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1

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

E-print Network

We present a time-dependent localized Hartree-Fock density-functional linear response approach for the treatment of photoionization of atomic systems. This approach employs a spin-dependent localized Hartree-Fock exchange ...

Zhou, Zhongyuan; Chu, Shih-I

2009-05-13

2

Lagrangian approach to molecular vibrational Raman intensities using time-dependent hybrid density functional theory.  

PubMed

The authors propose a new route to vibrational Raman intensities based on analytical derivatives of a fully variational polarizability Lagrangian. The Lagrangian is constructed to recover the negative frequency-dependent polarizability of time-dependent Hartree-Fock or adiabatic (hybrid) density functional theory at its stationary point. By virtue of the variational principle, first-order polarizability derivatives can be computed without using derivative molecular orbital coefficients. As a result, the intensities of all Raman-active modes within the double harmonic approximation are obtained at approximately the same cost as the frequency-dependent polarizability itself. This corresponds to a reduction of the scaling of computational expense by one power of the system size compared to a force constant calculation and to previous implementations. Since the Raman intensity calculation is independent of the harmonic force constant calculation more, computationally demanding density functionals or basis sets may be used to compute the polarizability gradient without much affecting the total time required to compute a Raman spectrum. As illustrated for fullerene C60, the present approach considerably extends the domain of molecular vibrational Raman calculations at the (hybrid) density functional level. The accuracy of absolute and relative Raman intensities of benzene obtained using the PBE0 hybrid functional is assessed by comparison with experiment. PMID:17552747

Rappoport, Dmitrij; Furche, Filipp

2007-05-28

3

Size dependence of yield strength simulated by a dislocation-density function dynamics approach  

NASA Astrophysics Data System (ADS)

The size dependence of the strength of nano- and micron-sized crystals is studied using a new simulation approach in which the dynamics of the density functions of dislocations are modeled. Since any quantity of dislocations can be represented by a density, this approach can handle large systems containing large quantities of dislocations, which may handicap discrete dislocation dynamics schemes due to the excessive computation time involved. For this reason, pillar sizes spanning a large range, from the sub-micron to micron regimes, can be simulated. The simulation results reveal the power-law relationship between strength and specimen size up to a certain size, beyond which the strength varies much more slowly with size. For specimens smaller than ?4000b, their strength is found to be controlled by the dislocation depletion condition, in which the total dislocation density remains almost constant throughout the loading process. In specimens larger than ?4000b, the initial dislocation distribution is of critical importance since the presence of dislocation entanglements is found to obstruct deformation in the neighboring regions within a distance of ?2000b. This length scale suggests that the effects of dense dislocation clusters are greater in intermediate-sized specimens (e.g. 4000b and 8000b) than in larger specimens (e.g. 16?000b), according to the weakest-link concept.

Leung, P. S. S.; Leung, H. S.; Cheng, B.; Ngan, A. H. W.

2015-04-01

4

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

5

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, ...

Chu, Shih-I; Zhou, Zhongyuan

2009-10-27

6

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

7

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

8

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

E-print Network

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.

Arianna Carbone; Arnau Rios; Artur Polls

2014-11-19

9

Intermolecular forces and nonbonded interactions: Superoperator nonlinear time-dependent density-functional-theory response approach  

E-print Network

and dispersive interactions of polarizable molecules are expressed in terms of generalized (nonretarded) charge originating from correlated charge density fluctuations [1­6] control many important chemical and biological on accurate predictions of intermolecular forces. The most widely studied and best un- derstood interactions

Mukamel, Shaul

10

Ion Collisions with Water Molecules: A Time-Dependent Density Functional Theory Approach  

NASA Astrophysics Data System (ADS)

Collisions of simple ions from water molecules in the energy range of 10-5000 keV/amu are considered within an independent electron model. The basis generator method applied in the past successfully to ion-atom collisions is adapted to deal with molecular targets. Cross sections for single- and multiple-electron processes (capture and transfer to the continuum) are obtained directly from solving time-dependent Kohn-Sham-type orbital equations and using a Slater determinant based analysis. Fragmentation yields are predicted on the basis of a semi-phenomenological model which uses the calculated cross sections as input. Comparison with experiment is made for proton and He+He+ impact collisions.

Kirchner, Tom; Murakami, Mitsuko; Horbatscha, Marko; Lüddeb, Hans Jürgen

2013-06-01

11

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

NASA Astrophysics Data System (ADS)

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.

Zhang, Xing; Herbert, John M.

2015-02-01

12

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

13

PERSPECTIVES Estimating delayed density-dependent mortality  

E-print Network

PERSPECTIVES Estimating delayed density-dependent mortality in sockeye salmon (Oncorhynchus nerka in many populations of sockeye salmon (Oncorhynchus nerka). We used a meta-analytical approach to test dans de nombreuses populations de saumon rouge (Oncorhynchus nerka). Les auteurs ont utilisé une

Myers, Ransom A.

14

Density-dependent covariant energy density functionals  

SciTech Connect

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

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

2012-10-20

15

A density-dependent stand-level optimization approach for deriving management prescriptions for interior northwest (USA) landscapes  

Microsoft Academic Search

Stand-level forest management prescriptions for federal forests in the interior northwest (USA) have changed emphasis in the past decade from being influenced by economic criteria to being influenced by ecological criteria. The design of forest management prescriptions that maintain stand density levels within a target range is now preferred over the design of prescriptions that maximize net present value. We

Pete Bettinger; David Graetz; John Sessions

2005-01-01

16

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 ...

Chu, Shih-I; Telnov, Dmitry A.

2009-04-03

17

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

18

Interspecific Competition Intraspecific competition = density dependence  

E-print Network

Biol 303 1 Interspecific Competition Outline Intraspecific competition = density dependence Intraspecific and interspecific competition Limiting resources Interference vs exploitation Effects lecture showed that intraspecific competition can decrease survival and reproduction as a population

Creel, Scott

19

Wildlife disease elimination and density dependence.  

PubMed

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

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

2012-08-22

20

Central Density Dependent Anisotropic Compact Stars  

E-print Network

Stars can be treated as self-gravitating fluid. In this connection, we propose a model for an anisotropic star under the relativistic framework of Krori-Barua (1975) spacetime. It is shown that the solutions are regular and singularity free. The uniqueness of the model is that interior physical properties of the star solely depend on the central density of the matter distribution.

Mehedi Kalam; Farook Rahaman; Sk. Monowar Hossein; Saibal Ray

2012-12-27

21

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

22

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

23

Effective pairing interactions with isospin density dependence  

SciTech Connect

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

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

2008-05-15

24

Patterns of density dependence in measles dynamics.  

PubMed Central

An important question in metapopulation dynamics is the influence of external perturbations on the population's long-term dynamic behaviour. In this paper we address the question of how spatiotemporal variations in demographic parameters affect the dynamics of measles populations in England and Wales. Specifically, we use nonparametric statistical methods to analyse how birth rate and population size modulate the negative density dependence between successive epidemics as well as their periodicity. For the observed spatiotemporal data from 60 cities, and for simulated model data, the demographic variables act as bifurcation parameters on the joint density of the trade-off between successive epidemics. For increasing population size, a transition occurs from an irregular unpredictable pattern in small communities towards a regular, predictable endemic pattern in large places. Variations in the birth rate parameter lead to a bifurcation from annual towards biennial cyclicity in both observed data and model data. PMID:9628034

Finkenstädt, B; Keeling, M; Grenfell, B

1998-01-01

25

CHAMP Density Dependence on Geomagnetic Indices  

NASA Astrophysics Data System (ADS)

The STAR accelerometer on the CHAMP satellite has made it possible to accumulate near-continuous records of thermosphere density at approximately 400 km altitude since May 2001. The response of the thermosphere under extremely quiet geomagnetic conditions, as well as for virtually every significant geomagnetic storm and all levels of activity in between, has been recorded during this period. The solar activity has decreased since the beginning of the mission, when the solar cycle was at its peak, to minimum conditions or nearly so at the end of 2006. CHAMP is in a near-polar and quasi-circular orbit, and complete local time sampling is obtained about every 4 months. Therefore, this density dataset offers unique opportunities to study the variability of the thermosphere due to geomagnetic disturbances in relation to solar activity, season, solar local time, and latitude. In the present study, the complete CHAMP density database is analyzed in terms of orbit-to-orbit variability (global response of the thermosphere) as a function of geomagnetic indices, such as ap and sectorial am (index given for 9 latitude-longitude sectors). Density residuals, obtained by de-trending the data using moving averaging windows, are computed to that purpose. The variability is characterized by the large-scale (600 - 5600 km) and medium-scale (160 - 600 km) disturbances, which are isolated by forming the residuals. Secondly, latitude- dependent (i.e., local response) variability is evaluated by binning the densities in pertinent latitude bands (e.g., equatorial, sub-auroral, auroral).

Bruinsma, S. L.; Forbes, J. M.

2007-12-01

26

Evolution of density- and patch-size-dependent dispersal rates.  

PubMed Central

Based on a marginal value approach, we derive a nonlinear expression for evolutionarily stable (ES) dispersal rates in a metapopulation with global dispersal. For the general case of density-dependent population growth, our analysis shows that individual dispersal rates should decrease with patch capacity and-beyond a certain threshold-increase with population density. We performed a number of spatially explicit, individual-based simulation experiments to test these predictions and to explore further the relevance of variation in the rate of population increase, density dependence, environmental fluctuations and dispersal mortality on the evolution of dispersal rates. They confirm the predictions of our analytical approach. In addition, they show that dispersal rates in metapopulations mostly depend on dispersal mortality and inter-patch variation in population density. The latter is dominantly driven by environmental fluctuations and the rate of population increase. These conclusions are not altered by the introduction of neighbourhood dispersal. With patch capacities in the order of 100 individuals, kin competition seems to be of negligible importance for ES dispersal rates except when overall dispersal rates are low. PMID:11916481

Poethke, Hans Joachim; Hovestadt, Thomas

2002-01-01

27

Electron correlation of one-dimensional H{sub 2} in intense laser fields: Time-dependent extended Hartree-Fock and time-dependent density-functional-theory approaches  

SciTech Connect

Ionization and high-order harmonic generation of the one-dimensional (1D) H{sub 2} molecule in intense ultrashort laser fields are investigated using several current approximations for electron dynamics. Single- and double-ionization probabilities are compared with exact results. It is found that for the ground state X {sup 1}{sigma}{sub g}{sup +}, time-dependent extended Hartree-Fock gives generally comparable results except in the plateau region. The adiabatic local density approximation and time-dependent optimized effective potential with self-interaction correction (TDKLI) methods underestimate the ionization probabilities with no plateau and knee for double ionization contrary to the exact results. For the triplet excited state A {sup 3}{sigma}{sub u}{sup +}, where exchange is important, the TDKLI results agree well with the exact results. The exact double-ionization probabilities suggest the need for accurate pair-correlation functions.

Nguyen, Nam A.; Bandrauk, Andre D. [Laboratoire de Chimie Theorique, Faculte des Sciences, Universite de Sherbrooke, QC J1K 2R1 (Canada)

2006-03-15

28

Selective fishing induces density-dependent growth.  

PubMed

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

Svedäng, Henrik; Hornborg, Sara

2014-01-01

29

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

30

Stochasticity and determinism: how density-independent and density-dependent processes affect population variability.  

PubMed

A persistent debate in population ecology concerns the relative importance of environmental stochasticity and density dependence in determining variability in adult year-class strength, which contributes to future reproduction as well as potential yield in exploited populations. Apart from the strength of the processes, the timing of density regulation may affect how stochastic variation, for instance through climate, translates into changes in adult abundance. In this study, we develop a life-cycle model for the population dynamics of a large marine fish population, Northeast Arctic cod, to disentangle the effects of density-independent and density-dependent processes on early life-stages, and to quantify the strength of compensatory density dependence in the population. The model incorporates information from scientific surveys and commercial harvest, and dynamically links multiple effects of intrinsic and extrinsic factors on all life-stages, from eggs to spawners. Using a state-space approach we account for observation error and stochasticity in the population dynamics. Our findings highlight the importance of density-dependent survival in juveniles, indicating that this period of the life cycle largely determines the compensatory capacity of the population. Density regulation at the juvenile life-stage dampens the impact of stochastic processes operating earlier in life such as environmental impacts on the production of eggs and climate-dependent survival of larvae. The timing of stochastic versus regulatory processes thus plays a crucial role in determining variability in adult abundance. Quantifying the contribution of environmental stochasticity and compensatory mechanisms in determining population abundance is essential for assessing population responses to climate change and exploitation by humans. PMID:24893001

Ohlberger, Jan; Rogers, Lauren A; Stenseth, Nils Chr

2014-01-01

31

Stochasticity and Determinism: How Density-Independent and Density-Dependent Processes Affect Population Variability  

PubMed Central

A persistent debate in population ecology concerns the relative importance of environmental stochasticity and density dependence in determining variability in adult year-class strength, which contributes to future reproduction as well as potential yield in exploited populations. Apart from the strength of the processes, the timing of density regulation may affect how stochastic variation, for instance through climate, translates into changes in adult abundance. In this study, we develop a life-cycle model for the population dynamics of a large marine fish population, Northeast Arctic cod, to disentangle the effects of density-independent and density-dependent processes on early life-stages, and to quantify the strength of compensatory density dependence in the population. The model incorporates information from scientific surveys and commercial harvest, and dynamically links multiple effects of intrinsic and extrinsic factors on all life-stages, from eggs to spawners. Using a state-space approach we account for observation error and stochasticity in the population dynamics. Our findings highlight the importance of density-dependent survival in juveniles, indicating that this period of the life cycle largely determines the compensatory capacity of the population. Density regulation at the juvenile life-stage dampens the impact of stochastic processes operating earlier in life such as environmental impacts on the production of eggs and climate-dependent survival of larvae. The timing of stochastic versus regulatory processes thus plays a crucial role in determining variability in adult abundance. Quantifying the contribution of environmental stochasticity and compensatory mechanisms in determining population abundance is essential for assessing population responses to climate change and exploitation by humans. PMID:24893001

Ohlberger, Jan; Rogers, Lauren A.; Stenseth, Nils Chr.

2014-01-01

32

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.

33

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

34

Statistical approach to nuclear level density  

SciTech Connect

We discuss the level density in a finite many-body system with strong interaction between the constituents. Our primary object of applications is the atomic nucleus but the same techniques can be applied to other mesoscopic systems. We calculate and compare nuclear level densities for given quantum numbers obtained by different methods, such as nuclear shell model (the most successful microscopic approach), our main instrument - moments method (statistical approach), and Fermi-gas model; the calculation with the moments method can use any shell-model Hamiltonian excluding the spurious states of the center-of-mass motion. Our goal is to investigate statistical properties of nuclear level density, define its phenomenological parameters, and offer an affordable and reliable way of calculation.

Sen'kov, R. A.; Horoi, M. [Department of Physics, Central Michigan University, Mount Pleasant, MI 48859 (United States); Zelevinsky, V. G. [Department of Physics and Astronomy and National Superconducting Cyclotron Laboratory, Michigan State University, East Lansing, MI 48824-1321 (United States)

2014-10-15

35

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

36

Density-dependent regulation of natural and laboratory rotifer populations  

Microsoft Academic Search

Density-dependent regulation of abundance is fundamentally important in the dynamics of most animal populations. Density effects, however, have rarely been quantified in natural populations, so population models typically have a large uncertainty in their predictions. We used models generated from time series analysis to explore the form and strength of density-dependence in several natural rotifer populations. Population growth rate (r)

Terry W. Snell; Brian J. Dingmann; Manuel Serra

2001-01-01

37

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. All of the current programs used to study this issue 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 (LBM) represents such a 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. A key advantage of lattice Boltzmann method is that it has the ability to solve the Navier-Stokes equations in larger conduits and pores. Recent advances in lattice Boltzmann modeling permit simulation of large-scale density-dependent ground water flow and heat/solute transport. These simulations can be accomplished while retaining the advantages of 'regular' lattice Boltzmann methods, such as solute/heat transport at high Reynolds numbers. Hence it allows for eddy diffusion brought on by inertial components of flow at higher Reynolds numbers, which may occur in some coastal aquifers. This may prove to be an advantage for freshwater/seawater interface simulations especially given the highly macroporous nature of the aquifers underlying south Florida. 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 LBM to fluid flow and contaminant transport problems. There are only a handful of scientists attempting to apply LBM to density-dependent flows in general; even fewer have considered seawater intrusion. We show how this method can be applied to density-dependent flows. We present two sets of results. The first are the results for the Horton-Rogers-Lapwood problem, where heat is the cause of varying density. The second are our results for a seawater intrusion simulation, a Henry-like problem.

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

2008-12-01

38

Density imaging using a multiple-frequency DBIM approach.  

PubMed

Current inverse scattering methods for quantitative density imaging have limitations that keep them from practical experimental implementations. In this work, an improved approach, termed the multiple-frequency distorted Born iterative method (MF-DBIM) algorithm, was developed for imaging density variations. The MF-DBIM approach consists of inverting the wave equation by solving for a single function that depends on both sound speed and density variations at multiple frequencies. Density information was isolated by using a linear combination of the reconstructed single-frequency profiles. Reconstructions of targets using MF-DBIM from simulated data were compared with reconstructions using methods currently available in the literature, i.e., the dual-frequency DBIM (DF-DBIM) and T-matrix approaches. Useful density reconstructions, i.e., root mean square errors (RMSEs) less than 30%, were obtained with MF-DBIM even with 2% Gaussian noise in the simulated data and using frequency ranges spanning less than an order of magnitude. Therefore, the MFDBIM approach outperformed both the DF-DBIM method (which has problems converging with noise even an order of magnitude smaller) and the T-matrix method (which requires a ka factor close to unity to achieve convergence). However, the convergence of all the density imaging algorithms was compromised when imaging targets with object functions exhibiting high spatial frequency content. PMID:21041134

Lavarello, Roberto; Oelze, Michael

2010-11-01

39

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

40

Limitations of methods to test density-dependent fecundity hypothesis.  

PubMed

1. Two main hypotheses are usually invoked to explain density dependence in fecundity: the habitat heterogeneity hypothesis (HHH) and the individual adjustment hypothesis (IAH). Although simple methods have been proposed to discriminate between the two hypotheses, their adequacy was tested for only a limited set of real and model populations. 2. In a computer simulation study based on a stochastic territory-based approach, Ferrer, Newton & Casado (2006, Journal of Animal Ecology, 75, 111-117) argued that a strong negative relationship between mean fecundity and its skewness in stable or increasing populations provides critical support for HHH, as this relationship should be lacking under IAH. A negative relationship between mean fecundity and its coefficient of variation (CV) was predicted under both hypotheses, although with a lower slope under IAH. 3. We used a comparable simulation approach, with model populations parameterized from an increasing Bonelli's eagle Hieraaetus fasciatus population (1992-2006), to show that both HHH and IAH can produce indistinguishable relationships between mean fecundity and both its CV and its skewness. 4. Strong negative correlations between the mean and both its CV and its skewness can emerge as statistical artifacts under biologically plausible assumptions, and so they may be largely inadequate to infer mechanisms underlying density dependence in demographic parameters. PMID:18254920

Beja, Pedro; Palma, Luis

2008-03-01

41

Very-high-order harmonic generation from Ar atoms and Ar+ ions in superintense pulsed laser fields: An ab initio self-interaction-free time-dependent density-functional approach  

E-print Network

We present an ab initio nonpertubative investigation of the mechanisms responsible for the production of very-high-order harmonic generation (HHG) from Ar atoms and Ar+ ions by means of the self-interaction-free time-dependent density...

Carrera, Juan J.; Chu, Shih-I; Tong, Xiao-Min

2005-06-21

42

Adaptive density dependence of avian clutch size  

Microsoft Academic Search

In birds, the annual mean clutch size is often negatively correlated with population density. This relationship is at least in part due to adjustment by individuals. We investigated whether this response is adaptive in two ways. First we used an optimality model to predict how optimal clutch size (the clutch size that maximizes the number of breeding birds [recruits and

Joost M. Tinbergen; Marcel E. Visser

2000-01-01

43

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

44

Skyrme energy-density functional approach to collective dynamics  

NASA Astrophysics Data System (ADS)

Our recent developments for the microscopic description of nuclear collective dynamics in the framework of the time-dependent density functional theory is presented. It is shown that the quasiparticle random-phase approximation is ready for the systematic investigation of the giant resonances in the entire mass region of nuclear chart and that the collective Hamiltonian approach gives the quantitative description of the low-lying states in transitional nuclei with the Skyrme and pairing energy density functionals as a microscopic input. We put emphasis on necessity of the massive use of high performance computers to carry out such microscopic calculations.

