–1 GeV nucleon?1) of the density and temperature reached during the collisions. The mass-asymmetry factor influences the reaction dynamics drastically at higher incident energies and central collisions. Owing to different density profiles, when subjected to different mass-asymmetry combinations, the role of density-dependent symmetry energy varies accordingly.

Singh Vinayak, Karan; Chaudhuri, Asis K.

2015-02-01

75

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

E-print Network

in the Connecticut River have demonstrated that larval and juvenile mortality rates decline with age (Crecco et a- and postrecruitment mortality data for American shad, AloBa sapidiBsima, in the Connecticut River to estimate density-dependent and density-independent mortality rates at the prejuvenile (age 1-100 days) and posijuvenile (age 101 days

76

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

77

Computational complexity of time-dependent density functional theory  

NASA Astrophysics Data System (ADS)

Time-dependent density functional theory (TDDFT) is rapidly emerging as a premier method for solving dynamical many-body problems in physics and chemistry. The mathematical foundations of TDDFT are established through the formal existence of a fictitious non-interacting system (known as the Kohn-Sham system), which can reproduce the one-electron reduced probability density of the actual system. We build upon these works and show that on the interior of the domain of existence, the Kohn-Sham system can be efficiently obtained given the time-dependent density. We introduce a V-representability parameter which diverges at the boundary of the existence domain and serves to quantify the numerical difficulty of constructing the Kohn-Sham potential. For bounded values of V-representability, we present a polynomial time quantum algorithm to generate the time-dependent Kohn-Sham potential with controllable error bounds.

Whitfield, J. D.; Yung, M.-H.; Tempel, D. G.; Boixo, S.; Aspuru-Guzik, A.

2014-08-01

78

Density dependence and the economic efficacy of marine reserves  

Microsoft Academic Search

Predictions on the efficacy of marine reserves for benefiting fisheries differ in large part due to considerations of models\\u000a of either intra- or inter-cohort population density regulating fish recruitment. Here, I consider both processes acting on\\u000a recruitment and show using a bioeconomic model how for many fisheries density dependent recruitment dynamics interact with\\u000a harvest costs to influence fishery profit with

Crow White

2009-01-01

79

Parity dependence of level densities in sup 49 V  

SciTech Connect

In this research, we have studied {sup 48}Ti(p, p{sub 1}) and {sup 48}(p, p{sub 1}{gamma}) in an effort to determine the dependence of level densities on parity in the compound nucleus {sup 49}V. This nuclide was chosen because of the high level density of the {sup 49}V system (leading to good statistical accuracy) and because the target is zero spin (making the assignment of J easier). 5 refs., 3 figs.

York, B.W.

1991-01-01

80

New Dependability Approach for Implanted Medical Devices  

E-print Network

New Dependability Approach for Implanted Medical Devices Fabien Soulier§, Fanny Le Floch§, Serge, this paper proposes a strategy for risk management at system level for FES medical implant. The idea a new strategy for risk manage- ment at system level for FES medical implant. The aim is to propose

Paris-Sud XI, Université de

81

Superlinear density dependence of singlet fission rate in tetracene films  

E-print Network

We experimentally show that the rate of singlet fission in tetracene films has a superlinear dependence on the density of photo-excited singlet excitons with ultrafast transient absorption spectroscopy. The spectrotemporal features of singlet and triplet dynamics can be disentangled from experimental data with the algorithm of singular value decomposition. The correlation between their temporal dynamics indicates a nonlinear density dependence of fission rate, which leads to a conjecture of coherent singlet fission process arising from superradiant excitons in crystalline tetracene. This hypothesis might be able to resolve some long-standing controversies.

Zhang, Bo; Wang, Rui; Tan, Zhanao; Liu, Yunlong; Guo, Wei; Zhai, Xiaoling; Wang, Xiaoyong; Xiao, Min

2014-01-01

82

Time-dependent density functional theory for quantum transport  

NASA Astrophysics Data System (ADS)

The rapid miniaturization of electronic devices motivates research interests in quantum transport. Recently time-dependent quantum transport has become an important research topic. Here we review recent progresses in the development of time-dependent density-functional theory for quantum transport including the theoretical foundation and numerical algorithms. In particular, the reduced-single electron density matrix based hierarchical equation of motion, which can be derived from Liouville-von Neumann equation, is reviewed in details. The numerical implementation is discussed and simulation results of realistic devices will be given.

Kwok, Yanho; Zhang, Yu; Chen, GuanHua

2014-12-01

83

Density-dependent nest predation in waterfowl: the relative importance of nest density versus nest dispersion  

USGS Publications Warehouse

When nest predation levels are very high or very low, the absolute range of observable nest success is constrained (a floor/ceiling effect), and it may be more difficult to detect density-dependent nest predation. Density-dependent nest predation may be more detectable in years with moderate predation rates, simply because there can be a greater absolute difference in nest success between sites. To test this, we replicated a predation experiment 10 years after the original study, using both natural and artificial nests, comparing a year when overall rates of nest predation were high (2000) to a year with moderate nest predation (2010). We found no evidence for density-dependent predation on artificial nests in either year, indicating that nest predation is not density-dependent at the spatial scale of our experimental replicates (1-ha patches). Using nearest-neighbor distances as a measure of nest dispersion, we also found little evidence for “dispersion-dependent” predation on artificial nests. However, when we tested for dispersion-dependent predation using natural nests, we found that nest survival increased with shorter nearest-neighbor distances, and that neighboring nests were more likely to share the same nest fate than non-adjacent nests. Thus, at small spatial scales, density-dependence appears to operate in the opposite direction as predicted: closer nearest neighbors are more likely to be successful. We suggest that local nest dispersion, rather than larger-scale measures of nest density per se, may play a more important role in density-dependent nest predation.

Ackerman, Joshua T.; Ringelman, KM; Eadie, J.M.

2012-01-01

84

Watching excitons move: the time-dependent transition density matrix  

NASA Astrophysics Data System (ADS)

Time-dependent density-functional theory allows one to calculate excitation energies and the associated transition densities in principle exactly. The transition density matrix (TDM) provides additional information on electron-hole localization and coherence of specific excitations of the many-body system. We have extended the TDM concept into the real-time domain in order to visualize the excited-state dynamics in conjugated molecules. The time-dependent TDM is defined as an implicit density functional, and can be approximately obtained from the time-dependent Kohn-Sham orbitals. The quality of this approximation is assessed in simple model systems. A computational scheme for real molecular systems is presented: the time-dependent Kohn-Sham equations are solved with the OCTOPUS code and the time-dependent Kohn-Sham TDM is calculated using a spatial partitioning scheme. The method is applied to show in real time how locally created electron-hole pairs spread out over neighboring conjugated molecular chains. The coupling mechanism, electron-hole coherence, and the possibility of charge separation are discussed.

Ullrich, Carsten

2012-02-01

85

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

2012-09-25

86

Time-dependent density-functional theory calculations of triplet-triplet absorption  

Microsoft Academic Search

We present density-functional theory calculations of triplet-triplet absorption by three different approaches based on time-dependent density-functional theory (DFT): unrestricted DFT linear response, open-shell restricted DFT linear response applied to the triplet state, and quadratic response with triplet excitations applied to the ground state. Comparison is also made with corresponding results obtained by Hartree-Fock and multiconfiguration self-consistent-field response theory. Two main

Peter Cronstrand; Zilvinas Rinkevicius; Yi Luo; Hans Gren

2005-01-01

87

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

88

A Morpho-Density Approach to Estimating Neural Connectivity  

PubMed Central

Neuronal signal integration and information processing in cortical neuronal networks critically depend on the organization of synaptic connectivity. Because of the challenges involved in measuring a large number of neurons, synaptic connectivity is difficult to determine experimentally. Current computational methods for estimating connectivity typically rely on the juxtaposition of experimentally available neurons and applying mathematical techniques to compute estimates of neural connectivity. However, since the number of available neurons is very limited, these connectivity estimates may be subject to large uncertainties. We use a morpho-density field approach applied to a vast ensemble of model-generated neurons. A morpho-density field (MDF) describes the distribution of neural mass in the space around the neural soma. The estimated axonal and dendritic MDFs are derived from 100,000 model neurons that are generated by a stochastic phenomenological model of neurite outgrowth. These MDFs are then used to estimate the connectivity between pairs of neurons as a function of their inter-soma displacement. Compared with other density-field methods, our approach to estimating synaptic connectivity uses fewer restricting assumptions and produces connectivity estimates with a lower standard deviation. An important requirement is that the model-generated neurons reflect accurately the morphology and variation in morphology of the experimental neurons used for optimizing the model parameters. As such, the method remains subject to the uncertainties caused by the limited number of neurons in the experimental data set and by the quality of the model and the assumptions used in creating the MDFs and in calculating estimating connectivity. In summary, MDFs are a powerful tool for visualizing the spatial distribution of axonal and dendritic densities, for estimating the number of potential synapses between neurons with low standard deviation, and for obtaining a greater understanding of the relationship between neural morphology and network connectivity. PMID:24489738

Tarigan, Bernadetta; van Pelt, Jaap; van Ooyen, Arjen; de Gunst, Mathisca

2014-01-01

89

DEVELOPMENT OF TIME-DEPENDENT DENSITY FUNCTIONAL THEORY IN  

E-print Network

DEVELOPMENT OF TIME-DEPENDENT DENSITY FUNCTIONAL THEORY IN CHEMICAL AND SOLID-STATE PHYSICS BY FAN functional theory in chemical and solid-state physics by Fan Zhang Dissertation Director: Professor Kieron of TDDFT in chemical and solid-state physics, we show how to use TDDFT to solve the double excitation

Burke, Kieron

90

Rapidity dependence of particle densities in pp and AA collisions  

E-print Network

We use multiple scattering and energy conservation arguments to describe $dn/d\\eta_{NANA}$ as a function of $dn/d\\eta_{pp}$ in the framework of string percolation. We discuss the pseudo-rapidity $\\eta$? and beam rapidity Y dependence of particle densities. We present our results for pp, Au- Au, and Pb-Pb collisions at RHIC and LHC.

Irais Bautista; Carlos Pajares; José Guilherme Milhano; Jorge Dias de Deus

2012-06-28

91

Do southern elephant seals show density dependence in fecundity?  

Microsoft Academic Search

Here we provide an alternative interpretation to that of Pistorius et al. (2001), concerning density-dependent increases in fecundity resulting in population regulation of the southern elephant seal population at Marion Island. We do not contradict the findings of Pistorius et al. (2001), because it does appear: (1) that a change in fecundity has been observed, and (2) that some factor

C. Bradshaw; C. McMahon; M. Hindell; P. Pistorius; M. Bester

2002-01-01

92

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

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

2014-01-01

93

Two Approaches for Tomographic Density Imaging Using Inverse Scattering  

E-print Network

to the scattered data. The DF- DBIM approach allowed separation of density and speed of sound profiles at low sound speed changes [4]. The effects of variable density in the reconstruction of sound speed were. The number of UCT studies that consider variable den- sity is limited. Variable density UCT was introduced

Illinois at Urbana-Champaign, University of

94

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

95

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

96

Density Functional Approaches In Condensed Matter Physics  

E-print Network

Density Approxima2on most oUen used in conjunc2on with homogenous electron gas · Applica2ons: · Entanglement Connec2ons to QPTs · Inhomogenous Hubbard Model #12;Densityo calcula2ons of electronic proper2es of solids · Promotes par2cle

97

The dependence of natural graphite anode performance on electrode density  

SciTech Connect

The effect of electrode density for lithium intercalation and irreversible capacity loss on the natural graphite anode in lithium ion batteries was studied by electrochemical methods. Both the first-cycle reversible and irreversible capacities of the natural graphite anode decreased with an increase in the anode density though compression. The reduction in reversible capacity was attributed to a reduction in the chemical diffusion coefficient for lithium though partially agglomerated particles with a larger stress. For the natural graphite in this study the potentials for Li (de)insertion shifted between the first and second formation cycles and the extent of this shift was dependent on electrode density. The relation between this peak shift and the irreversible capacity loss are probably both due to the decrease in graphite surface area with compression.

Shim, Joongpyo; Striebel, Kathryn A.

2003-11-01

98

DENSITY DEPENDENCE IN MARINE FISH POPULATIONS REVEALED AT SMALL AND LARGE SPATIAL SCALES  

Microsoft Academic Search

Experimental manipulation of population density has frequently been used to demonstrate demographic density dependence. However, such studies are usually small scale and typically provide evidence of spatial (within-generation) density dependence. It is often unclear whether small-scale, experimental tests of spatial density dependence will accurately describe temporal (between-generation) density dependence required for pop- ulation regulation. Understanding the mechanisms generating density dependence

DARREN W. J OHNSON

2006-01-01

99

Density-dependent predation by skunks using olfactory search images  

Microsoft Academic Search

The formation of search images can create density-dependent predation. Predators have been shown to form search images when\\u000a searching for many small prey items in one feeding session. This paper reports experiments that test whether striped skunks\\u000a can form olfactory search images in other situations: when prey are found over several days, when prey are large, and when\\u000a prey are

Vilis O. Nams

1997-01-01

100

Probing the density dependence of the symmetry potential at low and high densities  

SciTech Connect

We have investigated the influence of different forms of symmetry energies on various observables proposed to be sensitive to the symmetry energy at subnormal and supranormal densities for reactions {sup 208}Pb +{sup 208}Pb, {sup 132}Sn +{sup 124}Sn, and {sup 96}Zr +{sup 96}zR at E{sub b}=0.4A GeV within the microscopic transport model--ultrarelativistic quantum molecular dynamics model. The same systems are adopted for testing the probes of the symmetry potential at both subnormal and supranormal densities. It is shown that the differences between the predicted values of investigated quantities by different symmetry potentials have a close correspondence with the different behavior of the density dependence of the interaction part of the different symmetry energies. We also find that the comprehensive study with multiple probes provides a possibility for gaining the density dependence of the symmetry potential in a broad density region, which allows us to extract the information of the isospin-dependent part of the effective interaction.

Li Qingfeng [Frankfurt Institute for Advanced Studies, Johann Wolfgang Goethe-Universitaet, Max-von-Laue-Strasse 1, D-60438 Frankfurt (Germany); Li Zhuxia [China Institute of Atomic Energy, P.O. Box 275 (18), Beijing 102413 (China); Soff, Sven; Bleicher, Marcus [Institut fuer Theoretische Physik, Johann Wolfgang Goethe-Universitaet, Max-von-Laue-Strasse 1, D-60438 Frankfurt (Germany); Stoecker, Horst [Frankfurt Institute for Advanced Studies, Johann Wolfgang Goethe-Universitaet, Max-von-Laue-Strasse 1, D-60438 Frankfurt (Germany); Institut fuer Theoretische Physik, Johann Wolfgang Goethe-Universitaet, Max-von-Laue-Strasse 1, D-60438 Frankfurt (Germany)

2005-09-01

101

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

102

Cycles, stochasticity and density dependence in pink salmon population dynamics  

PubMed Central

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

Krkošek, Martin; Hilborn, Ray; Peterman, Randall M.; Quinn, Thomas P.

2011-01-01

103

Effect of tensor force on the density dependence of symmetry energy within the BHF framework  

NASA Astrophysics Data System (ADS)

The effect of tensor force on the density dependence of nuclear symmetry energy has been investigated within the framework of the Brueckner-Hartree-Fock (BHF) approach. It is shown that the tensor force manifests its effect via the tensor 3SD1 channel. The density dependence of symmetry energy Esym turns out to be determined essentially by the tensor force from the ? meson and ? meson exchanges via the 3SD1 coupled channel. Increasing the strength of the tensor component due to the ?-meson exchange tends to enhance the repulsion of the equation of state of symmetric nuclear matter and leads to the reduction of symmetry energy. The present results confirm the dominant role played by the tensor force in determining nuclear symmetry energy and its density dependence within the microscopic BHF framework.

Wang, Pei; Zuo, Wei

2015-01-01

104

Gauge approach to superfluid density in underdoped cuprates  

E-print Network

We prove that a gauge approach based on a composite structure of the hole in hole-doped cuprates is able to capture analytically many features of the experimental data on superfluid density in the moderate-underdoping to nearly-optimal doping region, including critical exponent, the Uemura relation and almost universality of the normalized superfluid density.

Marchetti, P A

2015-01-01

105

Angular-momentum-dependent orbital-free density functional theory.  

PubMed

Orbital-free (OF) density functional theory (DFT) directly solves for the electron density rather than the wave function of many electron systems, greatly simplifying and enabling large scale first principles simulations. However, the required approximate noninteracting kinetic energy density functionals and local electron-ion pseudopotentials severely restrict the general applicability of conventional OFDFT. Here, we present a new generation of OFDFT called angular-momentum-dependent (AMD)-OFDFT to harness the accuracy of Kohn-Sham DFT and the simplicity of OFDFT. The angular momenta of electrons are explicitly introduced within atom-centered spheres so that the important ionic core region can be accurately described. In addition to conventional OF total energy functionals, we introduce a crucial nonlocal energy term with a set of AMD energies to correct errors due to the kinetic energy density functional and the local pseudopotential. We find that our AMD-OFDFT formalism offers substantial improvements over conventional OFDFT, as we show for various properties of the transition metal titanium. PMID:23971595

Ke, Youqi; Libisch, Florian; Xia, Junchao; Wang, Lin-Wang; Carter, Emily A

2013-08-01

106

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

107

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

108

Nuclear 'pasta' phase within density dependent hadronic models  

SciTech Connect

In the present paper, we investigate the onset of the 'pasta' phase with different parametrizations of the density dependent hadronic model and compare the results with one of the usual parametrizations of the nonlinear Walecka model. The influence of the scalar-isovector virtual {delta} meson is shown. At zero temperature, two different methods are used, one based on coexistent phases and the other on the Thomas-Fermi approximation. At finite temperature, only the coexistence phases method is used. npe matter with fixed proton fractions and in {beta} equilibrium are studied. We compare our results with restrictions imposed on the values of the density and pressure at the inner edge of the crust, obtained from observations of the Vela pulsar and recent isospin diffusion data from heavy-ion reactions, and with predictions from spinodal calculations.

Avancini, S. S.; Marinelli, J. R.; Menezes, D. P.; Moraes, M. M. W. de [Depto de Fisica, CFM, Universidade Federal de Santa Catarina Florianopolis, SC-CP. 476, CEP 88.040-900 (Brazil); Brito, L.; Providencia, C.; Santos, A. M. [Centro de Fisica Computacional, Department of Physics, University of Coimbra, P-3004-516, Coimbra (Portugal)

2009-03-15

109

Subcycle dynamics of high-order-harmonic generation of He atoms excited by attosecond pulses and driven by near-infrared laser fields: A self-interaction-free time-dependent density-functional-theory approach  

NASA Astrophysics Data System (ADS)

In the framework of the self-interaction-free time-dependent density-functional theory, we have performed three-dimensional (3D) ab initio calculations of He atoms in near-infrared (NIR) laser fields subject to excitation by a single extreme ultraviolet (XUV) attosecond pulse (SAP). We have explored the dynamical behavior of the subcycle high harmonic generation (HHG) for transitions from the excited states to the ground state and found oscillation structures with respect to the time delay between the SAP and NIR fields. The oscillatory pattern in the photon emission spectra has a period of ˜1.3 fs which is half of the NIR laser optical cycle, similar to that recently measured in the experiments on transient absorption of He [M. Chini et al., Sci. Rep. 3, 1105 (2013), 10.1038/srep01105]. We present the photon emission spectra from 1s2p, 1s3p, 1s4p, 1s5p, and 1s6p excited states as functions of the time delay. We explore the subcycle Stark shift phenomenon in NIR fields and its influence on the photon emission process. Our analysis reveals several interesting features of the subcycle HHG dynamics and we identify the mechanisms responsible for the observed peak splitting in the photon emission spectra.

Heslar, John; Telnov, Dmitry A.; Chu, Shih-I.

2014-05-01

110

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

111

Time-dependent density-functional studies on strength functions in neutron-rich nuclei  

E-print Network

The electric dipole (E1) strength functions have been systematically calculated based on the time-dependent density functional theory (TDDFT), using the finite amplitude method and the real-time approach to the TDDFT with pairing correlations. The low-energy E1 strengths in neutron-rich isotopes show peculiar behaviors, such as sudden enhancement and reduction, as functions of the neutron numbers.They seem to be due to the interplay between the neutron shell effect and the deformation effect.

Shuichiro Ebata; Tsunenori Inakura; Takashi Nakatsukasa

2013-02-08

112

Current density partitioning in time-dependent current density functional theory  

SciTech Connect

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. [Department of Chemistry, Purdue University, West Lafayette, Indiana 47907 (United States)] [Department of Chemistry, Purdue University, West Lafayette, Indiana 47907 (United States); Wasserman, Adam, E-mail: awasser@purdue.edu [Department of Chemistry, Purdue University, West Lafayette, Indiana 47907 (United States) [Department of Chemistry, Purdue University, West Lafayette, Indiana 47907 (United States); Department of Physics, Purdue University, West Lafayette, Indiana 47907 (United States)

2014-05-14

113

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

114

Dye chemistry with time-dependent density functional theory.  

PubMed

In this perspective, we present an overview of the determination of excited-state properties of "real-life" dyes, and notably of their optical absorption and emission spectra, performed during the last decade with time-dependent density functional theory (TD-DFT). We discuss the results obtained with both vertical and adiabatic (vibronic) approximations, choosing relevant examples for several series of dyes. These examples include reproducing absorption wavelengths of numerous families of coloured molecules, understanding the specific band shape of amino-anthraquinones, optimising the properties of dyes used in solar cells, mimicking the fluorescence wavelengths of fluorescent brighteners and BODIPY dyes, studying optically active biomolecules and photo-induced proton transfer, as well as improving the properties of photochromes. PMID:24548975

Laurent, Adèle D; Adamo, Carlo; Jacquemin, Denis

2014-07-28

115

Variable age structure and apparent density dependence in survival of adult ungulates  

Microsoft Academic Search

Summary 1. Large herbivores have strongly age-structured populations. Because recruitment often decreases as population density increases, in unexploited populations the propor- tion of older adults may increase with density. Because survival senescence is typical of ungulates, ignoring density-dependent changes in age structure could lead to apparent density-dependence in adult survival. 2. To test for density dependence in adult survival, we

Marco Festa-Bianchet; Jean-Michel Gaillard; Steeve D. Cote

2003-01-01

116

Business Cycle Duration Dependence: A Parametric Approach  

Microsoft Academic Search

This paper reexamines duration dependence in U.S. business cycles using parametric hazard models. Positive duration dependence would indicate that expansions or contractions are more likely to end as they become \\

Daniel E Sichel

1991-01-01

117

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

118

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

119

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

E-print Network

Density-dependent carrier dynamics in a quantum dots-in-a-well heterostructure R. P. Prasankumar,1 transmission spectroscopy to temporally and spectrally resolve density-dependent carrier dynamics in a quantum levels at low densities, while at high densities we observe an anomalous induced absorption

Krishna, Sanjay

120

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

121

Asymmetric nuclear matter studied by time-dependent local isospin density approximation  

NASA Astrophysics Data System (ADS)

The dynamic response of asymmetric nuclear matter is studied by means of a time-dependent local isospin density approximation (TDLIDA) approach. Calculations are based on a local density energy functional derived by an auxiliary field diffusion Monte Carlo (AFDMC) calculation of bulk nuclear matter. Three types of excited states emerge: collective states, a continuum of quasiparticle-quasihole excitations and unstable solutions. These states are analyzed and discussed for different values of the nuclear density ? and isospin asymmetry ?=(N-Z)/A. An analytical expression of the compressibility as a function of ? and ? is derived which shows explicitly an instability of the neutron matter around ??0.09 fm-3 when a small fraction of protons are added to the system.

Lipparini, Enrico; Pederiva, Francesco

2013-08-01

122

FW518x Animal Population Dynamics February 9, 2006 Lab 2 Density dependence  

E-print Network

of columns. In this scenario, juveniles experience density dependence. Set up the same column headers, exceptFW518x Animal Population Dynamics February 9, 2006 Lab 2 Density dependence Objectives: The objective of this lab is to explore various forms of density dependence and their implications

Gervais, Jennifer

123

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

124

A density-dependent model of Cirsium vulgare population dynamics using field-estimated parameter values  

Microsoft Academic Search

Two versions of a stage-structured model of Cirsium vulgare population dynamics were developed. Both incorporated density dependence at one stage in the life cycle of the plant. In version 1 density dependence was assumed to operate during germination whilst in version 2 it was included at the seedling stage. Density-dependent parameter values for the model were estimated from annual census

M. Gillman; J. M. Bullock; J. Silvertown; B. Clear Hill

1993-01-01

125

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

126

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

127

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

128

Phenomenological approach for describing environment dependent growths  

E-print Network

Different classes of phenomenological universalities of environment dependent growths have been proposed. The logistic as well as environment dependent West-type allometry based biological growth can be explained in this proposed framework of phenomenological description. It is shown that logistic and environment dependent West-type growths are phenomenologically identical in nature. However there is a difference between them in terms of coefficients involved in the phenomenological descriptions. It is also established that environment independent and enviornment dependent biological growth processes lead to the same West-type biological growth equation. Involuted Gompertz function, used to describe biological growth processes undergoing atrophy or a demographic and economic system undergoing involution or regression, can be addressed in this proposed environment dependent description. In addition, some other phenomenological descriptions have been examined in this proposed framework and graphical representations of variation of different parameters involved in the description are executed.

Dibyendu Biswas; Swarup Poria

2014-12-22

129

Ensemble Density Functional Approach to the Quantum Hall Effect  

NASA Astrophysics Data System (ADS)

The fractional quantum Hall effect (FQHE) occurs in a two-dimensional electron gas of density n when a strong magnetic field perpendicular to the plane of the electron gas takes on certain strengths B(n). At these magnetic field strengths the system is incompressible, i.e., there is a finite cost in energy for creating charge density fluctuations in the bulk. Even so the boundary of the electron gas supports gapless modes of density waves. The bulk energy gap arises because of the strong electron-electron interactions. There are very good models for infinite homogeneous systems and for the gapless excitations of the boundary of the electron gas. But in order to explain experiments on quantum Hall systems, including Hall bars and quantum dots, new approaches are needed which can accurately describe inhomogeneous systems, including Landau level mixing and the spin degree of freedom. One possibility is an ensemble density functional theory approach that we have developed.(O. Heinonen, M.I. Lubin, and M.D. Johnson, Phys. Rev. Lett. 75), 4110 (1995)(O. Heinonen, M.I. Lubin, and M.D. Johnson, Int. J. Quant. Chem, December 1996) We have applied this to study edge reconstructions of spin-polarized quantum dots. The results for a six-electron test case are in excellent agreement with numerical diagonalizations. For larger systems, compressible and incompressible strips appear as the magnetic field is increased from the region in which a dot forms a compact so-called maximum density droplet. We have recently included spin degree of freedom to study the stability of a maximum density droplet, and charge-spin textures in inhomogeneous systems. As an example, when the Zeeman coupling is decreased, we find that the maximum density droplet develops a spin-structured edge instability. This implies that the spin degree of freedom may play a significant role in the study of edge modes at low or moderate magnetic fields.

Heinonen, O.

1997-03-01

130

Evolutionary Ecology, 1990, 4, 290-297 Density-dependent natural selection does not  

E-print Network

Evolutionary Ecology, 1990, 4, 290-297 Density-dependent natural selection does not increase- dependent natural selection, or r- and K-selection, as it is often called (Boyce, 1984). Their discussion-dependent natural selection have identified density-dependent rates of population growth as the phenotype

Rose, Michael R.

131

Cooling-rate dependence of the density of Pd40Ni10Cu30P20 bulk metallic glass  

NASA Astrophysics Data System (ADS)

The effect of cooling rate on the density of the Pd40Ni10Cu30P20 bulk metallic glass (BMG) have been investigated by over the cooling rate range from 0.63 to 500 K/s. The cooling rate dependence of density, microhardness and thermal properties of the BMG is determined. Based on the thermal-expansion coefficients measured in the amorphous and supercooled liquid states, respectively, it is deduced that the density of the BMG in the ``ideal'' state approaches that of its crystalline counterpart. The effect of cooling rate on the density is discussed in terms of structural relaxation.

Hu, X.; Ng, S. C.; Feng, Y. P.; Li, Y.

2001-11-01

132

Three- to two-dimensional crossover in time-dependent density-functional theory  

NASA Astrophysics Data System (ADS)

Quasi-two-dimensional (2D) systems, such as an electron gas confined in a quantum well, are important model systems for many-body theories. Earlier studies of the crossover from 3D to 2D in ground-state density-functional theory showed that local and semilocal exchange-correlation functionals which are based on the 3D electron gas are appropriate for wide quantum wells, but eventually break down as the 2D limit is approached. We now consider the dynamical case and study the performance of various linear-response exchange kernels in time-dependent density-functional theory. We compare approximate local, semilocal, and orbital-dependent exchange kernels, and analyze their performance for inter- and intrasubband plasmons as the quantum wells approach the 2D limit. 3D (semi)local exchange functionals are found to fail for quantum well widths comparable to the 2D Wigner-Seitz radius rs2 D, which implies in practice that 3D local exchange remains valid in the quasi-2D dynamical regime for typical quantum well parameters, except for very low densities.

Karimi, Shahrzad; Ullrich, Carsten A.

2014-12-01

133

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

134

Surface properties of liquid mercury: a comparison of density-dependent and density-independent force fields  

E-print Network

The surface properties of liquid mercury (Hg) at a temperature of 293 K are investigated by classical Molecular Dynamics simulation using density-independent (DI) and density-dependent (DD) force fields. The latter force fields were introduced to improve the description of surface properties. Both force fields yield lower values of the surface tension in comparison to experimental data. Moreover, the density-dependent force field results in an anomalous thermodynamic behavior. These findings are rationalized by liquid-state theory. An optimized, density-independent force field is proposed that yields a higher surface tension and, at the same time, provides an accurate description of the liquid-vapor coexistence.

A. Iakovlev; D. Bedrov; M. Müller

2014-12-08

135

Reduced density matrix approach to calculation of electronic structure  

NASA Astrophysics Data System (ADS)

The possibility of using the one-body and two-body reduced density matrices, rather than the many-body wavefunction, as the fundamental object of study for electronic structure calculations was actively explored in the 1960's and 1970's [1]-[3], but interest has waned since. In this approach the calculation of ground-state properties is reduced to a linear optimization problem subject to the representability conditions for the density matrices, which are a mixture of linear equalities and bounds on eigenvalues. We will review this approach, present a new family of representability conditions, and discuss our experience in using present-day methods for semi-definite programming for this application. [1] A. J. Coleman: Structure of fermion density matrices. Rev. Mod. Phys. 35 (1963) 668--689. [2] Claude Garrod and Jerome K. Percus: Reduction of the N-particle variational problem. J. Math. Phys. 5 (1964) 1756--1776. [3] M. Rosina and C. Garrod: The variational calculation of reduced density matrices. J. Comput. Phys. 18 (1975) 300--310.