Yoshida, Kenichi; Hinohara, Nobuo; Nakatsukasa, Takashi

2011-09-01

45

Density dependent friction of lipid monolayers.  

PubMed

We measure frictional properties of liquid-expanded and liquid-condensed phases of lipid Langmuir-Blodgett monolayers by interfacial force microscopy. We find that over a reasonably broad surface-density range, the friction shear strength of the lipid monolayer film is proportional to the surface area (42-74 A2/molecule) occupied by each molecule. The increase in frictional force (i.e., friction shear strength with molecular area can be attributed to the increased conformational freedom and the resulting increase in the number of available modes for energy dissipation. PMID:17655211

Goertz, M P; Stottrup, B L; Houston, J E; Zhu, X-Y

2007-12-13

46

An Infrastructureless Approach to Estimate Vehicular Density in Urban Environments  

PubMed Central

In Vehicular Networks, communication success usually depends on the density of vehicles, since a higher density allows having shorter and more reliable wireless links. Thus, knowing the density of vehicles in a vehicular communications environment is important, as better opportunities for wireless communication can show up. However, vehicle density is highly variable in time and space. This paper deals with the importance of predicting the density of vehicles in vehicular environments to take decisions for enhancing the dissemination of warning messages between vehicles. We propose a novel mechanism to estimate the vehicular density in urban environments. Our mechanism uses as input parameters the number of beacons received per vehicle, and the topological characteristics of the environment where the vehicles are located. Simulation results indicate that, unlike previous proposals solely based on the number of beacons received, our approach is able to accurately estimate the vehicular density, and therefore it could support more efficient dissemination protocols for vehicular environments, as well as improve previously proposed schemes. PMID:23435054

Sanguesa, Julio A.; Fogue, Manuel; Garrido, Piedad; Martinez, Francisco J.; Cano, Juan-Carlos; Calafate, Carlos T.; Manzoni, Pietro

2013-01-01

47

Density-dependent acoustic properties of PBX 9502  

SciTech Connect

We have measured the longitudinal and shear acoustic velocities of PBX 9502 as a function of density for die-pressed samples over the range 1.795 g/cc to 1.888 g/cc. The density dependence of the velocities is linear. Thermal cycling of PBX 9502 is known to induce irreversible volume growth. We have measured this volume growth dependence on density for a subset of the pressed parts and find that the most growth occurs for the samples with lowest initial density. The acoustic velocity changes due to the volume growth are significant and reflect damage in the samples.

Brown, Geoffrey W [Los Alamos National Laboratory; Thompson, Darla G [Los Alamos National Laboratory; Deluca, Racci [Los Alamos National Laboratory; Hartline, Ernest L [Los Alamos National Laboratory; Hagelberg, Stephanie I [Los Alamos National Laboratory

2009-07-31

48

Density dependence of reactor performance with thermal confinement scalings  

SciTech Connect

Energy confinement scalings for the thermal component of the plasma published thus far have a different dependence on plasma density and input power than do scalings for the total plasma energy. With such thermal scalings, reactor performance (measured by Q, the ratio of the fusion power to the sum of the ohmic and auxiliary input powers) worsens with increasing density. This dependence is the opposite of that found using scalings based on the total plasma energy, indicating that reactor operation concepts may need to be altered if this density dependence is confirmed in future research.

Stotler, D.P.

1992-03-01

49

Influence of density dependence of symmetry energy on fragmentation  

E-print Network

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

Karan Singh Vinayak; Mohinder Singh; Suneel Kumar

2011-02-09

50

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

E-print Network

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

Kobryn, A E; Hirata, F

2005-01-01

51

Density-dependent recruitment in grassland small mammals  

Microsoft Academic Search

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

Aaron W. Reed; Norman A. Slade

2008-01-01

52

Testing for density dependence allowing for weather effects  

Microsoft Academic Search

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

Peter Rothery; Ian Newton; Lois Dale; Tomasz Wesolowski

1997-01-01

53

Dynamical instabilities in density-dependent hadronic relativistic models  

SciTech Connect

Unstable modes in asymmetric nuclear matter (ANM) at subsaturation densities are studied in the framework of relativistic mean-field density-dependent hadron models. The size of the instabilities that drive the system are calculated and a comparison with results obtained within the nonlinear Walecka model is presented. The distillation and antidistillation effects are discussed.

Santos, A. M.; Brito, L.; Providencia, C. [Centro de Fisica Teorica, Department of Physics, University of Coimbra, P-3004-516 Coimbra (Portugal)

2008-04-15

54

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

55

Density-dependence as a size-independent regulatory mechanism  

Microsoft Academic Search

The growth function of populations is central in biomathematics. The main dogma is the existence of density-dependence mechanisms, which can be modelled with distinct functional forms that depend on the size of the population. One important class of regulatory functions is the ?-logistic, which generalizes the logistic equation. Using this model as a motivation, this paper introduces a simple dynamical

Harold P. de Vladar

2006-01-01

56

2005 Nature Publishing Group Density dependence explains tree species  

E-print Network

of the current neutral theory in ecology3­10 can easily be generalized to incorporate symmetric density is that seeds that disperse farther away from the maternal parent are more likely to escape mortality from host-dependent deaths are then exploited by less-common species. Therefore, among-species frequency dependence

He, Fangliang

57

A Wigner Monte Carlo approach to density functional theory  

SciTech Connect

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., E-mail: jeanmichel.sellier@gmail.com; Dimov, I.

2014-08-01

58

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

59

Dependence of polar hole density on magnetic and solar conditions  

NASA Technical Reports Server (NTRS)

Electron densities from the Langmuir probes on the Atmospheric Explorer C and Dynamics Explorer 2 are used for analyzing the behavior of the high-altitude night-side F region polar hole as a function of solar and magnetic activity and of universal time (UT). The polar region of invariant latitude from 70 deg to 80 deg and MLT from 22 to 03 hours is examined. The strongest dependencies are observed in F10.7 and UT; a strong hemispherical difference due to the offset of the magnetic poles from the earth's rotation axis is observed in the UT dependence of the ionization hole. A seasonal variation in the dependence of ion density on solar flux is indicated, and an overall asymmetry in the density level between hemispheres is revealed, with the winter-hole density about a factor of 10 greater in the north than in the south.

Hoegy, W. R.; Grebowsky, J. M.

1991-01-01

60

Founder takes all: density-dependent processes structure biodiversity.  

PubMed

Density-dependent processes play a key role in the spatial structuring of biodiversity. Specifically, interrelated demographic processes, such as gene surfing, high-density blocking, and competitive exclusion, can generate striking geographic contrasts in the distributions of genes and species. Here, we propose that well-studied evolutionary and ecological biogeographic patterns of postglacial recolonization, progressive island colonization, microbial sectoring, and even the 'Out of Africa' pattern of human expansion, are fundamentally similar, underpinned by a 'founder takes all' density-dependent principle. Additionally, we hypothesize that older historic constraints of density-dependent processes are seen today in the dramatic biogeographic shifts that occur in response to human-mediated extinction events, whereby surviving lineages rapidly expand their ranges to replace extinct sister taxa. PMID:23000431

Waters, Jonathan M; Fraser, Ceridwen I; Hewitt, Godfrey M

2013-02-01

61

Competition and the density dependence of metabolic rates.  

PubMed

Although mass and temperature are strong predictors of metabolic rates, there is considerable unexplained variation in metabolic rates both within and across species after body size and temperature are taken into account. Some of this variation may be due to changes in the rate of food intake with population density, as metabolism depends on the throughput of food to fuel biochemical reactions. Using data collected from the literature, we show that individual metabolic rates are negatively correlated with population density for a wide range of organisms including primary producers and consumers. Using new data for the zooplankter Daphnia ambigua, we also find genotypic variation in the relationship between metabolic rate and population density. The relationship between metabolic rate and population density generally follows a power law scaling, and within a population, density-correlated variation in metabolism can span two orders of magnitude. We suggest that density-dependent metabolic rates arise via competitive effects on foraging rates (both exploitation and interference competition), combined with an activity response to accommodate the resource constraint induced by competition. Standard ecological models predict the kind of density-dependent foraging patterns that could give rise to density-dependent metabolic rates, but this has generally not been investigated. Our results indicate that after body mass and temperature, population density represents an important third axis that may account for a large amount of unexplained variance in metabolic rates within and among species. The effect of population density on metabolism has implications for the scaling of metabolic rates from individuals to populations and the relative performance of species and genotypes and therefore also for community assembly and evolution. PMID:23565624

DeLong, John P; Hanley, Torrance C; Vasseur, David A

2014-01-01

62

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

63

Multivariate Density Estimation: An SVM Approach  

E-print Network

We formulate density estimation as an inverse operator problem. We then use convergence results of empirical distribution functions to true distribution functions to develop an algorithm for multivariate density estimation. ...

Mukherjee, Sayan

1999-04-01

64

Experimental investigations by using electrochemical, steady state and time resolved spectroscopic tools on the photoreactions of disubstituted indoles in presence of tetracyanoquinodimethane (TCNQ) and a theoretical approach by using time-dependent density functional theory  

Microsoft Academic Search

The spectroscopic and photophysical properties of some dimethylindoles, 1,2-dimethylindole (12DMI) and 2,3-dimethylindole (23DMI) were measured in presence of electron acceptor, tetracyanoquinodimethane (TCNQ) in solvents of varying polarity by using electrochemical, steady state and time resolved spectroscopic techniques. Both from the theoretical considerations made by using time-dependent density functional theory (TD-DFT) and steady state polarization spectral measurements, it reveals the possibility

Paulami Mandal; Tanushree Sahu; Tapas Misra; Suman K. Pal; Tapan Ganguly

2007-01-01

65

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

66

A Maximum Likelihood Approach to Density Estimation with Semidefinite Programming  

Microsoft Academic Search

Density estimation plays an important and fundamental role in pattern recognition, machine learning, and statistics. In this article, we develop a parametric approach to univariate (or low-dimensional) density estimation based on semidefinite programming (SDP). Our density model is expressed as the product of a nonnegative polynomial and a base density such as normal distribution, exponential distribution, and uniform distribution. When

Tadayoshi Fushiki; Shingo Horiuchi; Takashi Tsuchiya

2006-01-01

67

Plant defense and density dependence in the population growth of herbivores.  

PubMed

Long-standing theory has predicted that plant defensive and nutritional traits contribute to the population dynamics of insect herbivores. To examine the role of plant variation in density dependence, I took a comparative approach by conducting density manipulation experiments with the specialist aphid, Aphis nerii, on 18 species of milkweed (Asclepias spp.). The strength of density dependence varied on the plant species. Variation in plant secondary compounds (cardenolides), trichomes, leaf carbon and nitrogen concentrations, and seed mass of the milkweed species predicted the R(max) of aphid populations, while specific leaf weight, carbon concentration, latex, water content, and trichome density were significant predictors of the strength of density dependence. Thus, plant traits that probably evolved for primary and defensive functions contribute to the ecological dynamics of herbivore populations. PMID:15266375

Agrawal, Anurag A

2004-07-01

68

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

69

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

70

Gogny force with a finite-range density dependence  

NASA Astrophysics Data System (ADS)

In the present work, we have investigated an extension of the effective nucleon-nucleon Gogny interaction in which the zero-range density-dependent term has been replaced with a finite-range term. The parameters have been adjusted on both nuclear-matter properties and a few observables of stable nuclei. The traditional and unified fitting procedure of the Gogny force used in Bruyères-le-Châtel ensures common basic properties between the extended analytical form of the Gogny interaction, called D 2 , and the original one. In particular, symmetric infinite nuclear-matter and neutron-matter properties as well as pairing correlations have been investigated. A few static properties obtained in finite nuclei using the Hartree-Fock-Bogoliubov approach and the D 2 parametrization of the Gogny interaction are analyzed and compared to the results obtained with D 1 -type parametrizations and experiment when it is possible. The D 2 parametrization makes it possible to reproduce nuclear structure properties with improved accuracy.

Chappert, F.; Pillet, N.; Girod, M.; Berger, J.-F.

2015-03-01

71

Intercohort density dependence drives brown trout habitat selection  

NASA Astrophysics Data System (ADS)

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

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

2013-01-01

72

Carrier-Density-Dependent Lattice Stability in InSb  

NASA Astrophysics Data System (ADS)

The ultrafast decay of the x-ray diffraction intensity following laser excitation of an InSb crystal has been utilized to observe carrier dependent changes in the potential energy surface. For the first time, an abrupt carrier dependent onset for potential energy surface softening and the appearance of accelerated atomic disordering for a very high average carrier density have been observed. Inertial dynamics dominate the early stages of crystal disordering for a wide range of carrier densities between the onset of crystal softening and the appearance of accelerated atomic disordering.

Hillyard, P. B.; Gaffney, K. J.; Lindenberg, A. M.; Engemann, S.; Akre, R. A.; Arthur, J.; Blome, C.; Bucksbaum, P. H.; Cavalieri, A. L.; Deb, A.; Falcone, R. W.; Fritz, D. M.; Fuoss, P. H.; Hajdu, J.; Krejcik, P.; Larsson, J.; Lee, S. H.; Meyer, D. A.; Nelson, A. J.; Pahl, R.; Reis, D. A.; Rudati, J.; Siddons, D. P.; Sokolowski-Tinten, K.; von der Linde, D.; Hastings, J. B.

2007-03-01

73

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

74

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

PubMed

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. PMID:22179311

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

2012-07-01

75

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

76

Bridging density-functional and many-body perturbation theory: orbital-density dependence in electronic-structure functionals  

NASA Astrophysics Data System (ADS)

Energy functionals which depend explicitly on the orbital densities (ODD), instead of the total charge density, appear when applying self-interaction corrections to density-functional theory. In these cases (e.g. the Perdew-Zunger [1] and the non-Koopmans [2] approaches) the total energy loses invariance under unitary rotations of the orbitals, and the minimization of the functionals leads to orbital-dependent Hamiltonians. We show that it is possible to identify the orbital-dependency of densities and potentials with an effective and discretized frequency-dependency, in close analogy to the quasi-particle approximation of frequency-dependent self-energies and naturally oriented to interpret electronic spectroscopies [3]. Some of the existing ODD functionals are analyzed from this new perspective. Numerical results for the electronic structure of gas-phase molecules (within the Koopmans-corrected class of functionals) are computed and found in excellent agreement with photoemission (UPS) data. [1] J.-P. Perdew and A. Zunger, Phys. Rev. B 23, 5048 (1981). [2] I. Dabo, A. Ferretti, N. Poilvert, Y. Li, N. Marzari, M. Cococcioni, Phys. Rev. B 82, 115121 (2010). [3] M. Gatti, V. Olevano, L. Reining, I.-V. Tokatly, Phys. Rev. Lett. 99, 057401 (2007).

Ferretti, Andrea; Dabo, Ismaila; Cococcioni, Matteo; Marzari, Nicola

2012-02-01

77

Approach of high density coal preparation method  

SciTech Connect

Density difference of aged anthracite coal of high density and discard is less than that of general coal and discard; conventional separation methods are difficult to be used. For the special coal, coal dry beneficiation technology with air-dense medium fluidized bed has obvious superiority over other separation methods.

Yang, Y.; Chen, Q. [China Univ. of Mining and Technology, Xuzhou, Jiangsu (China). Mineral Processing Research Center

1996-12-31

78

Hard scale dependent gluon density, saturation, and forward-forward dijet production at the LHC  

NASA Astrophysics Data System (ADS)

We propose a method to introduce Sudakov effects to the unintegrated gluon density, promoting it to be hard scale dependent. The advantage of the approach is that it guarantees that the gluon density is positive definite and that the Sudakov effects cancel on the integrated level. As a case study, we apply the method to calculate angular correlations and the Rp A ratio for p +p vs p +Pb collision in the production of forward-forward dijets.

Kutak, Krzysztof

2015-02-01

79

A phenomenological density-scaling approach to lamellipodial actin dynamics(†).  

PubMed

The integration of protein function studied in vitro in a dynamic system like the cell lamellipodium remains a significant challenge. One reason is the apparent contradictory effect that perturbations of some proteins can have on the overall lamellipodium dynamics, depending on exact conditions. Theoretical modelling offers one approach for understanding the balance between the mechanisms that drive and regulate actin network growth and decay. Most models use a 'bottom-up' approach, involving explicitly assembling biochemical components to simulate observable behaviour. Their correctness therefore relies on both the accurate characterization of all the components and the completeness of the relevant processes involved. To avoid potential pitfalls due to this uncertainty, we used an alternative 'top-down' approach, in which measurable features of lamellipodium behaviour, here observed in two different cell types (HL60 and B16-F1), directly inform the development of a simple phenomenological model of lamellipodium dynamics. We show that the kinetics of F-actin association and dissociation scales with the local F-actin density, with no explicit location dependence. This justifies the use of a simplified kinetic model of lamellipodium dynamics that yields predictions testable by pharmacological or genetic intervention. A length-scale parameter (the lamellipodium width) emerges from this analysis as an experimentally accessible probe of network regulatory processes. PMID:25485077

Lewalle, Alexandre; Fritzsche, Marco; Wilson, Kerry; Thorogate, Richard; Duke, Tom; Charras, Guillaume

2014-12-01

80

A phenomenological density-scaling approach to lamellipodial actin dynamics†  

PubMed Central

The integration of protein function studied in vitro in a dynamic system like the cell lamellipodium remains a significant challenge. One reason is the apparent contradictory effect that perturbations of some proteins can have on the overall lamellipodium dynamics, depending on exact conditions. Theoretical modelling offers one approach for understanding the balance between the mechanisms that drive and regulate actin network growth and decay. Most models use a ‘bottom-up’ approach, involving explicitly assembling biochemical components to simulate observable behaviour. Their correctness therefore relies on both the accurate characterization of all the components and the completeness of the relevant processes involved. To avoid potential pitfalls due to this uncertainty, we used an alternative ‘top-down’ approach, in which measurable features of lamellipodium behaviour, here observed in two different cell types (HL60 and B16-F1), directly inform the development of a simple phenomenological model of lamellipodium dynamics. We show that the kinetics of F-actin association and dissociation scales with the local F-actin density, with no explicit location dependence. This justifies the use of a simplified kinetic model of lamellipodium dynamics that yields predictions testable by pharmacological or genetic intervention. A length-scale parameter (the lamellipodium width) emerges from this analysis as an experimentally accessible probe of network regulatory processes. PMID:25485077

Lewalle, Alexandre; Fritzsche, Marco; Wilson, Kerry; Thorogate, Richard; Duke, Tom; Charras, Guillaume

2014-01-01

81

Generalized Floquet Theoretical Formulation of Time-Dependent Density  

E-print Network

-dependent density functional theory TDDFT . While much progress has been made in the steady-state DFT since, St. Petersburg State University, 198904 St. Petersburg, Russia 2 Department of Chemistry, University recent. w xThe extension of the steady-state DFT 2 7 to the w xtime domain 8 13 is by no means

Chu, Shih-I

82

Simple density-dependent matrix model for population projection  

Microsoft Academic Search

A matrix model based on a discrete time form of the logistic equation and the Leslie matrix model was developed for density-dependent population growth; the model is simpler and more easily applied than the model developed by Liu and Cohen in 1987 using a different discrete time form of the logistic equation. The new model requires no additional parameters, matrices,

A. L. Jensen

1995-01-01

83

Density dependent matrix model for gray wolf population projection  

Microsoft Academic Search

A Leslie matrix model was developed for a small gray wolf (Canis lupus) population recolonizing an area with abundant resources and uncontrolled by humans. The model was modified to describe population growth in a limited environment using a discrete form of the logistic equation. The density dependent Leslie matrix model was applied to investigate gray wolf population recovery in the

David H Miller; Al L Jensen; James H Hammill

2002-01-01

84

FITNESS AND DENSITY-DEPENDENT POPULATION GROWTH IN DROSOPHILA MELANOGASTER  

E-print Network

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

Rose, Michael R.

85

Density-dependent effects of prey defenses and predator offenses.  

PubMed

Defenses protect prey, while offenses arm predators. Some defenses and offenses are constitutive (e.g. tortoise shells), while others are phenotypically plastic and not always expressed (e.g. neckteeth in water fleas). All of them are costly and only adaptive at certain prey densities. Here, I analyse such density-dependent effects, applying a functional response model to categorize defenses and offenses and qualitatively predict at which prey densities each category should evolve (if it is constitutive) or be expressed (if it is phenotypically plastic). The categories refer to the step of the predation cycle that a defense or offense affects: (1) search, (2) encounter, (3) detection, (4) attack, or (5) meal. For example, prey warning signals such as red coloration prevent predator attacks and are hence step 4 defenses, while sharp predator eyes enhance detection and are step 3 offenses. My theoretical analyses predict that step 1 defenses, which prevent predators from searching for their next meal (e.g. toxic substances), evolve or are expressed at intermediate prey densities. Other defenses, however, should be most beneficial at low prey densities. Regarding predators, step 1 offenses (e.g. immunity against prey toxins) are predicted to evolve or be expressed at high prey densities, other offenses at intermediate densities. I provide evidence from the literature that supports these predictions. PMID:16842823

Jeschke, Jonathan M

2006-10-21

86

Time-dependent density-matrix-functional theory  

NASA Astrophysics Data System (ADS)

Although good progress has been made in the calculation of correlation energies from total energy expressions which are implicit functionals of the one-particle reduced density matrix, and explicit functionals of the natural orbitals (NOs) and their occupation numbers, a formulation of the calculation of excitation energies in this so-called density-matrix-functional theory (DMFT) is still lacking. In this paper we propose a time-dependent density-matrix-functional theory (TDDMFT). It is based on the equation of motion (EOM) for the 1-matrix P(s)(t) in the representation of the stationary NOs. In the final form of the EOM, the rate of change of the P(s)(t) , ?P(s)(t)/?t , is determined by the commutator of the generalized time-dependent Fock matrix F(s)(t) with P(s)(t) plus an additional term D(s)(t) . The matrix F(s)(t) determines the evolution of the NOs in the time-dependent one-electron Schrödinger equations, while D(s)(t) determines the time evolution of the NO occupations. With the neglect of the electron Coulomb correlation, the time-dependent one-electron equations for the NOs reduce to those for the Hartree-Fock (HF) orbitals of time-dependent HF (TDHF) theory. The coupled-perturbed equations of TDDMF response theory (TDDMFRT) are derived for the linear response of the 1-matrix ?P(s)(t) to a time-dependent perturbation ?vext(t) of the external potential. The frequency-dependent changes ?P(s),ij(?) and ?P(s),kl(?) are coupled through the coupling matrix Kijkl(?) , which is produced with the derivatives of F(s)(t) and D(s)(t) with respect to Pkl(t') . Based on the response equations, TDDMFRT eigenvalue equations are derived for the electron excitations ?q .

Pernal, Katarzyna; Gritsenko, Oleg; Baerends, Evert Jan

2007-01-01

87

Large area photonic crystal cavities: a local density approach.  

PubMed

Large area photonic crystal cavities are devices of interest for photovoltaics, optoelectronics, and solid-state lighting. However, depending on their dimensions they pose a large computational challenge. Here, we use a local density approach to avoid direct simulation of the device. We capture the effect of both ideal and distorted photonic crystals in an effective mass and an effective potential. We use these to map the problem of calculating the electromagnetic field modes to solving a simple time-independent Schrödinger equation. We show that, in the case that the hole radius varies quadratically as a function of position, the eigenmodes of the photonic crystals can be described by the corresponding eigenmodes of the quantum harmonic oscillator with typical agreements well above 90%. PMID:25837088

Dobbelaar, M C F; Greveling, S; van Oosten, D

2015-03-23

88

Modelling interactions of toxicants and density dependence in wildlife populations  

USGS Publications Warehouse

1. A major challenge in the conservation of threatened and endangered species is to predict population decline and design appropriate recovery measures. However, anthropogenic impacts on wildlife populations are notoriously difficult to predict due to potentially nonlinear responses and interactions with natural ecological processes like density dependence. 2. Here, we incorporated both density dependence and anthropogenic stressors in a stage-based matrix population model and parameterized it for a density-dependent population of peregrine falcons Falco peregrinus exposed to two anthropogenic toxicants [dichlorodiphenyldichloroethylene (DDE) and polybrominated diphenyl ethers (PBDEs)]. Log-logistic exposure–response relationships were used to translate toxicant concentrations in peregrine falcon eggs to effects on fecundity. Density dependence was modelled as the probability of a nonbreeding bird acquiring a breeding territory as a function of the current number of breeders. 3. The equilibrium size of the population, as represented by the number of breeders, responded nonlinearly to increasing toxicant concentrations, showing a gradual decrease followed by a relatively steep decline. Initially, toxicant-induced reductions in population size were mitigated by an alleviation of the density limitation, that is, an increasing probability of territory acquisition. Once population density was no longer limiting, the toxicant impacts were no longer buffered by an increasing proportion of nonbreeders shifting to the breeding stage, resulting in a strong decrease in the equilibrium number of breeders. 4. Median critical exposure concentrations, that is, median toxicant concentrations in eggs corresponding with an equilibrium population size of zero, were 33 and 46 ?g g?1 fresh weight for DDE and PBDEs, respectively. 5. Synthesis and applications. Our modelling results showed that particular life stages of a density-limited population may be relatively insensitive to toxicant impacts until a critical threshold is crossed. In our study population, toxicant-induced changes were observed in the equilibrium number of nonbreeding rather than breeding birds, suggesting that monitoring efforts including both life stages are needed to timely detect population declines. Further, by combining quantitative exposure–response relationships with a wildlife demographic model, we provided a method to quantify critical toxicant thresholds for wildlife population persistence.

Schipper, Aafke M.; Hendriks, Harrie W.M.; Kauffman, Matthew J.; Hendriks, A. Jan; Huijbregts, Mark A.J.

2013-01-01

89

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

90

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

91

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

E-print Network

-dependent and density-independent mortality rates at the prejuvenile (age 1-100 days) and posijuvenile (age 101 days to 5 years) phases. Total post- juvenile mortality rates from 1967 through 1982 were dominated, and inversely related to mean June temperature, whereas mortality rates of all other life stages showed

92

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

93

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

94

Initial distribution spread: A density forecasting approach  

NASA Astrophysics Data System (ADS)

Ensemble forecasting of nonlinear systems involves the use of a model to run forward a discrete ensemble (or set) of initial states. Data assimilation techniques tend to focus on estimating the true state of the system, even though model error limits the value of such efforts. This paper argues for choosing the initial ensemble in order to optimise forecasting performance rather than estimating the true state of the system. Density forecasting and choosing the initial ensemble are treated as one problem. Forecasting performance can be quantified by some scoring rule. In the case of the logarithmic scoring rule, theoretical arguments and empirical results are presented. It turns out that, if the underlying noise dominates model error, we can diagnose the noise spread.

Machete, R. L.; Moroz, I. M.

2012-04-01

95

Initial Distribution Spread: A density forecasting approach  

E-print Network

Ensemble forecasting of nonlinear systems involves the use of a model to run forward a discrete ensemble (or set) of initial states. Data assimilation techniques tend to focus on estimating the true state of the system, even though model error limits the value of such efforts. This paper argues for choosing the initial ensemble in order to optimise forecasting performance rather than estimate the true state of the system. Density forecasting and choosing the initial ensemble are treated as one problem. Forecasting performance can be quantified by some scoring rule. In the case of the logarithmic scoring rule, theoretical arguments and empirical results are presented. It turns out that, if the underlying noise dominates model error, we can diagnose the noise spread.

Reason L. Machete; Irene M. Moroz

2012-07-18

96

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

97

Subsystem real-time Time Dependent Density Functional Theory  

E-print Network

We present the extension of Frozen Density Embedding (FDE) theory to real-time Time Dependent Density Functional Theory (rt-TDDFT). FDE a is DFT-in-DFT embedding method that allows to partition a larger Kohn-Sham system into a set of smaller, coupled Kohn-Sham systems. Additional to the computational advantage, FDE provides physical insight into the properties of embedded systems and the coupling interactions between them. The extension to rt-TDDFT is done straightforwardly by evolving the Kohn-Sham subsystems in time simultaneously, while updating the embedding potential between the systems at every time step. Two main applications are presented: the explicit excitation energy transfer in real time between subsystems is demonstrated for the case of the Na$_4$ cluster and the effect of the embedding on optical spectra of coupled chromophores. In particular, the importance of including the full dynamic response in the embedding potential is demonstrated.

Krishtal, Alisa; Pavanello, Michele

2015-01-01

98

Subcycle dynamics of high-harmonic generation in valence-shell and virtual states of Ar atoms: A self-interaction-free time-dependent density-functional-theory approach  

NASA Astrophysics Data System (ADS)

We perform an ab initio all-electron study of the subcycle structure, dynamics, and spectra of high harmonic generation (HHG) processes of Ar atoms in the presence of extreme ultraviolet (XUV) attosecond pulses and near-infrared (NIR) laser fields by means of the self-interaction-free time-dependent density functional theory (TDDFT). The TDDFT equations are solved accurately and efficiently via the time-dependent generalized pseudospectral (TDGPS) method. We focus on the subcycle (with respect to NIR field) temporal behavior of the level shift of the excited energy levels and related dynamics of harmonic photon emission. We observe and identify the subcycle shifts in the harmonic emission spectrum as a function of the time delay between the XUV and NIR pulses. In the region where the two pulses overlap, the photon emission peaks have an oscillatory structure with a period of ˜1.3 fs, which is half of the NIR laser optical cycle. We present and analyze the harmonic emission spectra from 3 s n p0,3 p0n s ,3 p1n d1,3 p1n p1,3 p0n d0,3 p0n p0 , and 3 p0n s excited states and the 3 p04 p0- virtual state as functions of the time delay. In addition, we explore the subcycle a.c. Stark shift phenomenon in NIR fields and its influence on the harmonic emission process. Our analysis reveals several features of the subcycle HHG dynamics and spectra as well as a temporal energy level shift.

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

2015-02-01

99

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

100

Wilson mass dependence of the overlap topological charge density  

NASA Astrophysics Data System (ADS)

The dependence of the overlap Dirac operator on the Wilson-mass regulator parameter is studied through calculations of the overlap topological charge densities at a variety of Wilson-mass values, using a Lüscher-Weisz gauge action. In this formulation, the Wilson-mass is used in the negative mass region and acts as a regulator governing the scale at which the Dirac operator is sensitive to topological aspects of the gauge field. We observe a clear dependence on the value of the Wilson-mass and demonstrate how these values can be calibrated against a finite number of stout-link smearing sweeps. The overlap topological charge density is also computed using a pre-smeared gauge field for the input kernel. We show how applying the overlap operator leads to further filtering of the gauge field. The results suggest that the freedom typically associated with smearing algorithms, through the variable number of sweeps, also exists in the overlap operator, through the variable Wilson-mass parameter.

Moran, Peter J.; Leinweber, Derek B.; Zhang, J. B.

2011-01-01

101

Altitude Dependence of Neutral Density Geomagnetic Storm Response  

NASA Astrophysics Data System (ADS)

New formulations for satellite neutral density response to geomagnetic activity developed for the Jacchia-Bowman 2008 empirical model were based on data at GRACE altitudes and are applicable for large geomagnetic storms (ap>75). Storm response from the Jacchia-Bowman 2008 and NRLMSIS empirical models are tested at low satellite altitudes using a unique historic set of accelerometer neutral density data from satellites flown in 1982-1983 with perigee altitudes near 170 km. These data are particularly important for evaluating the capability of modern empirical and first principles models to predict satellite drag affecting reentry. The model validations are compared with those of selected CHAMP and GRACE data, 2002-2005, for similar conditions of geomagnetic activity, solar flux, local time and day of year to validate the altitude dependence of geomagnetic heating. Results are interpreted in the framework of General Circulation Models. To further understand thermospheric responses to solar wind energy deposition, we estimate the energy input required to produce these neutral density enhancements.

Marcos, F. A.; Lin, C.; Noah, M.; Burke, W. J.; Cable, S. B.; Wise, J. O.; Sutton, E. K.

2010-12-01

102

Linear-response calculation in the time-dependent density functional theory  

SciTech Connect

Linear response calculations based on the time-dependent density-functional theory are presented. Especially, we report results of the finite amplitude method which we have recently proposed as an alternative and feasible approach to the (quasiparticle-)random-phase approximation. Calculated properties of the giant resonances and low-energy E1 modes are discussed. We found a universal linear correlation between the low-energy E1 strength and the neutron skin thickness.

Nakatsukasa, Takashi; Inakura, Tsunenori; Avogadro, Paolo; Ebata, Shuichiro; Sato, Koichi; Yabana, Kazuhiro [RIKEN Nishina Center, Wako-shi 351-0198, Japan and Center for Computational Sciences, University of Tsukuba, Tsukuba 305-8571 (Japan); RIKEN Nishina Center, Wako-shi 351-0198 (Japan); Departimento di Fisica, Universita degli Studi di Milano, via Celoria 16, 20133 Milan (Italy) and RIKEN Nishina Center, Wako-shi 351-0198 (Japan); Center for Nuclear Study, University of Tokyo, Bunkyo-ku, 113-0033, Japan and RIKEN Nishina Center, Wako-shi 351-0198 (Japan); RIKEN Nishina Center, Wako-shi 351-0198 (Japan); Center for Computational Sciences, University of Tsukuba, Tsukuba 305-8571 (Japan) and RIKEN Nishina Center, Wako-shi 351-0198 (Japan)

2012-11-12

103

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

104

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

PubMed Central

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

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

1998-01-01

105

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

106

Time-dependent local density measurements in unsteady flows  

NASA Technical Reports Server (NTRS)

A laser-induced fluorescence technique for measuring the relative time-dependent density fluctuations in unsteady or turbulent flows is demonstrated. Using a 1.5-W continuous-wave Kr(+) laser, measurements have been obtained in 0.1-mm-diameter by 1-mm-long sampling volumes in a Mach 3 flow of N2 seeded with biacetyl vapor. A signal amplitude resolution of 2% was achieved for a detection frequency bandwidth of 10 kHz. The measurement uncertainty was found to be dominated by noise behaving as photon statistical noise. The practical limits of signal-to-noise ratios have been characterized for a wide range of detection frequency bandwidths that encompasses those of interest in supersonic turbulence measurements.

Mckenzie, R. L.; Monson, D. J.; Exberger, R. J.

1979-01-01

107

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

108

Bone density in transfusion dependent thalassemia patients in Urmia, Iran  

PubMed Central

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

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

2014-01-01

109

Density-dependent mortality in an oceanic copepod population.  

PubMed

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

Ohman, M D; Hirche, H J

2001-08-01

110

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

111

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

112

Energy-density functional approach for non-spherical nuclei  

NASA Astrophysics Data System (ADS)

The extension of the self-consistent energy-density functional method to non-spherical nuclei is briefly described. A comparison of the spherical and the non-spherical approach in lead nuclei is given and first results for the chain of strongly deformed dysprosium isotopes are presented.

Krömer, E.; Tolokonnikov, S. V.; Fayans, S. A.; Zawischa, D.

1995-02-01

113

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

114

Determination of plasmaspheric electron density profile by a stochastic approach  

NASA Astrophysics Data System (ADS)

The determination of plasmaspheric electron density profiles has been attempted using a stochastic approach, using Omega signals observed on the Akebono satellite. Since the wave normal directions and delay times of Omega signals can be theoretically calculated by ray tracing under an appropriate electron density model, the density profile can be estimated by fitting the calculated directions and times to the observed values. In the present paper, we introduce a flexible model and a novel algorithm in the fitting method, taking into account the stochastic factors of the density distribution and wave propagation. The stochastic representations of directions and times enable us to separately estimate the effects of the ionosphere and the plasmasphere. The validity of the proposed method is examined using observational data collected during the recovery phase of a magnetic storm. In another example, an estimation of the asymmetry of the plasmasphere is attempted.

Goto, Y.; Kasahara, Y.; Sato, T.

2003-06-01

115

Towards time-dependent current-density-functional theory in the non-linear regime  

NASA Astrophysics Data System (ADS)

Time-Dependent Density-Functional Theory (TDDFT) is a well-established theoretical approach to describe and understand irradiation processes in clusters and molecules. However, within the so-called adiabatic local density approximation (ALDA) to the exchange-correlation (xc) potential, TDDFT can show insufficiencies, particularly in violently dynamical processes. This is because within ALDA the xc potential is instantaneous and is a local functional of the density, which means that this approximation neglects memory effects and long-range effects. A way to go beyond ALDA is to use Time-Dependent Current-Density-Functional Theory (TDCDFT), in which the basic quantity is the current density rather than the density as in TDDFT. This has been shown to offer an adequate account of dissipation in the linear domain when the Vignale-Kohn (VK) functional is used. Here, we go beyond the linear regime and we explore this formulation in the time domain. In this case, the equations become very involved making the computation out of reach; we hence propose an approximation to the VK functional which allows us to calculate the dynamics in real time and at the same time to keep most of the physics described by the VK functional. We apply this formulation to the calculation of the time-dependent dipole moment of Ca, Mg and Na2. Our results show trends similar to what was previously observed in model systems or within linear response. In the non-linear domain, our results show that relaxation times do not decrease with increasing deposited excitation energy, which sets some limitations to the practical use of TDCDFT in such a domain of excitations.

Escartín, J. M.; Vincendon, M.; Romaniello, P.; Dinh, P. M.; Reinhard, P.-G.; Suraud, E.

2015-02-01

116

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

117

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.

118

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

119

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

E-print Network

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, 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.

Shahrzad Karimi; Carsten A. Ullrich

2014-10-24

120

Predators reverse the direction of density dependence for juvenile salmon mortality  

Microsoft Academic Search

The effect of predators on prey populations depends on how predator-caused mortality changes with prey population density.\\u000a Predators can enforce density-dependent prey mortality and contribute to population stability, but only if they have a positive\\u000a numerical or behavioral response to increased prey density. Otherwise, predator saturation can result in inversely density-dependent\\u000a mortality, destabilizing prey populations and increasing extinction risk. Juvenile

Darren M. Ward; Keith H. Nislow; Carol L. Folt

2008-01-01

121

Multiphoton ionization and high-order harmonic generation of He, Ne, and Ar atoms in intense pulsed laser fields: Self-interaction-free time-dependent density-functional theoretical approach  

E-print Network

We present a detailed study of the multiphoton ionization and high-order harmonic generation (HHG) processes of rare-gas atoms (He, Ne, and Ar) in intense pulsed laser fields by means of a self-interaction-free time-dependent ...

Chu, Shih-I; Tong, Xiao-Min

2001-06-12

122

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

123

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

124

Phase-space explorations in time-dependent density functional theory A.K. Rajam a  

E-print Network

Phase-space explorations in time-dependent density functional theory A.K. Rajam a , Paul Hessler b and City University of New York, 695 Park Avenue, New York, NY 10065, USA a r t i c l e i n f o Article online xxxx Keywords: Time-dependent density functional theory Phase-space Momentum-distributions Density

125

Spin-dependent localized Hartree-Fock density-functional calculation of singly, doubly, and triply excited and Rydberg states of He- and Li-like ions  

E-print Network

A spin-dependent density-functional approach for the calculation of highly and multiply excited state of atomic system is proposed based on the localized Hartree-Fock density-functional method and Slater’s diagonal sum rule. In this approach...

Chu, Shih-I; Zhou, Zhongyuan

2005-02-28

126

Dynamics of an ant-ant obligate mutualism: colony growth, density dependence and frequency dependence.  