Braams, Bastiaan J.; Jiang, Shidong; Nayakkankuppam, Madhu; Overton, Michael L.; Percus, Jerome K.

1998-11-01

136

Influence of boundary condition types on unstable density-dependent flow.  

PubMed

Boundary conditions are required to close the mathematical formulation of unstable density-dependent flow systems. Proper implementation of boundary conditions, for both flow and transport equations, in numerical simulation are critical. In this paper, numerical simulations using the FEFLOW model are employed to study the influence of the different boundary conditions for unstable density-dependent flow systems. A similar set up to the Elder problem is studied. It is well known that the numerical simulation results of the standard Elder problem are strongly dependent on spatial discretization. This work shows that for the cases where a solute mass flux boundary condition is employed instead of a specified concentration boundary condition at the solute source, the numerical simulation results do not vary between different convective solution modes (i.e., plume configurations) due to the spatial discretization. Also, the influence of various boundary condition types for nonsource boundaries was studied. It is shown that in addition to other factors such as spatial and temporal discretization, the forms of the solute transport equation such as divergent and convective forms as well as the type of boundary condition employed in the nonsource boundary conditions influence the convective solution mode in coarser meshes. On basis of the numerical experiments performed here, higher sensitivities regarding the numerical solution stability are observed for the Adams-Bashford/Backward Trapezoidal time integration approach in comparison to the Euler-Backward/Euler-Forward time marching approach. The results of this study emphasize the significant consequences of boundary condition choice in the numerical modeling of unstable density-dependent flow. PMID:23659688

Ataie-Ashtiani, Behzad; Simmons, Craig T; Werner, Adrian D

2014-01-01

137

The electron density dependence of polar mesospheric summer echoes  

NASA Astrophysics Data System (ADS)

Many of the expressions for the reflectivity of polar mesospheric summer echoes (PMSE) predict that the echo strength is controlled by the electron density. We present several observations made using the Poker Flat Incoherent Scatter Radar which are difficult to explain based on those expressions, including observations at night with no detectable incoherent scatter below 90 km and during aurora where the reflectivity shows no response to abrupt changes in ionization. We derive a new expression for the reflectivity and are still able to explain all of these observations as scatter from free electrons. When the electron density is much smaller than the ice density the reflectivity is a strong function of electron density. In the opposite limit the reflectivity is solely controlled by the ice density because only a fraction of the electron density variance generated at the largest scales can get convected to sufficiently small scales.

Varney, Roger H.; Kelley, Michael C.; Nicolls, Michael J.; Heinselman, Craig J.; Collins, Richard L.

2011-09-01

138

Real-time linear response for time-dependent density-functional theory Department of Physical Chemistry and the Lise Meitner Minerva-Center for Quantum Chemistry,  

E-print Network

Real-time linear response for time-dependent density-functional theory Roi Baer Department a linear-response approach for time-dependent density-functional theories using time-adiabatic functionals. The resulting theory can be performed both in the time and in the frequency domain. The derivation considers

Baer, Roi

139

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

140

Adaptive landscapes and density-dependent selection in declining salmonid populations: going beyond numerical responses to human disturbance  

PubMed Central

Theory suggests an important role for population density in shaping adaptive landscapes through density-dependent selection. Here, we identify five methodological approaches for studying such selection, review the existing empirical evidence for it, and ask whether current declines in abundance can be expected to trigger evolutionary responses in salmonid fishes. Across taxa we find substantial amounts of evidence for population density influencing the location of adaptive peaks for a range of traits, and, in the presence of frequency dependence, changing the shape of selection (stabilizing versus disruptive). For salmonids, biological and theoretical considerations suggest that the optimal value of a number of traits associated with juvenile competitive ability (e.g. egg size, timing of emergence from nests, dominance ability), may depend on population density. For adults, more direct experimental and comparative evidence suggest that secondary sexual traits can be subject to density-dependent selection. There is also evidence that density affects the frequency-dependent selection likely responsible for the expression of alternative male reproductive phenotypes in salmon. Less is known however about the role of density in maintaining genetic variation among juveniles. Further efforts are required to elucidate the indirect evolutionary effects of declining population abundances, both in salmonids and in other anthropogenically challenged organisms. PMID:25567629

Einum, Sigurd; Robertsen, Grethe; Fleming, Ian A

2008-01-01

141

Mechanisms of density dependence in ducks: importance of space and per capita food.  

PubMed

The growth rate of populations usually varies over time, often in a density-dependent manner. Despite the large amount of literature on density dependence, relatively little is known of the mechanisms underlying the density-dependent processes affecting populations, especially per capita natality. We performed a 20-year study on the density dependence of brood production in two duck species differing in the stability of habitat use. Our study was conducted in a boreal watershed in southern Finland. We predicted that a diving duck common goldeneye Bucephala clangula, with more stable habitat use, would show stronger density dependence than a dabbling duck common teal Anas crecca. We investigated reproductive output in relation to the duck pair density per se as well as in relation to per capita food availability. As predicted, the reproductive output of the goldeneye showed a more density-dependent pattern than that of the teal. The number of goldeneye broods per pair decreased when the pair density increased. This was not the case with the teal. However, when the breeding success was measured by taking into account per capita food availability, both species showed density dependence. Our results imply that the occurrence of density dependent processes may vary even in sympatric ducks breeding in the same, relatively stable landscape. Our analysis also emphasizes that it is important to take into account per capita resource availability when studying the density dependence of breeding success. Both findings have important implications for the management and conservation of species. PMID:25398723

Nummi, Petri; Holopainen, Sari; Rintala, Jukka; Pöysä, Hannu

2014-11-15

142

Angular momentum dependent orbital-free density functional theory: Formulation and implementation  

NASA Astrophysics Data System (ADS)

Orbital-free density functional theory (OFDFT) directly solves for the ground-state electron density. It scales linearly with respect to system size, providing a promising tool for large-scale material simulations. Removal of the orbitals requires use of approximate noninteracting kinetic energy density functionals. If replacing ionic cores with pseudopotentials, removal of the orbitals also requires these pseudopotentials to be local. These are two severe challenges to the capabilities of conventional OFDFT. While main group elements are often well described within conventional OFDFT, transition metals remain intractable due to their localized d electrons. To advance the accuracy and general applicability of OFDFT, we have recently reported a general angular momentum dependent formulation as a next-generation OFDFT. In this formalism, we incorporate the angular momenta of electrons by devising a hybrid scheme based on a muffin tin geometry: inside spheres centered at the ionic cores, the electron density is expanded in a set of atom-centered basis functions combined with an onsite density matrix. The explicit treatment of the angular momenta of electrons provides an important basis for accurately describing the important ionic core region, which is not possible in conventional OFDFT. In addition to the conventional OFDFT total energy functional, we introduce a nonlocal energy term containing a set of angular momentum dependent energies to correct the errors due to the approximate kinetic energy density functional and local pseudopotentials. Our approach greatly increases the accuracy of OFDFT while largely preserving its numerical simplicity. Here, we provide details of the theoretical formulation and practical implementation, including the hybrid scheme, the derivation of the nonlocal energy term, the choice of basis functions, the direct minimization of the total energy, the procedure to determine the angular momentum dependent energies, the force formula with Pulay correction, and the solution to emerging numerical instability. To test the angular momentum dependent OFDFT formalism and its numerical implementations, we calculate a diverse set of properties of the transition metal Ti and compare with different levels of DFT approximation. The results suggest that angular momentum dependent OFDFT ultimately will extend the reliable reach of OFDFT to the rest of the periodic table.

Ke, Youqi; Libisch, Florian; Xia, Junchao; Carter, Emily A.

2014-04-01

143

Relativistic Coulomb Excitation within the Time Dependent Superfluid Local Density Approximation.  

PubMed

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 the superfluid local density approximation 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 compute 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 are excited during the process. The one-body dissipation of collective dipole modes is shown to lead a damping width ?_{?}?0.4??MeV and the number of preequilibrium neutrons emitted has been quantified. PMID:25615463

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

2015-01-01

144

Relativistic Coulomb Excitation within the Time Dependent Superfluid Local Density Approximation  

NASA Astrophysics Data System (ADS)

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 238U. The approach is based on the superfluid local density approximation 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 compute 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 are excited during the process. The one-body dissipation of collective dipole modes is shown to lead a damping width ???0.4 MeV and the number of preequilibrium neutrons emitted has been quantified.

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

2015-01-01

145

Determination of dependency parameter in joint distribution of dependent risks by fuzzy approach  

Microsoft Academic Search

Frechet bounds for distribution of sum of dependent risks and Farlie–Gumbel–Morgenstein (FGM) distribution family are suggested for the analysis of dependent risks. Behaviour of the coefficient of dependency in FGM distribution is investigated by fuzzy approach. Deliberations are made for the dependency and uncertainty context.

Fatih Tank; Omer L. Gebizlioglu; Aysen Apaydin

2006-01-01

146

A likelihood approach to estimating animal density from binary acoustic transects.  

PubMed

We propose an approximate maximum likelihood method for estimating animal density and abundance from binary passive acoustic transects, when both the probability of detection and the range of detection are unknown. The transect survey is purposely designed so that successive data points are dependent, and this dependence is exploited to simultaneously estimate density, range of detection, and probability of detection. The data are assumed to follow a homogeneous Poisson process in space, and a second-order Markov approximation to the likelihood is used. Simulations show that this method has small bias under the assumptions used to derive the likelihood, although it performs better when the probability of detection is close to 1. The effects of violations of these assumptions are also investigated, and the approach is found to be sensitive to spatial trends in density and clustering. The method is illustrated using real acoustic data from a survey of sperm and humpback whales. PMID:21039393

Horrocks, Julie; Hamilton, David C; Whitehead, Hal

2011-09-01

147

Estimation of neural firing rate: the wavelet density estimation approach.  

PubMed

The computation of neural firing rates based on spike sequences has been introduced as a useful tool for extraction of an animal's behavior. Different estimating methods of such neural firing rates have been developed by neuroscientists, and among these methods, time histogram and kernel estimators have been used more than other approaches. In this paper, the problem in the estimation of firing rates using wavelet density estimators has been considered. The results of simulation study in estimation of underlying rates based on spike sequences sampled from two different variable firing rates show that the proposed wavelet density method provides better and more accurate estimation of firing rates with smooth results compared to two other classical approaches. Furthermore, the performance of a different family of wavelet density estimators in the estimation of the underlying firing rate of biological data have been compared with results of both time histogram and kernel estimators. All in all, the results show that the proposed method can be useful in the estimation of firing rate of neural spike trains. PMID:23924519

Khorasani, Abed; Daliri, Mohammad Reza

2013-08-01

148

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

149

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...

150

Dependence of Modulation Amplitude on Electron Density in Unidirectional Lateral Superlattices  

E-print Network

Dependence of Modulation Amplitude on Electron Density in Unidirectional Lateral Superlattices 19, 2005; accepted March 23, 2005) The amplitude V0 of unidirectional periodic potential modulation, AlGaAs/GaAs single heterostructure, potential modulation, lateral superlattice, electron density

Katsumoto, Shingo

151

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

152

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

SciTech Connect

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; Corminboeuf, Clémence, E-mail: clemence.corminboeuf@epfl.ch [Laboratory for Computational Molecular Design, Institut des Sciences et Ingénierie Chimiques, Ecole Polytechnique Fédérale de Lausanne, CH-1015 Lausanne (Switzerland)] [Laboratory for Computational Molecular Design, Institut des Sciences et Ingénierie Chimiques, Ecole Polytechnique Fédérale de Lausanne, CH-1015 Lausanne (Switzerland); Golubev, Nikolay [Laboratory for Computational Molecular Design, Institut des Sciences et Ingénierie Chimiques, Ecole Polytechnique Fédérale de Lausanne, CH-1015 Lausanne (Switzerland) [Laboratory for Computational Molecular Design, Institut des Sciences et Ingénierie Chimiques, Ecole Polytechnique Fédérale de Lausanne, CH-1015 Lausanne (Switzerland); Department of Chemistry, M. V. Lomonosov Moscow State University, Moscow 119991 (Russian Federation); Steinmann, Stephan N., E-mail: sns25@duke.edu [Laboratory for Computational Molecular Design, Institut des Sciences et Ingénierie Chimiques, Ecole Polytechnique Fédérale de Lausanne, CH-1015 Lausanne (Switzerland); Department of Chemistry, Duke University, Durham, North Carolina 27708 (United States)

2014-05-14

153

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

154

Anomalous dependence of interface superconductivity on carrier density in metal-insulator bilayer  

NASA Astrophysics Data System (ADS)

The interface superconductivity is of particular interest for its higher superconducting temperature (Tc) and close connection to the HTS study. However, despite of the experimental and theoretical progress, the answer to the following key question still remains ambiguous: can the underlying mechanism of the interface superconductivity be interpreted solely in terms of charge transfer and Sr interdiffusion? One experimental approach to answer this question is to study the dependence of Tc on the carrier density of La2-xSrxCuO4/La2CuO4 (LSCO/LCO) bilayer by changing the doping level in the metallic LSCO layer. The charge transfer and cation interdiffusion at the interface is proportional to the carrier density in LSCO layer; therefore, Tc of the interface is expected to show a significant and non-monotonic dependence on the carrier density. To exam this prediction, we synthesized a series of La2-xSrxCuO4/La2CuO4 bilayers with x ranging in 0.27 to 0.47 by atomic layer-by-layer molecular beam epitaxy (ALL-MBE). The uniqueness of these samples is a 4% Sr doping gradient of the central doping level across the 10mm width of the substrate. These combinatorial samples realize an extremely fine tuning of doping and enable us to construct a phase diagram that is 30 times denser than a normal method can achieve.

Wu, Jie; Pelleg, Oshri; Logvenov, Gennady; Bollinger, Anthony; Boebinger, Greg; Bozovic, Ivan

2012-02-01

155

Phase- and density-dependent population dynamics in Norwegian lemmings: interaction between deterministic and stochastic processes.  

PubMed Central

We analysed two 26-year long (1970-1995) time-series on annual population growth rates of Norwegian lemmings (Lemmus lemmus) from Finse, south Norway, using a threshold autoregressive (TAR) approach. We demonstrate that the population dynamics is both phase- and density-dependent. The phase-dependence accounts for the observed nonlinearity. We used the deduced stochastic model structure as a basis for evaluating the dynamic properties of this system. The dynamics is characterized either by limit cycles or chaos (the latter with a strong semi-periodic component). Stochasticity is seen to play an important role in the determination of the periodicity. The ecological implications of these statistical and mathematical results are discussed. PMID:9821362

Stenseth, N C; Chan, K S; Framstad, E; Tong, H

1998-01-01

156

Brief review related to the foundations of time-dependent density functional theory  

Microsoft Academic Search

The electron density n(r,t), which is the central tool of time-dependent density functional theory, is presently considered to be derivable from a one-body\\u000a time-dependent potential V(r,t), via one-electron wave functions satisfying a time-dependent Schrödinger equation. This is here related via a generalized\\u000a equation of motion to a Dirac density matrix now involving t. Linear response theory is then surveyed, with

Thomas A. NiehausNorman; Norman H. March

2010-01-01

157

Quantifying Correlations Between Isovector Observables and the Density Dependence of Nuclear Symmetry Energy away from Saturation Density  

E-print Network

According to the Hugenholtz-Van Hove theorem, the nuclear symmetry energy $S(\\rho)$ and its slope $L(\\rho)$ at arbitrary densities can be decomposed in terms of the density and momentum dependence of the single-nucleon potentials in isospin-asymmetric nuclear matter which are potentially accessible to experiment. We quantify the correlations between several well-known isovector observables and $L(\\rho)$ to locate the density range in which each isovector observable is most sensitive to the density dependence of the $S(\\rho)$. We then study the correlation coefficients between those isovector observables and all the components of the $L(\\rho)$. The neutron skin thickness of $^{208}$Pb is found to be strongly correlated with the $L(\\rho)$ at a subsaturation density of $\\rho = 0.59 \\rho_0$ through the density dependence of the first-order symmetry potential. Neutron star radii are found to be strongly correlated with the $L(\\rho)$ over a wide range of supra-saturation densities mainly through both the density an...

Fattoyev, F J; Li, Bao-An

2014-01-01

158

Psychological Approaches for the Treatment of Cocaine Dependence-A Neurobehavioral Approach  

Microsoft Academic Search

A variety of psychological approaches have been utilized for the treatment of cocaine dependence. Most information has been presented in case report format. However, several investigations have established integrated outpatient approaches which are currently being systematically evaluated. One of these approaches, the neurobehavioral model of cocaine dependency treatment establishes a clear timetable for cocaine recovery and focusses attention on four

Richard A Rawson; Jeanne Obert; Michael J. McCann; Walter Ling

1992-01-01

159

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

160

Solvent density inhomogeneities and solvation free energies in supercritical diatomic fluids: a density functional approach.  

PubMed

Density functional theory is used to explore the solvation properties of a spherical solute immersed in a supercritical diatomic fluid. The solute is modeled as a hard core Yukawa particle surrounded by a diatomic Lennard-Jones fluid represented by two fused tangent spheres using an interaction site approximation. The authors' approach is particularly suitable for thoroughly exploring the effect of different interaction parameters, such as solute-solvent interaction strength and range, solvent-solvent long-range interactions, and particle size, on the local solvent structure and the solvation free energy under supercritical conditions. Their results indicate that the behavior of the local coordination number in homonuclear diatomic fluids follows trends similar to those reported in previous studies for monatomic fluids. The local density augmentation is particularly sensitive to changes in solute size and is affected to a lesser degree by variations in the solute-solvent interaction strength and range. The associated solvation free energies exhibit a nonmonotonous behavior as a function of density for systems with weak solute-solvent interactions. The authors' results suggest that solute-solvent interaction anisotropies have a major influence on the nature and extent of local solvent density inhomogeneities and on the value of the solvation free energies in supercritical solutions of heteronuclear molecules. PMID:17302486

Husowitz, B; Talanquer, V

2007-02-01

161

Reduced density matrix hybrid approach: Application to electronic energy transfer  

SciTech Connect

Electronic energy transfer in the condensed phase, such as that occurring in photosynthetic complexes, frequently occurs in regimes where the energy scales of the system and environment are similar. This situation provides a challenge to theoretical investigation since most approaches are accurate only when a certain energetic parameter is small compared to others in the problem. Here we show that in these difficult regimes, the Ehrenfest approach provides a good starting point for a dynamical description of the energy transfer process due to its ability to accurately treat coupling to slow environmental modes. To further improve on the accuracy of the Ehrenfest approach, we use our reduced density matrix hybrid framework to treat the faster environmental modes quantum mechanically, at the level of a perturbative master equation. This combined approach is shown to provide an efficient and quantitative description of electronic energy transfer in a model dimer and the Fenna-Matthews-Olson complex and is used to investigate the effect of environmental preparation on the resulting dynamics.

Berkelbach, Timothy C.; Reichman, David R. [Department of Chemistry, Columbia University, 3000 Broadway, New York, New York 10027 (United States); Markland, Thomas E. [Department of Chemistry, Stanford University, 333 Campus Drive, Stanford, California 94305 (United States)

2012-02-28

162

Application of the relativistic time-dependent density functional theory to the photoionization of xenon  

NASA Astrophysics Data System (ADS)

Absolute photoionization cross section profiles, branching ratios and asymmetry parameters of xenon have been calculated with the time-dependent density functional theory (TDDFT) approach in a wide photon energy range, from threshold up to 740 eV. The relativistic TDDFT (RTDDFT) equations have been implemented employing a B-spline finite basis set with a non-iterative algorithm for the calculation of the response-induced potential, thus eliminating the well known convergence difficulties associated with their iterative solution. The use of the gradient-dependent LB94 exchange-correlation potential, which allows the existence of bound Rydberg states, has permitted the description, for the first time at RTDDFT level, of the autoionization resonances. Generally, an excellent reproduction of experimental results is obtained, always at least as accurate as that obtained with the computationally more expensive relativistic random phase approximation approach, making the B-spline RTDDFT formulation a very promising approach for the calculation of photoionization processes in heavy-atom systems.

Toffoli, D.; Stener, M.; Decleva, P.

2002-03-01

163

Approaches to 100 Gbit/sq. in. recording density  

NASA Technical Reports Server (NTRS)

A recording density of 10 Gbit/sq. in. is being pursued by a number of companies and universities in the National Storage Industry Consortium. It is widely accepted that this goal will be achieved in the laboratory within a few years. In this paper approaches to achieving 100 Gbit/sq. in. storage densities are considered. A major obstacle to continued scaling of magnetic recording to higher densities is that as the bit size is reduced, the grain size in the magnetic media must be reduced in order that media noise does not become so large that the signal to noise ratio (SNR) degrades sufficiently to make detection impossible. At 100 Gbit/sq. in., the bit size is only 0.006 square micrometers, which, in order to achieve 30 dB SNR, requires a grain size of about 2.5 nm. Such small grains are subject to thermal instability, and the recorded information will degrade over time unless the magnetic anisotropy of the materials used is increased significantly, or the media thickness is made much larger than expected on the basis of scaling today's longitudinal media thickness.

Kryder, Mark H.

1994-01-01

164

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

165

Density-Dependent Effects on Group Size Are Sex-Specific in a Gregarious Ungulate  

PubMed Central

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/km2 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/km2 and we found evidence for a minimum group size at population density ?0.70 elk/km2. 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

166

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

PubMed

Although the dispersal of animals is influenced by a variety of factors, few studies have used a condition-dependent approach to assess it. The mechanisms underlying dispersal are thus poorly known in many species, especially in large mammals. We used 10 microsatellite loci to examine population density effects on sex-specific dispersal behavior in the American black bear, Ursus americanus. We tested whether dispersal increases with population density in both sexes. Fine-scale genetic structure was investigated in each of four sampling areas using Mantel tests and spatial autocorrelation analyses. Our results revealed male-biased dispersal pattern in low-density areas. As population density increased, females appeared to exhibit philopatry at smaller scales. Fine-scale genetic structure for males at higher densities may indicate reduced dispersal distances and delayed dispersal by subadults. PMID:22822432

Roy, Justin; Yannic, Glenn; Côté, Steeve D; Bernatchez, Louis

2012-03-01

167

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

PubMed Central

Although the dispersal of animals is influenced by a variety of factors, few studies have used a condition-dependent approach to assess it. The mechanisms underlying dispersal are thus poorly known in many species, especially in large mammals. We used 10 microsatellite loci to examine population density effects on sex-specific dispersal behavior in the American black bear, Ursus americanus. We tested whether dispersal increases with population density in both sexes. Fine-scale genetic structure was investigated in each of four sampling areas using Mantel tests and spatial autocorrelation analyses. Our results revealed male-biased dispersal pattern in low-density areas. As population density increased, females appeared to exhibit philopatry at smaller scales. Fine-scale genetic structure for males at higher densities may indicate reduced dispersal distances and delayed dispersal by subadults. PMID:22822432

Roy, Justin; Yannic, Glenn; Côté, Steeve D; Bernatchez, Louis

2012-01-01

168

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

169

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

170

Quark mass density- and temperature- dependent model for bulk strange quark matter  

E-print Network

It is shown that the quark mass density-dependent model can not be used to explain the process of the quark deconfinement phase transition because the quark confinement is permanent in this model. A quark mass density- and temperature-dependent model in which the quark confinement is impermanent has been suggested. We argue that the vacuum energy density B is a function of temperature. The dynamical and thermodynamical properties of bulk strange quark matter for quark mass density- and temperature-dependent model are discussed.

Yun Zhang et. al

2002-01-18

171

Kinetic modeling of rhamnolipid production by Pseudomonas aeruginosa PAO1 including cell density-dependent regulation.  

PubMed

The production of rhamnolipid biosurfactants by Pseudomonas aeruginosa is under complex control of a quorum sensing-dependent regulatory network. Due to a lack of understanding of the kinetics applicable to the process and relevant interrelations of variables, current processes for rhamnolipid production are based on heuristic approaches. To systematically establish a knowledge-based process for rhamnolipid production, a deeper understanding of the time-course and coupling of process variables is required. By combining reaction kinetics, stoichiometry, and experimental data, a process model for rhamnolipid production with P. aeruginosa PAO1 on sunflower oil was developed as a system of coupled ordinary differential equations (ODEs). In addition, cell density-based quorum sensing dynamics were included in the model. The model comprises a total of 36 parameters, 14 of which are yield coefficients and 7 of which are substrate affinity and inhibition constants. Of all 36 parameters, 30 were derived from dedicated experimental results, literature, and databases and 6 of them were used as fitting parameters. The model is able to describe data on biomass growth, substrates, and products obtained from a reference batch process and other validation scenarios. The model presented describes the time-course and interrelation of biomass, relevant substrates, and products on a process level while including a kinetic representation of cell density-dependent regulatory mechanisms. PMID:24770383

Henkel, Marius; Schmidberger, Anke; Vogelbacher, Markus; Kühnert, Christian; Beuker, Janina; Bernard, Thomas; Schwartz, Thomas; Syldatk, Christoph; Hausmann, Rudolf

2014-08-01

172

Density-dependence of functional spiking networks in vitro  

SciTech Connect

During development, the mammalian brain differentiates into specialized regions with unique functional abilities. While many factors contribute to this functional specialization, we explore the effect neuronal density can have on neuronal interactions. Two types of networks, dense (50,000 neurons and glia support cells) and sparse (12,000 neurons and glia support cells), are studied. A competitive first response model is applied to construct activation graphs that represent pairwise neuronal interactions. By observing the evolution of these graphs during development in vitro we observe that dense networks form activation connections earlier than sparse networks, and that link-!llltropy analysis of the resulting dense activation graphs reveals that balanced directional connections dominate. Information theoretic measures reveal in addition that early functional information interactions (of order 3) are synergetic in both dense and sparse networks. However, during development in vitro, such interactions become redundant in dense, but not sparse networks. Large values of activation graph link-entropy correlate strongly with redundant ensembles observed in the dense networks. Results demonstrate differences between dense and sparse networks in terms of informational groups, pairwise relationships, and activation graphs. These differences suggest that variations in cell density may result in different functional specialization of nervous system tissue also in vivo.

Ham, Michael I [Los Alamos National Laboratory; Gintautuas, Vadas [Los Alamos National Laboratory; Rodriguez, Marko A [Los Alamos National Laboratory; Bettencourt, Luis M A [Los Alamos National Laboratory; Bennett, Ryan [UNIV OF NORTH TEXAS; Santa Maria, Cara L [UNIV OF NORTH TEXAS

2008-01-01

173

Optical properties of Quantum Disks: Real density matrix approach  

NASA Astrophysics Data System (ADS)

We show how to compute the optical response of a Quantum Disk (QDisk) to an electromagnetic wave as a function of the incident wave polarization, in the energetic region of interband transitions. Both the TM and TE polarization in guided-wave geometry are analyzed. The method uses the microscopic calculation of Quantum Disk eigenfunctions and the macroscopic real density matrix approach to compute the effective QDisk susceptibility, taking into account the valence band structure of the QDisk material and the Coulomb interaction between the electron and the hole. Analytical expressions for the QDisk susceptibility are obtained for a certain model electron — hole potential. Using these expressions, all optical functions can be computed. Results for the absorption coefficient are computed for InAs/GaAs QDisks. Fair agreement with experiments is obtained.

Czajkowski, Gerard; Silvestri, Leonardo

2006-06-01

174

Local bias approach to the clustering of discrete density peaks  

NASA Astrophysics Data System (ADS)

Maxima of the linear density field form a point process that can be used to understand the spatial distribution of virialized halos that collapsed from initially overdense regions. However, owing to the peak constraint, clustering statistics of discrete density peaks are difficult to evaluate. For this reason, local bias schemes have received considerably more attention in the literature thus far. In this paper, we show that the two-point correlation function of maxima of a homogeneous and isotropic Gaussian random field can be thought of, up to second order at least, as arising from a local bias expansion formulated in terms of rotationally invariant variables. This expansion relies on a unique smoothing scale, which is the Lagrangian radius of dark matter halos. The great advantage of this local bias approach is that it circumvents the difficult computation of joint probability distributions. We demonstrate that the bias factors associated with these rotational invariants can be computed using a peak-background split argument, in which the background perturbation shifts the corresponding probability distribution functions. Consequently, the bias factors are orthogonal polynomials averaged over those spatial locations that satisfy the peak constraint. In particular, asphericity in the peak profile contributes to the clustering at quadratic and higher order, with bias factors given by generalized Laguerre polynomials. We speculate that our approach remains valid at all orders, and that it can be extended to describe clustering statistics of any point process of a Gaussian random field. Our results will be very useful to model the clustering of discrete tracers with more realistic collapse prescriptions involving the tidal shear for instance.

Desjacques, Vincent

2013-02-01

175

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

176

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

NASA Astrophysics Data System (ADS)

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.

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.

2012-06-01

177

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

178

Optical properties of triplex DNA from time-dependent density functional theory.  

PubMed

We present a combined investigation of the dynamics and optics of triplex DNA, based on classical molecular dynamics and time-dependent density functional theory. Our approach is devised to include the effects of conformational fluctuations on the electronic structure and optical excitations of the system. We find that the structural flexibility has a strong role in the determination of the optical signals. Our results allow us to unravel the peculiar fingerprints of Watson-Crick and Hoogsteen H-bonding in the optical absorption spectra. We find a specific optical absorption feature that is due to the simultaneous presence of the two H-bonding patterns in C(+)GC triplets. While this peculiar triplet signal is wiped out in some structures that are representative of the finite-temperature dynamics, it can be recovered in an average view, so that it is a pristine result of this work. PMID:22866829

Ghane, Tahereh; Brancolini, Giorgia; Varsano, Daniele; Di Felice, Rosa

2012-09-01

179

Liquid Water Simulations with Density Fragment Interaction Approach  

PubMed Central

We reformulate the density fragment interaction (DFI) approach [J. Chem. Phys. 129, 054102 (2008)] to achieve linear-scaling quantum mechanical calculations for large molecular systems. Two key approximations are developed to improve the DFI efficiency 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 good agreement with experimental measurements including the heat capacity, binding energy per water molecule, and the radial distribution functions of for 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

2013-01-01

180

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

181

Estimation of density-dependent mortality of juvenile bivalves in the Wadden Sea.  