PubMed

In insect societies, worker vs. queen development (reproductive caste) is typically governed by environmental factors, but many Pogonomyrmex seed-harvester ants exhibit strict genetic caste determination, resulting in an obligate mutualism between two reproductively isolated lineages. Same-lineage matings produce fertile queens while alternate-lineage matings produce sterile workers. Because new virgin queens mate randomly with multiple males of each lineage type, and both worker and queen phenotypes are required for colony growth and future reproduction, fitness is influenced by the relative frequency of each lineage involved in the mutualistic breeding system. While models based solely on frequency-dependent selection predict the convergence of lineage frequencies towards equal (0.5/0.5), we surveyed the lineage ratios of 49 systems across the range of the mutualism and found that the global lineage frequency differed significantly from equal. Multiple regression analysis of our system survey data revealed that the density and relative frequency of one lineage decreases at lower elevations, while the frequency of the alternate lineage increases with total colony density. While the production of the first worker cohort is largely frequency dependent, relying on the random acquisition of worker-biased sperm stores, subsequent colony growth is independent of lineage frequency. We provide a simulation model showing that a net ecological advantage held by one lineage can lead to the maintenance of stable but asymmetric lineage frequencies. Collectively, these findings suggest that a combination of frequency-dependent and frequency-independent mechanisms can generate many different localized and independently evolving system equilibria. PMID:21366750

Anderson, Kirk E; Wheeler, Diana E; Yang, Kimberly; Linksvayer, Timothy A

2011-04-01

127

Density functional and theoretical study of the temperature and pressure dependency of the plasmon energy of solids  

SciTech Connect

The temperature and pressure dependency of the volume plasmon energy of solids was investigated by density functional theory calculations. The volume change of crystal is the major factor responsible for the variation of valence electron density and plasmon energy in the free electron model. Hence, to introduce the effect of temperature and pressure for the density functional theory calculations of plasmon energy, the temperature and pressure dependency of lattice parameter was used. Also, by combination of the free electron model and the equation of state based on the pseudo-spinodal approach, the temperature and pressure dependency of the plasmon energy was modeled. The suggested model is in good agreement with the results of density functional theory calculations and available experimental data for elements with the free electron behavior.

Attarian Shandiz, M., E-mail: mohammad.attarianshandiz@mail.mcgill.ca; Gauvin, R. [Department of Materials Engineering, McGill University, Montreal, Quebec H3A 0C5 (Canada)

2014-10-28

128

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

129

Cell density-dependent oligopeptide production in cyanobacterial strains.  

PubMed

Cyanobacteria can form blooms and in these situations they dominate the phytoplanktonic community, reaching extremely high densities. In the domain Bacteria, high population densities can stimulate a phenomenon known as quorum sensing, which may produce several modifications in the cell physiology. Very little is known about quorum sensing in Cyanobacteria. Because of their planktonic way of life, quorum sensing should be more evident during a bloom event. In this work, we tested whether cell density could shape the production of bioactive compounds produced by Cyanobacteria. The experiments consisted of two treatments, where cultures of Cyanobacteria were maintained at low and high cellular densities through a semi-continuous set-up. Analyses were performed by HPLC-PDA and MALDI-TOF MS. Seventeen peptides were detected and 14 identified, including microcystins, aeruginosins, cyanopeptolins and microviridins. The results showed that cellular density seems to have a significant effect on the peptides production. Most of the compounds had significantly higher cellular quotas in the higher-density treatment, although microviridins and an unknown peptide were produced only at low density. These results may hint at a possible role for quorum sensing in triggering the production of several cyanobacterial peptides. PMID:24410818

Pereira, Daniel A; Giani, Alessandra

2014-04-01

130

Predators reverse the direction of density dependence for juvenile salmon mortality.  

PubMed

The effect of predators on prey populations depends on how predator-caused mortality changes with prey population density. Predators can enforce density-dependent prey mortality and contribute to population stability, but only if they have a positive numerical or behavioral response to increased prey density. Otherwise, predator saturation can result in inversely density-dependent mortality, destabilizing prey populations and increasing extinction risk. Juvenile salmon and trout provide some of the clearest empirical examples of density-dependent mortality in animal populations. However, although juvenile salmon are very vulnerable to predators, the demographic effects of predators on juvenile salmon are unknown. We tested the interactive effects of predators and population density on the mortality of juvenile Atlantic salmon (Salmo salar) using controlled releases of salmon in natural streams. We introduced newly hatched juvenile salmon at three population density treatments in six study streams, half of which contained slimy sculpin (Cottus cognatus), a common generalist predator (18 release sites in total, repeated over two summers). Sculpin reversed the direction of density dependence for juvenile salmon mortality. Salmon mortality was density dependent in streams with no sculpin, but inversely density dependent in streams where sculpin were abundant. Such predator-mediated inverse density dependence is especially problematic for prey populations suppressed by other factors, thereby presenting a fundamental challenge to persistence of rare populations and restoration of extirpated populations. PMID:18317816

Ward, Darren M; Nislow, Keith H; Folt, Carol L

2008-06-01

131

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

132

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

133

Benchmarking the performance of time-dependent density functional methods  

NASA Astrophysics Data System (ADS)

The performance of 24 density functionals, including 14 meta-generalized gradient approximation (mGGA) functionals, is assessed for the calculation of vertical excitation energies against an experimental benchmark set comprising 14 small- to medium-sized compounds with 101 total excited states. The experimental benchmark set consists of singlet, triplet, valence, and Rydberg excited states. The global-hybrid (GH) version of the Perdew-Burke-Ernzerhoff GGA density functional (PBE0) is found to offer the best overall performance with a mean absolute error (MAE) of 0.28 eV. The GH-mGGA Minnesota 2006 density functional with 54% Hartree-Fock exchange (M06-2X) gives a lower MAE of 0.26 eV, but this functional encounters some convergence problems in the ground state. The local density approximation functional consisting of the Slater exchange and Volk-Wilk-Nusair correlation functional (SVWN) outperformed all non-GH GGAs tested. The best pure density functional performance is obtained with the local version of the Minnesota 2006 mGGA density functional (M06-L) with an MAE of 0.41 eV.

Leang, Sarom S.; Zahariev, Federico; Gordon, Mark S.

2012-03-01

134

The viscosity of gaseous propane and its initial density dependence  

SciTech Connect

Results of five series of high-precision viscosity measurements on gaseous propane, each differing in density, are reported. The measurements were performed in a quartz oscillating-disk viscometer with small gaps from room temperature up to about 625 K and for densities between 0.01 and 0.05 mol {center_dot} L{sup -1}. The experimental data were evaluated with a first-order expansion, in terms of density, for the viscosity. Reduced values of the second viscosity virial coefficients deduced from the zero-density and initial-density viscosity coefficients for propane and for further n-alkanes are in close agreement with the theoretical representation of the Rainwater-Friend theory for the potential parameter ratios by Bich and Vogel. A new representation of the viscosity of propane in the limit of zero density is provided using the new experimental data and some data sets from literature. The universal correlation based on the extended principle of corresponding states extends over the temperature range 293 to 625 K with an uncertainty of {plus_minus}0.5% and deviates from earlier representations by about 1% at the upper temperature limit.

Vogel, E. [Universitaet Rostock (Germany)

1995-11-01

135

Average excitation energies from time-dependent density functional response theory  

NASA Astrophysics Data System (ADS)

The authors present an occupation number averaging scheme for time-dependent density functional response theory (TD-DFRT) in frequency domain. The known problem that TD-DFRT within the local (spin) density approximation (LDA/LSDA) inaccurately predicts Rydberg and charge-transfer excitation energies has been reexamined from the methodology of linear response, without explicit correction of the exchange-correlation potential. The working equations of TD-DFRT are adapted to treat arbitrary difference of orbital occupation numbers, using the nonsymmetric matrix form of Casida's formulation of TD-DFRT [M. E. Casida, in Recent Advances in Density Functional Methods, edited by D. P. Chong (World Scientific, Singapore, 1995), Pt. I, p. 155]. The authors' scheme is applied to typical closed-shell and open-shell molecular systems by examining the dependence of excitation energies on the fraction of excited electron. Good performance of this modified linear response scheme is shown, and is consistent with the authors' previous examination by the real-time propagation approach, suggesting that the calculation of average excitation energies might be one of the ways to better decode excitation energies from LDA/LSDA. Different techniques for treating singlet, triplet, and doublet states are discussed.

Hu, Chunping; Sugino, Osamu

2007-02-01

136

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

137

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

E-print Network

Motivated by an experimental interest we investigate by the means of atomistic Molecular Dynamics simulation the ability of density-independent, empiric density-dependent, and recently proposed embedded-atom force fields for liquid mercury to predict the surface tension of the free surface of liquid mercury at the temperature of 293~K. The effect of the density dependence of the studied models on the liquid-vapor coexistence and surface tension is discussed in detail. In view of computational efficiency of the density-independent model we optimize its functional form to obtain higher surface tension values in order to improve agreement with experiment. The results are also corroborated by Monte Carlo simulations and semi-analytic estimations of the liquid-vapor coexistence density.

A. Iakovlev; D. Bedrov; M. Müller

2015-03-14

138

Dynamics of kaonic nuclei in an improved quark mass density-dependent model  

NASA Astrophysics Data System (ADS)

The improved quark mass density-dependent model, which is able to provide a good description for the properties of both finite nuclei and bulk nuclear matter, is employed to investigate the properties of several possible light kaonic nuclei. The current approach is based on an extended version of the relativistic mean-field theory, where the kaon-nucleon and nucleon-nucleon interactions are treated on the same footing. In this work, the single K--nuclear states in the possible bubble nuclei are studied, with concentration on the experimentally accessible light nuclei. The K- binding energies, the decay widths, single-particle spectra, and nuclear and K- density distributions are evaluated. The calculations indicate that, when the K- meson is embedded in nuclei with speculated "bubble" structure, the depleted central nuclear density might be modified, and in certain cases, the bubble structure may even disappear. Furthermore, it is found that the properties of the kaonic nuclei are sensitive to the strength of the antikaon optical potential at saturation density.

Xu, R. L.; Wu, C.; Qian, W.-L.; Ren, Z. Z.

2015-02-01

139

Cell-density-dependent repression of discoidin in Dictyostelium discoideum.  

PubMed

When Dictyostelium discoideum cells are grown on bacteria, their natural food source, the discoidin genes are induced by cell-density-sensing factors before the food supply is exhausted [11, 18], and expression increases continuously thereafter. This regulation pattern is changed when cells are grown in axenic medium: the discoidins are induced at a considerably lower cell density and are no longer expressed in stationary phase [13]. We have investigated this phenomenon further and show that repression begins when cells are still in exponential growth. It occurs at the level of transcription and involves an element of the discoidin I gamma promoter for which no function has previously been described. Since the effect of high cell density can be mimicked by conditioned medium, it appears that the repression is due to an extracellular signal. This signal is neither ammonia, nor folate, nor cAMP, the known repressors of discoidin expression. PMID:8882814

Wetterauer, B W; Salger, K; Carballo-Metzner, C; MacWilliams, H K

1995-12-01

140

Scale Dependent Analysis Approach for Star Events  

NASA Astrophysics Data System (ADS)

This work presents a new approach to analyze multi-particle events in nucleus-nucleus collisions. Events multiplicity obtained recently at STAR detector gives us the possibility to estimate fractal dimension for each event and make classification of events based on this quantities. This analysis is applied for data from Au + Au collisions at ? {sNN} = 200 GeV and ? {sNN} = 62 and for different kinematic variables. It is shown that this analysis provides an information of the dynamical properties of events.

Rogachevsky, O. V.

2007-11-01

141

Fibroblast PER2 Circadian Rhythmicity Depends on Cell Density  

PubMed Central

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

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

2013-01-01

142

Bridging density-functional and many-body perturbation theory: Orbital-density dependence in electronic-structure functionals  

NASA Astrophysics Data System (ADS)

Energy functionals which depend explicitly on orbital densities, rather than on the total charge density, appear when applying self-interaction corrections to density-functional theory; this is, e.g., the case for Perdew-Zunger and Koopmans-compliant functionals. In these formulations the total energy is not invariant under unitary rotations of the orbitals, and local, orbital-dependent potentials emerge. We argue that this is not a shortcoming, and that instead these potentials can provide, in a functional form, a simplified quasiparticle approximation to the spectral potential, i.e., the local, frequency-dependent contraction of the many-body self-energy that is sufficient to describe exactly the spectral function. As such, orbital-density-dependent functionals have the flexibility to accurately describe both total energies and quasiparticle excitations in the electronic-structure problem. In addition, and at variance with the Kohn-Sham case, orbital-dependent potentials do not require nonanalytic derivative discontinuities. We present numerical solutions based on the frequency-dependent Sham-Schlüter equation to support this view, and examine some of the existing functionals in this perspective, highlighting the very close agreement between exact and approximate orbital-dependent potentials.

Ferretti, Andrea; Dabo, Ismaila; Cococcioni, Matteo; Marzari, Nicola

2014-05-01

143

Universality for Random Matrix Flows with Time-dependent Density  

E-print Network

We show that the Dyson Brownian Motion exhibits local universality after a very short time assuming that local rigidity and level repulsion hold. These conditions are verified, hence bulk spectral universality is proven, for a large class of Wigner-like matrices, including deformed Wigner ensembles and ensembles with non-stochastic variance matrices whose limiting densities differ from the Wigner semicircle law.

Laszlo Erdos; Kevin Schnelli

2015-04-02

144

Frequency and Density-Dependent Selection on Life-History Strategies – A Field Experiment  

PubMed Central

Negative frequency-dependence, which favors rare genotypes, promotes the maintenance of genetic variability and is of interest as a potential explanation for genetic differentiation. Density-dependent selection may also promote cyclic changes in frequencies of genotypes. Here we show evidence for both density-dependent and negative frequency-dependent selection on opposite life-history tactics (low or high reproductive effort, RE) in the bank vole (Myodes glareolus). Density-dependent selection was evident among the females with low RE, which were especially favored in low densities. Instead, both negative frequency-dependent and density-dependent selection were shown in females with high RE, which were most successful when they were rare in high densities. Furthermore, selection at the individual level affected the frequencies of tactics at the population level, so that the frequency of the rare high RE tactic increased significantly at high densities. We hypothesize that these two selection mechanisms (density- and negative frequency-dependent selection) may promote genetic variability in cyclic mammal populations. Nevertheless, it remains to be determined whether the origin of genetic variance in life-history traits is causally related to density variation (e.g. population cycles). PMID:18301764

Mappes, Tapio; Koivula, Minna; Koskela, Esa; Oksanen, Tuula A.; Savolainen, Tiina; Sinervo, Barry

2008-01-01

145

Transverse momentum dependent quark densities from Lattice QCD  

E-print Network

We study transverse momentum dependent parton distribution functions (TMDs) with non?local operators in lattice QCD, using MILC?LHPC lattices. We discuss the basic concepts of the method, including renormalization of the ...

Musch, B. U.

146

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

147

Spatially heterogeneous dynamics investigated via a time-dependent four-point density correlation function  

NASA Astrophysics Data System (ADS)

Relaxation in supercooled liquids above their glass transition and below the onset temperature of "slow" dynamics involves the correlated motion of neighboring particles. This correlated motion results in the appearance of spatially heterogeneous dynamics or "dynamical heterogeneity." Traditional two-point time-dependent density correlation functions, while providing information about the transient "caging" of particles on cooling, are unable to provide sufficiently detailed information about correlated motion and dynamical heterogeneity. Here, we study a four-point, time-dependent density correlation function g4(r,t) and corresponding "structure factor" S4(q,t) which measure the spatial correlations between the local liquid density at two points in space, each at two different times, and so are sensitive to dynamical heterogeneity. We study g4(r,t) and S4(q,t) via molecular dynamics simulations of a binary Lennard-Jones mixture approaching the mode coupling temperature from above. We find that the correlations between particles measured by g4(r,t) and S4(q,t) become increasingly pronounced on cooling. The corresponding dynamical correlation length ?4(t) extracted from the small-q behavior of S4(q,t) provides an estimate of the range of correlated particle motion. We find that ?4(t) has a maximum as a function of time t, and that the value of the maximum of ?4(t) increases steadily from less than one particle diameter to a value exceeding nine particle diameters in the temperature range approaching the mode coupling temperature from above. At the maximum, ?4(t) and the ? relaxation time ?? are related by a power law. We also examine the individual contributions to g4(r,t), S4(q,t), and ?4(t), as well as the corresponding order parameter Q(t) and generalized susceptibility ?4(t), arising from the self and distinct contributions to Q(t). These contributions elucidate key differences between domains of localized and delocalized particles.

La?evi?, N.; Starr, F. W.; Schrøder, T. B.; Glotzer, S. C.

2003-10-01

148

Cell-density-dependent Changes in the Metabolism of Chloronema Cell Cultures: I. Relationship between Cell Density and Enzymic Activities.  

PubMed

In the growing chloronema cell suspension cultures of the moss Funaria hygrometrica Hedw., activities of several enzymes have been found to be cell-density-dependent. Cyclic nucleotide phosphodiesterase (cNPDE), nitrate reductase (NR), and protein kinase showed highest activity at a low cell density (1 to 2 milligrams per milliliter) while indoleacetic acid (IAA) oxidase and peroxidase were highest at a high cell density (>10 milligrams per milliliter). 3'-Nucleotidase and the glycolytic enzymes (aldolase, hexokinase, phosphofructokinase, phosphoglucoisomerase, pyruvate kinase, and triose phosphate isomerase) showed no significant dependence on the cell density. Alternatively, if the NR and peroxidase activities were determined as a function of time in batch cultures, their levels were maximal 60 to 70 and 320 hours after subculture, respectively, the corresponding cell densities being 1 to 2 and 23 milligrams per milliliter. The relationship between cell density and NR and peroxidase activities is the same, whether these enzymes are measured in batch cultures during a growth cycle or in the cells cultured at different initial inoculum densities for a constant time. Conventionally enzymic changes have been correlated with growth phases; however, it is felt that the pattern of enzymic activities can also be interpreted as cell-density-dependent.In moss protonema, the dependence of cNPDE, IAA oxidase, and peroxidase on cell density may play an important role in modulating the endogenous levels of IAA and cAMP, both of which regulate the differentiation of specific cell types (Johri and Desai 1973 Nature New Biol 245: 223-224; and Handa and Johri 1976 Nature 259: 480-482). PMID:16660905

Sharma, S; Jayaswal, R K; Johri, M M

1979-07-01

149

High temperature intensifies negative density dependence of fitness in red flour beetles  

PubMed Central

Competition for food, space, or other depletable resources has strong impacts on the fitness of organisms and can lead to a pattern known as negative density dependence, where fitness decreases as population density increases. Yet, many resources that have strong impacts on fitness are nondepletable (e.g., moisture or temperature). How do these nondepletable resources interact with depletable resources to modify negative density dependence? We tested the hypothesis that negative density dependence is modulated by temperature in red flour beetles and tested the prediction that the strength of negative density dependence should decrease as temperature decreases. We measured the number of eggs laid, offspring development time, and the number of offspring that reached maturity at three temperatures and two food treatment combinations as we simultaneously manipulated adult population density. We demonstrated that low temperatures weaken negative density dependence in the number of eggs laid; this pattern was most evident when food was abundant. Density had no effect on development time, but low temperatures increased development time. The percent of eggs that emerged as adults decreased with both density and temperature and increased with food. Temperature, an abiotic driver, can thus modulate density-dependent processes in ectotherms. Therefore, models of population growth for ectotherms should incorporate the effects of temperature. PMID:25798223

Halliday, William D; Thomas, Alison S; Blouin-Demers, Gabriel

2015-01-01

150

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

151

Electronvibration coupling in time-dependent density-functional theory: Application to benzene  

E-print Network

Electron­vibration coupling in time-dependent density-functional theory: Application to benzene G://jcp.aip.org/about/rights_and_permissions #12;Electron­vibration coupling in time-dependent density-functional theory: Application to benzene G for electron­vibration coupling, we apply it to the optical properties of the ­ * transitions in benzene

Bertsch George F.

152

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

E-print Network

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

Wilde, Gene

153

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

Microsoft Academic Search

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

William E. Jensen; Jack F. Cully

2005-01-01

154

Density-Dependent Growth and Fecundity in the Paedomorphic Salamander Ambystoma Talpoideum  

E-print Network

Density-Dependent Growth and Fecundity in the Paedomorphic Salamander Ambystoma Talpoideum Raymond~caiSociety of America DENSITY-DEPENDENT GROWTH AND FECUNDITY IN THE PAEDOMORPHIC SALAMANDER AMBYSTOMA TALPOIDEUM1- ically affect the ultimate reproductive fitness of the next generation. Key rtjords: Ambystoma talpoideum

Dever, Jennifer A.