PubMed

We investigated density-dependent mortality within the early months of life of the bivalves Macoma balthica (Baltic tellin) and Cerastoderma edule (common cockle) in the Wadden Sea. Mortality is thought to be density-dependent in juvenile bivalves, because there is no proportional relationship between the size of the reproductive adult stocks and the numbers of recruits for both species. It is not known however, when exactly density dependence in the pre-recruitment phase occurs and how prevalent it is. The magnitude of recruitment determines year class strength in bivalves. Thus, understanding pre-recruit mortality will improve the understanding of population dynamics. We analyzed count data from three years of temporal sampling during the first months after bivalve settlement at ten transects in the Sylt-Rømø-Bay in the northern German Wadden Sea. Analyses of density dependence are sensitive to bias through measurement error. Measurement error was estimated by bootstrapping, and residual deviances were adjusted by adding process error. With simulations the effect of these two types of error on the estimate of the density-dependent mortality coefficient was investigated. In three out of eight time intervals density dependence was detected for M. balthica, and in zero out of six time intervals for C. edule. Biological or environmental stochastic processes dominated over density dependence at the investigated scale. PMID:25105293

Andresen, Henrike; Strasser, Matthias; van der Meer, Jaap

2014-01-01

182

Estimation of Density-Dependent Mortality of Juvenile Bivalves in the Wadden Sea  

PubMed Central

We investigated density-dependent mortality within the early months of life of the bivalves Macoma balthica (Baltic tellin) and Cerastoderma edule (common cockle) in the Wadden Sea. Mortality is thought to be density-dependent in juvenile bivalves, because there is no proportional relationship between the size of the reproductive adult stocks and the numbers of recruits for both species. It is not known however, when exactly density dependence in the pre-recruitment phase occurs and how prevalent it is. The magnitude of recruitment determines year class strength in bivalves. Thus, understanding pre-recruit mortality will improve the understanding of population dynamics. We analyzed count data from three years of temporal sampling during the first months after bivalve settlement at ten transects in the Sylt-Rømø-Bay in the northern German Wadden Sea. Analyses of density dependence are sensitive to bias through measurement error. Measurement error was estimated by bootstrapping, and residual deviances were adjusted by adding process error. With simulations the effect of these two types of error on the estimate of the density-dependent mortality coefficient was investigated. In three out of eight time intervals density dependence was detected for M. balthica, and in zero out of six time intervals for C. edule. Biological or environmental stochastic processes dominated over density dependence at the investigated scale. PMID:25105293

Andresen, Henrike; Strasser, Matthias; van der Meer, Jaap

2014-01-01

183

A high-performance Fortran code to calculate spin- and parity-dependent nuclear level densities  

NASA Astrophysics Data System (ADS)

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+g- model spaces. Examples of the calculations for 28Si (in the sd-model space) and 64Ge (in the pf+g-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+g, 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. Program summaryProgram title: MM Catalogue identifier: AENM_v1_0 Program summary URL:http://cpc.cs.qub.ac.uk/summaries/AENM_v1_0.html Program obtainable from: CPC Program Library, Queen's University, Belfast, N. Ireland Licensing provisions: Standard CPC licence, http://cpc.cs.qub.ac.uk/licence/licence.html No. of lines in distributed program, including test data, etc.: 193181 No. of bytes in distributed program, including test data, etc.: 1298585 Distribution format: tar.gz Programming language: Fortran 90, MPI. Computer: Any architecture with a Fortran 90 compiler and MPI. Operating system: Linux. RAM: Proportional to the system size, in our examples, up to 75Mb Classification: 17.15. External routines: MPICH2 (http://www.mcs.anl.gov/research/projects/mpich2/) Nature of problem: Calculating of the spin- and parity-dependent nuclear level density. Solution method: The algorithm implies exact calculation of the first and second Hamiltonian moments for different configurations at fixed spin and parity. The code is parallelized using the Message Passing Interface and a master-slaves dynamical load-balancing approach. Restrictions: The program uses two-body interaction in a restricted single-level basis. For example, GXPF1A in the pf-valence space. Running time: Depends on the system size and the number of processors used (from 1 min to several hours).

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

2013-01-01

184

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

185

Time-dependent Laudau theory of charge-density waves in transition-metal dichalcogenides  

Microsoft Academic Search

A time-dependent Landau theory is proposed for the transition-metal dichalcogenides coupling the motion of the charge-density waves to the electron liquid. The dissipative terms are related to the metallic resistance. The \\

W. L. McMillan

1975-01-01

186

A comprehensive approach for the assessment of in-situ pavement density using GPR technique  

NASA Astrophysics Data System (ADS)

Proper construction of the asphalt pavement is a prerequisite to developing a long lasting roadway that does not require extensive future maintenance. This goal is achieved by verifying that design specifications are met through the use of quality assurance (QA) practices. The in-situ density is regarded as one of the most important controls used to ensure that a pavement being placed is of high quality because it is a good indicator of future performance. In-situ density is frequently assessed utilizing one or more of the following three methods: cores, nuclear density gauge measurements or non-nuclear density gauge measurements. Each of the above mentioned methods, however, have their distinct disadvantages. Cores, for example, are generally considered to be the most accurate means of measuring in-situ density, however, they are a time consuming and destructive test that introduces a defect into asphalt pavements. Because of the destructive nature associated with coring, contractors and agencies have alternatively used non-destructive nuclear and non-nuclear density gauges for quality control purposes. These instruments allow for a more rapid assessment of the in-situ density, allowing measurements to be taken even during the pavement's construction. The disadvantage of these gauges are that they provide density readings only at discrete locations of the asphalt pavement mat, while no consensus exists among pavement researchers on the proper correlation between the gauges and core density. In recent years, numerous alternative methods have been introduced for the assessment of in-situ density, both during asphalt pavement construction and afterwards. These methods include, amongst others, intelligent compaction, thermal imaging and ground penetrating radar (GPR). Among these methods, GPR has been defined as both a technically feasible and promising method for the nondestructive, rapid, and continuous evaluation of in-situ asphalt pavement density based on electromagnetic mixing (EM) theory, through the utilization of proper models. These models enable the prediction of asphalt mixture density dependent on its bulk dielectric constant as measured by the GPR, the dielectric properties of the asphalt mix materials, as well as other material information. The goal of the present study is to attempt to verify the prediction performance of various density models. To accomplish this goal GPR surveys were carried out in the field during asphalt pavement construction to evaluate the density results due to different compaction modes. The GPR data was analyzed to calculate the appropriate asphalt mix dielectric properties needed for the activation of the considered density prediction models. Predicted densities were compared with densities of the field cores extracted from the as-built asphalt pavement prior to trafficking. It was found that the predicted density values were significantly lower when compared to the ground truth data. A further investigation of the effect of temperature on GPR readings showed that GPR seems to overestimate the in-situ density. However, this approach could be used effectively to evaluate the performance of different compaction methods and set up the compaction pattern that is needed to achieve the desired asphalt pavement density.

Plati, Christina; Georgiou, Panos; Loizos, Andreas

2013-04-01

187

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

188

Time-dependent density functional theory for molecules in liquid solutions  

Microsoft Academic Search

A procedure based on the polarizable continuum model (PCM) has been applied to reproduce solvent effects on electronic spectra in connection with the time-dependent density functional theory (TD-DFT). To account for solute-solvent interactions, a suitable operator has been defined, which depends on the solute electronic density and can be used to modify the TD-DFT equations for the calculation of molecular

Maurizio Cossi; Vincenzo Barone

2001-01-01

189

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

190

A novel approach to evaluating breast density using ultrasound tomography  

Microsoft Academic Search

Women with high mammographic breast density have a 4- to 5-fold increased breast cancer risk compared to women with fatty breasts. Current breast density estimation involves mammography, although a mammogram is a 2-D projection which does not provide an accurate volumetric density estimate due to breast thickness not being taken into account. Presumably, breast cancer risk would have a stronger

Carri K. Glide

2007-01-01

191

TimeDependent Variational Approach to BoseEinstein Condensation  

E-print Network

Time­Dependent Variational Approach to Bose­Einstein Condensation Luca Salasnich Istituto Nazionale­interacting Bose­Einstein condensate (BEC) and its connection with the exact many­ body problem by deriving the Gross­Pitaevskii action of the condensate. The mechanics of the BEC in a harmonic potential is studied

192

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

193

The effects of density dependent resource limitation on size of wild reindeer  

Microsoft Academic Search

A density-dependent decrement in size for wild reindeer from 12 different Norwegian herds at 16 different densities was shown using lower jawbone-length as the criterion of size. This criterion was tested and found to adequately predict body size of both bucks and does. Lactation in does did not affect jaw length but significantly affected dressed weights.

Terje Skogland

1983-01-01

194

Density-dependent mating tactic expression is linked to stress hormone in Woodhouse's toad  

Microsoft Academic Search

The social environment can alter mating tactic expression through modulatory effects on the endocrine system. In anuran amphibians (frogs and toads), males often adopt noncalling mating tactics in response to increased chorus density, but the underlying physiological factors mediating this response are poorly understood. Recent models suggest that density-dependent expression of alternative mating tactics in anurans is associated with a

Christopher J. Leary; Apryl M. Garcia; Rosemary Knapp

2008-01-01

195

Evolution of Competitive Ability in Drosophila by Density-Dependent Natural Selection  

Microsoft Academic Search

The theory of density-dependent natural selection predicts that populations kept at extreme densities should evolve different competitive abilities for limited resources. These predictions have been tested with laboratory populations of Drosophila melanogaster. Six independent populations were maintained in two environments, called r and K, for 128 generations. In the r environment, population sizes were small and resources for larvae and

Laurence D. Mueller

1988-01-01

196

Isospin-invariant Skyrme energy-density-functional approach with axial symmetry  

NASA Astrophysics Data System (ADS)

Background: Density functional theory (DFT) is the microscopic tool of choice to describe properties of nuclei over the entire nuclear landscape, with a focus on medium-mass and heavy complex systems. Modern energy density functionals (EDFs) often offer a level of accuracy typical of phenomenological approaches based on parameters locally fitted to the data. It is clear, however, that in order to achieve high quality of predictions to guide spectroscopic studies, current functionals must be improved, especially in the isospin channel. In this respect, experimental studies of short-lived nuclei far from stability offer a unique test of isospin aspects of the many-body theory. Purpose: We develop the isospin-invariant Skyrme-EDF method by considering local densities in all possible isospin channels and proton-neutron (p-n) mixing terms as mandated by the isospin symmetry. The EDF employed has the most general form that depends quadratically on the isoscalar and isovector densities. We test and benchmark the resulting p-n EDF approach, and study the general properties of the new scheme by means of the cranking in the isospin space. Methods: We extend the existing axial DFT solver hfbtho to the case of isospin-invariant EDF approach with all possible p-n mixing terms. Explicit expressions have been derived for all the densities and potentials that appear in the isospin representation. In practical tests, we consider the Skyrme EDF SkM* and, as a first application, concentrate on Hartree-Fock aspects of the problem, i.e., pairing has been disregarded. Results: Calculations have been performed for the (A =78,T?11), (A =40,T?8), and (A =48,T?4) isobaric analog chains. Isospin structure of self-consistent p-n mixed solutions has been investigated with and without the Coulomb interaction, which is the sole source of isospin symmetry breaking in our approach. The extended axial hfbtho solver has been benchmarked against the symmetry-unrestricted hfodd code for deformed and spherical states. Conclusions: We developed and tested a general isospin-invariant Skyrme-EDF framework. The new approach permits spin-isospin densities that may give rise to hitherto unexplored modes in the excitation spectrum. The new formalism has been tested in the Hartree-Fock limit. A systematic comparison between hfodd and hfbtho results show a maximum deviation of about 10 keV on the total binding energy for deformed nuclei when the Coulomb term is included. Without this term, the results of both solvers agree down to a ˜10 eV level.

Sheikh, J. A.; Hinohara, N.; Dobaczewski, J.; Nakatsukasa, T.; Nazarewicz, W.; Sato, K.

2014-05-01

197

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

198

Morphological Transitions Governed by Density Dependence and Lipoxygenase Activity in Aspergillus flavus? †  

PubMed Central

Aspergillus flavus differentiates to produce asexual dispersing spores (conidia) or overwintering survival structures called sclerotia. Results described here show that these two processes are oppositely regulated by density-dependent mechanisms and that increasing the cell density (from 101 to 107 cells/plate) results in the lowest numbers of sclerotial and the highest numbers of conidial. Extract from spent medium of low-cell-density cultures induced a high-sclerotium-number phenotype, whereas high-cell-density extract increased conidiation. Density-dependent development is also modified by changes in lipid availability. Exogenous linoleic acid increased sclerotial production at intermediate cell densities (104 and 105 cells/plate), whereas oleic and linolenic acids inhibited sclerotium formation. Deletion of Aflox encoding a lipoxygenase (LOX) greatly diminished density-dependent development of both sclerotia and conidia, resulting in an overall increase in the number of sclerotia and a decrease in the number of conidia at high cell densities (>105 cells/plate). Aflox mutants showed decreased linoleic acid LOX activity. Taken together, these results suggest that there is a quorum-sensing mechanism in which a factor(s) produced in dense cultures, perhaps a LOX-derived metabolite, activates conidium formation, while a factor(s) produced in low-density cultures stimulates sclerotium formation. PMID:18658287

Horowitz Brown, S.; Zarnowski, R.; Sharpee, W. C.; Keller, N. P.

2008-01-01

199

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

200

Dependence of the density limit on the toroidal magnetic field on FTU  

NASA Astrophysics Data System (ADS)

The capability of predicting the density limit of a magnetically confined burning plasma is of crucial importance to establish the ultimate performance of a fusion power plant. The Greenwald density limit, commonly used as an empirical scaling law, predicts that the maximum achievable central line-averaged density is given by the relation \\bar{n}_G = k \\bar{J} , where \\bar{J} is the average plasma current density and k is the plasma elongation. However, several experiments have pointed out that such a limit can be overcome in the presence of peaked density profiles. This paper proposes a new empirical scaling law for a limiter tokamak operating in the low-energy confinement mode (L-mode) concerning the case of peaked density profiles associated with the presence of multifaceted asymmetric radiation from the edges. This result is based on dedicated experiments performed on the Frascati Tokamak Upgrade (FTU) under extremely clean machine conditions (Zeff = 1.0-1.5), in which the high-density domain is explored in a wide range of values of plasma current (Ip = 500-900 kA) and toroidal magnetic field (BT = 4-8 T). It is found that the maximum achievable central line-averaged density essentially depends on the toroidal magnetic field only and does not depend on the average plasma current density: the behaviour is explained in terms of density profile peaking in the high-density domain. As a confirmation that the limit is an edge limit, it is also shown that a Greenwald-like scaling (i.e. depending on the current density) actually holds for the edge line-averaged density (at r/a ? 4/5).

Pucella, G.; Tudisco, O.; Apicella, M. L.; Apruzzese, G.; Artaserse, G.; Belli, F.; Bin, W.; Boncagni, L.; Botrugno, A.; Buratti, P.; Calabrò, G.; Castaldo, C.; Cianfarani, C.; Cocilovo, V.; Dimatteo, L.; Esposito, B.; Frigione, D.; Gabellieri, L.; Giovannozzi, E.; Granucci, G.; Marinucci, M.; Marocco, D.; Martines, E.; Mazzitelli, G.; Mazzotta, C.; Nowak, S.; Ramogida, G.; Romano, A.; Tuccillo, A. A.; Zeng, L.; Zuin, M.

2013-02-01

201

Time-dependent density functional methods for Raman spectra in open-shell systems.  

PubMed

We present an implementation of a time-dependent density functional theory (TD-DFT) linear response module in NWChem for unrestricted DFT calculations and apply it to the calculation of resonant Raman spectra in open-shell molecular systems using the short-time approximation. The new source code was validated and applied to simulate Raman spectra on several doublet organic radicals (e.g., benzyl, benzosemiquinone, TMPD, trans-stilbene anion and cation, and methyl viologen) and the metal complex copper phthalocyanine. We also introduce a divide-and-conquer approach for the evaluation of polarizabilities in relatively large systems (e.g., copper phthalocyanine). The implemented tool gives comparisons with experiment that are similar to what is commonly found for closed-shell systems, with good agreement for most features except for small frequency shifts, and occasionally large deviations for some modes that depend on the molecular system studied, experimental conditions not being accounted in the modeling such as solvation effects and extra solvent-based peaks, and approximations in the underlying theory. The approximations used in the quantum chemical modeling include (i) choice of exchange-correlation functional and basis set; (ii) harmonic approximation used in the frequency analysis to determine vibrational normal modes; and (iii) short-time approximation (omission of nuclear motion effects) used in calculating resonant Raman spectra. PMID:24380604

Aquino, Fredy W; Schatz, George C

2014-01-16

202

Demographic variability and density-dependent dynamics of a free-ranging rhesus macaque population  

PubMed Central

Density-dependence is hypothesized as the major mechanism of population regulation. However, the lack of long-term demographic data has hampered the use of density-dependent models in nonhuman primates. In this study, we make use of the long-term demographic data from Cayo Santiago’s rhesus macaques to parameterize and analyze both a density-independent and a density-dependent population matrix model, and compare their projections with the observed population changes. We also employ a retrospective analysis to determine how variance in vital rates, and covariance among them, contributed to the observed variation in long-term fitness across different levels of population density. The population exhibited negative density-dependence in fertility and the model incorporating this relationship accounted for 98% of the observed population dynamics. Variation in survival and fertility of sexually active individuals contributed the most to the variation in long-term fitness, while vital rates displaying high temporal variability exhibited lower sensitivities. Our findings are novel in describing density-dependent dynamics in a provisioned primate population, and in suggesting that selection is acting to lower the variance in the population growth rate by minimizing the variation in adult survival at high density. Because density-dependent mechanisms may become stronger in wild primate populations due to increasing habitat loss and food scarcity, our study demonstrates it is important to incorporate variation in population size, as well as demographic variability into population viability analyses for a better understanding of the mechanisms regulating the growth of primate populations. PMID:23847126

Hernández-Pacheco, Raisa; Rawlins, Richard G.; Kessler, Matthew J.; Williams, Lawrence E.; Ruiz-Maldonado, Tagrid M.; González-Martínez, Janis; Ruiz-Lambides, Angelina V.; Sabat, Alberto M.

2014-01-01

203

Sexual segregation in North American elk: the role of density dependence  

PubMed Central

We investigated how density-dependent processes and subsequent variation in nutritional condition of individuals influenced both timing and duration of sexual segregation and selection of resources. During 1999–2001, we experimentally created two population densities of North American elk (Cervus elaphus), a high-density population at 20 elk/km2, and a low-density population at 4 elk/km2 to test hypotheses relative to timing and duration of sexual segregation and variation in selection of resources. We used multi-response permutation procedures to investigate patterns of sexual segregation, and resource selection functions to document differences in selection of resources by individuals in high- and low-density populations during sexual segregation and aggregation. The duration of sexual segregation was 2 months longer in the high-density population and likely was influenced by individuals in poorer nutritional condition, which corresponded with later conception and parturition, than at low density. Males and females in the high-density population overlapped in selection of resources to a greater extent than in the low-density population, probably resulting from density-dependent effects of increased intraspecific competition and lower availability of resources. PMID:25691992

Stewart, Kelley M; Walsh, Danielle R; Kie, John G; Dick, Brian L; Bowyer, R Terry

2015-01-01

204

A physical-space approach for the probability hypothesis density and cardinalized probability hypothesis density filters  

NASA Astrophysics Data System (ADS)

The probability hypothesis density (PHD) filter, an automatically track-managed multi-target tracker, is attracting increasing but cautious attention. Its derivation is elegant and mathematical, and thus of course many engineers fear it; perhaps that is currently limiting the number of researchers working on the subject. In this paper, we explore a physical-space approach - a bin model - which leads us to arrive the same filter equations as the PHD. Unlike the original derivation of the PHD filter, the concepts used are the familiar ones of conditional probability. The original PHD suffers from a "target-death" problem in which even a single missed detection can lead to the apparent disappearance of a target. To obviate this, PHD originator Mahler has recently developed a new "cardinalized" version of PHD (CPHD). We are able to extend our physical-space derivation to the CPHD case as well. We stress that the original derivations are mathematically correct, and need no embellishment from us; our contribution here is to offer an alternative derivation, one that we find appealing.

Erdinc, Ozgur; Willett, Peter; Bar-Shalom, Yaakov

2006-05-01

205

Benchmarks for electronically excited states: Time-dependent density functional theory and density functional theory based multireference configuration interaction  

Microsoft Academic Search

Time-dependent density functional theory (TD-DFT) and DFT-based multireference configuration interaction (DFT\\/MRCI) calculations are reported for a recently proposed benchmark set of 28 medium-sized organic molecules. Vertical excitation energies, oscillator strengths, and excited-state dipole moments are computed using the same geometries (MP2\\/6-31G*) and basis set (TZVP) as in our previous ab initio benchmark study on electronically excited states. The results from

Mario R. Silva-Junior; Marko Schreiber; Stephan P. A. Sauer; Walter Thiel

2008-01-01

206

Benchmarks for electronically excited states: Time-dependent density functional theory and density functional theory based multireference configuration interaction  

Microsoft Academic Search

Time-dependent density functional theory (TD-DFT) and DFT-based multireference configuration interaction (DFT\\/MRCI) calculations are reported for a recently proposed benchmark set of 28 medium-sized organic molecules. Vertical excitation energies, oscillator strengths, and excited-state dipole moments are computed using the same geometries (MP2\\/6-31G*) and basis set (TZVP) as in our previous ab initio benchmark study on electronically excited states. The results from

Mario R. Silva-Junior; Marko Schreiber; Stephan P. A. Sauer; Walter Thiel

2008-01-01

207

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

USGS Publications Warehouse

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 (???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. ?? Springer-Verlag 2004.

Jensen, W.E.; Cully, J.F., Jr.

2005-01-01

208

Remarks on the density functional approach to the inhomogeneous electron gas  

Microsoft Academic Search

The density functional approach is reformulated by using the most general form of the Hohenberg-Kohn theorem based on p-particle densities. Comparison with the reduced density matrix theory is made, exhibiting fully the p-particle hierarchy of both theories. Some advantages and drawbacks of the generalized density functional approach are discussed. The 1-particle spin-polarized case is presented to indicate the place of

M. Tomásek; V. Mikolás

1979-01-01

209

Temperature dependence of vibrational lifetimes at the critical density in supercritical mixtures  

E-print Network

Temperature dependence of vibrational lifetimes at the critical density in supercritical mixtures D measurements are reported for the temperature dependence of the vibrational lifetime, T1 , of the asymmetric CO the critical temperature to substantially higher temperatures. T1 is found initially to increase

Fayer, Michael D.

210

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.

211

Time-dependent Stochastic Bethe-Salpeter Approach  

E-print Network

A time-dependent formulation for electron-hole excitations in extended finite systems, based on the Bethe-Salpeter equation (BSE), is developed using a stochastic wave function approach. The time-dependent formulation builds on the connection between time-dependent Hartree-Fock (TDHF) theory and configuration-interaction with single substitution (CIS) method. This results in a time dependent Schr\\"odinger-like equation for the quasiparticle orbital dynamics based on an effective Hamiltonian containing direct Hartree and screened exchange terms, where screening is described within the Random Phase Approximation (RPA). To solve for the optical absorption spectrum, we develop a stochastic formulation in which the quasiparticle orbitals are replaced by stochastic orbitals to evaluate the direct and exchange terms in the Hamiltonian as well as the RPA screening. This leads to an overall quadratic scaling, a significant improvement over the equivalent symplectic eigenvalue representation of the BSE. Application of ...

Rabani, Eran; Neuhauser, Daniel

2015-01-01

212

Phytotherapeutic approach to alcohol dependence: new old way?  

PubMed

Alcohol abuse and dependence represent a worldwide problem from both medical and social points of view. In Italy it is estimated that there are about one million alcohol-dependent subjects. The pharmacological treatment of patients with alcohol dependence plays a key role in order to achieve alcohol abstinence and prevent relapse. At present, the possible utility of the complementary medicines in the treatment of alcohol dependence is controversial. In the last years, pre-clinical and clinical data from traditional medicines suggest that novel pharmacological approaches for treatment of alcoholism and alcohol abuse may stem from natural substances. The present review summarizes the findings of the effects of phytotherapy in alcohol addiction. PMID:19216063

Abenavoli, Ludovico; Capasso, Francesco; Addolorato, Giovanni

2009-06-01

213

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

214

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

215

Ecological drivers of guanaco recruitment: variable carrying capacity and density dependence.  

PubMed

Ungulates living in predator-free reserves offer the opportunity to study the influence of food limitation on population dynamics without the potentially confounding effects of top-down regulation or livestock competition. We assessed the influence of relative forage availability and population density on guanaco recruitment in two predator-free reserves in eastern Patagonia, with contrasting scenarios of population density. We also explored the relative contribution of the observed recruitment to population growth using a deterministic linear model to test the assumption that the studied populations were closed units. The observed densities increased twice as fast as our theoretical populations, indicating that marked immigration has taken place during the recovery phase experienced by both populations, thus we rejected the closed-population assumption. Regarding the factors driving variation in recruitment, in the low- to medium-density setting, we found a positive linear relationship between recruitment and surrogates of annual primary production, whereas no density dependence was detected. In contrast, in the high-density scenario, both annual primary production and population density showed marked effects, indicating a positive relationship between recruitment and per capita food availability above a food-limitation threshold. Our results support the idea that environmental carrying capacity fluctuates in response to climatic variation, and that these fluctuations have relevant consequences for herbivore dynamics, such as amplifying density dependence in drier years. We conclude that including the coupling between environmental variability in resources and density dependence is crucial to model ungulate population dynamics; to overlook temporal changes in carrying capacity may even mask density dependence as well as other important processes. PMID:24899131

Marino, Andrea; Pascual, Miguel; Baldi, Ricardo

2014-08-01

216

Predicting Fish Densities in Lotic Systems: a Simple Modeling Approach  

EPA Science Inventory

Fish density models are essential tools for fish ecologists and fisheries managers. However, applying these models can be difficult because of high levels of model complexity and the large number of parameters that must be estimated. We designed a simple fish density model and te...

217

Universal time dependence of nighttime F region densities at high latitudes  

NASA Technical Reports Server (NTRS)

Coincident auroral-zone experiments using three incoherent-scatter radars at widely spaced longitudes are reported. The observational results demonstrate that, during the night, the F layer electron density is strongly dependent on the longitude of the observing site. Ionization patches were observed in the nighttime F region from the Chatanika and EISCAT radars, while densities observed from the Millstone radar were substantially smaller. The electron density within these maxima is larger at EISCAT than at Chatanika. When observed in the midnight sector auroral zone, these densities had a peak density at a high altitude of 360-475 km. The density was maximum when EISCAT was in the midnight sector and minimum when Millstone was in the midnight sector. A minimum in insolation in the auroral zone occurs at the UT when Millstone is in the midnight sector.

De La Beaujardiere, O.; Wickwar, V. B.; Caudal, G.; Holt, J. M.; Craven, J. D.; Frank, L. A.; Brace, L. H.

1985-01-01

218

Density-dependent intraspecific aggression regulates survival in northern Yellowstone wolves (Canis lupus).  

PubMed

Understanding the population dynamics of top-predators is essential to assess their impact on ecosystems and to guide their management. Key to this understanding is identifying the mechanisms regulating vital rates. Determining the influence of density on survival is necessary to understand the extent to which human-caused mortality is compensatory or additive. In wolves (Canis lupus), empirical evidence for density-dependent survival is lacking. Dispersal is considered the principal way in which wolves adjust their numbers to prey supply or compensate for human exploitation. However, studies to date have primarily focused on exploited wolf populations, in which density-dependent mechanisms are likely weak due to artificially low wolf densities. Using 13 years of data on 280 collared wolves in Yellowstone National Park, we assessed the effect of wolf density, prey abundance and population structure, as well as winter severity, on age-specific survival in two areas (prey-rich vs. prey-poor) of the national park. We further analysed cause-specific mortality and explored the factors driving intraspecific aggression in the prey-rich northern area of the park. Overall, survival rates decreased during the study. In northern Yellowstone, density dependence regulated adult survival through an increase in intraspecific aggression, independent of prey availability. In the interior of the park, adult survival was less variable and density-independent, despite reduced prey availability. There was no effect of prey population structure in northern Yellowstone, or of winter severity in either area. Survival was similar among yearlings and adults, but lower for adults older than 6 years. Our results indicate that density-dependent intraspecific aggression is a major driver of adult wolf survival in northern Yellowstone, suggesting intrinsic density-dependent mechanisms have the potential to regulate wolf populations at high ungulate densities. When low prey availability or high removal rates maintain wolves at lower densities, limited inter-pack interactions may prevent density-dependent survival, consistent with our findings in the interior of the park. PMID:24749694

Cubaynes, Sarah; MacNulty, Daniel R; Stahler, Daniel R; Quimby, Kira A; Smith, Douglas W; Coulson, Tim

2014-04-21

219

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

220

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

221

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

222

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

223

Harnessing the meta-generalized gradient approximation for time-dependent density functional theory.  

PubMed

Density functionals within the meta-generalized gradient approximation (MGGA) are widely used for ground-state electronic structure calculations. However, the gauge variance of the kinetic energy density ? confounds applications of MGGAs to time-dependent systems, excited states, magnetic properties, and states with strong spin-orbit coupling. Becke and Tao used the paramagnetic current density to construct a gauge invariant generalized kinetic energy density ?. We show that ?(W)??, where ?(W) is the von Weizsäcker kinetic energy density of a one-electron system. Thus, replacing ? by ? leads to current-dependent MGGAs (cMGGAs) that are not only gauge invariant but also restore the accuracy of MGGAs in iso-orbital regions for time-dependent and current-carrying states. The current dependence of cMGGAs produces a vector exchange-correlation (XC) potential in the time-dependent adiabatic Kohn-Sham (KS) equations. While MGGA response properties of current-free ground states become manifestly gauge-variant to second order, linear response properties are affected by a new XC kernel appearing in the cMGGA magnetic orbital rotation Hessian. This kernel reflects the first-order coupling of KS orbitals due to changes in the paramagnetic current density and has apparently been ignored in previous MGGA response implementations. Inclusion of the current dependence increases total computation times by less than 50%. Benchmark applications to 109 adiabatic excitation energies using the Tao-Perdew-Staroverov-Scuseria (TPSS) MGGA and its hybrid version TPSSh show that cMGGA excitation energies are slightly lower than the MGGA ones on average, but exhibit fewer outliers. Similarly, the optical rotations of 13 small organic molecules show a small but systematic improvement upon inclusion of the magnetic XC kernel. We conclude that cMGGAs should replace MGGAs in all applications involving time-dependent or current-carrying states. PMID:23126693

Bates, Jefferson E; Furche, Filipp

2012-10-28

224

Harnessing the meta-generalized gradient approximation for time-dependent density functional theory  

NASA Astrophysics Data System (ADS)

Density functionals within the meta-generalized gradient approximation (MGGA) are widely used for ground-state electronic structure calculations. However, the gauge variance of the kinetic energy density ? confounds applications of MGGAs to time-dependent systems, excited states, magnetic properties, and states with strong spin-orbit coupling. Becke and Tao used the paramagnetic current density to construct a gauge invariant generalized kinetic energy density hat{? }. We show that ? _W ? hat{? }, where ?W is the von Weizsäcker kinetic energy density of a one-electron system. Thus, replacing ? by hat{? } leads to current-dependent MGGAs (cMGGAs) that are not only gauge invariant but also restore the accuracy of MGGAs in iso-orbital regions for time-dependent and current-carrying states. The current dependence of cMGGAs produces a vector exchange-correlation (XC) potential in the time-dependent adiabatic Kohn-Sham (KS) equations. While MGGA response properties of current-free ground states become manifestly gauge-variant to second order, linear response properties are affected by a new XC kernel appearing in the cMGGA magnetic orbital rotation Hessian. This kernel reflects the first-order coupling of KS orbitals due to changes in the paramagnetic current density and has apparently been ignored in previous MGGA response implementations. Inclusion of the current dependence increases total computation times by less than 50%. Benchmark applications to 109 adiabatic excitation energies using the Tao-Perdew-Staroverov-Scuseria (TPSS) MGGA and its hybrid version TPSSh show that cMGGA excitation energies are slightly lower than the MGGA ones on average, but exhibit fewer outliers. Similarly, the optical rotations of 13 small organic molecules show a small but systematic improvement upon inclusion of the magnetic XC kernel. We conclude that cMGGAs should replace MGGAs in all applications involving time-dependent or current-carrying states.