155

Permanence of a delayed SIR epidemic model with density dependent birth rate  

Microsoft Academic Search

In this paper, we consider the permanence of a modified delayed SIR epidemic model with density dependent birth rate which is proposed in [M. Song, W. Ma, Asymptotic properties of a revised SIR epidemic model with density dependent birth rate and time delay, Dynamic of Continuous, Discrete and Impulsive Systems, 13 (2006) 199–208]. It is shown that global dynamic property

Mei Song; Wanbiao Ma; Yasuhiro Takeuchi

2007-01-01

156

Intra-and interspecific density-dependent effects on growth in helminth parasites of the cormorant,  

E-print Network

; accepted 4 December 2001) SUMMARY The action of intra- and interspecific competition, mediated by density their intraspecific effects, density-depen- dent processes can have interspecific influences, i.e. the number of worms537 Intra- and interspecific density-dependent effects on growth in helminth parasites

Poulin, Robert

157

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

158

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

159

Density-dependent life history and the dynamics of small populations.  

PubMed

1. Small population dynamics depend importantly on the strength and shape of density dependence. Unfortunately, the lack of reliable life-history data often prevents to make accurate demographic predictions for populations regulated by density dependence. 2. We created a gradient from low to high densities in small experimental populations of common lizards (Zootoca vivipara) and investigated the shape and strength of the density dependence of life-history traits during a yearly cycle. We then analysed stochastic population dynamics using one-sex and two-sex age-structured matrix models. 3. Body growth and reproductive performances decreased with density, yearling and adult survival and body size at birth were density-independent, and juvenile survival increased with density. The density dependence of reproduction was partly explained by positive effects of body size on age at first reproduction and clutch size. 4. Parturition date decreased with density in sparse populations and then increased, providing one of the first empirical evidence of a component Allee effect in the phenology of reproduction. 5. Population growth rate (?) was most affected by variations in juvenile and yearling survival. However, density at equilibrium was most affected by juvenile access to reproduction and yearling clutch size. 6. Stochastic simulations revealed that negative density dependence buffers the effects of initial density on extinction probability, has positive effects on the persistence of sparse populations and interacts with sex ratio fluctuations to shape extinction dynamics. 7. This study demonstrates that negative density dependence modifies the dynamics of small populations and should be investigated together with Allee effects to predict extinction risks. PMID:23859253

Mugabo, Marianne; Perret, Samuel; Legendre, Stéphane; Le Galliard, Jean-François

2013-11-01

160

Broadcasting but not receiving: density dependence considerations for SETI signals  

E-print Network

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

Reginald D. Smith

2010-02-07

161

Determining Locations for Wireless Sensor Node Placement with Problem Dependent Node Densities  

SciTech Connect

Fast, effective monitoring following airborne releases of toxic substances is critical to mitigate risks to threatened population areas. Wireless sensor nodes at fixed predetermined locations may monitor such airborne releases and provide early warnings to the public. A challenging algorithmic problem is determining the locations to place these sensor nodes while meeting several criteria: 1) provide complete coverage of the domain, and 2) create a topology with problem dependent node densities, while 3) minimizing the number of sensor nodes. This manuscript presents a novel approach to determining optimal sensor placement, Advancing Front mEsh generation with Constrained dElaunay Triangulation and Smoothing (AFECETS) that addresses these criteria. A unique aspect of AFECETS is the ability to determine wireless sensor node locations for areas of high interest (hospitals, schools, high population density areas) that require higher density of nodes for monitoring environmental conditions, a feature that is difficult to find in other research work. The AFECETS algorithm was tested on several arbitrary shaped domains. AFECETS simulation results show that the algorithm 1) provides significant reduction in the number of nodes, in some cases over 40%, compared to an advancing front mesh generation algorithm, 2) maintains and improves optimal spacing between nodes, and 3) produces simulation run times suitable for real-time applications.

Kurt Derr; Milos Manic

2015-01-01

162

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

163

Load balancing and density dependent jump markov processes  

SciTech Connect

We provide a new approach for analyzing both static and dynamic randomized load balancing strategies. We demonstrate the approach by providing the first analysis of the following model: customers arrive as a Poisson stream of rate {lambda}n, {lambda} < 1, at a collection of n servers. Each customer chooses some constant d servers independently and uniformly at random from the n servers, and waits for service at the one with the fewest customers. Customers are served according to the first-in first-out (FIFO) protocol, and the service time for a customer is exponentially distributed with mean 1. We call this problem the supermarket model. We wish to know how the system behaves, and in particular we are interested the expected time a customer spends in the system in equilibrium. The model provides a good abstraction of a simple, efficient load balancing scheme in the setting where jobs arrive at a large system of parallel processors. This model appears more realistic than similar models studied previously, in that it is both dynamic and open: that is, customers arrive over time, and the number of customers is not fixed. Our approach consists of two distinct stages: we first develop a limiting, deterministic model representing the behavior as n {r_arrow} {infinity}, and then show how to translate results from this model to results for large, but finite, values of n. The analysis of the deterministic model is interesting in its own right. This methodology proves effective for studying a number of similar problems, and simulations demonstrate that the method accurately predicts system behavior even for relatively small systems.

Mitzenmacher, M. [Univ. of California, Berkeley, CA (United States)

1996-12-31

164

Measuring the mobility of single crystalline wires and its dependence on temperature and carrier density  

NASA Astrophysics Data System (ADS)

Kinetic transport parameters are fundamental for the development of electronic nanodevices. We present new results for the temperature dependence of mobility and carrier density in single crystalline In2O3 samples and the method of extraction of these parameters which can be extended to similar systems. The data were obtained using a conventional Hall geometry and were quantitatively described by the semiconductor transport theory characterizing the electron transport as being controlled by the variable range hopping mechanism. A comprehensive analysis is provided showing the contribution of ionized impurities (low temperatures) and acoustic phonon (high temperatures) scattering mechanisms to the electron mobility. The approach presented here avoids common errors in kinetic parameter extraction from field effect data, serving as a versatile platform for direct investigation of any nanoscale electronic materials.

Amorim, Cleber A.; Berengue, Olivia M.; Kamimura, Hanay; Leite, Edson R.; Chiquito, Adenilson J.

2011-05-01

165

Troubleshooting time-dependent density-functional theory for photochemical applications: Oxirane  

SciTech Connect

The development of analytic-gradient methodology for excited states within conventional time-dependent density-functional theory (TDDFT) would seem to offer a relatively inexpensive alternative to better established quantum-chemical approaches for the modeling of photochemical reactions. However, even though TDDFT is formally exact, practical calculations involve the use of approximate functional, in particular the TDDFT adiabatic approximation, the use of which in photochemical applications must be further validated. Here, we investigate the prototypical case of the symmetric CC ring opening of oxirane. We demonstrate by direct comparison with the results of high-quality quantum Monte Carlo calculations that, far from being an approximation on TDDFT, the Tamm-Dancoff approximation is a practical necessity for avoiding triplet instabilities and singlet near instabilities, thus helping maintain energetically reasonable excited-state potential energy surfaces during bond breaking. Other difficulties one would encounter in modeling oxirane photodynamics are pointed out.

Cordova, Felipe; Doriol, L. Joubert; Ipatov, Andrei; Casida, Mark E.; Filippi, Claudia; Vela, Alberto [Laboratoire de Chimie Theorique, Departement de Chimie Molecularie (DCM, UMR CNRS/UJF 5250), Institut de Chimie Moleculaire de Grenoble (ICMG, FR2607), Universite Joseph Fourier - Grenoble I, 301 rue de la Chimie, BP 53, F-38041 Grenoble Cedex 9 (France); Instituut-Lorentz for Theoretical Physics, Universiteit Leiden, Niels Bohrweg 2, Leiden NL-2333 CA Netherlands (Netherlands); Departamento de Quimica, Cinvestav, Avenida Instituto Politecnico Nacional 2508, A.P. 14-740, Mexico D.F. 07000 (Mexico)

2007-10-28

166

Density dependent effective interactions and recollections of the Rutgers-Princeton years  

NASA Astrophysics Data System (ADS)

The density-dependent effective interactions given by the new Brown-Rho (new- BR) scalings and chiral three-nucleon force V3N are compared with the empirical density- dependent force of the Skyrme interactions. The new-BR scaling is based on a Skyrmion- half-Skyrmion model where nuclear matter is treated as a Skyrmion matter for density smaller than a transition density n1/2 ? 0.32 fm?3 while a half-Skyrmion matter for density greater. In this model, the meson mass, nucleon mass and meson-nucleon coupling are all scaled with density, making the resulting two-nucleon interaction density dependent. By integrating out a participating nucleon over the Fermi sea, Holt, Kaiser and Weise have obtained an effective three-nucleon force bar V3N which is also a density-dependent two-nucleon interaction. The equation of state for symmetric nuclear matter given by the new-BR-scaled V2N, bar V3N, and (unscaled-V2N + a Skyrme-type density-dependent force) are all found to be closely similar to each other.

Kuo, T. T. S.

2015-02-01

167

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

168

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

169

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

170

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

171

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

NASA Astrophysics Data System (ADS)

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

Ullrich, Carsten A.; D'Amico, Irene

2013-03-01

172

A Laplace-like formula for the energy dependence of the nuclear level density parameter  

E-print Network

Collective effects in the level density are not well understood, and including these effects as enhancement factors to the level density does not produce sufficiently consistent predictions of observables. Therefore, collective effects are investigated in the level density parameter instead of treating them as a final factor in the level density. A new Laplace-like formula is proposed for the energy dependence of the level density parameter, including collective effects. A significant improvement has been achieved in agreement between observed and predicted energy levels. This new model can also be used in both structure and reaction calculations of the nuclei far from stability, especially near the drip lines.

Bora Canbula; Ramazan Bulur; Deniz Canbula; Halil Babacan

2014-06-05

173

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

PubMed

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

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

2009-01-01

174

Exchange-correlation and QED effects from a density functional based level shift approach  

NASA Astrophysics Data System (ADS)

Perturbation theory based on the auxiliary noninteracting Kohn-Sham Hamiltonian allows a systematic study of the exchange-correlation (xc) energy functional of nonrelativistic density functional theory. In particular, it serves as the starting point for the derivation of implicit (orbital-dependent) density functionals for the xc-functional. Within the framework of quantum electrodynamics, a relativistic variant of Kohn-Sham perturbation theory has been formulated in Phys. Rev. A 58 (1998) 964. In this approach, a coupling constant integration technique is used for the exact representation of the relativistic xc-functional. In the present contribution an alternative systematic approach to this functional is put forward, which is based on a density functional version of Sucher's level shift formula. As in the case of the coupling constant integration form, an exact relation for the xc-functional can be established. This relation provides the basis for a perturbation expansion to second order, including all inherent radiative corrections. A detailed comparison with the coupling constant integration result verifies the equivalence of both approaches. Finally, the first order level shift is analyzed in order to identify the Lamb shift within the Kohn-Sham scheme.

Engel, E.; Lechner, U.

2005-04-01

175

Relativistic mean field interaction with density dependent meson-nucleon vertices based on microscopical calculations  

E-print Network

Although ab-initio calculations of relativistic Brueckner theory lead to large scalar isovector fields in nuclear matter, at present, successful versions of covariant density functional theory neglect the interactions in this channel. A new high precision density functional DD-ME$\\delta$ is presented which includes four mesons $\\sigma$, $\\omega$, $\\delta$, and $\\rho$ with density dependent meson-nucleon couplings. It is based to a large extent on microscopic ab-initio calculations in nuclear matter. Only four of its parameters are determined by adjusting to binding energies and charge radii of finite nuclei. The other parameters, in particular the density dependence of the meson-nucleon vertices, are adjusted to non-relativistic and relativistic Brueckner calculations of symmetric and asymmetric nuclear matter. The isovector effective mass $m_{p}^{\\ast}-m_{n}^{\\ast}$ derived from relativistic Brueckner theory is used to determine the coupling strength of the $\\delta$-meson and its density dependence.

X. Roca-Maza; X. Viñas; M. Centelles; P. Ring; P. Schuck

2011-11-16

176

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

E-print Network

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

Karan Singh Vinayak; Suneel Kumar

2011-12-22

177

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

E-print Network

. Gross Institut für Theoretische Physik, Freie Universität Berlin, Arnimallee 14, 14195 Berlin, Germany function, particle statistics, and interaction, a given time-dependent density n rt can arise from at most

Burke, Kieron

178

Coarse-grained, density dependent implicit solvent model reliably reproduces behavior of a model surfactant system  

E-print Network

Density dependent, implicit solvent (DDIS) potentials, the generation of which has been described previously [ E. C. Allen and G. C. Rutledge, J. Chem. Phys. 128, 154115 (2008) ; E. C. Allen and G. C. Rutledge, J. Chem. ...

Allen, Erik C.

179

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

E-print Network

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. 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. 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. 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.

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

2014-05-20

180

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

Microsoft Academic Search

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

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

2006-01-01

181

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

182

On the existence of effective potentials in time-dependent density functional theory  

E-print Network

We investigate the existence and properties of effective potentials in time-dependent density functional theory. We outline conditions for a general solution of the corresponding Sturm-Liouville boundary value problems. We define the set of potentials and v-representable densities, give a proof of existence of the effective potentials under certain restrictions, and show the set of v-representable densities to be independent of the interaction.

M. Ruggenthaler; M. Penz; D. Bauer

2009-11-10

183

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

184

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

Microsoft Academic Search

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

J. Van Buskirk

1987-01-01

185

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

E-print Network

Invasions with density-dependent ecological parameters Sanjeeva Balasuriya a,b,Ã? a School to consumption, dispersal adaptation due to population pressure, biological control agents, and a range fauna and flora from invasive species. Factors influencing this include the species' range, density

Balasuriya, Sanjeeva

186

Magnetic Field Dependence of Critical Current Density of Polycrystalline Tl2223 Wire  

Microsoft Academic Search

The magnetic field dependence of critical current density Jc for polycrystalline Tl-2223 tape-shaped wire was examined in terms of a model regarding a sample as a bulk which consists of an anisotropic superconductor with an incomplete crystal alignment. We assumed that the critical current density in each grain would be governed only by the perpendicular component of magnetic field to

Michiya Okada; Toshihide Nabatame; Toyotaka Yuasa; Katsuzo Aihara; Masahiro Seido; Shinpei Matsuda

1991-01-01

187

ANALYSIS OF DENSITY-DEPENDENT SURVIVAL OF DIABROTICA (COLEOPTERA: CHRYSOMELIDAE) IN CORN FIELDS  

Technology Transfer Automated Retrieval System (TEKTRAN)

We analyzed published field data concerning rootworm survival from egg to adult to create general relationships between density and survival. A series of equations were generated that best fit the data and these equations may be used to help understand density-dependent survival of future studies. ...

188

Time-dependent series variance learning with recurrent mixture density networks  

E-print Network

heteroskedastic model, as well as with the nonlinear mixture density network trained with static derivatives, our non-linear GARCH based on neural networks [5,14]. However, this approach to volatility modeling using Keywords: Mixture density neural networks GARCH models Real-time recurrent learning algorithm a b s t r a c

Tino, Peter

189

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

190

Morphological transitions governed by density dependence and lipoxygenase activity in Aspergillus flavus.  

PubMed

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 10(1) to 10(7) 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 (10(4) and 10(5) 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 (>10(5) 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

2008-09-01

191

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

192

Time-dependent density functional theory for open systems with a positivity-preserving decomposition scheme for environment spectral functions.  

PubMed

Understanding electronic dynamics on material surfaces is fundamentally important for applications including nanoelectronics, inhomogeneous catalysis, and photovoltaics. Practical approaches based on time-dependent density functional theory for open systems have been developed to characterize the dissipative dynamics of electrons in bulk materials. The accuracy and reliability of such approaches depend critically on how the electronic structure and memory effects of surrounding material environment are accounted for. In this work, we develop a novel squared-Lorentzian decomposition scheme, which preserves the positive semi-definiteness of the environment spectral matrix. The resulting electronic dynamics is guaranteed to be both accurate and convergent even in the long-time limit. The long-time stability of electronic dynamics simulation is thus greatly improved within the current decomposition scheme. The validity and usefulness of our new approach are exemplified via two prototypical model systems: quasi-one-dimensional atomic chains and two-dimensional bilayer graphene. PMID:25877567

Wang, RuLin; Zheng, Xiao; Kwok, YanHo; Xie, Hang; Chen, GuanHua; Yam, ChiYung

2015-04-14

193

Spatial, Temporal, and Density-Dependent Components of Habitat Quality for a Desert Owl  

PubMed Central

Spatial variation in resources is a fundamental driver of habitat quality but the realized value of resources at any point in space may depend on the effects of conspecifics and stochastic factors, such as weather, which vary through time. We evaluated the relative and combined effects of habitat resources, weather, and conspecifics on habitat quality for ferruginous pygmy-owls (Glaucidium brasilianum) in the Sonoran Desert of northwest Mexico by monitoring reproductive output and conspecific abundance over 10 years in and around 107 territory patches. Variation in reproductive output was much greater across space than time, and although habitat resources explained a much greater proportion of that variation (0.70) than weather (0.17) or conspecifics (0.13), evidence for interactions among each of these components of the environment was strong. Relative to habitat that was persistently low in quality, high-quality habitat buffered the negative effects of conspecifics and amplified the benefits of favorable weather, but did not buffer the disadvantages of harsh weather. Moreover, the positive effects of favorable weather at low conspecific densities were offset by intraspecific competition at high densities. Although realized habitat quality declined with increasing conspecific density suggesting interference mechanisms associated with an Ideal Free Distribution, broad spatial heterogeneity in habitat quality persisted. Factors linked to food resources had positive effects on reproductive output but only where nest cavities were sufficiently abundant to mitigate the negative effects of heterospecific enemies. Annual precipitation and brooding-season temperature had strong multiplicative effects on reproductive output, which declined at increasing rates as drought and temperature increased, reflecting conditions predicted to become more frequent with climate change. Because the collective environment influences habitat quality in complex ways, integrated approaches that consider habitat resources, stochastic factors, and conspecifics are necessary to accurately assess habitat quality. PMID:25786257

Flesch, Aaron D.; Hutto, Richard L.; van Leeuwen, Willem J. D.; Hartfield, Kyle; Jacobs, Sky

2015-01-01

194

Spatial, temporal, and density-dependent components of habitat quality for a desert owl.  

PubMed

Spatial variation in resources is a fundamental driver of habitat quality but the realized value of resources at any point in space may depend on the effects of conspecifics and stochastic factors, such as weather, which vary through time. We evaluated the relative and combined effects of habitat resources, weather, and conspecifics on habitat quality for ferruginous pygmy-owls (Glaucidium brasilianum) in the Sonoran Desert of northwest Mexico by monitoring reproductive output and conspecific abundance over 10 years in and around 107 territory patches. Variation in reproductive output was much greater across space than time, and although habitat resources explained a much greater proportion of that variation (0.70) than weather (0.17) or conspecifics (0.13), evidence for interactions among each of these components of the environment was strong. Relative to habitat that was persistently low in quality, high-quality habitat buffered the negative effects of conspecifics and amplified the benefits of favorable weather, but did not buffer the disadvantages of harsh weather. Moreover, the positive effects of favorable weather at low conspecific densities were offset by intraspecific competition at high densities. Although realized habitat quality declined with increasing conspecific density suggesting interference mechanisms associated with an Ideal Free Distribution, broad spatial heterogeneity in habitat quality persisted. Factors linked to food resources had positive effects on reproductive output but only where nest cavities were sufficiently abundant to mitigate the negative effects of heterospecific enemies. Annual precipitation and brooding-season temperature had strong multiplicative effects on reproductive output, which declined at increasing rates as drought and temperature increased, reflecting conditions predicted to become more frequent with climate change. Because the collective environment influences habitat quality in complex ways, integrated approaches that consider habitat resources, stochastic factors, and conspecifics are necessary to accurately assess habitat quality. PMID:25786257

Flesch, Aaron D; Hutto, Richard L; van Leeuwen, Willem J D; Hartfield, Kyle; Jacobs, Sky

2015-01-01

195

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

196

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

USGS Publications Warehouse

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

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

2009-01-01

197

Skyrme Energy-Density Functional Approach to Collective Modes of Excitation in Exotic Nuclei  

NASA Astrophysics Data System (ADS)

Our recent developments for the microscopic description of nuclear collective dynamics in the framework of the time-dependent density functional theory is presented. We show two examples; one is for small-amplitude collective dynamics and the other for the large-amplitude collective dynamics. First, it is shown that the quasiparticle random-phase approximation is ready for the systematic investigation of the giant resonances in the entire mass region of nuclear chart. Secondly, we show that the shape-phase transition investigated by employing the collective Hamiltonian approach gives the quantitative description of the low-lying states in transitional nuclei with the Skyrme and pairing energy density functionals as a microscopic input.