Bates, Jefferson E.; Furche, Filipp

2012-10-01

225

Tight-binding approach to strain-dependent DNA electronics  

NASA Astrophysics Data System (ADS)

Small mechanical strain perturbations are considered in calculations of the poly(G)-poly(C) DNA molecular electronic structure, using a tight-binding framework in conjunction with the theories of Slater-Koster and linear elasticity. Results reveal a strain-induced band gap for DNA which is linearly dependent on the induced strain. Local density of states calculations expose that the contribution of the guanine-cytosine base pairs in the charge transport mechanism is significantly enhanced relative to the backbones when DNA is compressed. Transport investigations also disclose a strain-induced metal-semiconductor transition for the DNA molecule, which suggests possible potential uses for sensing applications.

Malakooti, Sadeq; Hedin, Eric; Joe, Yong

2013-07-01

226

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

227

A novel approach to evaluating breast density using ultrasound tomography  

NASA Astrophysics Data System (ADS)

Women with high mammographic breast density have a 4- to 5-fold increased breast cancer risk compared to women with fatty breasts. Current breast density estimation involves mammography, although a mammogram is a 2-D projection which does not provide an accurate volumetric density estimate due to breast thickness not being taken into account. Presumably, breast cancer risk would have a stronger relationship to the volume of dense tissue as opposed to projected area. The purpose of this work was to investigate the feasibility of assessing breast density with ultrasound tomography. First, we evaluated a breast-mimicking phantom, and sound speed measurements were well-correlated with mass density and CT number of the phantom material. Approximately 40 m/s difference in sound speed was observed between fatty and fibroglandular components, a critical result for breast density characterization. Next, we investigated the extension of our results in vivo for a sample of ˜100 patients. We observed significant differences in global sound speed between every BI-RADS compositional category, showing that our technique was consistent with the current standard of care. A strong, positive association was revealed between breast sound speed and quantitative mammographic percent density (MPD). We also assessed volumetric ultrasound percent density (USPD) using sound speed tomograms. The USPD results were also consistent with BI-RADS Categories. Further, an increase in USPD was observed with increased MPD for both mammographic views. Next, we introduced texture analysis, and moderate negative associations between sound speed skewness and MPD, and fractal dimension and MPD, were observed. This work suggests that non-invasive and non-ionizing evaluation of breast density is achievable, and global sound speed and USPD appear to be the most promising indicators of breast density. These applications may play an integral role in monitoring treatment, tracking chemoprevention response, and identifying high-risk patients. Overall, potential benefits of quantifying a relationship between acoustical properties and breast density introduce a novel application of clinical ultrasound, one that be significant in breast cancer risk assessment.

Glide, Carri K.

2007-12-01

228

Density dependence of hydrogen bonding and the translational-orientational structural order in supercritical water: A molecular dynamics study  

NASA Astrophysics Data System (ADS)

Molecular dynamics simulation have been performed with a wide range of densities along a near critical isotherm of supercritical water (SCW) in order to study the density dependence of the structure order and hydrogen bonding (HB). It is revealed that the translational structure order is nearly invariant while the orientational tetrahedral structure order is very sensitive to the bulk density under supercritical conditions. Meanwhile, some energetically unfavorable intermediate water dimer structures are found to appear under supercritical conditions due to the reduced energy difference and the enhanced energy fluctuation. As a consequence, a general geometrical criterion or the inclusion of a energy-based criterion instead of currently widely adopted pure rOH-based geometric criterion is suggested to be used in the HB statistics under supercritical conditions. It is found that the average HB number per H2O molecule (nHB) reduces with the decreasing SCW bulk density although a given pair of H2O molecules are shown to have a stronger ability to form a hydrogen bond under lower SCW bulk densities. Accordingly, the orientational tetrahedral structure order q decreases with the reducing bulk density under supercritical conditions. However, when the fluid is dilute with ? ? 0.19?c (?c = 0.322 g/cm3), the energy fluctuation increases sharply and the short-range order is destroyed, signifying the supercritical fluid (SCF)-gas state transition. Accordingly, the orientational tetrahedral structure order q gets reversal around ? = 0.19?c and approaches zero under very dilute conditions. The sensitivity of the orientational order to the density implies the microscopic origin of the significant dependence of SCF's physicochemical properties on the pressure.

Ma, Haibo; Ma, Jing

2011-08-01

229

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

230

Mechanical properties of silicon carbide nanowires: effect of size-dependent defect density.  

PubMed

This paper reports quantitative mechanical characterization of silicon carbide (SiC) nanowires (NWs) via in situ tensile tests inside scanning electron microscopy using a microelectromechanical system. The NWs are synthesized using the vapor-liquid-solid process with growth direction of ?111?. They consist of three types of structures, pure face-centered cubic (3C) structure, 3C structure with an inclined stacking fault (SF), and highly defective structure, in a periodic fashion along the NW length. The SiC NWs are found to deform linear elastically until brittle fracture. Their fracture origin is identified in the 3C structures with inclined SFs, rather than the highly defective structures. The fracture strength increases as the NW diameter decreases from 45 to 17 nm, approaching the theoretical strength of 3C SiC. The size effect on fracture strength of SiC NWs is attributed to the size-dependent defect density rather than the surface effect that is dominant for single crystalline NWs. PMID:24382314

Cheng, Guangming; Chang, Tzu-Hsuan; Qin, Qingquan; Huang, Hanchen; Zhu, Yong

2014-02-12

231

Time-dependent density-functional theory for electronic excitations in materials  

Microsoft Academic Search

There is currently an intense effort underway to study the optical properties of bulk and nanostructured materials using time-dependent density-functional theory (TDDFT). This talk will discuss challenges and recent advances of TDDFT in this area, and present some new applications to excitonic effects in bulk insulators and to collective charge- and spin-density excitations in doped quantum wells. A TDDFT version

Carsten A. Ullrich

2009-01-01

232

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

233

Explaining the dark energy, baryon and dark matter coincidence via domain-dependent random densities  

SciTech Connect

The dark energy, dark matter and baryon densities in the Universe are observed to be similar, with a factor of no more than 20 between the largest and smallest densities. We show that this coincidence can be understood via superhorizon domains of randomly varying densities when the baryon density at initial collapse of galaxy-forming perturbations is determined by anthropic selection. The baryon and dark matter densities are assumed to be dependent on random variables ?{sub d} and ?{sub b} according to ?{sub dm}??{sub d}{sup ?} and ?{sub b}??{sub b}{sup ?}, while the effectively constant dark energy density is dependent upon a random variable ?{sub Q} according to ?{sub Q}??{sub Q}{sup n}. The ratio of the baryon density to the dark energy density at initial collapse, r{sub Q}, and the baryon-to-dark matter ratio, r, are then determined purely statistically, with no dependence on the anthropically-preferred baryon density. We compute the probability distribution for r{sub Q} and r and show that the observed values of r{sub Q} and r can be naturally understood within this framework. In particular, for the case ? = 2, ? = 1 and n = 4, which can be physically realized via a combination of axion dark matter, Affleck-Dine baryogenesis and frozen quintessence with a ?{sub Q}{sup 4} potential, the range of r{sub Q} and r which corresponds to the observed Universe is a quite natural, with a probability which is broadly similar to other ranges of r{sub Q} and r.

McDonald, John, E-mail: j.mcdonald@lancaster.ac.uk [Lancaster-Manchester-Sheffield Consortium for Fundamental Physics, Cosmology and Astroparticle Physics Group, Dept. of Physics, University of Lancaster, Lancaster LA1 4YB (United Kingdom)

2013-05-01

234

Bayesian Inference on the Effect of Density Dependence and Weather on a Guanaco Population from Chile  

PubMed Central

Understanding the mechanisms that drive population dynamics is fundamental for management of wild populations. The guanaco (Lama guanicoe) is one of two wild camelid species in South America. We evaluated the effects of density dependence and weather variables on population regulation based on a time series of 36 years of population sampling of guanacos in Tierra del Fuego, Chile. The population density varied between 2.7 and 30.7 guanaco/km2, with an apparent monotonic growth during the first 25 years; however, in the last 10 years the population has shown large fluctuations, suggesting that it might have reached its carrying capacity. We used a Bayesian state-space framework and model selection to determine the effect of density and environmental variables on guanaco population dynamics. Our results show that the population is under density dependent regulation and that it is currently fluctuating around an average carrying capacity of 45,000 guanacos. We also found a significant positive effect of previous winter temperature while sheep density has a strong negative effect on the guanaco population growth. We conclude that there are significant density dependent processes and that climate as well as competition with domestic species have important effects determining the population size of guanacos, with important implications for management and conservation. PMID:25514510

Zubillaga, María; Skewes, Oscar; Soto, Nicolás; Rabinovich, Jorge E.; Colchero, Fernando

2014-01-01

235

Bayesian inference on the effect of density dependence and weather on a guanaco population from chile.  

PubMed

Understanding the mechanisms that drive population dynamics is fundamental for management of wild populations. The guanaco (Lama guanicoe) is one of two wild camelid species in South America. We evaluated the effects of density dependence and weather variables on population regulation based on a time series of 36 years of population sampling of guanacos in Tierra del Fuego, Chile. The population density varied between 2.7 and 30.7 guanaco/km2, with an apparent monotonic growth during the first 25 years; however, in the last 10 years the population has shown large fluctuations, suggesting that it might have reached its carrying capacity. We used a Bayesian state-space framework and model selection to determine the effect of density and environmental variables on guanaco population dynamics. Our results show that the population is under density dependent regulation and that it is currently fluctuating around an average carrying capacity of 45,000 guanacos. We also found a significant positive effect of previous winter temperature while sheep density has a strong negative effect on the guanaco population growth. We conclude that there are significant density dependent processes and that climate as well as competition with domestic species have important effects determining the population size of guanacos, with important implications for management and conservation. PMID:25514510

Zubillaga, María; Skewes, Oscar; Soto, Nicolás; Rabinovich, Jorge E; Colchero, Fernando

2014-01-01

236

Lonely hearts or sex in the city? Density-dependent effects in mating systems  

PubMed Central

Two very basic ideas in sexual selection are heavily influenced by numbers of potential mates: the evolution of anisogamy, leading to sex role differentiation, and the frequency dependence of reproductive success that tends to equalize primary sex ratios. However, being explicit about the numbers of potential mates is not typical to most evolutionary theory of sexual selection. Here, we argue that this may prevent us from finding the appropriate ecological equilibria that determine the evolutionary endpoints of selection. We review both theoretical and empirical advances on how population density may influence aspects of mating systems such as intrasexual competition, female choice or resistance, and parental care. Density can have strong effects on selective pressures, whether or not there is phenotypic plasticity in individual strategies with respect to density. Mating skew may either increase or decrease with density, which may be aided or counteracted by changes in female behaviour. Switchpoints between alternative mating strategies can be density dependent, and mate encounter rates may influence mate choice (including mutual mate choice), multiple mating, female resistance to male mating attempts, mate searching, mate guarding, parental care, and the probability of divorce. Considering density-dependent selection may be essential for understanding how populations can persist at all despite sexual conflict, but simple models seem to fail to predict the diversity of observed responses in nature. This highlights the importance of considering the interaction between mating systems and population dynamics, and we strongly encourage further work in this area. PMID:16612890

Kokko, Hanna; Rankin, Daniel J

2006-01-01

237

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

PubMed

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. PMID:24028103

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

2013-09-01

238

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

239

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

240

Quantum mechanical/molecular mechanical/continuum style solvation model: time-dependent density functional theory.  

PubMed

A combined quantum mechanical/molecular mechanical/continuum (QM/MMpol/C) style method is developed for time-dependent density functional theory (TDDFT, including long-range corrected TDDFT) method, induced dipole polarizable force field, and induced surface charge continuum model. Induced dipoles and induced charges are included in the TDDFT equations to solve for the transition energies, relaxed density, and transition density. Analytic gradient is derived and implemented for geometry optimization and molecular dynamics simulation. QM/MMpol/C style DFT and TDDFT methods are used to study the hydrogen bonding of the photoactive yellow protein chromopore in ground state and excited state. PMID:24006973

Thellamurege, Nandun M; Cui, Fengchao; Li, Hui

2013-08-28

241

Harnessing the Meta-Generalized Gradient Approximation for Time-Dependent Density Functional Theory  

NASA Astrophysics Data System (ADS)

Density functionals within the meta-generalized gradient approximation (MGGA) are widely used for ground-state electronic structure calculations. However, the gauge variance of the kinetic energy density tau confounds applications of MGGAs to time-dependent systems, excited states, magnetic properties, and states with strong spin-orbit coupling. Becke and Tao used the paramagnetic current density to construct a gauge invariant generalized kinetic energy density t? . We show that tauW ? t? , where tauW is the von Weizsacker kinetic energy density of a one-electron system. Thus, replacing tau by t? leads to current-dependent MGGAs (cMGGAs) that are not only gauge invariant, but also restore the accuracy of MGGAs in iso-orbital regions for time-dependent and current-carrying states. The current dependence of cMGGAs produces a vector exchange-correlation (XC) potential in the time-dependent adiabatic Kohn-Sham (KS) equations. While response properties of current-free ground states become manifestly gauge-variant to second order, linear response properties are affected by a new XC kernel appearing in the cMGGA magnetic orbital rotation Hessian. Inclusion of the current dependence increases total computation times by less than 50%. Benchmark applications to 109 adiabatic excitation energies using the Tao-Perdew-Staroverov-Scuseria (TPSS) MGGA and its hybrid version TPSSh show that cMGGA excitation energies are slightly lower than the MGGA ones on average, but exhibit fewer outliers. Similarly, the optical rotations of 13 small organic molecules show a small but systematic improvement upon inclusion of the magnetic XC kernel. We conclude that cMGGAs should replace MGGAs in all applications involving time-dependent or current-carrying states.

Bates, Jefferson Earl

242

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

243

An information theory approach to the density of the earth  

NASA Technical Reports Server (NTRS)

Information theory can develop a technique which takes experimentally determined numbers and produces a uniquely specified best density model satisfying those numbers. A model was generated using five numerical parameters: the mass of the earth, its moment of inertia, three zero-node torsional normal modes (L = 2, 8, 26). In order to determine the stability of the solution, six additional densities were generated, in each of which the period of one of the three normal modes was increased or decreased by one standard deviation. The superposition of the seven models is shown. It indicates that current knowledge of the torsional modes is sufficient to specify the density in the upper mantle but that the lower mantle and core will require smaller standard deviations before they can be accurately specified.

Graber, M. A.

1977-01-01

244

Density dependence in group dynamics of a highly social mongoose, Suricata suricatta.  

PubMed

1.?For social species, the link between individual behaviour and population dynamics is mediated by group-level demography. 2.?Populations of obligate cooperative breeders are structured into social groups, which may be subject to inverse density dependence (Allee effects) that result from a dependence on conspecific helpers, but evidence for population-wide Allee effects is rare. 3.?We use field data from a long-term study of cooperative meerkats (Suricata suricatta; Schreber, 1776) - a species for which local Allee effects are not reflected in population-level dynamics - to empirically model interannual group dynamics. 4.?Using phenomenological population models, modified to incorporate environmental conditions and potential Allee effects, we first investigate overall patterns of group dynamics and find support only for conventional density dependence that increases after years of low rainfall. 5.?To explain the observed patterns, we examine specific demographic rates and assess their contributions to overall group dynamics. Although per-capita meerkat mortality is subject to a component Allee effect, it contributes relatively little to observed variation in group dynamics, and other (conventionally density dependent) demographic rates - especially emigration - govern group dynamics. 6.?Our findings highlight the need to consider demographic processes and density dependence in subpopulations before drawing conclusions about how behaviour affects population processes in socially complex systems. PMID:22117843

Bateman, Andrew W; Ozgul, Arpat; Coulson, Tim; Clutton-Brock, Tim H

2012-05-01

245

Time-dependent quantum-fluid density-functional study of high-energy proton-helium collisions  

SciTech Connect

A quantum-fluid density-functional theory (QF DFT) is proposed, yielding a time-dependent (TD) generalized nonlinear Schroedinger equation (GNLSE) as the equation of motion (EOM) for dealing with proton--helium-atom collisions from start'' to finish.'' The EOM contains the Weizsaecker term as the kinetic-energy functional, apart from local exchange and correlation functionals. The GNLSE is numerically solved in cylindrical polar coordinates by a leapfrog-type finite-difference algorithm. Various TD quantities such as difference density (DD), induced-dipole moment (IDM), dipole polarizability tensor component, reaction probability, etc., have been studied to obtain physical insights into the mechanism of the TD collision process. In particular, the DD and the oscillating IDM permit a natural partitioning of the {ital p}-He collision process into {ital approach}, {ital encounter}, and {ital departure} regimes. The TD DD profiles reveal that, as a result of the interaction, {ital p}{sigma} densities mix substantially into the 1{ital s} density of the He atom. Critical comments are made on the usefulness of the QF DFT approach for understanding TD processes.

Deb, B.M.; Chattaraj, P.K.; Mishra, S. (Theoretical Chemistry Group, Department of Chemistry, Panjab University, Chandigarh 160014, India (IN))

1991-02-01

246

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

247

Local environment and density-dependent feedbacks determine population growth in a forest herb.  

PubMed

Linking spatial variation in environmental factors to variation in demographic rates is essential for a mechanistic understanding of the dynamics of populations. However, we still know relatively little about such links, partly because feedbacks via intraspecific density make them difficult to observe in natural populations. We conducted a detailed field study and investigated simultaneous effects of environmental factors and the intraspecific density of individuals on the demography of the herb Lathyrus vernus. In regression models of vital rates we identified effects associated with spring shade on survival and growth, while density was negatively correlated with these vital rates. Density was also negatively correlated with average individual size in the study plots, which is consistent with self-thinning. In addition, average plant sizes were larger than predicted by density in plots that were less shaded by the tree canopy, indicating an environmentally determined carrying capacity. A size-structured integral projection model based on the vital rate regressions revealed that the identified effects of shade and density were strong enough to produce differences in stable population sizes similar to those observed in the field. The results illustrate how the local environment can determine dynamics of populations and that intraspecific density may have to be more carefully considered in studies of plant demography and population viability analyses of threatened species. We conclude that demographic approaches incorporating information about both density and key environmental factors are powerful tools for understanding the processes that interact to determine population dynamics and abundances. PMID:25224800

Dahlgren, Johan P; Ostergård, Hannah; Ehrlén, Johan

2014-12-01

248

Time-dependent Kohn-Sham approach to quantum electrodynamics  

SciTech Connect

We prove a generalization of the van Leeuwen theorem toward quantum electrodynamics, providing the formal foundations of a time-dependent Kohn-Sham construction for coupled quantized matter and electromagnetic fields. We circumvent the symmetry-causality problems associated with the action-functional approach to Kohn-Sham systems. We show that the effective external four-potential and four-current of the Kohn-Sham system are uniquely defined and that the effective four-current takes a very simple form. Further we rederive the Runge-Gross theorem for quantum electrodynamics.

Ruggenthaler, M. [Institut fuer Physik, Universitaet Rostock, DE-18051 Rostock (Germany); Department of Physics, Nanoscience Center, University of Jyvaeskylae, FI-40014 Jyvaeskylae (Finland); Mackenroth, F. [Max-Planck-Institut fuer Kernphysik, Postfach 103980, DE-69029 Heidelberg (Germany); Bauer, D. [Institut fuer Physik, Universitaet Rostock, DE-18051 Rostock (Germany)

2011-10-15

249

Orbital-free kinetic-energy density functionals with a density-dependent kernel Yan Alexander Wang, Niranjan Govind, and Emily A. Carter  

E-print Network

-density-dependent kernel, can repro- duce the Kohn-Sham results almost exactly for several aluminum bulk phases. We further-index aluminum surfaces, where the density variations are very large, can be reproduced to within reasonable at a price: accu- rate, transferable orbital-free kinetic-energy density function- als do not, as yet, exist

Carter, Emily A.

250

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

251

Invasions with density-dependent ecological parameters Sanjeeva Balasuriya a,b,  

E-print Network

and the structure of population dispersal is an important area of study, in particular in the protection of native fauna and flora from invasive species. Factors influencing this include the species' range, density to logistic growth: a sigmoidal curve which has the population approaching the carrying capacity as time

Balasuriya, Sanjeeva

252

Symmetry energy effects on isovector properties of neutron rich nuclei with a density functional approach  

E-print Network

We employ a variational method to study the effect of the symmetry energy on the neutron skin thickness and the symmetry energy coefficients of various neutron rich nuclei. We concentrate our interest on $^{208}$Pb, $^{124}$Sn, $^{90}$Zr, and $^{48}$Ca, although the method can be applied in the totality of medium and heavy neutron rich nuclei. Our approach has the advantage that the isospin asymmetry function $\\alpha(r)$, which is the key quantity to calculate isovector properties of various nuclei, is directly related with the symmetry energy as a consequence of the variational principle. Moreover, the Coulomb interaction is included in a self-consistent way and its effects can be separated easily from the nucleon-nucleon interaction. We confirm, both qualitatively and quantitatively, the strong dependence of the symmetry energy on the various isovector properties for the relevant nuclei, using possible constraints between the slope and the value of the symmetry energy at the saturation density.

Papazoglou, M C

2014-01-01

253

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! ...

Miss Witcher

2011-10-06

254

Local-scale density-dependent survival of mobile organisms in continuous habitats: an experimental test using Atlantic salmon  

Microsoft Academic Search

For organisms with restricted mobility, density dependence may occur on spatial scales much smaller than that of the whole population. Averaging densities over whole populations in such organisms gives a more or less inaccurate description of the real variation in competitive intensity over time and space. The potential for local density dependence in more mobile organisms is less well understood,

Sigurd Einum; Keith H. Nislow

2005-01-01

255

Density dependent state space model for population abundance data with unequal time intervals  

PubMed Central

The Gompertz state-space (GSS) model is a stochastic model for analyzing time series observations of population abundances. The GSS model combines density dependence, environmental process noise, and observation error toward estimating quantities of interest in biological monitoring and population viability analysis. However, existing methods for estimating the model parameters apply only to population data with equal time intervals between observations. In the present paper, we extend the GSS model to data with unequal time intervals, by embedding it within a state-space version of the Ornstein-Uhlenbeck process, a continuous-time model of an equilibrating stochastic system. Maximum likelihood and restricted maximum likelihood calculations for the Ornstein-Uhlenbeck state-space model involve only numerical maximization of an explicit multivariate normal likelihood, and so the extension allows for easy bootstrapping, yielding confidence intervals for model parameters, statistical hypothesis testing of density dependence, and selection among sub-models using information criteria. Ecologists and managers previously drawn to models lacking density dependence or observation error because such models accommodated unequal time intervals (for example, due to missing data) now have an alternative analysis framework incorporating density dependence, process noise and observation error. PMID:25230459

Dennis, Brian; Ponciano, José Miguel

2014-01-01

256

Density-dependent state-space model for population-abundance data with unequal time intervals.  

PubMed

The Gompertz state-space (GSS) model is a stochastic model for analyzing time-series observations of population abundances. The GSS model combines density dependence, environmental process noise, and observation error toward estimating quantities of interest in biological monitoring and population viability analysis. However, existing methods for estimating the model parameters apply only to population data with equal time intervals between observations. In the present paper, we extend the GSS model to data with unequal time intervals, by embedding it within a state-space version of the Ornstein-Uhlenbeck process, a continuous-time model of an equilibrating stochastic system. Maximum likelihood and restricted maximum likelihood calculations for the Ornstein-Uhlenbeck state-space model involve only numerical maximization of an explicit multivariate normal likelihood, and so the extension allows for easy bootstrapping, yielding confidence intervals for model parameters, statistical hypothesis testing of density dependence, and selection among sub-models using information criteria. Ecologists and managers previously drawn to models lacking density dependence or observation error because such models accommodated unequal time intervals (for example, due to missing data) now have an alternative analysis framework incorporating density dependence, process noise, and observation error. PMID:25230459

Dennis, Brian; Ponciano, José Miguel

2014-08-01

257

Density dependence in a fluctuating wild reindeer herd; maternal vs. offspring effects  

Microsoft Academic Search

The Hardangervidda wild reindeer herd in Norway is the largest in Western Europe. It has fluctuated between 7000 and 32000 animals during the last 35 years. Four density-dependent effects were found: 1. A food limitation effect due to a shift in diet after overgrazing lichen on the winter range. This led to increased tooth wear and lowered body size and

Terje Skogland

1990-01-01

258

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

259

The invisible niche: weakly density-dependent mortality and the coexistence of species  

E-print Network

on species-area laws and species-abundance curves and should therefore receive more attention. Corresponding such as species-abundance curves and species-area laws. We illustrate this point by investigating the effectThe invisible niche: weakly density-dependent mortality and the coexistence of species Thilo Gross

260

Density dependence and climate effects in Rocky Mountain elk: an application of regression with  

E-print Network

Density dependence and climate effects in Rocky Mountain elk: an application of regression) presence had much weaker effects on elk (Cervus elaphus) dynamics [though limitation by wolves is strong in some elk popula- tions with well-established wolf populations (Creel et al. 2007; Creel & Christianson

Creel, Scott

261

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.

262

Toxicology Letters 161 (2006) 135142 Functionalization density dependence of single-walled  

E-print Network

Toxicology Letters 161 (2006) 135­142 Functionalization density dependence of single-walled carbon.toxlet.2005.08.011 #12;136 C.M. Sayes et al. / Toxicology Letters 161 (2006) 135­142 are of considerable interest for biological applications, and a full toxicological evaluation can only be performed

263

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

264

Time-Dependent Density-Functional Calculations of Photoabsorption Spectra of Carbon Nanostructures  

Microsoft Academic Search

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

Tomoyuki Noguchi; Masaaki Araidai; Kazuyuki Watanabe

2005-01-01

265

The Polarization of Fluorescence of DNA Stains Depends on the Incorporation Density of the  

E-print Network

The Polarization of Fluorescence of DNA Stains Depends on the Incorporation Density of the Dye: The fluorescence induced by polarized light sources, such as the lasers that are used in flow cytometry, is often polarized and anisotropic. In addition, most optical detector systems are sensitive to the direc- tion

Asbury, Chip

266

A mass-dependent density profile for dark matter haloes including the influence of galaxy formation  

NASA Astrophysics Data System (ADS)

We introduce a mass-dependent density profile to describe the distribution of dark matter within galaxies, which takes into account the stellar-to-halo mass dependence of the response of dark matter to baryonic processes. The study is based on the analysis of hydrodynamically simulated galaxies from dwarf to Milky Way mass, drawn from the Making Galaxies In a Cosmological Context project, which have been shown to match a wide range of disc scaling relationships. We find that the best-fitting parameters of a generic double power-law density profile vary in a systematic manner that depends on the stellar-to-halo mass ratio of each galaxy. Thus, the quantity M?/Mhalo constrains the inner (?) and outer (?) slopes of dark matter density, and the sharpness of transition between the slopes (?), reducing the number of free parameters of the model to two. Due to the tight relation between stellar mass and halo mass, either of these quantities is sufficient to describe the dark matter halo profile including the effects of baryons. The concentration of the haloes in the hydrodynamical simulations is consistent with N-body expectations up to Milky Way-mass galaxies, at which mass the haloes become twice as concentrated as compared with pure dark matter runs. This mass-dependent density profile can be directly applied to rotation curve data of observed galaxies and to semi-analytic galaxy formation models as a significant improvement over the commonly used NFW profile.

Di Cintio, Arianna; Brook, Chris B.; Dutton, Aaron A.; Macciò, Andrea V.; Stinson, Greg S.; Knebe, Alexander

2014-07-01

267

Anomalous Concentration Dependences of Optical Density of Stable Nanodisperse Titanium Dioxide Hydrosols  

Microsoft Academic Search

Optical properties of two aggregative-stable nanodisperse TiO2 hydrosols differed in sizes of crystalline particles were studied. These sols were found to have the anomalous concentration dependences of the optical density. It was suggested that these peculiarities are caused by an increase in structural order of the sols during their concentration.

O. B. Pavlova-Verevkina; V. V. Nazarov

2002-01-01

268

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

269

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

E-print Network

Host-parasite population dynamics under combined frequency- and density-dependent transmission, Scotland, EH14 4AS. Many host-parasite models assume that transmission increases linearly with host alternative (usually applied to sexually transmitted parasites) assumes instead that the rate at which hosts

White, Andrew

270

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...

271

Density-dependent response of an ultracold plasma to few-cycle radio-frequency pulses  

NASA Astrophysics Data System (ADS)

Ultracold neutral plasmas exhibit a density-dependent resonant response to applied radio-frequency (rf) fields in the frequency range of several to hundreds of megahertz for achievable densities. We have conducted measurements where short bursts of an rf fieldwere applied to these plasmas, with pulse durations as short as two cycles. We still observed a density-dependent resonant response to these short pulses, but the time scale of the response is too short to be consistent with local heating of electrons in the plasma from collisions under a range of experimental parameters. Instead, our results are consistent with rapid energy transfer to individual electrons from electric fields resulting from an overall displacement of the electron cloud from the ions during the collective motion of the electrons. This collective motion was also observed by applying two sharp electric field pulses separated in time to the plasma. These measurements demonstrate the importance of collective motion in the energy transport in these systems.

Wilson, Truman M.; Chen, Wei-Ting; Roberts, Jacob L.

2013-01-01

272

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

273

Collisionality Dependence of Multi-species Density Peaking in Turbulence Simulations of C-Mod Plasmas  

NASA Astrophysics Data System (ADS)

In nonlinear GYRO simulations of C-Mod plasmas, a turbulently driven pinch produces modest density peaking of all species. The ratio of density at r/a=0.44 and 0.74 is 1.2 for the majority and minority D & H (and electrons), and increases with impurity Z: 1.1 for helium, 1.15 for boron, 1.29 for neon, 1.36 for argon, 1.47 for molybdenum. Density peaking is only weakly affected when the ion temperature profile is varied to align the predicted heat flux to the experimental transport analysis. New simulations will extend the collisionality to the lower part of the experimentally accessible range in C-Mod to study the collisionality dependence of density peaking, and to establish whether much stronger peaking is predicted for lower collisionalities. Simulations based on measured I-mode ion and electron temperature profiles will also be presented.