Yoshida, K.

2014-09-01

198

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

199

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

PubMed

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/km(2), and a low-density population at 4 elk/km(2) 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

2015-02-01

200

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

PubMed

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

Jensen, William E; Cully, Jack F

2005-01-01

201

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

202

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

203

Time-dependent first-principles approaches to PV materials  

SciTech Connect

Computational scheme for designing photovoltaic (PV) materials is presented. First-principles electron dynamics of photo-excitation and subsequent electron-hole splitting is performed based on the time-dependent density functional theory. Photo-induced enhancement of dipole moment was observed in a polar crystal and a donor-acceptor molecular pair. These experiences will pave a way to design PV material from first-principles simulations.

Miyamoto, Yoshiyuki [Nanosystem Research Institute, AIST, Central 2, 1-1-1, Umezono, Tsukuba, 305-8568 (Japan)

2013-12-10

204

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

E-print Network

of anisogamy, leading to sex role differentiation, and the frequency dependence of reproductive successLonely hearts or sex in the city? Density-dependent effects in mating systems Hanna Kokko that tends to equalize primary sex ratios. However, being explicit about the numbers of potential mates

Rankin, Daniel

205

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.

206

Finite metapopulation models with density-dependent migration and stochastic local dynamics  

PubMed Central

The effects of small density-dependent migration on the dynamics of a metapopulation are studied in a model with stochastic local dynamics. We use a diffusion approximation to study how changes in the migration rate and habitat occupancy affect the rates of local colonization and extinction. If the emigration rate increases or if the immigration rate decreases with local population size, a positive expected rate of change in habitat occupancy is found for a greater range of habitat occupancies than when the migration is density-independent. In contrast, the reverse patterns of density dependence in respective emigration and immigration reduce the range of habitat occupancies where the metapopulation will be viable. This occurs because density-dependent migration strongly influences both the establishment and rescue effects in the local dynamics of metapopulations.

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

1999-01-01

207

Investigations of turbulent scalar fields using probability density function approach  

NASA Technical Reports Server (NTRS)

Scalar fields undergoing random advection have attracted much attention from researchers in both the theoretical and practical sectors. Research interest spans from the study of the small scale structures of turbulent scalar fields to the modeling and simulations of turbulent reacting flows. The probability density function (PDF) method is an effective tool in the study of turbulent scalar fields, especially for those which involve chemical reactions. It has been argued that a one-point, joint PDF approach is the one to choose from among many simulation and closure methods for turbulent combustion and chemically reacting flows based on its practical feasibility in the foreseeable future for multiple reactants. Instead of the multi-point PDF, the joint PDF of a scalar and its gradient which represents the roles of both scalar and scalar diffusion is introduced. A proper closure model for the molecular diffusion term in the PDF equation is investigated. Another direction in this research is to study the mapping closure method that has been recently proposed to deal with the PDF's in turbulent fields. This method seems to have captured the physics correctly when applied to diffusion problems. However, if the turbulent stretching is included, the amplitude mapping has to be supplemented by either adjusting the parameters representing turbulent stretching at each time step or by introducing the coordinate mapping. This technique is still under development and seems to be quite promising. The final objective of this project is to understand some fundamental properties of the turbulent scalar fields and to develop practical numerical schemes that are capable of handling turbulent reacting flows.

Gao, Feng

1991-01-01

208

Keystone and intraguild predation, intraspecific density dependence, and a guild of coexisting consumers.  

PubMed

Previous models of diamond-shaped and intraguild predation community modules have represented the essence of the trade-off necessary for a top predator to prevent competitive exclusion among a set of resource-limited consumers. However, at most two consumers can coexist in these models. In this article, I show how intraspecific density dependence in the consumers can permit many more than two consumers to coexist in these community modules. Moreover, responses of the community to removal of the top predator depend on the patterns of the strengths of species interactions relative to the strengths of intraspecific density dependence. If the consumers experience similar strengths of intraspecific density dependence, removing the top predator will in most cases have little effect on consumer species richness. A substantial reduction in consumer species richness with predator removal (i.e., the keystone predation effect) will typically occur only when the consumer that can support a population at the lowest resource abundance also (1) experiences substantially weaker intraspecific density dependence than other consumers and (2) experiences significantly higher levels of mortality from the predator. These results identify how intraspecific density dependence fosters the coexistence of multiple consumers in two important community modules and shapes the responses of these community modules to perturbations such as predator removal. PMID:24334745

McPeek, Mark A

2014-01-01

209

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

210

A New Approach of Designing Superalloys for Low Density  

NASA Technical Reports Server (NTRS)

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

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

2010-01-01

211

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

SciTech Connect

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

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

2009-02-12

212

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

213

Negative density dependence of seed dispersal and seedling recruitment in a neotropical palm.  

PubMed

Negative density dependence (NDD) of recruitment is pervasive in tropical tree species. We tested the hypotheses that seed dispersal is NDD, due to intraspecific competition for dispersers, and that this contributes to NDD of recruitment. We compared dispersal in the palm Attalea butyracea across a wide range of population density on Barro Colorado Island in Panama and assessed its consequences for seed distributions. We found that frugivore visitation, seed removal and dispersal distance all declined with population density of A. butyracea, demonstrating NDD of seed dispersal due to competition for dispersers. Furthermore, as population density increased, the distances of seeds from the nearest adult decreased, conspecific seed crowding increased and seedling recruitment success decreased, all patterns expected under poorer dispersal. Unexpectedly, however, our analyses showed that NDD of dispersal did not contribute substantially to these changes in the quality of the seed distribution; patterns with population density were dominated by effects due solely to increasing adult and seed density. PMID:25039608

Jansen, Patrick A; Visser, Marco D; Joseph Wright, S; Rutten, Gemma; Muller-Landau, Helene C

2014-09-01

214

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...

215

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

216

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

217

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

218

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

PubMed Central

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

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

2008-01-01

219

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

220

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

221

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

222

Effects of hepatocyte growth factor on glutathione synthesis, growth, and apoptosis is cell density-dependent  

SciTech Connect

Hepatocyte growth factor (HGF) is a potent hepatocyte mitogen that exerts opposing effects depending on cell density. Glutathione (GSH) is the main non-protein thiol in mammalian cells that modulates growth and apoptosis. We previously showed that GSH level is inversely related to cell density of hepatocytes and is positively related to growth. Our current work examined whether HGF can modulate GSH synthesis in a cell density-dependent manner and how GSH in turn influence HGF's effects. We found HGF treatment of H4IIE cells increased cell GSH levels only under subconfluent density. The increase in cell GSH under low density was due to increased transcription of GSH synthetic enzymes. This correlated with increased protein levels and nuclear binding activities of c-Jun, c-Fos, p65, p50, Nrf1 and Nrf2 to the promoter region of these genes. HGF acts as a mitogen in H4IIE cells under low cell density and protects against tumor necrosis factor {alpha} (TNF{alpha})-induced apoptosis by limiting JNK activation. However, HGF is pro-apoptotic under high cell density and exacerbates TNF{alpha}-induced apoptosis by potentiating JNK activation. The increase in cell GSH under low cell density allows HGF to exert its full mitogenic effect but is not necessary for its anti-apoptotic effect.

Yang Heping; Magilnick, Nathaniel; Xia Meng [Division of Gastroenterology and Liver Diseases, USC Research Center for Liver Diseases, USC-UCLA Research Center for Alcoholic Liver and Pancreatic Diseases, Keck School of Medicine USC, Los Angeles, CA 90033 (United States); Lu, Shelly C. [Division of Gastroenterology and Liver Diseases, USC Research Center for Liver Diseases, USC-UCLA Research Center for Alcoholic Liver and Pancreatic Diseases, Keck School of Medicine USC, Los Angeles, CA 90033 (United States)], E-mail: shellylu@usc.edu

2008-01-15

223

Propagator corrections to adiabatic time-dependent density-functional theory linear response theory  

NASA Astrophysics Data System (ADS)

It has long been known that only one-electron excitations are available from adiabatic time-dependent density functional theory (TDDFT). This is particularly clear in Casida's formulation of TDDFT linear response theory [M. E. Casida, in Recent Advances in Density Functional Methods, Part I, edited by D. P. Chong (World Scientific, Singapore, 1995), p. 155]. Nevertheless the explicit inclusion of two- and higher-electron excitations is necessary for an adequate description of some excited states, notably the first excited singlet states of butadiene and quartet excited states of molecules with a doublet ground state. The equation-of-motion superoperator approach is used here to derive a Casida-like propagator equation which can be clearly separated into an adiabatic part and a nonadiabatic part. The adiabatic part is identified as corresponding to Casida's equation for adiabatic TDDFT linear response theory. This equivalence is confirmed by deriving a general formula which includes the result that Gonze and Scheffler derived to show the equivalence of TDDFT and Görling-Levy adiabatic connection perturbation theory for the exchange-only optimized effective potential [X. Gonze and M. Scheffler, Phys. Rev. Lett. 82, 4416 (1999)]. The nonadiabatic part explicitly corrects adiabatic TDDFT for two- and higher-electron excitations. The "dressed TDDFT" of Maitra, Zhang, Cave, and Burke is obtained as a special case where the ground state is closed shell [N. T. Maitra, F. Zhang, R. J. Cave, and K. Burke, J. Chem. Phys. 120, 5932 (2004)]. The extension of dressed TDDFT to the case where the ground state is an open-shell doublet is presented, highlighting the importance of correctly accounting for symmetry in this theory. The extension to other ground state spin symmetries is a straightforward consequence of the present work.

Casida, Mark E.

2005-02-01

224

A geometric approach to dislocation densities in semiconductors  

NASA Astrophysics Data System (ADS)

Dislocation densities threading semiconductor crystals are a problem for device developers. Among the issues presented by the defect density is the appearance of the so-called shallow levels. In this work, we introduce a geometric model to explain the origin of the observed shallow levels. We show that a uniform distribution of screw dislocations acts as an effective uniform magnetic field which yields electronic bound states even in the presence of a repulsive Coulomb-like potential. This introduces energy levels within the band gap, increasing the carrier concentration in the region threaded by the dislocation density and adding additional recombination paths other than the near band-edge recombination. Our results suggest that one might use a magnetic field to destroy the dislocation density bound states and therefore minimize its effects on the charge carriers.

Bakke, K.; Moraes, F.

2014-05-01

225

A Geometric Approach to Dislocation Densities in Semiconductors  

NASA Astrophysics Data System (ADS)

Dislocation densities threading semiconductor crystals are a problem for device developers. Among the issues presented by the defect density is the appearance of the so called shallow levels. In this work we introduce a geometric model to explain the origin of the observed shallow levels. We show that a uniform distribution of screw dislocations acts as an effective uniform magnetic field which yields electronic bound states even in the presence of a repulsive Coulomb-like potential. This introduces energy levels within the band gap, increasing the carrier concentration in the region threaded by the dislocation density and adding additional recombination paths other than the near band-edge recombination. Our results suggest that one might use a magnetic field to destroy the dislocation density bound states and therefore minimize its effects on the charge carriers.

Bakke, Knut; Moraes, Fernando

2015-10-01

226

Inversely density-dependent natal dispersal in brown bears Ursus arctos  

Microsoft Academic Search

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

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

2006-01-01

227

Wave fronts in a bistable reaction-diffusion system with density-dependent diffusivity  

Microsoft Academic Search

We obtain wave-front solutions for a one-dimensional bistable reaction-diffusion model with density-dependent diffusivity. These solutions - which are expected to stand for the asymptotic behaviour of a wide class of initial conditions - should describe the evolution of the walls of constant density domains, spontaneously formed in this system. The piecewise linearized form of the reaction terms and of the

D. E. Strier; D. H. Zanette; Horacio S. Wio

1996-01-01

228

Wave fronts in a bistable reaction-diffusion system with density-dependent diffusivity  

Microsoft Academic Search

We obtain wave-front solutions for a one-dimensional bistable reaction-diffusion model with density-dependent diffusivity. These solutions — which are expected to stand for the asymptotic behaviour of a wide class of initial conditions — should describe the evolution of the walls of constant density domains, spontaneously formed in this system. The piecewise linearized form of the reaction terms and of the

D. E. Strier; D. H. Zanette; Horacio S. Wio

1996-01-01

229

Estimating Density Dependence from Population Time Series Using Demographic Theory and Life?History Data  

Microsoft Academic Search

abstract:,For populations,with a density-dependent life history reproducing at discrete annual intervals, we analyze small or mod- erate fluctuations in population size around a stable equilibrium, which,is applicable to many,vertebrate populations. Using a life history having age at maturity a, with stochasticity and density de- pendence in adult recruitment and mortality, we derive a linearized autoregressive equation,with time lags from,1 to

R. Lande; S. Engen; F. Filli; E. Matthysen; H. Weimerskirch

2002-01-01

230

Density-mediated, context-dependent consumer-resource interactions between ants and extrafloral nectar plants.  

PubMed

Interspecific interactions are often mediated by the interplay between resource supply and consumer density. The supply of a resource and a consumer's density response to it may in turn yield context-dependent use of other resources. Such consumer-resource interactions occur not only for predator-prey and competitive interactions, but for mutualistic ones as well. For example, consumer-resource interactions between ants and extrafloral nectar (EFN) plants are often mutualistic, as EFN resources attract and reward ants which protect plants from herbivory. Yet, ants also commonly exploit floral resources, leading to antagonistic consumer-resource interactions by disrupting pollination and plant reproduction. EFN resources associated with mutualistic ant-plant interactions may also mediate antagonistic ant-flower interactions through the aggregative density response of ants on plants, which could either exacerbate ant-flower interactions or alternatively satiate and distract ants from floral resources. In this study, we examined how EFN resources mediate the density response of ants on senita cacti in the Sonoran Desert and their context-dependent use of floral resources. Removal of EFN resources reduced the aggregative density of ants on plants, both on hourly and daily time scales. Yet, the increased aggregative ant density on plants with EFN resources decreased rather than increased ant use of floral resources, including contacts with and time spent in flowers. Behavioral assays showed no confounding effect of floral deterrents on ant-flower interactions. Thus, ant use of floral resources depends on the supply of EFN resources, which mediates the potential for both mutualistic and antagonistic interactions by increasing the aggregative density of ants protecting plants, while concurrently distracting ants from floral resources. Nevertheless, only certain years and populations of study showed an increase in plant reproduction through herbivore protection or ant distraction from floral resources. Despite pronounced effects of EFN resources mediating the aggregative density of ants on plants and their context-dependent use of floral resources, consumer-resource interactions remained largely commensalistic. PMID:18543629

Chamberlain, Scott A; Holland, J Nathaniel

2008-05-01

231

Prevalence of Furunculosis in Chinook Salmon Depends on Density of the Host Exposed by Cohabitation  

Microsoft Academic Search

Chinook salmon Oncorhynchus tshawytscha were experimentally infected with Aeromonas salmonicida to determine the dependence of initiation and spread of a furunculosis epizootic on host density. Groups of 30 and 60 recipient Chinook salmon (1.7 ± 0.14 g) were held in various volumes of water at a wide range of densities (15.52, 7.76, 1.23, 0.61, 0.32, 0.15, and 0.047 g fish\\/L)

Hamdi Ogut; Paul Reno

2004-01-01

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

Exposing extinction risk analysis to pathogens: Is disease just another form of density dependence?  

USGS Publications Warehouse

In the United States and several other countries, the development of population viability analyses (PVA) is a legal requirement of any species survival plan developed for threatened and endangered species. Despite the importance of pathogens in natural populations, little attention has been given to host-pathogen dynamics in PVA. To study the effect of infectious pathogens on extinction risk estimates generated from PVA, we review and synthesize the relevance of host-pathogen dynamics in analyses of extinction risk. We then develop a stochastic, density-dependent host-parasite model to investigate the effects of disease on the persistence of endangered populations. We show that this model converges on a Ricker model of density dependence under a suite of limiting assumptions, including a high probability that epidemics will arrive and occur. Using this modeling framework, we then quantify: (1) dynamic differences between time series generated by disease and Ricker processes with the same parameters; (2) observed probabilities of quasi-extinction for populations exposed to disease or self-limitation; and (3) bias in probabilities of quasi-extinction estimated by density-independent PVAs when populations experience either form of density dependence. Our results suggest two generalities about the relationships among disease, PVA, and the management of endangered species. First, disease more strongly increases variability in host abundance and, thus, the probability of quasi-extinction, than does self-limitation. This result stems from the fact that the effects and the probability of occurrence of disease are both density dependent. Second, estimates of quasi-extinction are more often overly optimistic for populations experiencing disease than for those subject to self-limitation. Thus, although the results of density-independent PVAs may be relatively robust to some particular assumptions about density dependence, they are less robust when endangered populations are known to be susceptible to disease. If potential management actions involve manipulating pathogens, then it may be useful to model disease explicitly. ?? 2005 by the Ecological Society of America.

Gerber, L.R.; McCallum, H.; Lafferty, K.D.; Sabo, J.L.; Dobson, A.

2005-01-01

234

Density-dependent speciation alters the structure and dynamics of neutral communities.  

PubMed

The neutral theory of biodiversity (NTB) provides an individual-based modeling framework to study eco-evolutionary dynamics. Previous NTB models usually assumed the same per capita rate of speciation across lineages. However, population dynamics may induce macroevolutionary feedbacks that can result in variable per capita speciation rates across lineages. In this paper, with analytical and simulation approaches, we explore how different scenarios of density-dependent speciation may impact the diversity and phylogenetic patterns of neutral communities, and compare the results to predictions of the original NTB model with an invariant speciation rate. Our results show that positive per capita speciation rate-abundance relationships could result in higher species richness and evenness, enhanced stability (evidenced by higher post-disturbance recovery rates and lower temporal variability in species diversity), and higher imbalance in phylogenetic trees. The opposite patterns are predicted when per capita speciation rates decrease with abundance. Particularly, strong negative speciation rate-abundance relationships can generate a positive correlation between phylogenetic age and abundance, which has been observed in Panamanian tree species. Our findings demonstrate the importance of eco-evolutionary feedbacks for understanding long-term diversity and phylogenetic patterns in ecological communities. PMID:25701450

Wang, Shaopeng; Chen, Anping; Pacala, Stephen W; Fang, Jingyun

2015-05-01

235

Density-dependent gastroretentive microparticles motion in human gastric emptying studied using computer simulation.  