Mikkelsen, D. R.; Bitter, M.; Delgado-Aparicio, L.; Hill, K. W.; Greenwald, M.; Howard, N.; Podpaly, Y.; Reinke, M.; Rice, J. E.; Hughes, J. W.; Ma, Y.; Candy, J.; Waltz, R. E.

2012-10-01

274

Dependence of interstellar depletion on hydrogen column density - Possibilities and implications  

NASA Technical Reports Server (NTRS)

A reexamination of the observed column densities of various elements in diffuse clouds suggests that almost all elements including oxygen, nitrogen, sulfur, and argon may be depleted with respect to hydrogen in interstellar clouds with large hydrogen column density. The amount of depletion varies from element to element and increases with increasing column density of hydrogen nuclei. This result is in qualitative agreement with the depletion of oxygen and sulfur independently inferred from the gas phase chemistry of sulfur in dense clouds. The rate of increase of depletion with hydrogen column density implied by the present study is large. It is possible that observational selection effects may have amplified the real dependence on N(H). A broad spectrum of C/O ratios ranging from values greater than unity to values less than unity appears possible for interstellar clouds, which would have the effect of a large variation in chemical composition from cloud to cloud.

Tarafdar, S. P.; Prasad, S. S.; Huntress, W. T., Jr.

1983-01-01

275

Density- and Size-Dependent Winter Mortality and Growth of Late Chaoborus flavicans Larvae  

PubMed Central

Winter processes such as overwinter survival and growth of individuals can have wide-ranging consequences for population dynamics and communities within and across seasons. In freshwater organisms winter processes have been mainly studied in fish despite that invertebrates also have substantial impacts on lake and pond food webs. One of the major invertebrate consumers in lake and ponds is the planktonic larvae of the dipteran insect Chaoborus spec. However, while much is known about Chaoborus feeding ecology, behaviour and structuring role in food webs, its winter ecology and how it affects its populations are poorly understood. Here size- and density-dependent winter mortality and body growth of late Chaoborus flavicans larvae were quantified over naturally occurring size and density ranges in autumn and under natural winter conditions using two field enclosure experiments. Winter mortality increased with autumn density but decreased with autumn body size while winter growth rates decreased with autumn density and body sizes. There was also a density- and size-independent background mortality component. The proportion of pupae found in spring decreased strongly and exponentially with autumn density. These results may explain the commonly observed univoltine life cycle and multi-annual density fluctuations in northern Chaoborus populations. They further demonstrate the relevance of winter processes and conditions for freshwater invertebrates and ecosystems. PMID:24124517

Schröder, Arne

2013-01-01

276

Optimization of human corneal endothelial cell culture: density dependency of successful cultures in vitro  

PubMed Central

Background Global shortage of donor corneas greatly restricts the numbers of corneal transplantations performed yearly. Limited ex vivo expansion of primary human corneal endothelial cells is possible, and a considerable clinical interest exists for development of tissue-engineered constructs using cultivated corneal endothelial cells. The objective of this study was to investigate the density-dependent growth of human corneal endothelial cells isolated from paired donor corneas and to elucidate an optimal seeding density for their extended expansion in vitro whilst maintaining their unique cellular morphology. Results Established primary human corneal endothelial cells were propagated to the second passage (P2) before they were utilized for this study. Confluent P2 cells were dissociated and seeded at four seeding densities: 2,500 cells per cm2 (‘LOW’); 5,000 cells per cm2 (‘MID’); 10,000 cells per cm2 (‘HIGH’); and 20,000 cells per cm2 (‘HIGH×2’), and subsequently analyzed for their propensity to proliferate. They were also subjected to morphometric analyses comparing cell sizes, coefficient of variance, as well as cell circularity when each culture became confluent. At the two lower densities, proliferation rates were higher than cells seeded at higher densities, though not statistically significant. However, corneal endothelial cells seeded at lower densities were significantly larger in size, heterogeneous in shape and less circular (fibroblastic-like), and remained hypertrophic after one month in culture. Comparatively, cells seeded at higher densities were significantly homogeneous, compact and circular at confluence. Potentially, at an optimal seeding density of 10,000 cells per cm2, it is possible to obtain between 10 million to 25 million cells at the third passage. More importantly, these expanded human corneal endothelial cells retained their unique cellular morphology. Conclusions Our results demonstrated a density dependency in the culture of primary human corneal endothelial cells. Sub-optimal seeding density results in a decrease in cell saturation density, as well as a loss in their proliferative potential. As such, we propose a seeding density of not less than 10,000 cells per cm2 for regular passage of primary human corneal endothelial cells. PMID:23641909

2013-01-01

277

Predicting critical temperatures of iron(II) spin crossover materials: Density functional theory plus U approach  

NASA Astrophysics Data System (ADS)

A first-principles study of critical temperatures (Tc) of spin crossover (SCO) materials requires accurate description of the strongly correlated 3d electrons as well as much computational effort. This task is still a challenge for the widely used local density or generalized gradient approximations (LDA/GGA) and hybrid functionals. One remedy, termed density functional theory plus U (DFT+U) approach, introduces a Hubbard U term to deal with the localized electrons at marginal computational cost, while treats the delocalized electrons with LDA/GGA. Here, we employ the DFT+U approach to investigate the Tc of a pair of iron(II) SCO molecular crystals (? and ? phase), where identical constituent molecules are packed in different ways. We first calculate the adiabatic high spin-low spin energy splitting ?EHL and molecular vibrational frequencies in both spin states, then obtain the temperature dependent enthalpy and entropy changes (?H and ?S), and finally extract Tc by exploiting the ?H/T - T and ?S - T relationships. The results are in agreement with experiment. Analysis of geometries and electronic structures shows that the local ligand field in the ? phase is slightly weakened by the H-bondings involving the ligand atoms and the specific crystal packing style. We find that this effect is largely responsible for the difference in Tc of the two phases. This study shows the applicability of the DFT+U approach for predicting Tc of SCO materials, and provides a clear insight into the subtle influence of the crystal packing effects on SCO behavior.

Zhang, Yachao

2014-12-01

278

Predicting critical temperatures of iron(II) spin crossover materials: Density functional theory plus U approach.  

PubMed

A first-principles study of critical temperatures (Tc) of spin crossover (SCO) materials requires accurate description of the strongly correlated 3d electrons as well as much computational effort. This task is still a challenge for the widely used local density or generalized gradient approximations (LDA/GGA) and hybrid functionals. One remedy, termed density functional theory plus U (DFT+U) approach, introduces a Hubbard U term to deal with the localized electrons at marginal computational cost, while treats the delocalized electrons with LDA/GGA. Here, we employ the DFT+U approach to investigate the Tc of a pair of iron(II) SCO molecular crystals (? and ? phase), where identical constituent molecules are packed in different ways. We first calculate the adiabatic high spin-low spin energy splitting ?EHL and molecular vibrational frequencies in both spin states, then obtain the temperature dependent enthalpy and entropy changes (?H and ?S), and finally extract Tc by exploiting the ?H/T - T and ?S - T relationships. The results are in agreement with experiment. Analysis of geometries and electronic structures shows that the local ligand field in the ? phase is slightly weakened by the H-bondings involving the ligand atoms and the specific crystal packing style. We find that this effect is largely responsible for the difference in Tc of the two phases. This study shows the applicability of the DFT+U approach for predicting Tc of SCO materials, and provides a clear insight into the subtle influence of the crystal packing effects on SCO behavior. PMID:25481157

Zhang, Yachao

2014-12-01

279

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

2013-03-12

280

Theoretical analysis of the worthiness of Henry and Elder problems as benchmarks of density-dependent groundwater  

E-print Network

-dependent groundwater flow models M.J. Simpson a , T.P. Clement a,b,* a Centre for Water Research, Department the availability of benchmark problems for testing density-dependent groundwater models is limited, one should: Groundwater-modeling; Density-dependent flow; Unsaturated flow; Contaminant transport 1. Introduction

Clement, Prabhakar

281

Nuclear clustering in the Energy Density Functional Approach  

NASA Astrophysics Data System (ADS)

Nuclear Energy Density Functionals (EDFs) are a microscopic tool of choice extensively used over the whole chart to successfully describe the properties of atomic nuclei ensuing from their quantum liquid nature. In the last decade, they also have proved their ability to deal with the cluster phenomenon, shedding a new light on its fundamental understanding by treating on an equal footing both quantum liquid and cluster aspects of nuclei. Such a unified microscopic description based on nucleonic degrees of freedom enables to tackle the question pertaining to the origin of the cluster phenomenon and emphasizes intrinsic mechanisms leading to the emergence of clusters in nuclei.

Ebran, J.-P.; Khan, E.; Nikši?, T.; Vretenar, D.

2014-12-01

282

An evolutionary maximum principle for density-dependent population dynamics in a fluctuating environment  

PubMed Central

The evolution of population dynamics in a stochastic environment is analysed under a general form of density-dependence with genetic variation in r and K, the intrinsic rate of increase and carrying capacity in the average environment, and in ?e2, the environmental variance of population growth rate. The continuous-time model assumes a large population size and a stationary distribution of environments with no autocorrelation. For a given population density, N, and genotype frequency, p, the expected selection gradient is always towards an increased population growth rate, and the expected fitness of a genotype is its Malthusian fitness in the average environment minus the covariance of its growth rate with that of the population. Long-term evolution maximizes the expected value of the density-dependence function, averaged over the stationary distribution of N. In the ?-logistic model, where density dependence of population growth is a function of N?, long-term evolution maximizes E[N?]=[1??e2/(2r)]K?. While ?e2 is always selected to decrease, r and K are always selected to increase, implying a genetic trade-off among them. By contrast, given the other parameters, ? has an intermediate optimum between 1.781 and 2 corresponding to the limits of high or low stochasticity. PMID:19414466

Lande, Russell; Engen, Steinar; Sæther, Bernt-Erik

2009-01-01

283

Relativistic time-dependent density functional calculations for the excited states of the cadmium dimer  

NASA Astrophysics Data System (ADS)

In this paper we present a time-dependent density functional study for the ground-state as well the 20-lowest laying excited states of the cadmium dimer Cd2, we analyze its spectrum obtained from all electrons calculations performed with time-depended density functional for the relativistic Dirac-Coulomb- and relativistic spin-free-Hamiltonian as implemented in DIRAC-PACKAGE. The calculations were obtained with different density functional approximations, and a comparison with the literature is given as far as available. Our result is very encouraging, especially for the lowest excited states of this dimer, and is expected to be enlightened for similar systems. The result shows that only long-range corrected functionals such as CAMB3LYP, gives the correct asymptotic behavior for the higher states. A comparable but less satisfactory results were obtained with B3LYP and PBE0 functionals. Spin-free-Hamiltonian is shown to be very efficient for systems containing heavy elements such as Cd2 in frameworks of (time-dependent) density functional without introducing large errors.

Kullie, Ossama

2013-03-01

284

Local control theory using trajectory surface hopping and linear-response time-dependent density functional theory.  

PubMed

The implementation of local control theory using nonadiabatic molecular dynamics within the framework of linear-response time-dependent density functional theory is discussed. The method is applied to study the photoexcitation of lithium fluoride, for which we demonstrate that this approach can efficiently generate a pulse, on-the-fly, able to control the population transfer between two selected electronic states. Analysis of the computed control pulse yields insights into the photophysics of the process identifying the relevant frequencies associated to the curvature of the initial and final state potential energy curves and their energy differences. The limitations inherent to the use of the trajectory surface hopping approach are also discussed. PMID:23967692

Curchod, Basile F E; Penfold, Thomas J; Rothlisberger, Ursula; Tavernelli, Ivano

2013-01-01

285

Structure-dependent threshold current density in InGaAsP quantum well lasers  

SciTech Connect

Threshold current density is studied theoretically for InGaAsP multiple quantum well lasers by including the Auger recombination process. All the possible transitions between quantized subbands of two-dimensional carriers are taken into account in evaluating radiative and Auger processes. The Auger recombination current depends strongly on the quantum well structure, resulting in the necessity for an elaborate structure design. A design procedure is elucidated for the structure which gives the lowest threshold current density for InGaAsP multiple quantum well lasers.

Sugimura, A.

1983-01-01

286

Regulation of local populations of a coral reef fish via joint effects of density- and number-dependent mortality  

Microsoft Academic Search

Density-dependent mortality can regulate local populations - effectively minimizing the likelihood of local extinctions and unchecked population growth. It is considered particularly important for many marine reef organisms with demographically open populations that lack potential regulatory mechanisms tied to local reproduction. While density-dependent mortality has been documented frequently for reef fishes, few studies have explored how the strength of density-dependence

Jeffrey S. Shima

2001-01-01

287

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

288

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

PubMed Central

In populations of colony-breeding marine animals, foraging around colonies can lead to intraspecific competition. This competition affects individual foraging behavior and can cause density-dependent population growth. Where behavioral data are available, it may be possible to infer the mechanism of intraspecific competition. If these mechanics are understood, they can be used to predict the population-level functional response resulting from the competition. Using satellite relocation and dive data, we studied the use of space and foraging behavior of juvenile and adult gray seals (Halichoerus grypus) from a large (over 200,000) and growing population breeding at Sable Island, Nova Scotia (44.0 oN 60.0 oW). These data were first analyzed using a behaviorally switching state-space model to infer foraging areas followed by randomization analysis of foraging region overlap of competing age classes. Patterns of habitat use and behavioral time budgets indicate that young-of-year juveniles (YOY) were likely displaced from foraging areas near (<10 km) the breeding colony by adult females. This displacement was most pronounced in the summer. Additionally, our data suggest that YOY are less capable divers than adults and this limits the habitat available to them. However, other segregating mechanisms cannot be ruled out, and we discuss several alternate hypotheses. Mark–resight data indicate juveniles born between 1998 and 2002 have much reduced survivorship compared with cohorts born in the late 1980s, while adult survivorship has remained steady. Combined with behavioral observations, our data suggest YOY are losing an intraspecific competition between adults and juveniles, resulting in the currently observed decelerating logistic population growth. Competition theory predicts that intraspecific competition resulting in a clear losing competitor should cause compensatory population regulation. This functional response produces a smooth logistic growth curve as carrying capacity is approached, and is consistent with census data collected from this population over the past 50 years. The competitive mechanism causing compensatory regulation likely stems from the capital-breeding life-history strategy employed by gray seals. This strategy decouples reproductive success from resources available around breeding colonies and prevents females from competing with each other while young are dependent. PMID:24198943

Breed, Greg A; Don Bowen, W; Leonard, Marty L

2013-01-01

289

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 ...

Mr. Hansen

2010-10-26

290

Density functional approach for the magnetism of ?-TeVO[subscript 4  

E-print Network

Density functional calculations have been carried out to investigate the microscopic origin of the magnetic properties of ?-TeVO[subscript 4]. Two different approaches, based either on a perturbative treatment of the ...

Radtke, G.

291

A Probability Density Function Approach to Distributed Sensors' Path S. Ferrari, G. Foderaro, and A. Tremblay  

E-print Network

A Probability Density Function Approach to Distributed Sensors' Path Planning S. Ferrari, G, an approximately optimal sensors' distribution can be determined in the form of a S. Ferrari and G. Foderaro

Ferrari, Silvia

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

Lateral density variations in elastic Earth models from an extended minimum energy approach  

NASA Technical Reports Server (NTRS)

Kaula's minimum energy approach was extended to include the nonhydrostatic gravitational potential energy and the density perturbation field was obtained to degree and order eight. The depth profiles for the density perturbation show a stratification with density excesses and deficiencies alternating with depth. The addition of the gravitational potential energy in the minimization process does not change significantly the conclusions based on results for the minimum shear strain energy case, concerning the inability of the mantle to withstand the lateral loading elastically.

Sanchez, B. V.

1980-01-01

294

A comprehensive approach to age-dependent dosimetric modeling  

SciTech Connect

In the absence of age-specific biokinetic models, current retention models of the International Commission on Radiological Protection (ICRP) frequently are used as a point of departure for evaluation of exposures to the general population. These models were designed and intended for estimation of long-term integrated doses to the adult worker. Their format and empirical basis preclude incorporation of much valuable physiological information and physiologically reasonable assumptions that could be used in characterizing the age-specific behavior of radioelements in humans. In this paper we discuss a comprehensive approach to age-dependent dosimetric modeling in which consideration is given not only to changes with age in masses and relative geometries of body organs and tissues but also to best available physiological and radiobiological information relating to the age-specific biobehavior of radionuclides. This approach is useful in obtaining more accurate estimates of long-term dose commitments as a function of age at intake, but it may be particularly valuable in establishing more accurate estimates of dose rate as a function of age. Age-specific dose rates are needed for a proper analysis of the potential effects on estimates or risk of elevated dose rates per unit intake in certain stages of life, elevated response per unit dose received during some stages of life, and age-specific non-radiogenic competing risks.

Leggett, R.W.; Cristy, M.; Eckerman, K.F.

1986-01-01

295

Density matrix expansion for the isospin- and momentum-dependent MDI interaction  

E-print Network

PHYSICAL REVIEW C 82, 044311 (2010) Density matrix expansion for the isospin- and momentum-dependent MDI interaction Jun Xu1 and Che Ming Ko2 1Cyclotron Institute, Texas A&M University, College Station, Texas 77843-3366, USA 2Cyclotron Institute...-dependent Boltzmann-Uhling-Uhlenbeck (IBUU) transport model, an extensive amount of work has been carried out to study various isospin-sensitive observables in intermediate-energy heavy-ion collisions (for recent reviews, see Refs. [2,3]). From comparisons...

Xu, Jun; Ko, Che Ming.

2010-01-01

296

Angular momentum dependence of the nuclear level density in the A ?170 -200 region  

NASA Astrophysics Data System (ADS)

Neutron evaporation spectra along with ? multiplicity has been measured from 201Tl*,185Re*, and 169Tm* compound nuclei at the excitation energies of ˜27 and 37 MeV. Statistical model analysis of the experimental data has been carried out to extract the value of the inverse level density parameter k at different angular-momentum (J ) regions corresponding to different ? multiplicities. It is observed that, for the present systems the value of k remains almost constant for different J . The present results for the angular-momentum dependence of the nuclear level density (NLD) parameter a ˜(=A /k ) , for nuclei with A ˜180 are quite different from those obtained in earlier measurements in the case of light- and medium-mass systems. The present study provides useful information to understand the angular-momentum dependence of the NLD at different nuclear mass regions.

Gohil, M.; Roy, Pratap; Banerjee, K.; Bhattacharya, C.; Kundu, S.; Rana, T. K.; Ghosh, T. K.; Mukherjee, G.; Pandey, R.; Pai, H.; Srivastava, V.; Meena, J. K.; Banerjee, S. R.; Mukhopadhyay, S.; Pandit, D.; Pal, S.; Bhattacharya, S.

2015-01-01

297

Two-component hybrid time-dependent density functional theory within the Tamm-Dancoff approximation  

NASA Astrophysics Data System (ADS)

We report the implementation of a two-component variant of time-dependent density functional theory (TDDFT) for hybrid functionals that accounts for spin-orbit effects within the Tamm-Dancoff approximation (TDA) for closed-shell systems. The influence of the admixture of Hartree-Fock exchange on excitation energies is investigated for several atoms and diatomic molecules by comparison to numbers for pure density functionals obtained previously [M. Kühn and F. Weigend, J. Chem. Theory Comput. 9, 5341 (2013)]. It is further related to changes upon switching to the local density approximation or using the full TDDFT formalism instead of TDA. Efficiency is demonstrated for a comparably large system, Ir(ppy)3 (61 atoms, 1501 basis functions, lowest 10 excited states), which is a prototype molecule for organic light-emitting diodes, due to its "spin-forbidden" triplet-singlet transition.

Kühn, Michael; Weigend, Florian

2015-01-01

298

Two-component hybrid time-dependent density functional theory within the Tamm-Dancoff approximation.  

PubMed

We report the implementation of a two-component variant of time-dependent density functional theory (TDDFT) for hybrid functionals that accounts for spin-orbit effects within the Tamm-Dancoff approximation (TDA) for closed-shell systems. The influence of the admixture of Hartree-Fock exchange on excitation energies is investigated for several atoms and diatomic molecules by comparison to numbers for pure density functionals obtained previously [M. Kühn and F. Weigend, J. Chem. Theory Comput. 9, 5341 (2013)]. It is further related to changes upon switching to the local density approximation or using the full TDDFT formalism instead of TDA. Efficiency is demonstrated for a comparably large system, Ir(ppy)3 (61 atoms, 1501 basis functions, lowest 10 excited states), which is a prototype molecule for organic light-emitting diodes, due to its "spin-forbidden" triplet-singlet transition. PMID:25612698

Kühn, Michael; Weigend, Florian

2015-01-21

299

Folding model analysis of alpha-particle elastic scattering with a semirealistic density dependent effective interaction  

SciTech Connect

The aim of this study was to examine the density dependent effective interaction, DDEI, applied in the double-folding model and to investigate whether folding model analyses of alpha particle elastic scattering could be used to calculate nuclear matter densities. Accordingly, differential cross-sections for the elastic scattering of alpha particles from (40)Ca, (46,48,50)Ti, (58)Ni, (90)Zr and (208)Pb at 140 MeV and from (58,60,62,64)Ni at 172 MeV were analyzed using the model with a semirealistic DDEI based on the M3Y interaction for the real potential and various phenomenological imaginary potentials. Good fits with consistent parameters of the model were obtained. The application of folding model analyses of alpha particle elastic scattering to the determination of nuclear matter density distribution was critically examined.

Kobos, A.M.; Brown, B.A.; Hodgson, P.E.; Satchler, G.R.; Budzanowski, A.

1982-01-01

300

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

301

NUCLEAR AND HEAVY ION PHYSICS: Density-dependent potential for multi-neutron halo nuclei  

NASA Astrophysics Data System (ADS)

We apply a simple density-dependent potential model to the three-body calculation of the ground-state structure of drip-line nuclei with a weakly bound core. The hyperspherical harmonics method is used to solve the Faddeev equations. There are no undetermined potential parameters in this calculation. We find that for the halo nuclei with a weakly-bound core, the calculated properties of the ground-state structure are in better agreement with experimental data than the results calculated from the standard Woods-Saxon and Gauss type potentials. We also successfully reproduce the experimental cross sections by using the density calculated from this method. This may be explained by the fact that the simple Fermi or Gaussian function can not exactly describe the density distribution of the drip-line nuclei.

Chen, Shuang; Chu, Yan-Yun; Ren, Zhong-Zhou

2009-08-01

302

Protein adsorption on poly(N-isopropylacrylamide) brushes: dependence on grafting density and chain collapse.  

PubMed

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 poly(N-isopropyl acrylamide) brushes. PMID:21662243

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

2011-07-19

303

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

304

Bone mineral density and fractures among alcohol-dependent women in treatment and in recovery  

Microsoft Academic Search

Women are at higher risk for osteoporosis, but most of the literature examining the effect of alcohol abuse on bone mineral density (BMD) has been in men. The aim of this study was to determine differences in BMD and fracture prevalence among women in treatment for alcohol abuse, in recovery and non-alcohol-dependent women. This cross-sectional study was completed at two

M. Kathleen Clark; MaryFran R. Sowers; Farideh Dekordi; Sara Nichols

2003-01-01

305

New relativistic mean-field interaction with density-dependent meson-nucleon couplings  

SciTech Connect

We adjust a new improved relativistic mean-field effective interaction with explicit density dependence of the meson-nucleon couplings. The effective interaction DD-ME2 is tested in relativistic Hartree-Bogoliubov and quasiparticle random-phase approximation (QRPA) calculations of nuclear ground states and properties of excited states, in calculation of masses, and it is applied to the analysis of very recent data on superheavy nuclei.

Lalazissis, G.A.; Niksic, T.; Vretenar, D.; Ring, P. [Department of Theoretical Physics, Aristotle University of Thessaloniki, GR-54124 (Greece); Physik-Department der Technischen Universitaet Muenchen, D-85748 Garching (Germany); Physics Department, Faculty of Science, University of Zagreb, Croatia and Physik-Department der Technischen Universitaet Muenchen, D-85748 Garching (Germany); Physik-Department der Technischen Universitaet Muenchen, D-85748 Garching (Germany)

2005-02-01

306

Time-dependent density functional study of excimers and exciplexes of organic molecules  

Microsoft Academic Search

Organic excimers of pyrene and benzene and an exciplex of styrene with trimethylamine are studied by time-dependent density functional theory (TD-DFT). The performance of the method was first benchmarked on noble gas excimers. Potential curves for low-lying excited states of Ne2 and Kr2 are computed using the hybrid functional BH-LYP and the semi-local B-LYP GGA functional. In order to include

Robert Huenerbein; Stefan Grimme

2008-01-01

307

Time-Dependent Density Functional Theory Study of Structure-Property Relationships in Diarylethene Photochromic Compounds  

Microsoft Academic Search

Photochromic compounds exhibit reversible transition between closed and open isomeric forms upon irradiation accompanied by\\u000a change in their color. The two isomeric forms differ not only in absorption spectra, but also in various physical and chemical\\u000a properties and find applications as optical switching and data storage materials. In this contribution we apply Density Functional\\u000a Theory (DFT) and Time-Dependent DFT (TD-DFT)

Pansy D. Patel; Artëm E. Masunov

2009-01-01

308

Time-Dependent Density Functional Theory for Molecular Multiphoton Processes in Intense Laser Fields  

Microsoft Academic Search

We present a new time-dependent density functional theory (TDDFT) with optimized effective potential (OEP) and self-interaction-correction (SIC) for the first nonperturbative treatment of multiphoton and nonlinear optical processes of many-electron molecular systems in intense laser fields(X. Chu and S. I. Chu, Phys. Rev. A (submitted).). The theory allows the elimination of the spurious self-interaction energy in the conventional DFT and

Xi Chu; Shih-I. Chu

2000-01-01

309

Time dependent density functional photoionization of CH 4, NH 3, H 2O and HF  

Microsoft Academic Search

The time dependent density functional theory (TD-DFT) and the Kohn–Sham (KS) schemes have been employed to calculate the cross section and the asymmetry parameter profiles of all the orbitals (from outer valence to core 1s) of CH4, NH3, H2O and HF, employing a one centre expansion of B-spline functions. The comparison between the KS and TD-DFT results shows that the

M. Stener; G. Fronzoni; D. Toffoli; P. Decleva

2002-01-01

310

Time dependent density functional study of the symmetry resolved N 1s photoionization in N 2  

Microsoft Academic Search

The time dependent-density functional theory (TD-DFT) has been applied to study the photoionization from the two symmetry resolved initial states (1?g and 1?u) of the core N 1s in N2. The results are compared with very recent experimental data and with previous ab initio calculations. The calculated intensity ratio is in excellent agreement with the experiment and performs better than

M. Stener; G. Fronzoni; P. Decleva

2002-01-01

311

Dose effects and density-dependent regulation of two microparasites of Daphnia magna  

Microsoft Academic Search

Individual hosts constitute a limited resource for parasites, suggesting that density-dependent effects may play a role in\\u000a within-host growth and parasite regulation. This hypothesis has been tested for several helminth parasites, but not for microparasites.\\u000a We therefore examined dose-response patterns for the microparasitic bacterium Pasteuria ramosa and the fungus Metschnikowiella biscuspidata infecting the planktonic crustacean Daphnia magna. With increasing numbers

Dieter Ebert; Christina D. Zschokke-Rohringer; H. J. Carius

2000-01-01

312

High-spin states in heavy nuclei with the density dependent Gogny force  

SciTech Connect

The high-spin properties of the density dependent Gogny force are analyzed for the first time. The self-consistent cranking Hartree-Fock-Bogoliubov equations have been solved to analyze the rotational nucleus [sup 164]Er. We study pairing properties, moments of inertia, [ital E]2 transitions, and gyromagnetic factors, among others, as a function of the angular momentum. The agreement with the available experimental results is very satisfactory.

Egido, J.L.; Robledo, L.M. (Departamento de Fisica Teorica C-XI, Universidad Autonoma de Madrid, E-28049 Madrid (Spain))

1993-05-10

313

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

314

An irradiation density dependent energy relaxation in plant photosystem II antenna assembly.  

PubMed

Plant photosystem II (PSII) is a multicomponent pigment-protein complex that harvests sunlight via pigments photoexcitation, and converts light energy into chemical energy. Against high light induced photodamage, excess light absorption of antenna pigments triggers the operation of photoprotection mechanism in plant PSII. Non-photochemical energy relaxation as a major photoprotection way is essentially correlated to the excess light absorption. Here we investigate the energy relaxation of plant PSII complexes with varying incident light density, by performing steady-state and transient chlorophyll fluorescence measurements of the grana membranes (called as BBY), functional moiety PSII reaction center and isolated light-harvesting complex LHCII under excess light irradiation. Based on the chlorophyll fluorescence decays of these samples, it is found that an irradiation density dependent energy relaxation occurs in the LHCII assemblies, especially in the antenna assembly of PSII supercomplexes in grana membrane, when irradiation increases to somewhat higher density levels. Correspondingly, the average chlorophyll fluorescence lifetime of the highly isolated BBY fragments gradually decreases from ~1680 to ~1360ps with increasing the irradiation density from 6.1×10(9) to 5.5×10(10) photon cm(-2) pulse(-1). Analysis of the relation of fluorescence decay change to the aggregation extent of LHCIIs suggests that a dense arrangement of trimeric LHCIIs is likely the structural base for the occurrence of this irradiation density dependent energy relaxation. Once altering the irradiation density, this energy relaxation is quickly reversible, implying that it may play an important role in photoprotection of plant PSII. PMID:25482259

Tian, Wenming; Chen, Jun; Deng, Liezheng; Yao, Mingdong; Yang, Heping; Zheng, Yang; Cui, Rongrong; Sha, Guohe

2015-02-01

315

Ionisation density dependence of the optically and thermally stimulated luminescence from Al2O3:C.  

PubMed

This paper presents an overview of recent results on ionisation density dependence of the thermally stimulated luminescence (TL) and optically stimulated luminescence (OSL) signals from Al2O3:C, with emphasis on the sensitivity, efficiency, shape of the TL/OSL curves and the emission spectrum. High-ionisation densities are created uniformly by accumulated high doses of low-linear energy transfer radiation (gamma, beta, X rays) or non-uniformly in heavy charged particle tracks, even at low fluences, as in the case of space radiation fields. Significant deep trap filling, which occurs at these high-ionisation densities, ultimately results in changes in the concentration of recombination centres (F+-centres) and, consequently, in sensitivity changes and other effects. An OSL emission band at 335 nm has been observed in addition to the main F-centre luminescence band, and the relative intensities of these bands have been observed to be dependent on the ionisation density. The implications of these results and open issues are discussed. PMID:16585262

Yukihara, Eduardo G; McKeever, Stephen W S

2006-01-01

316

Experimental Examination of Intraspecific Density-Dependent Competition during the Breeding Period in Monarch Butterflies (Danaus plexippus)  

PubMed Central

A central goal of population ecology is to identify the factors that regulate population growth. Monarch butterflies (Danaus plexippus) in eastern North America re-colonize the breeding range over several generations that result in population densities that vary across space and time during the breeding season. We used laboratory experiments to measure the strength of density-dependent intraspecific competition on egg laying rate and larval survival and then applied our results to density estimates of wild monarch populations to model the strength of density dependence during the breeding season. Egg laying rates did not change with density but larvae at high densities were smaller, had lower survival, and weighed less as adults compared to lower densities. Using mean larval densities from field surveys resulted in conservative estimates of density-dependent population reduction that varied between breeding regions and different phases of the breeding season. Our results suggest the highest levels of population reduction due to density-dependent intraspecific competition occur early in the breeding season in the southern portion of the breeding range. However, we also found that the strength of density dependence could be almost five times higher depending on how many life-stages were used as part of field estimates. Our study is the first to link experimental results of a density-dependent reduction in vital rates to observed monarch densities in the wild and show that the effects of density dependent competition in monarchs varies across space and time, providing valuable information for developing robust, year-round population models in this migratory organism. PMID:22984614

Flockhart, D. T. Tyler; Martin, Tara G.; Norris, D. Ryan

2012-01-01

317

Time dependent density functional study of the photoionization dynamics of SF6  

NASA Astrophysics Data System (ADS)

The B-spline linear combination of atomic orbitals method has been employed to study the valence and core photoionization dynamics of SF6. The cross section and asymmetry parameter profiles calculated at the time dependent density functional theory level have been found to be in fairly nice agreement with the experimental data, with the quality of the exchange-correlation statistical average of orbital potential results superior to the Van Leeuwen-Baerends 94 (LB94) ones [Phys. Rev. A 49, 2421 (1994)]. The role of response effects has been identified by a comparison of the time dependent density functional theory results with the Kohn-Sham ones interchannel coupling effects and autoionization resonances play an important role at low kinetic energies. Prominent shape resonances features have been analyzed in terms of "dipole prepared" continuum orbitals and interpreted as due to a large angular momentum centrifugal barrier as well as anisotropic (nonspherical) molecular effective potential. Finally, the method has been proven numerically stable, robust, and efficient, thanks to a noniterative implementation of the time dependent density functional theory equations and suitability of the multicentric B-spline basis set to describe continuum states from outer valence to deep core states.

Stener, M.; Toffoli, D.; Fronzoni, G.; Decleva, P.

2006-03-01

318

Time dependent density functional study of the photoionization dynamics of SF6.  

PubMed

The B-spline linear combination of atomic orbitals method has been employed to study the valence and core photoionization dynamics of SF6. The cross section and asymmetry parameter profiles calculated at the time dependent density functional theory level have been found to be in fairly nice agreement with the experimental data, with the quality of the exchange-correlation statistical average of orbital potential results superior to the Van Leeuwen-Baerends 94 (LB94) ones [Phys. Rev. A 49, 2421 (1994)]. The role of response effects has been identified by a comparison of the time dependent density functional theory results with the Kohn-Sham ones interchannel coupling effects and autoionization resonances play an important role at low kinetic energies. Prominent shape resonances features have been analyzed in terms of "dipole prepared" continuum orbitals and interpreted as due to a large angular momentum centrifugal barrier as well as anisotropic (nonspherical) molecular effective potential. Finally, the method has been proven numerically stable, robust, and efficient, thanks to a noniterative implementation of the time dependent density functional theory equations and suitability of the multicentric B-spline basis set to describe continuum states from outer valence to deep core states. PMID:16555887

Stener, M; Toffoli, D; Fronzoni, G; Decleva, P

2006-03-21

319

Temperature-Dependent Total Emission Spectra of Azulene in Polymers: Modeling Using Spectral Densities  

NASA Astrophysics Data System (ADS)

Narrowband excited, temperature dependent (1.4-150 K) emission spectra of the S1-S0 transition of azulene in different amorphous polymers are used to explore the matrix modes that contribute to dephasing of the electronic transition. The low- temperature spectra in all polymers consist of sharp zero-phonon lines (ZPL) whose frequencies represent the ground state vibrations of azulene, and broad phonon wings (PW) on the lower-energy side of the ZPL which carry information about optically active matrix modes. As the temperature is raised the ratio of ZPL to PW intensity decreases and the PW broadens until the spectrum resembles that in room temperature liquids, with sharp Raman-like peaks on a broad fluorescence background. The temperature-dependent emission spectra are simulated by modeling the azulene vibrations as undamped, displaced harmonic oscillators while the effect of the matrix is incorporated as a spectral density function that represents the optically active modes weighted by their coupling strengths. Spectral densities extracted from other experiments in the same polymers are used to evaluate the dependence of the relevant spectral density on the chromophore and the type of measurement.

Gupta, Vinita; Myers, Anne B.

1998-03-01

320

Native defects in oxide semiconductors: a density functional approach.  

PubMed

We report a semilocal and hybrid Hartree-Fock density functional study of native defects in three oxide semiconductors: ZnO, SrTiO(3), and SnO. The defect that is responsible for the n-type conductivity of ZnO has been debated, in which the O vacancy, Zn interstitial, their complexes, and residual H impurity are considered candidates. Our results indicate that the O vacancy induces a deep and localized in-gap state, whereas the Zn interstitial is a shallow donor and hence can be a source of the carriers. In view of the formation energies, the O vacancy is likely to form with a substantial concentration under O-poor conditions, but the Zn interstitial is unlikely. We thus propose that the O vacancy is relevant to the nonstoichiometry of ZnO and that a source other than the native defects, such as the H impurity, needs to be considered for the n-type conductivity. For SrTiO(3), the O vacancy and its complexes have been regarded as the origins of some of the remarkable electrical and optical properties. We suggest significant roles of the Ti antisite for a new insight into the defect-induced properties. Two types of Ti antisite, both of which are off-centered from the Sr site but toward different directions, exhibit low formation energies under Ti-rich conditions as does the O vacancy. They can explain optical properties such as visible-light emission, deep-level absorption, and the ferroelectricity observed in reduced SrTiO(3). As an example of p-type conductors, SnO has been investigated with a focus on the acceptor-like native defects. Under O-rich conditions, the Sn vacancy and O interstitial are found to be energetically favorable. The Sn vacancy induces shallow acceptor levels and can therefore be a source of carriers. The O interstitial shows no in-gap levels and hence it is inactive in terms of the carrier generation and compensation. However, this defect is a key to the understanding of the structures of intermediate compounds between SnO and SnO(2). PMID:21386545

Oba, Fumiyasu; Choi, Minseok; Togo, Atsushi; Seko, Atsuto; Tanaka, Isao

2010-09-29

321

Simulations and Theory of Density-Dependent Dispersion in Weakly Heterogeneous Porous Media  

NASA Astrophysics Data System (ADS)

The effect of density gradients on dispersive mixing of miscible fluids is studied. Density gradients not only affect dispersive transport for unstable flow, but also can significantly affect dispersion under stable conditions. For vertical displacements, laboratory experiments show a reduction of the longitudinal dispersivity under the influence of stabilizing density gradients. Linear Fick's law for the dispersive flux is inadequate to model these experiments. Therefore, alternative theories have been developed that incorporate the effect of density gradients. In this study, accurate numerical simulations of vertical displacements in weakly heterogeneous porous columns are performed. The numerical results confirm experimental observations. Furthermore, the computed concentration profiles are used to validate nonlinear dispersion theories applicable for high density gradients. A comparison is made with the stochastic theory of Welty and Gelhar, with homogenization theory, and with the nonlinear dispersion theory of Hassanizadeh and Leijnse. For small variances in log k, the stochastic and homogenization theory lead to reasonably good predictions of the computed concentration profiles and variances, without any fitting. In the theory of Hassanizadeh and Leijnse a fitting parameter is involved. This nonlinear dispersion parameter is not a true medium parameter, as it is found to be dependent on the flow rate and on the travel distance.

Landman, A.; Schotting, R. J.

2004-12-01

322

vol. 164, no. 3 the american naturalist september 2004 Density-Dependent Sexual Selection in External Fertilizers  

E-print Network

vol. 164, no. 3 the american naturalist september 2004 Density-Dependent Sexual Selection of sexual selection mirror those typically noted in internally fertilizing taxa. * E-mail: levitan-0147/2004/16403-40159$15.00. All rights reserved. Keywords: sexual selection, density dependence, echinoid, micro- satellites

Sura, Philip

323

Density-Dependent Competitive Suppression of Sylvatic Dengue Virus by Endemic Dengue Virus in Cultured Mosquito Cells  

E-print Network

Density-Dependent Competitive Suppression of Sylvatic Dengue Virus by Endemic Dengue Virus University, Las Cruces, New Mexico Abstract Mosquito-borne dengue viruses are maintained in two discrete Dengue virus; Competition; Density-dependent; Superinfection; Sylvatic; Aedes Introduction THE FOUR

Hanley, Kathryn A.

324

Mechanisms of density dependence in stream fish: exploitation competition for food reduces growth of adult European bullheads ( Cottus gobio )  

Microsoft Academic Search

In field experiments using cage enclosures, exploitation competition for invertebrate prey reduced individual growth of adult European bullheads (Cottus gobio) despite a dietary shift that maintained total prey biomass consump- tion. Growth of bullheads was negatively density dependent over a range of densities found in the field and total bio- mass production per enclosure was unrelated to stocking density. Individuals

Andrew J. H. Davey; George F. Turner; Stephen J. Hawkins; C. Patrick Doncaster

2006-01-01

325

Time-dependent density functional theory for calculating origin-independent optical rotation and rotatory strength tensors.  

PubMed

An approach to calculate origin-independent electronic chiroptical property tensors using time-dependent density functional theory (TDDFT) and gauge-including atomic orbital (GIAO) basis sets is evaluated. Computations of origin-dependent optical rotation tensors and of rotatory strengths needed to simulate circular dichroism spectra are presented. The optical rotation tensor computations employ solutions of coupled perturbed Kohn-Sham equations for a dynamic electric field and a static magnetic field. Because the magnetic field is time independent, the GIAO treatment is somewhat simplified compared to a previously reported method, at some added computational cost if hybrid functionals are employed. GIAO rotatory strengths are also calculated, using transition density matrices from a standard TDDFT excitation energy module. A new implementation in the NWChem quantum chemistry package is employed for representative computations of origin-invariant chiroptical response tensors for methyloxirane, norbornenone, and the ketosteroid androstadienone. For the steroid molecule the vibrational structure of the CD spectrum is modeled explicitly by using calculated Franck-Condon factors. The agreement with experiment is favorable. PMID:21948480

Autschbach, Jochen

2011-12-01

326

Distribution of postsynaptic density (PSD)-95 and Ca2+/calmodulin-dependent protein kinase II at the PSD.  

PubMed

Postsynaptic densities (PSDs) contain proteins that regulate synaptic transmission. We determined the positions of calcium/calmodulin-dependent protein kinase II (CaMKII) and PSD-95 within the three-dimensional structure of isolated PSDs using immunogold labeling, rotary shadowing, and electron microscopic tomography. The results show that all PSDs contain a central mesh immediately underlying the postsynaptic membrane. Label for PSD-95 is found on both the cytoplasmic and cleft sides of this mesh, averaging 12 nm from the cleft side. All PSDs label for PSD-95. The properties of CaMKII labeling are quite different. Label is virtually absent on the cleft sides of PSDs, but can be heavy on the cytoplasmic side at a mean distance of 25 nm from the cleft. In tomograms, CaMKII holoenzymes can be visualized directly, appearing as labeled, tower-like structures reflecting the 20 nm diameter of the holoenzyme. These towers protrude from the cytoplasmic side of the central mesh. There appears to be a local organization of CaMKII, as judged by fact that the nearest-neighbor distances are nearly invariant over a wide range of labeling density for CaMKII. The average density of CaMKII holoenzymes is highly variable, ranging from zero to values approaching a tightly packed state. This variability is significantly higher than that for PSD-95 and is consistent with an information storage role for CaMKII. PMID:14657186

Petersen, Jennifer D; Chen, Xiaobing; Vinade, Lucia; Dosemeci, Ayse; Lisman, John E; Reese, Thomas S

2003-12-01

327

Transverse momentum dependent parton densities in associated real and virtual photon and jet production at the LHC  

NASA Astrophysics Data System (ADS)

We study the associated production of real (isolated) or virtual photons (with their subsequent leptonic decay) and hadronic jets in proton-proton collisions at the LHC using the kT-factorization approach of QCD. The consideration is based on the off-shell quark-gluon QCD Compton scattering subprocesses. In the case of virtual photon production, the contributions from Z boson exchange as well as ?*-Z interference with the full spin correlations are included. The transversemomentum-dependent (TMD) quark and gluon densities in a proton are determined from the Kimber-Martin-Ryskin prescription or Catani-Ciafoloni-Fiorani-Marchesini (CCFM) equation. In the latter, we restrict to the case where the gluon-to-quark splitting occurs at the last evolution step and calculate the sea quark density as a convolution of the CCFM-evolved gluon distribution and the TMD gluon-to-quark splitting function. Our numerical predictions are compared with the recent experimental data taken by the ATLAS Collaboration. We discuss the theoretical uncertainties of our calculations and argue that further studies are capable of constraining the TMD parton densities in the proton.

Lipatov, A. V.; Zotov, N. P.

2014-11-01

328

Empirical density-dependent effective interaction for nucleon-nucleus scattering at 500 MeV  

SciTech Connect

We report new cross-section and analyzing-power data for the excitation by 498-MeV protons of all narrow normal-parity states of {sup 16}O below 12.1-MeV excitation. In addition, spin-rotation measurements for elastic scattering and depolarization measurements for the 1{sub 1}{sup {minus}}, 2{sub 1}{sup +}, and 3{sub 1}{sup {minus}} states of {sup 16}O have been performed. These data are used in conjunction with existing data for {sup 40}Ca to study medium corrections to the effective interaction for nucleon-nucleus scattering at 500 MeV. Systematic differences between the data and nonrelativistic impulse approximation calculations based upon either the free {ital t} matrix or a recent density-dependent effective interaction are interpreted within the framework of the local-density approximation. An empirical effective interaction has been constructed which parametrizes the density dependence of the medium modifications in a simple form amenable to phenomenological analysis of data.

Flanders, B.S. (Department of Physics, American University, Washington, D.C. 20016 (USA)); Kelly, J.J.; Seifert, H. (Department of Physics and Astronomy, University of Maryland, College Park, Maryland 20742 (USA)); Lopiano, D.; Aas, B.; Azizi, A.; Igo, G.; Weston, G.; Whitten, C.; Wong, A. (Department of Physics, University of California at Los Angeles, Los Angeles, California 90024 (USA)); Hynes, M.V.; McClelland, J. (Los Alamos National Laboratory, Los Alamos, New Mexico 87545 (USA)); Bertozzi, W.; Finn, J.M.; Hyde-Wright, C.E.; Lourie, R.W.; Norum, B.E.; Ulmer, P. (Department of Physics and Laboratory for Nuclear Science, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 (USA)); Berman, B.L. (Department of Physics, George Washington University, Washington, D.C. 20052 (USA))

1991-05-01

329

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

330

A density-functional approach to polarizable models: A Kim-Gordon response density interaction potential for molecular simulations  

NASA Astrophysics Data System (ADS)

A combined linear-response-frozen electron-density model has been implemented in a molecular-dynamics scheme derived from an extended Lagrangian formalism. This approach is based on a partition of the electronic charge distribution into a frozen region described by Kim-Gordon theory [J. Chem. Phys. 56, 3122 (1972); J. Chem. Phys. 60, 1842 (1974)] and a response contribution determined by the instantaneous ionic configuration of the system. The method is free from empirical pair potentials and the parametrization protocol involves only calculations on properly chosen subsystems. We apply this method to a series of alkali halides in different physical phases and are able to reproduce experimental structural and thermodynamic properties with an accuracy comparable to Kohn-Sham density-functional calculations.

Tabacchi, Gloria; Hutter, Jürg; Mundy, Christopher J.

2005-08-01

331

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

332

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

333

Quorum Sensing-Mediated, Cell Density-Dependent Regulation of Growth and Virulence in Cryptococcus neoformans  

PubMed Central

ABSTRACT Quorum sensing (QS) is a cell density-dependent mechanism of communication between microorganisms, characterized by the release of signaling molecules that affect microbial metabolism and gene expression in a synchronized way. In this study, we investigated cell density-dependent behaviors mediated by conditioned medium (CM) in the pathogenic encapsulated fungus Cryptococcus neoformans. CM produced dose-dependent increases in the growth of planktonic and biofilm cells, glucuronoxylomannan release, and melanin synthesis, important virulence attributes of this organism. Mass spectrometry revealed the presence of pantothenic acid (PA) in our samples, and commercial PA was able to increase growth and melanization, although not to the same extent as CM. Additionally, we found four mutants that were either unable to produce active CM or failed to respond with increased growth in the presence of wild-type CM, providing genetic evidence for the existence of intercellular communication in C. neoformans. C. neoformans CM also increased the growth of Cryptococcus albidus, Candida albicans, and Saccharomyces cerevisiae. Conversely, CM from Cryptococcus albidus, C. albicans, S. cerevisiae, and Sporothrix schenckii increased C. neoformans growth. In summary, we report the existence of a new QS system regulating the growth and virulence factor expression of C. neoformans in vitro and, possibly, also able to regulate growth in other fungi. PMID:24381301

Albuquerque, Patrícia; Nicola, André M.; Nieves, Edward; Paes, Hugo Costa; Williamson, Peter R.; Silva-Pereira, Ildinete; Casadevall, Arturo

2013-01-01

334

Thickness Dependent Carrier Density at the Surface of SrTiO3 (111) Slabs  

SciTech Connect

We investigate the surface electronic structure and thermodynamic stability of the SrTiO3 (111) slabs using density functional theory. We observe that, for Ti-terminated slabs it is indeed possible to create a two-dimensional electron gas (2DEG). However, the carrier density of the 2DEG displays a strong thickness dependence due to the competition between electronic reconstruction and polar distortions. As expected, having a surface oxygen atom at the Ti termination can stabilize the system, eliminating any electronic reconstruction, thereby making the system insulating. An analysis of the surface thermodynamic stability suggests that the Ti terminated (111) surface should be experimentally realizable. This surface may be useful for exploring the behavior of electrons in oxide (111) interfaces and may have implications for modern device applications.

Sivadas, Mr. Nikhil [Carnegie Mellon University (CMU)] [Carnegie Mellon University (CMU); Dixit, Hemant M [ORNL] [ORNL; Cooper, Valentino R [ORNL] [ORNL; Xiao, Di [Carnegie Mellon University (CMU)] [Carnegie Mellon University (CMU)

2014-01-01

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

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

337

Evolution of flowering decisions in a stochastic, density-dependent environment  

PubMed Central

Demography is central to both ecology and evolution, and characterizing the feedback between ecology and evolution is critical for understanding organisms' life histories and how these might evolve through time. Here, we show how, by combining a range of theoretical approaches with the statistical analysis of individually structured databases, accurate prediction of life history decisions is possible in natural density-regulated populations undergoing large fluctuations in demographic rates from year to year. Our predictions are remarkably accurate and statistically well defined. In addition, we show that the predicted trait values are evolutionarily and convergence stable and that protected polymorphisms are possible. PMID:18641119

Metcalf, C. J. E.; Rose, K. E.; Childs, D. Z.; Sheppard, A. W.; Grubb, P. J.; Rees, M.

2008-01-01

338

Bone mineral density and fractures among alcohol-dependent women in treatment and in recovery.  

PubMed

Women are at higher risk for osteoporosis, but most of the literature examining the effect of alcohol abuse on bone mineral density (BMD) has been in men. The aim of this study was to determine differences in BMD and fracture prevalence among women in treatment for alcohol abuse, in recovery and non-alcohol-dependent women. This cross-sectional study was completed at two residential substance abuse centers in Iowa (USA). The patients were Caucasian women, aged 18-70 years, in treatment for alcohol abuse and dependence ( n=228); in recovery and abstaining from alcohol ( n=156); and women with no history of alcohol abuse ( n=447). The main outcome measures were femoral neck and lumbar spine BMD measured by dual-energy X-ray absorptiometry (DXA); self-reported lifetime fracture prevalence. After adjusting for age and menopausal status, women in treatment had BMDs that were 7.7% ( p<0.01) and 6.3% ( p<0.01) lower at the femoral neck and lumbar spine, respectively, than non-alcohol-abusing women, and 4.8% lower at both bone sites ( p<0.01) than women in recovery. Femoral neck BMD of women in recovery was 3.1% lower ( p<0.01) than in non-alcohol-dependent women; however, the difference was not significant following multivariate analysis. Women in treatment and recovery reported more fractures during childhood and early adolescence than non-alcohol-dependent women ( p<0.01). Women in recovery also reported significantly greater numbers of fractures following sobriety than their paired non-alcohol-dependent counterparts. Alcohol abuse and dependence was associated with lower femoral neck and lumbar spine BMD. Women with histories of alcohol dependence had a higher lifetime prevalence of fractures, including time periods before the onset of problem drinking and following abstinence, suggesting that factors other than acute intoxication contributed to the greater fracture prevalence. PMID:12730765

Clark, M Kathleen; Sowers, Mary Fran R; Dekordi, Farideh; Nichols, Sara

2003-06-01

339

Soliton solutions of the improved quark mass density-dependent model at finite temperature  

E-print Network

The improved quark mass density-dependent model (IQMDD) based on soliton bag model is studied at finite temperature. Appling the finite temperature field theory, the effective potential of the IQMDD model and the bag constant $B(T)$ have been calculated at different temperatures. It is shown that there is a critical temperature $T_{C}\\simeq 110 \\mathrm{MeV}$. We also calculate the soliton solutions of the IQMDD model at finite tmperature. It turns out that when $Tsoliton solutions are stable. However, when $T>T_{C}$ the bag constant $B(T)=0$ and there is no soliton solution, therefore, the confinement of quarks are removed quickly.

Hong Mao; Ru-Keng Su; Wei-Qin Zhao

2006-10-03

340

Spectral properties of bipyridyl ligands by time-dependent density functional theory  

Microsoft Academic Search

The properties of a series of bipyridyl ligands, which take an active part in numerous inorganic dyes designed for photovoltaic applications, are studied by the means of density functional theory and time-dependent DFT. In particular, the properties of the 2,2?-bipyridine (bpy) ligand and those of the 4,4?-dicarboxy-2,2?-bipyridine (4,4?-dcbpyH2) and 5,5?-dicarboxy-2,2?-bipyridine (5,5?-dcbpyH2) derivatives (in their neutral and anionic forms) were computed.

Frédéric Labat; Philippe P. Lainé; Ilaria Ciofini; Carlo Adamo

2006-01-01

341

Dibaryon systems in the quark mass density- and temperature-dependent model  

E-print Network

Using the quark mass density- and temperature-dependent model, we have studied the properties of the dibaryon systems. The binding energy, radius and mean lifetime of Omega-Omega and Omega-Xi are given. We find the dibaryons Omega-Omega, Omega-Xi are metastable at zero temperature, but the strong decay channel for Omega-Omega opens when temperature arrives at 129.3MeV. It is shown that our results are in good agreement with those given by the chiral S(3) quark model.

Yun Zhang; Ru-Keng Su

2003-04-15

342

Pressure Dependence of the Charge-Density-Wave Gap in Rare-Earth Tri-Tellurides  

SciTech Connect

We investigate the pressure dependence of the optical properties of CeTe{sub 3}, which exhibits an incommensurate charge-density-wave (CDW) state already at 300 K. Our data are collected in the mid-infrared 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 RTe{sub 3}.

Sacchetti, A.; /Zurich, ETH; Arcangeletti, E.; Perucchi, A.; Baldassarre, L.; Postorino, P.; Lupi, S.; /Rome U.; Ru, N.; Fisher, I.R.; /Stanford U., Geballe Lab.; Degiorgi, L.; /Zurich, ETH

2009-12-14

343

Density-dependent development of Anopheles gambiae (Diptera: Culicidae) larvae in artificial habitats.  

PubMed

The growth and development of Anopheles gambiae Giles larvae were studied in artificial habitats in western Kenya. Larvae responded to increasing densities by extending their development time and by emerging as smaller adults, although survival was not significantly affected. Addition of nutrients in the form of cow dung collected near the study site had no impact on larval growth and development. Regression analysis showed that female development time increased by 0.020 d and female dry mass decreased by 0.74 microg with each additional larva. By fitting the data to the pupation window model, the estimated minimum dry mass to achieve pupation was 0.130 mg and the estimated minimum time to pupation was 5 d. The most likely food source for An. gambiae larvae was algal growth, which was significantly reduced by the presence of larvae. Bacterial densities were not significantly affected by the presence of larvae although total bacteria counts were lower at the higher densities indicating they may provide a secondary food source when algal resources are depleted. Similarly, the levels of nitrogen and phosphorus in the habitats were not significantly affected by the presence of larvae although there was evidence of decreasing nitrogen levels occurring with increasing larval densities suggesting that nitrogen may be a limiting resource in the larval environment. The data indicate that competition within the larval environment may indirectly regulate An. gambiae populations by reducing adult body size, which may in turn reduce adult survivorship and fecundity. The potential impact of density-dependent interactions among An. gambiae larvae on the transmission of Plasmodium falciparum is discussed. PMID:11931252

Gimnig, John E; Ombok, Maurice; Otieno, Samson; Kaufman, Michael G; Vulule, John M; Walker, Edward D

2002-01-01

344

Density dependent mechanical properties and structures of a freeze dried biopharmaceutical excipient - Sucrose.  

PubMed

Knowledge of the mechanical behaviour of freeze dried biopharmaceutical products is essential for designing of products with physical robustness that will not to crack, crumble or collapse during processing or transportation. The compressive mechanical deformation behaviour for freeze-dried sucrose cakes has been experimentally studied from a relative density (?f/?s) of 0.01-0.30 using a novel in-vial indentation test. Cakes exhibited more open like structures at lower densities and more closed structures at higher densities with some faces being present at all densities, as confirmed by SEM. The reduced elastic modulus Ef/Es=0.0044(?f/?s)(1) for all cake densities, indicating that face stretching was the dominant deformation mode assuming Gibson and Ashby's closed cell model. This linear scaling for the reduced elastic modulus is in line with various theoretical treatments based on tetrakaidecahedral cells and other experimental studies. Consistently, the wall thickness to cell diameter ratio scaled ?f/?s with a power constant of 1.05. The maximum crushing stress was given by ?max=3800(?f/?s)(1.48) which agrees with a strut bending failure stress, assuming Gibson and Ashby's open cell model. Overall, the freeze dried cakes behaved as neither classic closed cell nor open cell materials, with their compressive elastic moduli reflecting a closed cell elastic response whilst their failure stresses reflecting an open cell failure mode. It was concluded that the mechanical response of freeze dried cellular materials depends upon their complex cellular structures and morphologies, and they cannot be rationalised using simple limiting case models of open or closed cell solids. PMID:25019365

Devi, Sharmila; Williams, Daryl R

2014-10-01

345

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

346

Density  

NSDL National Science Digital Library

This web page introduces the concepts of density and buoyancy. The discovery in ancient Greece by Archimedes is described. The densities of various materials are given and temperature effects introduced. Links are provided to news and other resources related to mass density. This is part of the Vision Learning collection of short online modules covering topics in a broad range of science and math topics.

Day, Martha M.

2008-05-26

347

Phytotherapeutic approach to alcohol dependence: New old way?  

Microsoft Academic Search

Alcohol abuse and dependence represent a worldwide problem from both medical and social points of view. In Italy it is estimated that there are about one million alcohol-dependent subjects. The pharmacological treatment of patients with alcohol dependence plays a key role in order to achieve alcohol abstinence and prevent relapse. At present, the possible utility of the complementary medicines in

Ludovico Abenavoli; Francesco Capasso; Giovanni Addolorato

2009-01-01

348

Plant growth responses to inorganic environmental contaminants are density-dependent: experiments with copper sulfate, barley and lettuce.  

PubMed

The density-dependence of terrestrial plant-plant interactions in the presence of toxins has previously been explored using biodegradable compounds. We exposed barley and lettuce to four copper concentrations at four stand densities. We hypothesized that toxin effects would decrease and Cu uptake would increase at increasing plant densities. We analyzed toxin effects by (a) comparing plant biomasses and (b) using a recent regression model that has a separate parameter for the interaction of resource competition and toxin interference. Plant response to Cu was density-dependent in both experiments. Total Cu uptake by barley increased and the dose per plant decreased as plant density increased. This study is the first to demonstrate that plant density mediates plant response to metals in soil in a predictable way. This highlights the need to explore the mechanisms for and consequences of these effects, and to integrate the use of several plant densities into standard ecotoxicological testing. PMID:24121419

Hansi, Mari; Weidenhamer, Jeffrey D; Sinkkonen, Aki

2014-01-01

349

Density dependence of the room temperature thermal conductivity of atomic layer deposition-grown amorphous alumina (Al2O3)  

NASA Astrophysics Data System (ADS)

We report on the thermal conductivity of atomic layer deposition-grown amorphous alumina thin films as a function of atomic density. Using time domain thermoreflectance, we measure the thermal conductivity of the thin alumina films at room temperature. The thermal conductivities vary ˜35% for a nearly 15% change in atomic density and are substrate independent. No density dependence of the longitudinal sound speeds is observed with picosecond acoustics. The density dependence of the thermal conductivity agrees well with a minimum limit to thermal conductivity model that is modified with a differential effective-medium approximation.

Gorham, Caroline S.; Gaskins, John T.; Parsons, Gregory N.; Losego, Mark D.; Hopkins, Patrick E.

2014-06-01

350

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

351

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

352

Warming and nitrogen affect size structuring and density dependence in a host–parasitoid food web  

PubMed Central

Body size is a major factor constraining the trophic structure and functioning of ecological communities. Food webs are known to respond to changes in basal resource abundance, and climate change can initiate compounding bottom-up effects on food-web structure through altered resource availability and quality. However, the effects of climate and co-occurring global changes, such as nitrogen deposition, on the density and size relationships between resources and consumers are unknown, particularly in host–parasitoid food webs, where size structuring is less apparent. We use a Bayesian modelling approach to explore the role of consumer and resource density and body size on host–parasitoid food webs assembled from a field experiment with factorial warming and nitrogen treatments. We show that the treatments increased resource (host) availability and quality (size), leading to measureable changes in parasitoid feeding behaviour. Parasitoids interacted less evenly within their host range and increasingly focused on abundant and high-quality (i.e. larger) hosts. In summary, we present evidence that climate-mediated bottom-up effects can significantly alter food-web structure through both density- and trait-mediated effects. PMID:23007092

de Sassi, Claudio; Staniczenko, Phillip P. A.; Tylianakis, Jason M.