PubMed

Density-dependent gastroretentive drug delivery systems have been used to prolong the gastric retention time of drugs since the 1960s. The design of density-dependent gastroretentive dosage forms, however, usually focuses on specific parameters rather than combines with the fluid dynamics of dosage form in the gastric emptying. Therefore, the purpose of the present study was to develop a 2-D model of multiple-phase flows for the simulation of gastric emptying and gastroretentive microparticles motion, and the influence of microparticle density, microparticle viscosity, and gastric juice viscosity on the gastric retention were studied. The recirculating flows, formed in the gastric emptying, could mix the conventional-density microparticles and transport them to the pylorus. However, the low-density microparticles remained floating on the surface of gastric juice, while the high-density microparticles could sink and deposit in the bottom of the stomach. The remaining integral area of microparticles was higher than 90% after 18.33min of simulation when the density of microparticles was lower than 550kg/m(3) or higher than 2500kg/m(3), which was higher compared to conventional-density microparticles (67.05%). These results are in good agreement with experimental data previously reported. In addition, the viscosity of microparticle and gastric juice also influenced the remaining integral area of gastroretentive microparticles. This study shows that the multiple-phase computational fluid dynamics models could provide detailed insights into the fluid dynamics of density-dependent gastroretentive microparticles in gastric emptying, which offers a powerful tool to further understand the mechanism of gastric retention for gastroretentive dosage forms and study the influence of different parameters on their ability for gastric retention. PMID:25640913

Hao, Shilei; Wang, Bochu; Wang, Yazhou

2015-04-01

236

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

237

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

PubMed

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

2006-02-28

238

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

239

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

240

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

241

2. Orbital-Dependent Functionals for the Exchange-Correlation Energy: A Third Generation of Density Functionals  

NASA Astrophysics Data System (ADS)

Contents: 2.1 Introduction 2.1.1 Preliminaries and Notation 2.1.2 Motivation for Orbital-Dependent Functionals 2.1.3 Basic Concept of Orbital-Dependent Functionals 2.2 Optimized Potential Method (OPM) 2.2.1 Direct Functional Derivative 2.2.2 Total Energy Minimization 2.2.3 Invariance of the Density 2.2.4 Exact Relations Related to OPM 2.2.5 Krieger-Li-Iafrate Approximation 2.3 Exchange-Only Results 2.3.1 Accuracy of the KLI Approximation 2.3.2 Properties of the Exact Exchange: Comparison with Explicit Density Functionals 2.4 First-Principles Implicit Correlation Functionals 2.4.1 Many-Body Theory on the Basis of the Kohn-Sham System: Exact Expression for Exc 2.4.2 Perturbative Approach to the Sham-Schlüter Equation: Second Order Correlation Functional 2.4.3 Extensions of the Second Order Functional 2.5 Semi-empirical Orbital-Dependent Exchange-Correlation Functionals 2.5.1 Self-interaction Corrected LDA 2.5.2 Colle-Salvetti Functional 2.6 Analysis of the Orbital-Dependent Correlation 2.6.1 Description of Dispersion Forces by Second Order Correlation Functional 2.6.2 Comparison of Available Orbital-Dependent Approximations for Ec 2.6.3 Analysis of the Second Order Correlation Potential 2.7 Final Remarks

Engel, E.

242

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

243

Embedded density functional approach for calculations of adsorption on ionic crystals  

E-print Network

Embedded density functional approach for calculations of adsorption on ionic crystals Eugene V Received 10 August 1995; accepted 17 November 1995 We present an embedded density functional approach uses the Green function formalism1,2 and the embedded cluster method which uses the concept

Truong, Thanh N.

244

LES/probability density function approach for the simulation of an ethanol spray flame  

E-print Network

LES/probability density function approach for the simulation of an ethanol spray flame Colin Heye a an experimental pilot-stabilized ethanol spray flame. In this particular flame, droplet evaporation occurs away: Large-eddy simulation; Probability density function; Flamelet/progress variable approach; Ethanol

Raman, Venkat

245

Linear and Non-Linear Optical Response using Real-Time Time-Dependent Density Functional Theory  

NASA Astrophysics Data System (ADS)

We present an approach for the calculation of the frequency- dependent response of nano-scale organic molecules for non-linear optical (NLO) devices. These calculations are performed using an efficient implementation of real-time, time-dependent density functional theory (RT-TDDFT) ootnotetextY. Takimoto, F. D. Vila, and J. J. Rehr, J. Chem. Phys. 127, 154114 (2007), and an adaptation of the SIESTA electronic structure code. This method yields frequency dependent nonlinear optical properties of large organic molecules, which have been difficult to obtain with frequency domain calculations. Here we discuss the efficiency of the method and compare the results against frequency-domain TDDFT methods and with experiment. Solvent effects on the NLO properties of photonic molecules are also briefly discussed.

Takimoto, Y.; Vila, F. D.; Rehr, J. J.

2008-03-01

246

A real-time time-dependent density functional theory method for calculating linear and nonlinear dynamic optical response  

NASA Astrophysics Data System (ADS)

We present an approach for the calculation of the frequency-dependent response of nano-scale organic molecules for non-linear optical (NLO) devices. These calculations are performed using an efficient implementation of real-time, time-dependent density functional theory (RT-TDDFT)1 and an adaptation of the SIESTA electronic structure code. This method yields frequency-dependent nonlinear optical properties of large organic molecules, which have been difficult to obtain with frequency domain calculations. Here we discuss the efficiency of the method and compare the results against frequency-domain TDDFT methods and to experiments. Solvent effects on the NLO properties of photonic molecules are also briefly discussed. 1Y. Takimoto, F. D. Vila, and J. J. Rehr, J. Chem. Phys. 127, 154114 (2007)

Takimoto, Yoshinari

2008-10-01

247

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

PubMed Central

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

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

2002-01-01

248

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

249

Exploration of Plasma Jets Approach to High Energy Density Physics  

SciTech Connect

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

Chen, Chiping [Massachusetts Institute of Technology

2013-08-26

250

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

251

Threading dislocation densities in semiconductor crystals: A geometric approach  

NASA Astrophysics Data System (ADS)

In this Letter, we introduce a geometric model to explain the origin of the observed shallow levels in semiconductors threaded by a dislocation density. We show that a uniform distribution of screw dislocations acts as an effective uniform magnetic field which yields bound states for a spin-half quantum particle, even in the presence of a repulsive Coulomb-like potential. This introduces energy levels within the band gap, increasing the carrier concentration in the region threaded by the dislocation density and adding additional recombination paths other than the near band-edge recombination.

Bakke, K.; Moraes, F.

2012-10-01

252

Simulation of heavy ion collision using a time-dependent density functional theory including nuclear superfluidity  

E-print Network

We carried out a simulation of heavy ion collision using a time-dependent density functional theory. We call it the canonical-basis time-dependent Hartree-Fock-Bogoliubov theory (Cb-TDHFB) which can describe nuclear dynamics in three-dimensional coordinate space, treating nuclear pairing correlation. We simulate 20O+20O collision using the Cb-TDHFB with a contact-type pairing functional, and show the behavior of gap energy which is decreasing and vibrating while colliding.

Shuichiro Ebata

2012-11-29

253

Field and frequency dependence of charge-density-wave conduction in NbSe3  

Microsoft Academic Search

Frequency (omega) - and field (E)-dependent conductivity (sigma) measurements are reported in both charge-density-wave (CDW) states of the linear-chain compound NbSe3. There is a direct scaling between the observed E and omega dependence for parameters corresponding to E>2ET, where ET is the threshold field for the onset of nonlinear conductivity. The functional form of sigma(E) is in agreement with a

G. Grüner; A. Zettl; W. G. Clark; John Bardeen

1981-01-01

254

Density-Dependent Natal Dispersal Patterns in a Leopard Population Recovering from Over-Harvest  

PubMed Central

Natal dispersal enables population connectivity, gene flow and metapopulation dynamics. In polygynous mammals, dispersal is typically male-biased. Classically, the ‘mate competition’, ‘resource competition’ and ‘resident fitness’ hypotheses predict density-dependent dispersal patterns, while the ‘inbreeding avoidance’ hypothesis posits density-independent dispersal. In a leopard (Panthera pardus) population recovering from over-harvest, we investigated the effect of sex, population density and prey biomass, on age of natal dispersal, distance dispersed, probability of emigration and dispersal success. Over an 11-year period, we tracked 35 subadult leopards using VHF and GPS telemetry. Subadult leopards initiated dispersal at 13.6 ± 0.4 months. Age at commencement of dispersal was positively density-dependent. Although males (11.0 ± 2.5 km) generally dispersed further than females (2.7 ± 0.4 km), some males exhibited opportunistic philopatry when the population was below capacity. All 13 females were philopatric, while 12 of 22 males emigrated. Male dispersal distance and emigration probability followed a quadratic relationship with population density, whereas female dispersal distance was inversely density-dependent. Eight of 12 known-fate females and 5 of 12 known-fate male leopards were successful in settling. Dispersal success did not vary with population density, prey biomass, and for males, neither between dispersal strategies (philopatry vs. emigration). Females formed matrilineal kin clusters, supporting the resident fitness hypothesis. Conversely, mate competition appeared the main driver for male leopard dispersal. We demonstrate that dispersal patterns changed over time, i.e. as the leopard population density increased. We conclude that conservation interventions that facilitated local demographic recovery in the study area also restored dispersal patterns disrupted by unsustainable harvesting, and that this indirectly improved connectivity among leopard populations over a larger landscape. PMID:25875293

Fattebert, Julien; Balme, Guy; Dickerson, Tristan; Slotow, Rob; Hunter, Luke

2015-01-01

255

Density-dependent natal dispersal patterns in a leopard population recovering from over-harvest.  

PubMed

Natal dispersal enables population connectivity, gene flow and metapopulation dynamics. In polygynous mammals, dispersal is typically male-biased. Classically, the 'mate competition', 'resource competition' and 'resident fitness' hypotheses predict density-dependent dispersal patterns, while the 'inbreeding avoidance' hypothesis posits density-independent dispersal. In a leopard (Panthera pardus) population recovering from over-harvest, we investigated the effect of sex, population density and prey biomass, on age of natal dispersal, distance dispersed, probability of emigration and dispersal success. Over an 11-year period, we tracked 35 subadult leopards using VHF and GPS telemetry. Subadult leopards initiated dispersal at 13.6 ± 0.4 months. Age at commencement of dispersal was positively density-dependent. Although males (11.0 ± 2.5 km) generally dispersed further than females (2.7 ± 0.4 km), some males exhibited opportunistic philopatry when the population was below capacity. All 13 females were philopatric, while 12 of 22 males emigrated. Male dispersal distance and emigration probability followed a quadratic relationship with population density, whereas female dispersal distance was inversely density-dependent. Eight of 12 known-fate females and 5 of 12 known-fate male leopards were successful in settling. Dispersal success did not vary with population density, prey biomass, and for males, neither between dispersal strategies (philopatry vs. emigration). Females formed matrilineal kin clusters, supporting the resident fitness hypothesis. Conversely, mate competition appeared the main driver for male leopard dispersal. We demonstrate that dispersal patterns changed over time, i.e. as the leopard population density increased. We conclude that conservation interventions that facilitated local demographic recovery in the study area also restored dispersal patterns disrupted by unsustainable harvesting, and that this indirectly improved connectivity among leopard populations over a larger landscape. PMID:25875293

Fattebert, Julien; Balme, Guy; Dickerson, Tristan; Slotow, Rob; Hunter, Luke

2015-01-01

256

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

257

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

258

Disclosure Control of Business Microdata: A Density-Based Approach  

Microsoft Academic Search

For continuous key variables, a measure of the individual risk of disclosure is proposed. This risk measure, the local outlier factor, estimates the density around a unit. A selective masking method based on the nearest-neighbour principle and microaggregation is also introduced. Some results of an application to the Italian sample of the Community Innovation Survey are presented. Copyright (c) 2009

Daniela Ichim

2009-01-01

259

Alternative approaches to the calculation of nutrient density  

Technology Transfer Automated Retrieval System (TEKTRAN)

Over thirty years ago researchers developed a variety of different methods for rating or measuring the nutritional quality of foods. Nutrient density as the initial concept emerged was most commonly defined as the ratio of the amount of nutrients in a food to the energy provided. The nutrient dens...

260

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

261

The density of states approach for the simulation of finite density quantum field theories  

E-print Network

Finite density quantum field theories have evaded first principle Monte-Carlo simulations due to the notorious sign-problem. The partition function of such theories appears as the Fourier transform of the generalised density-of-states, which is the probability distribution of the imaginary part of the action. With the advent of Wang-Landau type simulation techniques and recent advances, the density-of-states can be calculated over many hundreds of orders of magnitude. Current research addresses the question whether the achieved precision is high enough to reliably extract the finite density partition function, which is exponentially suppressed with the volume. In my talk, I review the state-of-play for the high precision calculations of the density-of-states as well as the recent progress for obtaining reliable results from highly oscillating integrals. I will review recent progress for the $Z_3$ quantum field theory for which results can be obtained from the simulation of the dual theory, which appears to free of a sign problem.

K. Langfeld; B. Lucini; A. Rago; R. Pellegrini; L. Bongiovanni

2015-03-02

262

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

263

Effect of electron temperature and electron density on topography dependent charging (TDC) damage in inductively coupled plasma etching tool  

Microsoft Academic Search

We investigated the correlation between electron temperature, electron density and topography dependent charging (TDC) damage in an inductively coupled plasma (ICP) metal etching tool. TDC damage was evaluated by controlling both electron temperature and density. The primary result is that TDC damage depends more strongly on electron density than on electron temperature. This remarkable result suggests that TDC damage could

K. Tokashiki; M. Araki; M. Nagase; K. Noguchi; H. Miyamoto; T. Horiuchi

1998-01-01

264

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

265

DENSITY-DEPENDENT RESPONSES OF GRAY-TAILED VOLES TO MOWING  

EPA Science Inventory

Voles (Microtus spp.) commonly inhabit forage crops and may cause excessive damage to these crops. owever, cover removal by mowing or haying may cause vole populations to decline. o determine if gray-tailed voles responded to mowing of alfalfa in a density-dependent manner, the a...

266

Electronvibration coupling in time-dependent density-functional theory: Application to benzene  

E-print Network

Electron­vibration coupling in time-dependent density-functional theory: Application to benzene G for electron­vibration coupling, we apply it to the optical properties of the ­ * transitions in benzene with the electronic excitations. In this work, we have chosen the benzene model for an exploratory study

Bertsch George F.

267

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

268

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

Microsoft Academic Search

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

MIGUEL FERRER; IAN NEWTON; EVA CASADO

2006-01-01

269

Heliocentric distance and temporal dependence of the interplanetary density-magnetic field magnitude correlation  

Microsoft Academic Search

The Helios, IMP 8, ISEE 3, and Voyager 2 spacecraft are used to examine the solar cycle and heliocentric distance dependence of the correlation between density n and magnetic field magnitude B in the solar wind. previous work had suggested that this correlation becomes progressively more negative with heliocentric distance out to 9.5 AU. Here they show that this evolution

D. A. Roberts

1990-01-01

270

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

271

Density dependence of the symmetry energy from neutron skin thickness in finite nuclei  

SciTech Connect

The density dependence of the symmetry energy, characterized by the parameter L, is studied using information provided by the neutron skin thickness in finite nuclei. An estimate of L is obtained from experimental data of antiprotonic atoms. We also discuss the ability of parity violating electron scatering to obtain information about the neutron skin thickness in {sup 208}Pb.

Vinas, X.; Centelles, M.; Roca-Maza, X.; Warda, M. [Departament d'Estructura i Conastituents de la Materia and Institut de Ciencies del Cosmos, Facultat de Fisica, Universitat de Barcelona, Marti i Franques 1, 08028, Barcelona (Spain); Instituto Nazionale di Fisica Nucleare, Sezione di Milano , Via Celoria 16, I-20133 Milano (Italy); Katedra Fizyki Teoretycznej, Uniwersytet Marii Curie-Skodowskiej ul. Radziszewskiego 10, 20-031 Lublin (Poland)

2012-10-20

272

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

273

Continuum states from time-dependent density functional theory Adam Wasserman  

E-print Network

.g., Ref. 13 . Since continuum states are the current-carrying states in mo- lecular electronic devices, we interacting electrons with that of its ground-state KS analog, s r,r ; .6 In operator form indicates spatial-dependent density functional theory is used to study low-lying electronic continuum states of targets that can bind

274

Demonstrating the Temperature Dependence of Density via Construction of a Galilean Thermometer  

ERIC Educational Resources Information Center

A method for the construction of a Galilean thermometer out of common chemistry glassware is described. Students in a first-semester physical chemistry (thermodynamics) class can construct the Galilean thermometer as an investigation of the thermal expansivity of liquids and the temperature dependence of density. This is an excellent first…

Priest, Marie A.; Padgett, Lea W.; Padgett, Clifford W.

2011-01-01

275

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...

276

Frequency and Density-dependent Selection in The Diploid Population with Only Two Pure Strategies  

Microsoft Academic Search

In this paper, the dynamical behavior of the frequency- and density-dependent diploid selection system with only two pure strategies is investigated. The results show that: (i) The genetic equilibrium point of the population is locally asymptotically stable if and only if there is heterozygotic advantage at this point. If there is not, then the genetic equilibrium point must be an

Tao Yi; Wang Zuwang

1995-01-01

277

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

E-print Network

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

Gotie, Robert Francis

1972-01-01

278

The scale-dependence of population density–body mass allometry: Statistical artefact or biological mechanism?  

Microsoft Academic Search

The relation between population density and body mass has vexed ecologists for nearly 30 years as a consequence of high variability in the observed slope of the relation: No single generalisation of the relation has been accepted as universally representative. Here, we use a simple computational approach to examine how observational scale (the body mass range considered) determines variation in

April Hayward; Jurek Kolasa; Jonathon R. Stone

2010-01-01

279

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

PubMed

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

Krüger, Oliver

2007-06-01

280

Plot the Dot: A Graphical Approach to Density  

NSDL National Science Digital Library

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

2012-07-11

281

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

2009-01-28

282

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

PubMed

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

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

2014-12-01

283

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

284

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

Microsoft Academic Search

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

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

2011-01-01

285

Shape of the liquid-vapor coexistence curve for temperature and density dependent effective interactions  

NASA Astrophysics Data System (ADS)

The asymmetry of the coexistence curve that is observed in several micellar systems is discussed in relation with the dependence of the effective interaction on temperature and density. Standard results for the diameter of the coexistence curve in the van der Waals theory are generalized so as to deal with this combined dependence. The qualitative trends so deduced are assessed by comparison with coexistence curves of Yukawa fluids computed with integral equation theories. The role of the variables used to plot the coexistence curve and the nonlinear behavior of its diameter beyond the critical region are discussed in relation with the decrease of the interaction strength with density. The possibility of using the asymmetry of the coexistence curve as an indicator of the state dependence of the effective interaction is finally discussed.

Amokrane, S.; Bouaskarne, M.

2002-05-01

286

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

287

A new weight-dependent direct statistical approach model  

SciTech Connect

A weight-dependent capability is inserted into the direct statistical approach (DSA) to optimize splitting and Russian roulette (RR) parameters in Monte Carlo particle transport calculations. In the new model, splitting or RR is carried out on a progenitor arriving at a surface in such a way that the weight of the progeny is fixed (for the particular surface). Thus, the model is named the DSA weight line model. In the presence of weight-dependent games, all components of the second moment, and the time, are not separable. In the absence of weight-dependent games, the component of the second moment describing the weight-dependent splitting or RR is still not separable. Two approximations are examined to render this component separable under these circumstances. One of these approximations, named the noninteger approximation, looks promising. The new DSA model with the noninteger approximation is tested on four sample problems. Comparisons with the previous weight-independent DSA model and with the MCNP (version 4a) weight window generator are made.

Burn, K.W. [ENEA, Bologna (Italy). Energy Dept.

1997-02-01

288

Time-dependent Kohn-Sham approach to quantum electrodynamics  

E-print Network

We prove a generalization of the van Leeuwen theorem towards quantum electrodynamics, providing the formal foundations of a time-dependent Kohn-Sham construction for coupled quantized matter and electromagnetic fields. Thereby 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.

M. Ruggenthaler; F. Mackenroth; D. Bauer

2011-10-10

289

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

290

The density and temperature dependence of the cooling timescale for fragmentation of self-gravitating disks  

NASA Astrophysics Data System (ADS)

The purpose of this paper is to explore the influences of cooling timescale on fragmentation of self-gravitating protoplanetary disks. We assume the cooling timescale, expressed in terms of the dynamical timescale ? tcool, has a power-law dependence on temperature and density, ? tcool ? ?-aT-b, where a and b are constants. We use this cooling timescale in a simple prescription for the cooling rate, du/dt = -u/tcool, where u is the internal energy. We perform our simulations using the smoothed particle hydrodynamics method. The simulations demonstrate that the disk is very sensitive to the cooling timescale, which depends on density and temperature. Under such a cooling timescale, the disk becomes gravitationally unstable and clumps form in the disk. This property even occurs for cooling timescales which are much longer than the critical cooling timescale, ? tcool ? 7. We show that by adding the dependence of a cooling timescale on temperature and density, the number of clumps increases and the clumps can also form at smaller radii. The simulations imply that the sensitivity of a cooling timescale to density is more than to temperature, because even for a small dependence of the cooling timescale on density, clumps can still form in the disk. However, when the cooling timescale has a large dependence on temperature, clumps form in the disk. We also consider the effects of artificial viscosity parameters on fragmentation conditions. This consideration is performed in two cases, where ? tcool is a constant and ? tcool is a function of density and temperature. The simulations consider both cases, and results show the artificial viscosity parameters have rather similar effects. For example, using too small of values for linear and quadratic terms in artificial viscosity can suppress the gravitational instability and consequently the efficiency of the clump formation process decreases. This property is consistent with recent simulations of self-gravitating disks. We perform simulations with and without the Balsara form of artificial viscosity. We find that in the cooling and self-gravitating disks without the Balsara switch, the clumps can form more easily than those with the Balsara switch. Moreover, in both cases where the Balsara switch is present or absent, the simulations show that the cooling timescale strongly depends on density and temperature.