2012-01-01

353

Time-dependent internal density functional theory formalism and Kohn-Sham scheme for self-bound systems  

SciTech Connect

The stationary internal density functional theory (DFT) formalism and Kohn-Sham scheme are generalized to the time-dependent case. It is proven that, in the time-dependent case, the internal properties of a self-bound system (such as an atomic nuclei or a helium droplet) are all defined by the internal one-body density and the initial state. A time-dependent internal Kohn-Sham scheme is set up as a practical way to compute the internal density. The main difference from the traditional DFT formalism and Kohn-Sham scheme is the inclusion of the center-of-mass correlations in the functional.

Messud, Jeremie [Universite de Toulouse, UPS, Laboratoire de Physique Theorique (IRSAMC), F-31062 Toulouse, France, and CNRS, LPT (IRSAMC), F-31062 Toulouse (France)

2009-11-15

354

Exact-exchange time-dependent density-functional theory for static and dynamic polarizabilities  

SciTech Connect

Time-dependent density-functional theory (TDDFT) employing the exact-exchange functional has been formulated on the basis of the optimized-effective-potential (OEP) method of Talman and Shadwick for second-order molecular properties and implemented into a Gaussian-basis-set, trial-vector algorithm. The only approximation involved, apart from the lack of correlation effects and the use of Gaussian-type basis functions, was the consistent use of the adiabatic approximation in the exchange kernel and in the linear response function. The static and dynamic polarizabilities and their anisotropy predicted by the TDDFT with exact exchange (TDOEP) agree accurately with the corresponding values from time-dependent Hartree-Fock theory, the exact-exchange counterpart in the wave function theory. The TDOEP is free from the nonphysical asymptotic decay of the exchange potential of most conventional density functionals or from any other manifestations of the incomplete cancellation of the self-interaction energy. The systematic overestimation of the absolute values and dispersion of polarizabilities that plagues most conventional TDDFT cannot be seen in the TDOEP.

Hirata, So; Ivanov, Stanislav; Bartlett, Rodney J.; Grabowski, Ireneusz [Quantum Theory Project, Department of Chemistry, University of Florida, Gainesville, Florida 32611-8435 (United States); Institute of Physics, Nicolaus Copernicus University, Torun 87-100 (Poland)

2005-03-01

355

Time-dependent density functional theory for ion diffusion in electrochemical systems  

NASA Astrophysics Data System (ADS)

We introduce a generic form of time-dependent density functional theory (TDDFT) to describe ion diffusion in electrochemical systems to account for steric effects and electrostatic correlations neglected in the Poisson-Nernst-Planck equations. An efficient numerical algorithm is proposed to analyze the charging kinetics of electric double layers in model electrochemical systems that consist of spherical ions in a dielectric continuum confined between two planar electrodes. By comparing the theoretical predictions from TDDFT and conventional electrokinetic methods for constant-voltage charging of the model electrochemical cells, we demonstrate that thermodynamic non-ideality plays a pivotal role in electrodiffusion even at relatively low electrolyte concentrations, and this effect cannot be captured by the lattice-gas model for the excluded volume effects. In particular, TDDFT predicts ‘wave-like’ variation of the ionic density profiles that has not been identified in previous investigations. At conditions where there are no significant correlations between electric double layers from opposite electrodes, the charging kinetics follows an exponential behavior with a linear dependence of the relaxation time on the cell thickness in excellent agreement with the equivalent circuit model. However, the conventional electrokinetic model breaks down when the electrodes are at small separation, in particular for systems with low ionic strength or high charging voltage. We also find that ionic screening retards the charging kinetics at low salt concentrations, but has the opposite effect at large salt concentrations.

Jiang, Jian; Cao, Dapeng; Jiang, De-en; Wu, Jianzhong

2014-07-01

356

Spin- and Parity-Dependent Level Densities in Exotic Calcium Isotopes?  

NASA Astrophysics Data System (ADS)

Level densities of 2+ and 1- states in 40Ca have been extracted from high energy-resolution (p,p') experiments performed using the K600 magnetic spectrometer at iThemba LABS, South Africa. The success of the method depends on the accurate determination of various background components underneath the giant resonance. It was found that using the nearly model-independent method of Discrete Wavelet Transform (DWT) background determination using bi-orthogonal wavelets of high order of vanishing moments proved sufficient for the extraction of level density in the excitation energy region of the Isoscalar Giant Quadrupole Resonance (ISGQR) and Isovector Giant Dipole Resonance (IVGDR). Comparison of the experimentally extracted 2+ and 1- level densities with Back-Shifted Fermi Gas Model (BSFG), Hartree-Fock Bogoliubov (HFB) and Hartree-Fock-Bardeen-Cooper Schriffer (HF-BCS) theoretical model predictions were made. Future interests in the use of radioactive-ion beams of different intensities will provide opportunities in elucidating this important property experimentally for more exotic calcium isotopes.

Usman, I. T.; Carter, J.; Sideras-Haddad, E.; Jingo, M.; Kureba, C. O.; Donaldson, L.; Latif, M.; Neveling, R.; Smit, F. D.; Swartz, C.; Nemulodi, F.; von Neumann-Cosel, P.; Richter, A.; Fearick, R. W.; Fujita, H.; Fulita, Y.; Tamii, A.

2015-11-01

357

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

358

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

359

Hybrid finite-element\\/molecular-dynamics\\/electronic-density-functional approach to materials simulations on parallel computers  

Microsoft Academic Search

A hybrid simulation approach is developed to study chemical reactions coupled with long-range mechanical phenomena in materials. The finite-element method for continuum mechanics is coupled with the molecular dynamics method for an atomic system that embeds a cluster of atoms described quantum-mechanically with the electronic density-functional method based on real-space multigrids. The hybrid simulation approach is implemented on parallel computers

Shuji Ogata; Elefterios Lidorikis; Fuyuki Shimojo; Aiichiro Nakano; Priya Vashishta; Rajiv K. Kalia

2001-01-01

360

A consistent approach for the treatment of Fermi acceleration in time-dependent billiards  

E-print Network

The standard description of Fermi acceleration, developing in a class of time-dependent billiards, is given in terms of a diffusion process taking place in momentum space. Within this framework the evolution of the probability density function (PDF) of the magnitude of particle velocities as a function of the number of collisions $n$ is determined by the Fokker-Planck equation (FPE). In the literature the FPE is constructed by identifying the transport coefficients with the ensemble averages of the change of the magnitude of particle velocity and its square in the course of one collision. Although this treatment leads to the correct solution after a sufficiently large number of collisions has been reached, the transient part of the evolution of the PDF is not described. Moreover, in the case of the Fermi-Ulam model (FUM), if a stadanrd simplification is employed, the solution of the FPE is even inconsistent with the values of the transport coefficients used for its derivation. The goal of our work is to provide a self-consistent methodology for the treatment of Fermi acceleration in time-dependent billiards. The proposed approach obviates any assumptions for the continuity of the random process and the existence of the limits formally defining the transport coefficients of the FPE. Specifically, we suggest, instead of the calculation of ensemble averages, the derivation of the one-step transition probability function and the use of the Chapman-Kolmogorov forward equation. This approach is generic and can be applied to any time-dependent billiard for the treatment of Fermi-acceleration. As a first step, we apply this methodology to the FUM, being the archetype of time-dependent billiards to exhibit Fermi acceleration.

A. K. Karlis; F. K. Diakonos; V. Constantoudis

2012-01-07

361

A consistent approach for the treatment of Fermi acceleration in time-dependent billiards  

NASA Astrophysics Data System (ADS)

The standard description of Fermi acceleration, developing in a class of time-dependent billiards, is given in terms of a diffusion process taking place in momentum space. Within this framework, the evolution of the probability density function (PDF) of the magnitude of particle velocities as a function of the number of collisions n is determined by the Fokker-Planck equation (FPE). In the literature, the FPE is constructed by identifying the transport coefficients with the ensemble averages of the change of the magnitude of particle velocity and its square in the course of one collision. Although this treatment leads to the correct solution after a sufficiently large number of collisions have been reached, the transient part of the evolution of the PDF is not described. Moreover, in the case of the Fermi-Ulam model (FUM), if a standard simplification is employed, the solution of the FPE is even inconsistent with the values of the transport coefficients used for its derivation. The goal of our work is to provide a self-consistent methodology for the treatment of Fermi acceleration in time-dependent billiards. The proposed approach obviates any assumptions for the continuity of the random process and the existence of the limits formally defining the transport coefficients of the FPE. Specifically, we suggest, instead of the calculation of ensemble averages, the derivation of the one-step transition probability function and the use of the Chapman-Kolmogorov forward equation. This approach is generic and can be applied to any time-dependent billiard for the treatment of Fermi-acceleration. As a first step, we apply this methodology to the FUM, being the archetype of time-dependent billiards to exhibit Fermi acceleration.

Karlis, A. K.; Diakonos, F. K.; Constantoudis, V.

2012-06-01

362

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

363

{sigma}{sup -}/{sigma}{sup +} ratio as a candidate for probing the density dependence of the symmetry potential at high nuclear densities  

SciTech Connect

Based on the UrQMD (ultrarelativistic quantum molecular dynamics) model, we have investigated the influence of the symmetry potential on the negatively and positively charged {pi} and {sigma} hyperon production ratios in heavy ion collisions at the SIS (SchwerIonen Synchrotron) energies. We find that, in addition to {pi}{sup -}/{pi}{sup +} ratio, the {sigma}{sup -}/{sigma}{sup +} ratio can be taken as a sensitive probe for investigating the density dependence of the symmetry potential of nuclear matter at high densities (1-4 times normal baryon density). This sensitivity of the symmetry potential to both the {pi}{sup -}/{pi}{sup +} and {sigma}{sup -}/{sigma}{sup +} ratios is found to depend strongly on the incident beam energy. Furthermore, the {sigma}{sup -}/{sigma}{sup +} ratio is shown to carry the information about the isospin-dependent part of the {sigma} hyperon single-particle potential.

Li Qingfeng [Frankfurt Institute for Advanced Studies (FIAS), Johann Wolfgang Goethe-Universitaet, Max-von-Laue-Str. 1, D-60438 Frankfurt (Germany); Institute of Theoretical Physics, Chinese Academy of Sciences, P.O. Box 2735, Beijing 100080 (China); Li Zhuxia [Frankfurt Institute for Advanced Studies (FIAS), Johann Wolfgang Goethe-Universitaet, Max-von-Laue-Str. 1, D-60438 Frankfurt (Germany); China Institute of Atomic Energy, P.O. Box 275 (18), Beijing 102413 (China); Center of Theoretical Nuclear Physics, National Laboratory of Lanzhou Heavy Ion Accelerator, Lanzhou 730000 (China); Institute of Theoretical Physics, Chinese Academy of Sciences, P.O. Box 2735, Beijing 100080 (China); Zhao Euguang [Institute of Theoretical Physics, Chinese Academy of Sciences, P.O. Box 2735, Beijing 100080 (China); Gupta, Raj K. [Frankfurt Institute for Advanced Studies (FIAS), Johann Wolfgang Goethe-Universitaet, Max-von-Laue-Str. 1, D-60438 Frankfurt (Germany); Department of Physics, Panjab University, Chandigarh - 160014 (India)

2005-05-01

364

Parity dependence of the level densities of 53Cr and 55Cr at high excitation  

NASA Astrophysics Data System (ADS)

The neutron total cross sections of 52Cr and 54Cr have been measured in the energy range from a few tens of keV to about 900 keV using a neutron time-of-flight technique and a pulsed electron linear accelerator. The nominal resolution of the measurements was about 0.06 ns/m. The total cross-section data have been analyzed using an R-matrix multilevel multichannel code to determine values of the resonance parameters (E0, g?n, and J?) and to investigate the parity dependence of the level densities of 53Cr and 55Cr at high excitation. From these analyses we obtain the following values for the average properties of the resonance parameters for s-wave resonances up to about 900 keV and for p-wave resonances up to 600 keV: D0=(45+/-6) keV, S0(×104)=(3.0+/-1.0), D1=(8.5+/-0.6) keV, S1(×104)=(0.70+/-0.12) for 52Cr; and D0=(60+/-9) keV, S0(×104)=(2.6+/-0.9), D1=(9.2+/-0.5) keV, S1(×104)=(0.67+/-0.11) for 54Cr. The distributions of reduced neutron widths for s- and p-wave resonances for each isotope show good agreement with the Porter-Thomas distribution. The values of the ?3 statistic for the long-range correlation of s-wave resonances for both 52Cr and 54Cr and found to be in reasonable agreement with the theoretical prediction of Dyson and Metha. The values for the ratio D0D1 for both nuclides are larger than expected from a (2J+1) level dependence, indicative of a parity dependence of the level densities of 53Cr and 55Cr at high excitation.

Agrawal, H. M.; Garg, J. B.; Harvey, J. A.

1984-12-01

365

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

366

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

367

Adaptive Time-Dependent Density-Matrix Renormalization-Group Technique for Calculating the Conductance of Strongly Correlated Nanostructures  

SciTech Connect

A procedure based on the recently developed ''adaptive'' time-dependent density-matrix-renormalization-group (DMRG) technique is presented to calculate the zero temperature conductance of nanostructures, such as quantum dots (QDs) or molecular conductors, when represented by a small number of active levels. The leads are modeled using noninteracting tight-binding Hamiltonians. The ground state at time zero is calculated at zero bias. Then, a small bias is applied between the two leads, the wave function is DMRG evolved in time, and currents are measured as a function of time. Typically, the current is expected to present periodicities over long times, involving intermediate well-defined plateaus that resemble steady states. The conductance can be obtained from those steady-state-like currents. To test this approach, several cases of interacting and noninteracting systems have been studied. Our results show excellent agreement with exact results in the noninteracting case. More importantly, the technique also reproduces quantitatively well-established results for the conductance and local density of states in both the cases of one and two coupled interacting QDs. The technique also works at finite bias voltages, and it can be extended to include interactions in the leads.

Al Hassanieh, Khaled A [ORNL; Feiguin, Adrian E [ORNL; Riera, Jose Alejandro [ORNL; Busser, Carlos A [ORNL; Dagotto, Elbio R [ORNL

2006-01-01

368

The Dependency Structure of Coordinate Phrases: A Corpus Approach  

ERIC Educational Resources Information Center

Hudson (1990) proposes that each conjunct in a coordinate phrase forms dependency relations with heads or dependents outside the coordinate phrase (the "multi-head" view). This proposal is tested through corpus analysis of Wall Street Journal text. For right-branching constituents (such as direct-object NPs), a short-long preference for conjunct…

Temperley, David

2005-01-01

369

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

370

Toward a model-independent constraint of the high-density dependence of the symmetry energy  

NASA Astrophysics Data System (ADS)

Neutron-proton elliptic flow difference and ratio have been shown to be promising observables in the attempt to constrain the density dependence of the symmetry energy above the saturation point from heavy-ion collision data. Their dependence on model parameters such as microscopic nucleon-nucleon cross sections, compressibility of nuclear matter, optical potential, and symmetry energy parametrization is thoroughly studied. By using a parametrization of the symmetry energy derived from the momentum-dependent Gogny force in conjunction with the Tübingen quantum molecular dynamics (QMD) model and comparing the results with the experimental FOPI-LAND data for 197Au +197Au collisions at 400 MeV/nucleon, a moderately stiff (Lsym=122±57 MeV and Ksym=229±363 MeV) symmetry energy is extracted, a result that agrees with that of a similar study that employed the UrQMD transport model and a power-law parametrization of the symmetry energy. This contrasts with diverging results extracted from the FOPI ?-/?+ ratio available in the literature.

Cozma, M. D.; Leifels, Y.; Trautmann, W.; Li, Q.; Russotto, P.

2013-10-01

371

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 ...

372

Temperature dependence of the generalized vibrational density of states of sodium bismuth titanate in the ferroelectric phase  

E-print Network

Temperature dependence of the generalized vibrational density of states of sodium bismuth titanate disordered with respect to the B sublattice. Sodium bismuth titanate Na1/2Bi1/2TiO3 (NBT) is among the group

373

Testing for Convergence Clubs in Income per-capita: A Predictive Density Approach  

Microsoft Academic Search

The paper proposes a technique to test jointly for groupings of unknown size in the cross-sectional dimension of a panel and estimates the parameters of each group, applying it to identifying convergence clubs in income per-capita. The approach uses the predictive density of the data, conditional on the parameters of the model. The steady state distribution of European regional data

Fabio Canova

1999-01-01

374

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

375

Collective enhancement of nuclear state densities by the shell model Monte Carlo approach  

E-print Network

The shell model Monte Carlo (SMMC) approach allows for the microscopic calculation of statistical and collective properties of heavy nuclei using the framework of the configuration-interaction shell model in very large model spaces. We present recent applications of the SMMC method to the calculation of state densities and their collective enhancement factors in rare-earth nuclei.

C. Özen; Y. Alhassid; H. Nakada

2015-01-22

376

Collective enhancement of nuclear state densities by the shell model Monte Carlo approach  

E-print Network

The shell model Monte Carlo (SMMC) approach allows for the microscopic calculation of statistical and collective properties of heavy nuclei using the framework of the configuration-interaction shell model in very large model spaces. We present recent applications of the SMMC method to the calculation of state densities and their collective enhancement factors in rare-earth nuclei.

Özen, C; Nakada, H

2015-01-01

377

Clustering Spatial Data in the Presence of Obstacles: a Density-Based Approach Osmar R. Zaiane  

E-print Network

[8, 2, 13], Probabilistic techniques [6], Graph theo- retic [31], Fuzzy methods [4, 23], Grid-basedClustering Spatial Data in the Presence of Obstacles: a Density-Based Approach Osmar R. Za during the clustering process is costly and the modeling of the constraints is paramount for good

Zaiane, Osmar R.

378

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

379

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

380

Eulerian Mapping Closure Approach for Probability Density Function of Concentration in Shear Flows  

NASA Technical Reports Server (NTRS)

The Eulerian mapping closure approach is developed for uncertainty propagation in computational fluid mechanics. The approach is used to study the Probability Density Function (PDF) for the concentration of species advected by a random shear flow. An analytical argument shows that fluctuation of the concentration field at one point in space is non-Gaussian and exhibits stretched exponential form. An Eulerian mapping approach provides an appropriate approximation to both convection and diffusion terms and leads to a closed mapping equation. The results obtained describe the evolution of the initial Gaussian field, which is in agreement with direct numerical simulations.

He, Guowei; Bushnell, Dennis M. (Technical Monitor)

2002-01-01

381

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

382

Density-dependent prophylaxis in crowded beet webworm, Loxostege sticticalis (Lepidoptera: Pyralidae) larvae to a parasitoid and a fungal pathogen  

Technology Transfer Automated Retrieval System (TEKTRAN)

Transmission of parasites and pathogens is generally positively density-dependent. Thus, as an insect population's density increases, the risk of an individual becoming attacked or infected increases. In some insect species, individuals experiencing crowded conditions are more resistant to natural e...

383

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

384

Experimental investigation on the temperature dependence of the nuclear level density parameter  

E-print Network

The effect of temperature (T) and angular momentum (J) on the inverse level density parameter (k) has been studied by populating the compound nucleus $^{97}$Tc in the reaction $^{4}$He + $^{93}$Nb at four incident beam energies of 28, 35, 42 and 50 MeV. For all the four energies, the value of k decreases with increasing J. The T dependence of k has been compared for two angular momentum windows with different theoretical predictions as well as with FTBCS1 calculation which takes into account the quasiparticle-number fluctuations in the pairing field. Interestingly, the experimental data are in good agreement with the theoretical calculations at higher J but deviate from all the calculations at lower J.

Balaram Dey; Deepak Pandit; Srijit Bhattacharya; K. Banerjee; N. Quang Hung; N. Dinh Dang; Debasish Mondal; S. Mukhopadhyay; Surajit Pal; A. De; S. R. Banerjee

2015-01-07

385

Isospin effects and the density dependence of the nuclear symmetry energy  

SciTech Connect

The density dependence of the nuclear symmetry energy is inspected using the statistical multifragmentation model with Skyrme effective interactions. The model consistently considers the expansion of the fragments' volumes at finite temperature at the freeze-out stage. By selecting parametrizations of the Skyrme force that lead to very different equations of state for the symmetry energy, we investigate the sensitivity of the isoscaling parameter and the isotopic distributions to differences in the symmetry energy. Our results suggest that, in spite of being sensitive to the thermal dilation of the fragments' volumes, it is difficult to distinguish among the Skyrme forces from the isoscaling analysis. On the other hand, the isotopic distribution of the emitted fragments turns out to be very sensitive to the force employed in the calculation.

Souza, S. R. [Instituto de Fisica, Universidade Federal do Rio de Janeiro, Cidade Universitaria, CP 68528, 21941-972, Rio de Janeiro (Brazil); Instituto de Fisica, Universidade Federal do Rio Grande do Sul, Av. Bento Goncalves 9500, CP 15051, 91501-970, Porto Alegre (Brazil); Tsang, M. B.; Lynch, W. G.; Steiner, A. W. [Joint Institute for Nuclear Astrophysics, National Superconducting Cyclotron Laboratory, and the Department of Physics and Astronomy, Michigan State University, East Lansing, Michigan 48824 (United States); Carlson, B. V. [Departamento de Fisica, Instituto Tecnologico de Aeronautica-CTA, 12228-900 Sao Jose dos Campos (Brazil); Donangelo, R. [Instituto de Fisica, Universidade Federal do Rio de Janeiro, Cidade Universitaria, CP 68528, 21941-972, Rio de Janeiro (Brazil); Instituto de Fisica, Facultad de Ingenieria, Universidad de la Republica, Julio Herrera y Reissig 565, 11.300 Montevideo (Uruguay)

2009-10-15

386

The initial density dependence of the viscosity of organic vapours: Cyclohexane and neopentane  

NASA Astrophysics Data System (ADS)

This paper presents new measurements on the viscosity of the vapours of cyclohexane and of neopentane from near room temperature to 630 K and for densities to 4.8 kg m -3 and 0.057 mol 1 -1 (cyclohexane) and to 5.0 kg m -3 and 0.069 mol 1 -1 (neopentane). Two all-quartz oscillating-disk viscometers with small gaps were used in the investigation. The data for cyclohexane and neopentane as well as earlier results for benzene show that the negative and positive values of the second viscosity virial coefficient B?, dependent on temperature represent the normal behaviour for all substances. The experimental data on the three hydrocarbons are suitable for the further development of the Rainwater-Friend theory concerning the transport properties of moderately dense gases in respect to their temperature range and their limitation to the Lennard-Jones 12-6 potential.

Vogel, E.; Holdt, B.; Strehlow, T.

1988-02-01

387

Temperature-dependent nonlinear phonon behavior in high-density carbon nanotube thin films  

NASA Astrophysics Data System (ADS)

We report the temperature-dependent Raman spectra for high-density single-walled carbon nanotube thin films. We show that the position of the main Raman mode (G) softens as the temperature increases and is nonlinear in the range of 70-270 K. This effect is explained by optical phonon decay. In the linear regime, the first-order temperature coefficient (?T) equals -0.02 cm-1/K, which is lower than for any other carbon nanotubes. Importantly, we found that local laser-induced temperature change shows a nonlinear trend as a function of global temperature with a minimum at 270 K. Our results contribute to understand the thermal properties of carbon nanotube thin films that could be applied, for example, in photovoltaic or thermoelectric devices.

Duzynska, A.; Judek, J.; Zdrojek, M.

2014-11-01

388

Experimental investigation on the temperature dependence of the nuclear level density parameter  

E-print Network

The effect of temperature (T) and angular momentum (J) on the inverse level density parameter (k) has been studied by populating the compound nucleus $^{97}$Tc in the reaction $^{4}$He + $^{93}$Nb at four incident beam energies of 28, 35, 42 and 50 MeV. For all the four energies, the value of k decreases with increasing J. The T dependence of k has been compared for two angular momentum windows with different theoretical predictions as well as with FTBCS1 calculation which takes into account the quasiparticle-number fluctuations in the pairing field. Interestingly, the experimental data are in good agreement with the theoretical calculations at higher J but deviate from all the calculations at lower J.

Dey, Balaram; Bhattacharya, Srijit; Banerjee, K; Hung, N Quang; Dang, N Dinh; Mondal, Debasish; Mukhopadhyay, S; Pal, Surajit; De, A; Banerjee, S R

2015-01-01

389

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

390

Investigation of a grid-free density functional theory (DFT) approach  

NASA Astrophysics Data System (ADS)

Density functional theory (DFT) has gained popularity, because it can frequently give accurate energies and geometries. Because evaluating DFT integrals fully analytically is usually impossible, most implementations use numerical quadrature over grid points, which can lead to numerical instabilities. To avoid these instabilities, the Almlöf-Zheng (AZ) grid-free approach was developed. This approach involves application of the resolution of the identity (RI) to evaluate the integrals. The focus of the current work is on the implementation of the AZ approach into the electronic structure code GAMESS, and on the convergence of the resolution of the identity with respect to basis set in the grid-free approach. Both single point energies and gradients are calculated for a variety of functionals and molecules. Conventional atomic basis sets are found to be inadequate for fitting the RI, particularly for gradient corrected functionals. Further work on developing auxiliary basis set approaches is warranted.

Glaesemann, Kurt R.; Gordon, Mark S.

1998-06-01

391

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

392

A relativistic time-dependent density functional study of the excited states of the mercury dimer  

NASA Astrophysics Data System (ADS)

In previous works on Zn2 and Cd2 dimers we found that the long-range corrected CAMB3LYP gives better results than other density functional approximations for the excited states, especially in the asymptotic region. In this paper, we use it to present a time-dependent density functional (TDDFT) study for the ground-state as well as the excited states corresponding to the (6s2 + 6s6p), (6s2 + 6s7s), and (6s2 + 6s7p) atomic asymptotes for the mercury dimer Hg2. We analyze its spectrum obtained from all-electron calculations performed with the relativistic Dirac-Coulomb and relativistic spinfree Hamiltonian as implemented in DIRAC-PACKAGE. A comparison with the literature is given as far as available. Our result is excellent for the most of the lower excited states and very encouraging for the higher excited states, it shows generally good agreements with experimental results and outperforms other theoretical results. This enables us to give a detailed analysis of the spectrum of the Hg2 including a comparative analysis with the lighter dimers of the group 12, Cd2, and Zn2, especially for the relativistic effects, the spin-orbit interaction, and the performance of CAMB3LYP and is enlightened for similar systems. The result shows, as expected, that spinfree Hamiltonian is less efficient than Dirac-Coulomb Hamiltonian for systems containing heavy elements such as Hg2.

Kullie, Ossama

2014-01-01

393

Performance of recent and high-performance approximate density functionals for time-dependent density functional theory calculations of valence and Rydberg electronic transition energies  

NASA Astrophysics Data System (ADS)

We report a test of 30 density functionals, including several recent ones, for their predictions of 69 singlet-to-singlet excitation energies of 11 molecules. The reference values are experimental results collected by Caricato et al. for 30 valence excitations and 39 Rydberg excitations. All calculations employ time-dependent density functional theory in the adiabatic, linear-response approximation. As far as reasonable, all of the assignments are performed by essentially the same protocol as used by Caricato et al., and this allows us to merge our mean unsigned errors (MUEs) with the ones they calculated for both density functional and wave function methods. We find 21 of the 30 density functionals calculated here have smaller MUEs for the 30 valence states than what they obtained (0.47 eV) for the state-of-the-art EOM-CCSD wave function. In contrast, for all of density functionals the MUE for 39 Rydberg states is larger than that (0.11 eV) of EOM-CCSD. Merging the 30 density functionals calculated here with the 26 calculated by Caricato et al. makes a set of 56 density functionals. Averaging the unsigned errors over both the valence excitations and the Rydberg excitations, none of the 56 density functionals shows a lower mean unsigned error than that (0.27 eV) of EOM-CCSD. Nevertheless, two functionals are successful in having an overall mean unsigned error of 0.30 eV, and another nine are moderately successful in having overall mean unsigned errors in the range 0.32-0.36 eV. Successful or moderately successful density functionals include seven hybrid density functionals with 41% to 54% Hartree-Fock exchange, and four range-separated hybrid density functionals in which the percentage of Hartree-Fock exchange increases from 0% to 19% at small interelectronic separation to 65%-100% at long range.

Isegawa, Miho; Peverati, Roberto; Truhlar, Donald G.

2012-12-01

394

Density-dependent regulation of brook trout population dynamics along a core-periphery distribution gradient in a central Appalachian watershed.  

PubMed

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 km(2) 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

395

Time-dependent Green's functions approach to nuclear reactions  

E-print Network

Nonequilibrium Green's functions represent underutilized means of studying the time evolution of quantum many-body systems. In view of a rising computer power, an effort is underway to apply the Green's functions formalism to the dynamics of central nuclear reactions. As the first step, mean-field evolution for the density matrix for colliding slabs is studied in one dimension. The strategy to extend the dynamics to correlations is described.

Arnau Rios; Pawel Danielewicz

2008-01-28

396

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

397

Age-specific, density-dependent and environment-based mortality of a short-lived perennial herb.  

PubMed

Density-independent and density-dependent processes affect plant mortality. Although less well understood, age-specific mortality can also play an important role in plant mortality. The goal of this study was to analyse several factors accounting for mortality in the Mediterranean short-lived perennial herb Lobularia maritima. We followed three cohorts of plants (from emergence to death) during 4 years in field conditions. We collected data on plant mortality of the effect of biotic agents (moth larvae and mycoplasma-like organisms, MLOs) and environmental variables. We also estimated density-dependent relationships affecting the fate of seedlings and adults. Results show that cohorts differed in their survival curves and ageing significantly increased mortality risk. Seedling mortality was density-dependent whereas adult mortality was not affected by density. MLO infection led to higher plant mortality whereas moth larvae attack did not affect plant mortality. In general, seedlings and adult plants experienced the highest mortality events in summer. We found, however, weak relationships between weather records and plant mortality. Age and size structures were not correlated. Overall, this study provides a comprehensive review of age-specific, density-dependent and density-independent factors that account for mortality of L. maritima plants throughout their life cycle in field conditions, highlighting the fact that age is an important factor in determining plant population dynamics. PMID:18426484

Picó, F X; Retana, J

2008-05-01

398

Rapid induction of immune density-dependent prophylaxis in adult social insects  

PubMed Central

The innate immune system provides defence against parasites and pathogens. This defence comes at a cost, suggesting that immune function should exhibit plasticity in response to variation in environmental threats. Density-dependent prophylaxis (DDP) has been demonstrated mostly in phase-polyphenic insects, where larval group size determines levels of immune function in either adults or later larval instars. Social insects exhibit extreme sociality, but DDP has been suggested to be absent from these ecologically dominant taxa. Here we show that adult bumble-bee workers (Bombus terrestris) exhibit rapid plasticity in their immune function in response to social context. These results suggest that DDP does not depend upon larval conditions, and is likely to be a widespread and labile response to rapidly changing conditions in adult insect populations. This has obvious ramifications for experimental analysis of immune function in insects, and serious implications for our understanding of the epidemiology and impact of pathogens and parasites in spatially structured adult insect populations. PMID:19656864

Ruiz-González, Mario X.; Moret, Yannick; Brown, Mark J. F.

2009-01-01

399

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

400

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

401

Density-dependent recruitment rates in great tits: the importance of being heavier  

PubMed Central

In birds, individuals with a higher mass at fledging have a higher probability of recruiting into the breeding population. This can be because mass is an indicator of general condition and thereby of the ability to survive adverse circumstances and/or because fledging mass is positively related to competitive strength in interactions with other fledglings. This latter explanation leads to two testable predictions: (i) there is stronger selection for fledging mass when there is more severe competition (i.e. at higher densities); and (ii) that besides absolute fledging mass, relative mass of fledglings within a cohort is important. We test these two predictions in two great tit (Parus major) populations. The first prediction was met for one of the populations, showing that competition affects the importance of mass-dependent recruitment. The second prediction, that fledglings recruit relatively well if they are heavy compared to the other fledglings, is met for both populations. The consequence of the importance of relative rather than absolute fledging mass is that the fitness consequences of reproductive decisions affecting fledging mass, such as clutch size, depend on the decisions of the other individuals in the population.

Both, C.; Visser, M. E.; Verboven, N.

1999-01-01

402

Synaptic State-Dependent Functional Interplay between Postsynaptic Density-95 and Synapse-Associated Protein 102  

PubMed Central

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

2013-01-01

403

Density-dependent lineage instability of MDA-MB-435 breast cancer cells  

PubMed Central

The use of cell lines in cancer research is strongly dependent on the avoidance of contaminations, the correct attribution of a cell line to the initial primary tumor and stability. Previous studies have identified expression of melanocytic molecular markers in the widely used breast cancer cell line, MDA-MB-435. In the present study the three breast cancer cell lines, MCF-7, MDA-MB-231 and MDA-MB-435, were systematically analyzed for mRNA and protein expression of major epithelial (cytokeratin isoforms), mammary (mammaglobin) and melanocytic (melan A and S100-protein) markers. Protein expression was identified by immunocytochemistry and quantitative RT-PCR was used to determine mRNA levels. While MCF-7 and MDA-MB-231 cells unambiguously revealed an epithelial/mammary phenotype, MDA-MB-435 cells were found to exhibit epithelial/mammary and melanocytic features dependent on cell density. Subconfluent cells demonstrated epithelial characteristics only, however, densely growing, confluent cells also expressed melanocytic markers. Consistent with gain of melanocytic features, the expression levels of mammaglobin mRNA decreased in these cells. These results indicate that the three cell lines are primarily of epithelial phenotype, however, MDA-MB-435 cells revealed lineage infidelity in dense cultures with a gain in melanocytic phenotype. These characteristics must be taken into consideration when analyzing cancer-relevant genes and their expression profiles in vitro. PMID:23599796

NERLICH, ANDREAS G.; BACHMEIER, BEATRICE E.

2013-01-01

404

A density-dependant finite element model for analysis of saltwater intrusion in coastal aquifers  

NASA Astrophysics Data System (ADS)

SummarySaltwater intrusion is a serious problem in coastal regions all over the world. It is one of the processes that degrade water-quality by raising salinity to levels exceeding acceptable drinking water standards. It may occur due to human activities and/or by natural events. Over-abstraction is considered the main cause of saltwater intrusion. Moreover, climate change and sea level rise speed up saltwater intrusion. This paper presents the development and validation of a coupled transient finite element model for simulation of fluid flow and solute transport in saturate and unsaturated soils with application to study saltwater intrusion in coastal aquifers. The model includes coupling of water flow, air flow, heat flow and solute transport. Furthermore, transient density-dependent flow is included in the model and the dependency of dispersion on velocity is considered. Different mechanisms that govern solute transport in porous media including, advection, diffusion, dispersion, adsorption, chemical reactions and biological degradation are included in the model. The governing equation of the solute transport is solved together with three balance equations for water flow, air flow and heat transfer. The nonlinear system of governing differential equations is solved using the finite element method in the space domain and a finite difference scheme in the time domain. The model is validated by application to a standard case study from the literature (Henry's problem) and then applied to predict saltwater intrusion in a coastal aquifer. The results of the model predictions are presented and discussed.

Abd-Elhamid, H. F.; Javadi, A. A.

2011-05-01

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

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

407

Three-dimensional density structure of the Earth: Limits to astrophysical and seismological approaches  

NASA Astrophysics Data System (ADS)

Large scale density structure of the Earth's interior is investigated. Two widely differing approaches to retrieving density structure are examined: through astrophysical and geophysical means. The theoretical framework of measuring the Earth's radial density profile from very high energy (TeV) neutrino attenuation measurements is presented. Celestial objects such as active galaxies and quasars are assumed to be sources of very high energy neutrinos, and theoretical fluxes are calculated. Using detector parameters, attenuation values of the very high neutrinos after they have traversed through the Earth are estimated. On the order of two decades' worth of measurements from the proposed neutrino sources would be required for a statistically significant observation of the Earth's core, given the current neutrino detector capabilities. Next, the more traditional approach of seismology is utilized. Earth normal mode theory is presented and the linearization of the seismogram relation to Earth structure is formulated, including corrections for mode coupling. Inversions of normal mode spectra directly for Earth structure are performed. Earth models, parameterized in terms of seismic velocity and density perturbations and topography on the core-mantle boundary, are presented. Comparisons of the seismic velocity models with those derived from body wave studies show good agreement. The core-mantle boundary topography pattern indicates correspondence with PcP and PKPab analyses of undulations at this boundary. The resolution for density perturbations is analyzed carefully in the last chapter. Contamination effects between seismic velocity model parameters and density model parameters are examined by using resolution matrices computed from data kernels, and by inverting synthetic spectra computed from realistic input Earth models. These resolution tests indicate that density structure retrieved from normal mode data are not reliable, to date.

Kuo, Chaincy

408

The dependency structure of coordinate phrases: a corpus approach.  

PubMed

Hudson (1990) proposes that each conjunct in a coordinate phrase forms dependency relations with heads or dependents outside the coordinate phrase (the "multi-head" view). This proposal is tested through corpus analysis of Wall Street Journal text. For right-branching constituents (such as direct-object NPs), a short-long preference for conjunct ordering is observed; this is predicted by the multi-head view, under the assumption that structures resulting in shorter dependencies are preferred. A short-long preference is also observed for left-branching constituents (such as subject NPs), which is less obviously accommodated by the multi-head view but not incompatible with it. The repetition of determiners was also examined (the dog and cat versus the dog and the cat), and a stronger preference was found for repetition with singular count nouns as opposed to mass or plural nouns; this accords well with the multi-head view, under the reasoning that single-determiner constructions require crossing dependencies with count nouns but not with plural or mass nouns. PMID:16341915

Temperley, David

2005-11-01

409

Reactivation-Dependent Amnesia in Pavlovian Approach and Instrumental Transfer  

ERIC Educational Resources Information Center

The theory of memory reconsolidation relates to the hypothesized restabilisation process that occurs following the reactivation of a memory through retrieval. Thus, the demonstration of reactivation-dependent amnesia for a previously acquired memory is a prerequisite for showing that such a memory undergoes reconsolidation. Here, we show that the…

Lee, Jonathan L. C.; Everitt, Barry J.

2008-01-01

410

Effect of measurement error on tests of density dependence of catchability for walleyes in northern Wisconsin angling and spearing fisheries  

USGS Publications Warehouse

We sought to determine how much measurement errors affected tests of density dependence of spearing and angling catchability for walleye Sander vitreus by quantifying relationships between spearing and angling catch rates (catch/h) and walleye population density (number/acre) in northern Wisconsin lakes. The mean measurement error of spearing catch rates was 43.5 times greater than the mean measurement error of adult walleye population densities, whereas the mean measurement error of angling catch rates was only 5.6 times greater than the mean measurement error of adult walleye population densities. The bias-corrected estimate of the relationship between spearing catch rate and adult walleye population density was similar to the ordinary-least-squares regression estimate but differed significantly from the geometric mean (GM) functional regression estimate. In contrast, the bias-corrected estimate of the relationship between angling catch rate and total walleye population density was intermediate between ordinary-least-squares and GM functional regression estimates. Catch rates of walleyes in both spearing and angling fisheries were not linearly related to walleye population density, which indicated that catch rates in both fisheries were hyperstable in relation to walleye population density. For both fisheries, GM functional regression overestimated the degree of hyperdepletion in catch rates and ordinary-least-squares regression overestimated the degree of hyperstability in catch rates. However, ordinary-least-squares regression induced significantly less bias in tests of density dependence than GM functional regression, so it may be suitable for testing the degree of density dependence in fisheries for which fish population density is estimated with mark-recapture methods similar to those used in our study. ?? Copyright by the American Fisheries Society 2005.

Hansen, M.J.; Beard, T.D., Jr.; Hewett, S.W.

2005-01-01

411

A new approach for analyzing bird densities from variable circular-plot counts  

USGS Publications Warehouse

An approach for calculating bird densities from variable circular-plot counts is described. The approach differs from previous methods in that data from several surveys are pooled and detection distances are adjusted as if all distances were recorded by a single observer under a given set of field conditions. Adjustments for covariates that affect detection distances such as observer, weather, time of day, and vegetation type are made using coefficients calculated by multiple linear regression. The effective area surveyed under standard conditions is calculated from the pooled data set and then used to determine the effective area surveyed at each sampling station under the actual conditions when the station was sampled. The method was validated in two field studies where the density of birds could be determined by independent methods. Computer software for entering and analyzing data by this method is described.

Fancy, S.G.

1997-01-01

412

A relativistic time-dependent density functional study of the excited states of the mercury dimer  

SciTech Connect

In previous works on Zn{sub 2} and Cd{sub 2} dimers we found that the long-range corrected CAMB3LYP gives better results than other density functional approximations for the excited states, especially in the asymptotic region. In this paper, we use it to present a time-dependent density functional (TDDFT) study for the ground-state as well as the excited states corresponding to the (6s{sup 2} + 6s6p), (6s{sup 2} + 6s7s), and (6s{sup 2} + 6s7p) atomic asymptotes for the mercury dimer Hg{sub 2}. We analyze its spectrum obtained from all-electron calculations performed with the relativistic Dirac-Coulomb and relativistic spinfree Hamiltonian as implemented in DIRAC-PACKAGE. A comparison with the literature is given as far as available. Our result is excellent for the most of the lower excited states and very encouraging for the higher excited states, it shows generally good agreements with experimental results and outperforms other theoretical results. This enables us to give a detailed analysis of the spectrum of the Hg{sub 2} including a comparative analysis with the lighter dimers of the group 12, Cd{sub 2}, and Zn{sub 2}, especially for the relativistic effects, the spin-orbit interaction, and the performance of CAMB3LYP and is enlightened for similar systems. The result shows, as expected, that spinfree Hamiltonian is less efficient than Dirac-Coulomb Hamiltonian for systems containing heavy elements such as Hg{sub 2}.

Kullie, Ossama, E-mail: kullie@uni-kassel.de, E-mail: ossama.kullie@unistra.fr [Institute de Chimie de Strasbourg, CNRS et Université de Strasbourg, Laboratoire de Chimie Quantique, 4 rue Blaise Pascal, 67070 Strasbourg, France and Theoretical Physics, Institute for Physics, Department of Mathematics and Natural Science, University of Kassel, D-34127 Kassel (Germany)] [Institute de Chimie de Strasbourg, CNRS et Université de Strasbourg, Laboratoire de Chimie Quantique, 4 rue Blaise Pascal, 67070 Strasbourg, France and Theoretical Physics, Institute for Physics, Department of Mathematics and Natural Science, University of Kassel, D-34127 Kassel (Germany)

2014-01-14

413

Applications of Time-Dependent and Time-Independent Density Functional Theory to Rydberg Transitions.  

PubMed

We have benchmarked the performance of time-independent density functional theory (?SCF and RSCF-CV-DFT) in studies on Rydberg transitions employing five different standard functionals and a diffuse basis. Our survey is based on 71 triplet or singlet Rydberg transitions distributed over nine different species: CO (7), CH2O (8), C2H2 (8), H2O (10), C2H4 (13), Be (6), Mg (6), and Zn (8). The best performance comes from the long-range corrected functional LCBP86 (? = 0.4.) with an average root-mean-square deviation (RMSD) of 0.23 eV. Of similar accuracy are LDA and B3LYP, both with a RMSD of 0.24 eV. The largest RMSD of 0.32 eV comes from BP86 and LCBP86* (? = 0.75). The performance of ?SCF is considerably better than that of adiabatic time-dependent density functional theory (ATDDFT) and matches that of highly optimized long-range corrected functionals. However, it is not as accurate as ATDDFT based on highly tuned functionals. The reasonable success of ?SCF is based on its well-documented ability to afford good estimates of ionization potentials (IP) and electron affinities (EA) even for simple local functionals after orbital relaxation has been taken into account. In ATDDFT based on semilocal functionals, both IP and -EA are poorly described, with errors of up to 5 eV. In the transition energy (?E = IP - EA), these errors are canceled to some degree. However, ?E still carries an error exceeding 1 eV. PMID:25322281

Seidu, Issaka; Krykunov, Mykhaylo; Ziegler, Tom

2014-10-27

414

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

415

The structure of mixed 3He-4He droplets doped with OCS: A density functional approach  

NASA Astrophysics Data System (ADS)

We have investigated the structure and energetics of mixed 3He-4He droplets doped with a carbonyl sulfide molecule within a density functional approach considering a small but finite temperature of 0.1 K. The molecule is treated as an external field to which the helium droplet is attached. The energetics and appearance of these droplets are discussed for selected numbers of helium atoms, identifying the first magic numbers of the fermionic component.

Leal, Antonio; Mateo, David; Pi, Martí; Barranco, Manuel; Navarro, Jesús

2013-11-01

416

The structure of mixed 3He-4He droplets doped with OCS: a density functional approach.  

PubMed

We have investigated the structure and energetics of mixed (3)He-(4)He droplets doped with a carbonyl sulfide molecule within a density functional approach considering a small but finite temperature of 0.1 K. The molecule is treated as an external field to which the helium droplet is attached. The energetics and appearance of these droplets are discussed for selected numbers of helium atoms, identifying the first magic numbers of the fermionic component. PMID:24206299

Leal, Antonio; Mateo, David; Pi, Martí; Barranco, Manuel; Navarro, Jesús

2013-11-01

417

A Conductivity Approach to Measuring Fixed Charge Density in Intervertebral Disc Tissue  

Microsoft Academic Search

A new method for measuring the fixed charge density (FCD) in intervertebral disc (IVD) tissues employing a two-point electrical\\u000a conductivity approach was developed. In this technique, the tissue is first confined and equilibrated in a potassium chloride\\u000a (KCl) solution, and the tissue conductivity is then measured. This is then repeated with a second concentration of KCl solution.\\u000a The FCD can

Alicia R. Jackson; Tai-Yi Yuan; Chun-Yuh Huang; Wei Yong Gu

2009-01-01

418

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

419

A New Robust Approach for Highway Traffic Density Estimation Fabio Morbidi, Luis Leon Ojeda, Carlos Canudas de Wit, Iker Bellicot  

E-print Network

A New Robust Approach for Highway Traffic Density Estimation Fabio Morbidi, Luis Le´on Ojeda for the uncertain graph-constrained Switching Mode Model (SMM), which we use to describe the highway traffic density density reconstruction via a switching observer, in an instrumented 2.2 km highway section of Grenoble

Paris-Sud XI, Université de

420

State-dependent approach to entropic measurement-disturbance relations  

NASA Astrophysics Data System (ADS)

Heisenberg's intuition was that there should be a trade-off between measuring a particle's position with greater precision and disturbing its momentum. Recent formulations of this idea have focused on the question of how well two complementary observables can be jointly measured. Here, we provide an alternative approach based on how enhancing the predictability of one observable necessarily disturbs a complementary one. Our measurement-disturbance relation refers to a clear operational scenario and is expressed by entropic quantities with clear statistical meaning. We show that our relation is perfectly tight for all measurement strengths in an existing experimental setup involving qubit measurements.

Coles, Patrick J.; Furrer, Fabian

2015-01-01

421

Two-electron Rabi oscillations in real-time time-dependent density-functional theory  

NASA Astrophysics Data System (ADS)

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.

Habenicht, Bradley F.; Tani, Noriyuki P.; Provorse, Makenzie R.; Isborn, Christine M.

2014-11-01

422

DFPT approach to the temperature dependence of electronic band energies  

NASA Astrophysics Data System (ADS)

The energy bands of semiconductors exhibit significant shifts and broadening with temperature at constant volume. This is an effect of the direct renormalization of band energies due to electron-phonon interactions. In search of an efficient linear response DFT approach to this effect, beyond semi-empirical approximation or frozen- phonon DFT, we have implemented formulas derived by Allen and Heine [J. Phys. C 9, 2305 (1976)] inside the ABINIT package. We have found that such formulas need a great number of bands, O(1000), to properly converge the thermal corrections of deep potential well atoms, i.e. elements of the first row. This leads to heavy computational costs even for simple systems like diamond. The DFPT formalism can be used to circumvent entirely the need for conduction bands by computing the first-order wave-functions using the self-consistent Sternheimer equation. We will compare the results of both formalism demonstrating that the DFPT approach reproduces the correct converged results of the formulas of Allen and Heine.

Boulanger, Paul; Cote, Michel; Gonze, Xavier

2010-03-01

423

White noise approach to the low density limit of a quantum particle in a gas  

E-print Network

The white noise approach to the investigation of the dynamics of a quantum particle interacting with a dilute and in general non-equilibrium gaseous environment in the low density limit is outlined. The low density limit is the kinetic Markovian regime when only pair collisions (i.e., collisions of the test particle with one particle of the gas at one time moment) contribute to the dynamics. In the white noise approach one first proves that the appropriate operators describing the gas converge in the sense of appropriate matrix elements to certain operators of quantum white noise. Then these white noise operators are used to derive quantum white noise and quantum stochastic equations describing the approximate dynamics of the total system consisting of the particle and the gas. The derivation is given ab initio, starting from the exact microscopic quantum dynamics. The limiting dynamics is described by a quantum stochastic equation driven by a quantum Poisson process. This equation then applied to the derivation of quantum Langevin equation and linear Boltzmann equation for the reduced density matrix of the test particle. The first part of the paper describes the approach which was developed by L. Accardi, I.V. Volovich and the author and uses the Fock-antiFock (or GNS) representation for the CCR algebra of the gas. The second part presents the approach to the derivation of the limiting equations directly in terms of the correlation functions, without use of the Fock-antiFock representation. This approach simplifies the derivation and allows to express the strength of the quantum number process directly in terms of the one-particle $S$-matrix.

Alexander Pechen

2006-07-19

424

Electron dynamics in complex environments with real-time time dependent density functional theory in a QM-MM framework  

NASA Astrophysics Data System (ADS)

This article presents a time dependent density functional theory (TDDFT) implementation to propagate the Kohn-Sham equations in real time, including the effects of a molecular environment through a Quantum-Mechanics Molecular-Mechanics (QM-MM) hamiltonian. The code delivers an all-electron description employing Gaussian basis functions, and incorporates the Amber force-field in the QM-MM treatment. The most expensive parts of the computation, comprising the commutators between the hamiltonian and the density matrix—required to propagate the electron dynamics—, and the evaluation of the exchange-correlation energy, were migrated to the CUDA platform to run on graphics processing units, which remarkably accelerates the performance of the code. The method was validated by reproducing linear-response TDDFT results for the absorption spectra of several molecular species. Two different schemes were tested to propagate the quantum dynamics: (i) a leap-frog Verlet algorithm, and (ii) the Magnus expansion to first-order. These two approaches were confronted, to find that the Magnus scheme is more efficient by a factor of six in small molecules. Interestingly, the presence of iron was found to seriously limitate the length of the integration time step, due to the high frequencies associated with the core-electrons. This highlights the importance of pseudopotentials to alleviate the cost of the propagation of the inner states when heavy nuclei are present. Finally, the methodology was applied to investigate the shifts induced by the chemical environment on the most intense UV absorption bands of two model systems of general relevance: the formamide molecule in water solution, and the carboxy-heme group in Flavohemoglobin. In both cases, shifts of several nanometers are observed, consistently with the available experimental data.

Morzan, Uriel N.; Ramírez, Francisco F.; Oviedo, M. Belén; Sánchez, Cristián G.; Scherlis, Damián A.; Lebrero, Mariano C. González

2014-04-01

425

Electron dynamics in complex environments with real-time time dependent density functional theory in a QM-MM framework  

SciTech Connect

This article presents a time dependent density functional theory (TDDFT) implementation to propagate the Kohn-Sham equations in real time, including the effects of a molecular environment through a Quantum-Mechanics Molecular-Mechanics (QM-MM) hamiltonian. The code delivers an all-electron description employing Gaussian basis functions, and incorporates the Amber force-field in the QM-MM treatment. The most expensive parts of the computation, comprising the commutators between the hamiltonian and the density matrix—required to propagate the electron dynamics—, and the evaluation of the exchange-correlation energy, were migrated to the CUDA platform to run on graphics processing units, which remarkably accelerates the performance of the code. The method was validated by reproducing linear-response TDDFT results for the absorption spectra of several molecular species. Two different schemes were tested to propagate the quantum dynamics: (i) a leap-frog Verlet algorithm, and (ii) the Magnus expansion to first-order. These two approaches were confronted, to find that the Magnus scheme is more efficient by a factor of six in small molecules. Interestingly, the presence of iron was found to seriously limitate the length of the integration time step, due to the high frequencies associated with the core-electrons. This highlights the importance of pseudopotentials to alleviate the cost of the propagation of the inner states when heavy nuclei are present. Finally, the methodology was applied to investigate the shifts induced by the chemical environment on the most intense UV absorption bands of two model systems of general relevance: the formamide molecule in water solution, and the carboxy-heme group in Flavohemoglobin. In both cases, shifts of several nanometers are observed, consistently with the available experimental data.

Morzan, Uriel N.; Ramírez, Francisco F.; Scherlis, Damián A., E-mail: damian@qi.fcen.uba.ar, E-mail: mcgl@qb.ffyb.uba.ar [Departamento de Química Inorgánica, Analítica y Química Física/INQUIMAE, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Ciudad Universitaria, Pab. II, Buenos Aires (C1428EHA) (Argentina); Oviedo, M. Belén; Sánchez, Cristián G. [Departamento de Matemática y Física, Facultad de Ciencias Químicas, INFIQC, Universidad Nacional de Córdoba, Ciudad Universitaria, X5000HUA Córdoba (Argentina)] [Departamento de Matemática y Física, Facultad de Ciencias Químicas, INFIQC, Universidad Nacional de Córdoba, Ciudad Universitaria, X5000HUA Córdoba (Argentina); Lebrero, Mariano C. González, E-mail: damian@qi.fcen.uba.ar, E-mail: mcgl@qb.ffyb.uba.ar [Instituto de Química y Fisicoquímica Biológicas, IQUIFIB, CONICET (Argentina)] [Instituto de Química y Fisicoquímica Biológicas, IQUIFIB, CONICET (Argentina)

2014-04-28

426

A blind deconvolution approach for improving the resolution of cryo-EM density maps.  

PubMed

Cryo-electron microscopy (cryo-EM) plays an increasingly prominent role in structure elucidation of macromolecular assemblies. Advances in experimental instrumentation and computational power have spawned numerous cryo-EM studies of large biomolecular complexes resulting in the reconstruction of three-dimensional density maps at intermediate and low resolution. In this resolution range, identification and interpretation of structural elements and modeling of biomolecular structure with atomic detail becomes problematic. In this article, we present a novel algorithm that enhances the resolution of intermediate- and low-resolution density maps. Our underlying assumption is to model the low-resolution density map as a blurred and possibly noise-corrupted version of an unknown high-resolution map that we seek to recover by deconvolution. By exploiting the nonnegativity of both the high-resolution map and blur kernel, we derive multiplicative updates reminiscent of those used in nonnegative matrix factorization. Our framework allows for easy incorporation of additional prior knowledge such as smoothness and sparseness, on both the sharpened density map and the blur kernel. A probabilistic formulation enables us to derive updates for the hyperparameters; therefore, our approach has no parameter that needs adjustment. We apply the algorithm to simulated three-dimensional electron microscopic data. We show that our method provides better resolved density maps when compared with B-factor sharpening, especially in the presence of noise. Moreover, our method can use additional information provided by homologous structures, which helps to improve the resolution even further. PMID:21385038

Hirsch, Michael; Schölkopf, Bernhard; Habeck, Michael

2011-03-01

427

Eigenvalue density of linear stochastic dynamical systems: A random matrix approach  

NASA Astrophysics Data System (ADS)

Eigenvalue problems play an important role in the dynamic analysis of engineering systems modeled using the theory of linear structural mechanics. When uncertainties are considered, the eigenvalue problem becomes a random eigenvalue problem. In this paper the density of the eigenvalues of a discretized continuous system with uncertainty is discussed by considering the model where the system matrices are the Wishart random matrices. An analytical expression involving the Stieltjes transform is derived for the density of the eigenvalues when the dimension of the corresponding random matrix becomes asymptotically large. The mean matrices and the dispersion parameters associated with the mass and stiffness matrices are necessary to obtain the density of the eigenvalues in the frameworks of the proposed approach. The applicability of a simple eigenvalue density function, known as the Mar?enko-Pastur (MP) density, is investigated. The analytical results are demonstrated by numerical examples involving a plate and the tail boom of a helicopter with uncertain properties. The new results are validated using an experiment on a vibrating plate with randomly attached spring-mass oscillators where 100 nominally identical samples are physically created and individually tested within a laboratory framework.

Adhikari, S.; Pastur, L.; Lytova, A.; Du Bois, J.

2012-02-01

428

Density-dependent recruitment after winter disturbance on tidal flats by the lugworm Arenicola marina  

NASA Astrophysics Data System (ADS)

The polychaete Arenicola marina is abundant and widespread on intertidal sand flats in Königshafen (island of Sylt, North Sea). Juveniles overwinter in subtidal channels and then colonize the upper tidal zone above the range of the adults. In the summers of 1995 and 1997, after a mild and a moderate winter, a distinct nursery belt fringing the shoreline was apparent. The severe winter of 1995/96 changed this pattern. Population size of the adults was halved, and in summer 1996 the juveniles were no longer restricted to an upper intertidal belt and settled over a wide range of tidal flats where adults were diminished. This spatial expansion of recruits is assumed to be a density-dependent response. An overcompensation by one-third of the previous population size did not carry over into the next year. Compared to some other species of the tidal-flat fauna, the disturbance effect by the severe winter on the lugworm population was small and brief.

Reise, K.; Simon, M.; Herre, E.

2001-07-01

429

Negative Density Dependence Regulates Two Tree Species at Later Life Stage in a Temperate Forest  

PubMed Central

Numerous studies have demonstrated that tree survival is influenced by negative density dependence (NDD) and differences among species in shade tolerance could enhance coexistence via resource partitioning, but it is still unclear how NDD affects tree species with different shade-tolerance guilds at later life stages. In this study, we analyzed the spatial patterns for trees with dbh (diameter at breast height) ?2 cm using the pair-correlation g(r) function to test for NDD in a temperate forest in South Korea after removing the effects of habitat heterogeneity. The analyses were implemented for the most abundant shade-tolerant (Chamaecyparis obtusa) and shade-intolerant (Quercus serrata) species. We found NDD existed for both species at later life stages. We also found Quercus serrata experienced greater NDD compared with Chamaecyparis obtusa. This study indicates that NDD regulates the two abundant tree species at later life stages and it is important to consider variation in species' shade tolerance in NDD study. PMID:25058660

Piao, Tiefeng; Chun, Jung Hwa; Yang, Hee Moon; Cheon, Kwangil

2014-01-01

430

Depth Dependency of Neutron Density Produced by Galactic Cosmic Rays in the Lunar Subsurface  

NASA Astrophysics Data System (ADS)

Neutron production by galactic cosmic rays (GCR) in the lunar subsurface plays an important role in the radiation environment on the surface and subsurface of the Moon and to make its accurate estimation is therefore very important for lunar and planetary nuclear spectroscopy and space dosimetry. In this work, the depth dependency of neutron production from protons and alpha particles in galactic cosmic ray (GCR) was estimated using the three-dimensional particle and heavy ion transport simulation code PHITS incorporating the latest high energy nuclear data, JENDL-HE. Our estimation of equilibrium neutron density profiles in the lunar subsurface was compared with the experimental data obtained by Apollo 17 Lunar Neutron Probe Experiment (LNPE). In our former study, it was shown that the proton-induced neutron production cross sections had the largest influence on the accuracy of the neutron production estimation. Therefore, we made an extensive comparison of proton-nucleus reaction models incorporated in PHITS using some experimental data and selected the best combination of JENDL-HE at the energy < 3 GeV and JAM model above it. As the results, our calculation successfully reproduced the LNPE data within experimental errors of 15% (measurement) + 30% (systematic).

Ota, Shuya

2012-07-01

431

Optimal floating and queuing strategies: consequences for density dependence and habitat loss.  

PubMed

Field studies of many vertebrates show that some individuals (floaters) do not defend territories even when there is space for them to do so. We show that the evolutionarily stable strategy (ESS) for the threshold territory quality at which floating takes place is that which maximizes the size of the floating population (but not the total population, breeding population, or reproductive output). The ESS is solved separately for two assumptions: whether individuals wait to occupy a single territory or multiple territories and whether queuing rules are strict or if all waiting individuals are equally likely to obtain the next territory. The four combinations of these assumptions all give the same evolutionarily stable population size of both floaters and breeders. At the ESS, only territories with expected lifetime reproductive success (LRS) exceeding 1 should be occupied, which introduces a limit to ideal habitat selection. The behavioral decision to float alters the shape of the density-dependent response, reduces the equilibrium population size, and affects the response of the population to habitat loss. Specifically, the floater: breeder ratio is directly related to average breeding habitat quality, and the floater population size will decrease more than the breeding population size if better than average quality habitat is lost. PMID:18811444

Kokko, H; Sutherland, W J

1998-09-01

432

Resonance Raman Scattering of Rhodamine 6G as Calculated Using 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. In this work, we present the first calculation of the resonance Raman scattering (RRS) spectrum of rhodamine 6G (R6G) which is a prototype molecule in surface-enhanced Raman scattering (SERS). The calculation is done using a recently developed time-dependent density functional theory (TDDFT) method, which uses a short-t