Faghei, Kazem

2014-06-01

291

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

292

Density-Dependent Response of an Ultracold Plasma to Few-Cycle Radio-Frequency Pulses  

E-print Network

Ultracold neutral plasmas exhibit a density-dependent resonant response to applied radio-frequency (RF) fields in the frequency range of several MHz to hundreds of MHz for achievable densities. We have conducted measurements where short bursts of RF were applied to these plasmas, with pulse durations as short as two cycles. We still observed a density-dependent resonant response to these short pulses. However, the too rapid timescale of the response, the dependence of the response on the sign of the driving field, the response as the number of pulses was increased, and the difference in plasma response to radial and axially applied RF fields are inconsistent with the plasma response being due to local resonant heating of electrons in the plasma. Instead, our results are consistent with rapid energy transfer from collective motion of the entire electron cloud to electrons in high-energy orbits. In addition to providing a potentially more robust way to measure ultracold neutral plasma densities, these measureme...

Wilson, Truman; Roberts, Jacob

2012-01-01

293

Meta-GGA-based adiabatic time-dependent density-functional theory  

NASA Astrophysics Data System (ADS)

The local-density approximation (LDA) to the ground-state density functional theory (DFT) is well known to allow for a generalization to the time-dependent case [1]. The assumption of the adiabaticity of the process greatly simplifies the theory. The further extension of the time-dependent DFT (TDDFT) to the generalized gradient approximation (GGA) is trivial. Here we address lifting the adiabatic TDDFT to the third rung of the ``Jacobs ladder'' [2] : We work out the kinetic energy density dependent (meta-GGA) TDDFT formalism. The new theory possesses remarkable properties not present in LDA and GGA: (i) It is non-local with respect to the particle density; (ii) In the case of bulk semiconductors, it supports the 1/q^2 singularity of the exchange-correlation kernel, where q is the wave-vector, the latter being important to reproduce the excitonic effect. We also present illustrative calculations of the optical absorption in semiconductors [3]. [4pt] [1] A. Zangwill and P. Soven, Phys. Rev. A, 21, 1561 (1980).[0pt] [2] J. Tao, J. P. Perdew, V. N. Staroverov, and G. E. Scuseria, Phys. Rev. Lett. 91, 146401 (2003).[0pt] [3] V. U. Nazarov and G. Vignale, Phys. Rev. Lett. 107, 216402(2011).

Nazarov, Vladimir; Vignale, Giovanni

2012-02-01

294

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

295

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

NASA Astrophysics Data System (ADS)

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 Ne 2 and Kr 2 are computed using the hybrid functional BH-LYP and the semi-local B-LYP GGA functional. In order to include dispersion (van der Waals) effects, a previously developed R-6 type correction (DFT-D) was applied. The TD-DFT/BH-LYP results are in good agreement with those from CC2 or older MRD-CI treatments while pure GGAs fail miserably. The approach is then used to fully optimise the organic complexes in their ground and excited states. Excited state dissociation energies ( D0) of 0.6, 1.2 and 0.8 eV are obtained for benzene and pyrene excimers and the organic exciplex of styrene and trimethylamine, respectively. For the latter system, the theoretical and experimental values agree to each other within 20%. For the excimers of pyrene and benzene, which have been measured in solution, the deviations are larger but within the range of expected solvent effects. In the case of the pyrene excimer, the TD-DFT/BH-LYP computed order of the low-lying excimer states is in agreement with that predicted by the simple exciton coupling model. The significant decrease of inter-molecular distances for all excited complexes compared to the ground state (0.07-0.4 Å) and the reasons for occurring rearrangements (favourable orbital interactions and charge transfer) are discussed.

Huenerbein, Robert; Grimme, Stefan

2008-01-01

296

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

NASA Astrophysics Data System (ADS)

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

Zhou, Shiqi

2009-01-01

297

Evaluating systematic dependencies of type Ia supernovae : the influence of deflagration to detonation density.  

SciTech Connect

We explore the effects of the deflagration to detonation transition (DDT) density on the production of {sup 56}Ni in thermonuclear supernova (SN) explosions (Type Ia supernovae). Within the DDT paradigm, the transition density sets the amount of expansion during the deflagration phase of the explosion and therefore the amount of nuclear statistical equilibrium (NSE) material produced. We employ a theoretical framework for a well-controlled statistical study of two-dimensional simulations of thermonuclear SNe with randomized initial conditions that can, with a particular choice of transition density, produce a similar average and range of {sup 56}Ni masses to those inferred from observations. Within this framework, we utilize a more realistic 'simmered' white dwarf progenitor model with a flame model and energetics scheme to calculate the amount of {sup 56}Ni and NSE material synthesized for a suite of simulated explosions in which the transition density is varied in the range (1-3) x 10{sup 7} g cm{sup -3}. We find a quadratic dependence of the NSE yield on the log of the transition density, which is determined by the competition between plume rise and stellar expansion. By considering the effect of metallicity on the transition density, we find the NSE yield decreases by 0.055 {+-} 0.004 M {circle_dot} for a 1 Z {circle_dot} increase in metallicity evaluated about solar metallicity. For the same change in metallicity, this result translates to a 0.067 {+-} 0.004 M {circle_dot} decrease in the {sup 56}Ni yield, slightly stronger than that due to the variation in electron fraction from the initial composition. Observations testing the dependence of the yield on metallicity remain somewhat ambiguous, but the dependence we find is comparable to that inferred from some studies.

Jackson, A. P.; Calder, A. C.; Townsley, D. M.; Chamulak, D. A.; Brown, E. F.; Timmes, F. X. (Physics); (State Univ. of New York); (Univ. of Alabama); (Michigan State Univ.); (Arizona State Univ.); (Joint Inst. for Nuclear Astrophysics)

2010-09-01

298

EVALUATING SYSTEMATIC DEPENDENCIES OF TYPE Ia SUPERNOVAE: THE INFLUENCE OF DEFLAGRATION TO DETONATION DENSITY  

SciTech Connect

We explore the effects of the deflagration to detonation transition (DDT) density on the production of {sup 56}Ni in thermonuclear supernova (SN) explosions (Type Ia supernovae). Within the DDT paradigm, the transition density sets the amount of expansion during the deflagration phase of the explosion and therefore the amount of nuclear statistical equilibrium (NSE) material produced. We employ a theoretical framework for a well-controlled statistical study of two-dimensional simulations of thermonuclear SNe with randomized initial conditions that can, with a particular choice of transition density, produce a similar average and range of {sup 56}Ni masses to those inferred from observations. Within this framework, we utilize a more realistic 'simmered' white dwarf progenitor model with a flame model and energetics scheme to calculate the amount of {sup 56}Ni and NSE material synthesized for a suite of simulated explosions in which the transition density is varied in the range (1-3) x10{sup 7} g cm{sup -3}. We find a quadratic dependence of the NSE yield on the log of the transition density, which is determined by the competition between plume rise and stellar expansion. By considering the effect of metallicity on the transition density, we find the NSE yield decreases by 0.055 {+-} 0.004 M {sub sun} for a 1 Z{sub sun} increase in metallicity evaluated about solar metallicity. For the same change in metallicity, this result translates to a 0.067 {+-} 0.004 M{sub sun} decrease in the {sup 56}Ni yield, slightly stronger than that due to the variation in electron fraction from the initial composition. Observations testing the dependence of the yield on metallicity remain somewhat ambiguous, but the dependence we find is comparable to that inferred from some studies.

Jackson, Aaron P.; Calder, Alan C. [Department of Physics and Astronomy, State University of New York-Stony Brook, Stony Brook, NY (United States); Townsley, Dean M. [Department of Physics and Astronomy, University of Alabama, Tuscaloosa, AL (United States); Chamulak, David A. [Argonne National Laboratory, Argonne, IL (United States); Brown, Edward F.; Timmes, F. X. [Joint Institute for Nuclear Astrophysics, Michigan State University, East Lansing, MI 48824 (United States)

2010-09-01

299

Ignitor and the High Density Approach for Fusion*  

NASA Astrophysics Data System (ADS)

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

Bombarda, F.; Coppi, B.

2010-11-01

300

Dependencies of Ultrasonic Properties on Apparent Bone Density in Trabecular Bone  

NASA Astrophysics Data System (ADS)

The present study aims to provide insight into the dependencies of ultrasonic properties, such as speed of sound (SOS), broadband ultrasonic attenuation (BUA), and integrated reflection coefficient (IRC), on apparent bone density in trabecular bone. SOS, BUA, and IRC were measured in 19 bovine trabecular bone specimens with apparent densities from 0.411 to 0.928 g/cm3, using a matched pair of transducers with a diameter of 12.7 mm and a center frequency of 1.0 MHz. They were also compared with the predictions obtained from the modified Biot-Attenborough (MBA) model for propagation in fluid-saturated porous media.

Lee, Kang Il

2008-07-01

301

Time-Dependent Density Functional Theory and the Real-Time Dynamics of Fermi Superfluids  

NASA Astrophysics Data System (ADS)

I describe the time-dependent superfluid local density approximation, which is an adiabatic extension of the density functional theory to superfluid Fermi systems and their real-time dynamics. This new theoretical framework has been used to describe several phenomena in cold atomic gases and nuclear collective motion: excitation of the Higgs modes in strongly interacting Fermi superfluids, generation of quantized vortices, crossing and reconnection of vortices, excitation of the superflow at velocities above the critical velocity, excitation of quantum shock waves and domain walls in the collisions of superfluid atomic clouds, and excitation of collective states in nuclei.

Bulgac, Aurel

2013-10-01

302

Lengthscale-Dependent Solvation and Density Fluctuations in n-Octane.  

PubMed

Much attention has been focused on the solvation and density fluctuations in water over the past decade. These studies have brought to light interesting physical features of solvation in condensed media, especially the dependence of solvation on the solute lengthscale, which may be general to many fluids. Here, we focus on the lengthscale-dependent solvation and density fluctuations in n-octane, a simple organic liquid. Using extensive molecular simulations, we show a crossover in the solvation of solvophobic solutes with increasing size in n-octane, with the specifics of the crossover depending on the shape of the solute. Large lengthscale solvation, which is dominated by interface formation, emerges over subnanoscopic lengthscales. The crossover in n-octane occurs at smaller lengthscales than that in water. We connect the lengthscale of crossover to the range of attractive interactions in the fluid. The onset of the crossover is accompanied by the emergence of non-Gaussian tails in density fluctuations in solute shaped observation volumes. Simulations over a range of temperatures highlight a corresponding thermodynamic crossover in solvation. Qualitative similarities between lengthscale-dependent solvation in water, n-octane, and Lennard-Jones fluids highlight the generality of the underlying physics of solvation. PMID:25402732

Wu, Eugene; Garde, Shekhar

2014-12-01

303

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

304

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

305

Pharmacological approaches to methamphetamine dependence: a focused review  

PubMed Central

Methamphetamine dependence is a serious worldwide public health problem with major medical, psychiatric, socioeconomic and legal consequences. Various neuronal mechanisms implicated in methamphetamine dependence have suggested several pharmacological approaches. A literature search from a range of electronic databases (PubMed, EMBASE, PsycInfo, the NIDA research monograph index and the reference list of clinicaltrials.gov) was conducted for the period from January 1985 to October 2009. There were no restrictions on the identification or inclusion of studies in terms of publication status, language and design type. A variety of medications have failed to show efficacy in clinical trials, including a dopamine partial agonist (aripiprazole), GABAergic agents (gabapentin) and serotonergic agents (SSRI, ondansetron, mirtazapine). Three double-blind placebo-controlled trials using modafinil, bupropion and naltrexone have shown positive results in reducing amphetamine or methamphetamine use. Two studies employing agonist replacement medications, one with d-amphetamine and the other with methylphenidate, have also shown promise. Despite the lack of success in most studies to date, increasing efforts are being made to develop medications for the treatment of methamphetamine dependence and several promising agents are targets of further research. PMID:20565449

Karila, Laurent; Weinstein, Aviv; Aubin, Henri-Jean; Benyamina, Amine; Reynaud, Michel; Batki, Steven L

2010-01-01

306

Density dependent expression of PDGF-A modulates the angiotensin II dependent proliferation of rat cardiac fibroblasts.  

PubMed

The proliferative effect of angiotensin II (Ang-II) on primary cardiac fibroblasts is not well understood and controversially discussed. Results described here show that fibroblasts from adult rat hearts exhibit a cell density dependent Ang-II induced cell proliferation. Whereas we could not detect a proliferative effect of Ang-II on confluent cells, which are still able to divide as shown by stimulation with platelet derived growth factor (PDGF), we observed an Ang-II induced cell division of approximately 20 % in non-confluent cells. At both densities, signal transduction molecules such as the mitogen activated protein kinases (MAPK) are activated. There has been substantial evidence that Ang-II may induce the expression and secretion of several growth factors. We demonstrated an approximately fivefold increase in platelet derived growth factor-A (PDGF-A) chain and transforming growth factor beta1 (TGF-beta1) mRNA-expression within non-confluent cardiac fibroblasts by semiquantitative reverse polymerase chain reaction, RT-PCR. In contrast, the mRNA-expression of basic fibroblast growth factor (bFGF), insulin-like growth factor 1 (IGF-1), PDGF-A/-B chains, and TGF-beta1 remained unchanged within confluent cardiac fibroblasts. Experiments with neutralizing antibodies showed that PDGF-AA is an important growth factor regulating cell proliferation, whereas TGF-beta1 interferes with cell size regulation. In summary, we could show that the Ang-II induced cell proliferation in adult cardiac fibroblasts is mainly due to cell density dependent expression of growth factors like PDGF-AA. PMID:10651158

Simm, A; Diez, C

1999-12-01

307

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

308

Density  

NSDL National Science Digital Library

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

Mrs. Petersen

2013-10-28

309

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

310

Permanence of a delayed SIR epidemic model with density dependent birth rate  

NASA Astrophysics Data System (ADS)

In this paper, we consider the permanence of a modified delayed SIR epidemic model with density dependent birth rate which is proposed in [M. Song, W. Ma, Asymptotic properties of a revised SIR epidemic model with density dependent birth rate and time delay, Dynamic of Continuous, Discrete and Impulsive Systems, 13 (2006) 199-208]. It is shown that global dynamic property of the modified delayed SIR epidemic model is very similar as that of the model in [W. Ma, Y. Takeuchi, T. Hara, E. Beretta, Permanence of an SIR epidemic model with distributed time delays, Tohoku Math. J. 54 (2002) 581-591; W. Ma, M. Song, Y. Takeuchi, Global stability of an SIR epidemic model with time delay, Appl. Math. Lett. 17 (2004) 1141-1145].

Song, Mei; Ma, Wanbiao; Takeuchi, Yasuhiro

2007-04-01

311

Prediction of excitation energies for conjugated polymers using time-dependent density functional theory  

NASA Astrophysics Data System (ADS)

Excitation energies of light-emitting conjugated polymers have been investigated with time-dependent density functional theory (TDDFT) within the adiabatic approximation. Our calculations show that the accuracy of the calculated TDDFT excitation energies largely depends on the dihedral angles obtained by the ground-state DFT geometry optimization. We find that, when the DFT torsional dihedral angles are close to experimental estimates, the TDDFT excitation energies agree well with experiments. This trend is observed based on calculations of eight different polymeric systems considered here. We further show that while hybrid density functionals can respect the thumb rule of ET?2ES/3 , where ES is the singlet-singlet excitation energy and ET the singlet-triplet excitation energy, nonhybrid functionals do not.

Tao, Jianmin; Tretiak, Sergei; Zhu, Jian-Xin

2009-12-01

312

The current density in quantum electrodynamics in time-dependent external potentials and the Schwinger effect  

E-print Network

In the framework of quantum electrodynamics (QED) in external potentials, we introduce a method to compute the time-dependence of the expectation value of the current density for time-dependent homogeneous external electric fields. We apply it to the so-called Sauter pulse. For late times, our results agree with the asymptotic value due to electron-positron pair production. For sub-critical peak field strengths, or results agree very well with the general expression derived by Serber for the linearization in the external field. In particular, the expectation value of the current density at intermediate times can be much greater than at asymptotic times. We comment on consequences of these findings for recent proposals to test the Schwinger effect with high intensity lasers using processes at intermediate times.

Zahn, Jochen

2015-01-01

313

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

314

Laboratory calibration of density-dependent lines in the extreme ultraviolet spectral region  

SciTech Connect

We have been making spectral measurements in the extreme ultraviolet (EUV) from different laboratory sources in order to investigate the electron density dependence of various astrophysically important emission lines and to test the atomic models underlying the diagnostic line ratios. The measurement are being performed at the Livermore EBIT-I electron beam ion trap, the National Spherical Torus Experiment (NSTX) at Princeton, and the Alcator C-Mod tokamak at the Massachusetts Institute of Technology, which together span an electron density of four orders of magnitude and which allow us to test the various models at high and low density limits. Here we present measurements of Fe XXII and Ar XIV, which include new data from an ultra high resolution ({lambda}/{Delta}{lambda} >4000) spectrometer at the EBIT-I facility. We found good agreement between the measurements and modeling calculations for Fe XXII, but poorer agreement for Ar XIV.

Lepson, J. K.; Beiersdorfer, P.; Gu, M. F.; Desai, P.; Bitter, M.; Roquemore, L.; Reinke, M. L. [Space Sciences Laboratory, University of California, Berkeley, CA 94720 (United States); Lawrence Livermore National Laboratory, Livermore, CA 94550 (United States); Space Sciences Laboratory, University of California, Berkeley, CA 94720 (United States); Princeton Plasma Physics Laboratory, Princeton, NJ 08543 (United States); Plasma Science and Fusion Center, Cambridge, MA 02139 (United States)

2012-05-25

315

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

316

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

Microsoft Academic Search

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

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

2007-01-01

317

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

Microsoft Academic Search

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

Leila Belguendouz; Lucie Fremont; Alain Linard

1997-01-01

318

Prospective Bone Mineral Density Evaluation in Patients With Insulin-Dependent Diabetes Mellitus  

Microsoft Academic Search

The bone mineral density (BMD) in patients with insulin-dependent diabetes mellitus (IDDM) was evaluated prospectively to assess the course of osteopenia in IDDM. We measured BMD in the lumbar spine, femoral region, and total body calcium in 23 patients aged 21–53 years with IDDM for 2.3 to 20 years using a dual energy X-ray absorptiometry. A second BMD measurement was

Marcia J. Kayath; Edelweiss F. Tavares; Sérgio A. Dib; José Gilberto H. Vieira

1998-01-01

319

Density-dependent switches in diet: a likely mechanism for negative feedbacks on goose population increase?  

Microsoft Academic Search

Goose grazing on arctic tundra vegetation has shown both positive and negative effects on subsequent foraging conditions.\\u000a To understand the potential of a density-dependent feedback on herbivore population size, the relation between grazing pressure\\u000a and future foraging conditions is essential. We studied the effect of increasing grazing pressure of barnacle geese (Branta leucopsis) on Spitsbergen. During the establishment of a

Dries Pieter Jan Kuijper; R. Ubels; M. J. J. E. Loonen

2009-01-01

320

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

321

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

Microsoft Academic Search

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

Oliver Krüger

2007-01-01

322

Global stability of a delayed SIR epidemic model with density dependent birth and death rates  

Microsoft Academic Search

An SIR epidemic model with density dependent birth and death rates is formulated. In our model it is assumed that the total number of the population is governed by logistic equation. The transmission of infection is assumed to be of the standard form, namely proportional to I(t-h)\\/N(t-h) where N(t) is the total (variable) population size, I(t) is the size of

Naoki Yoshida; Tadayuki Hara

2007-01-01

323

Path Dependence of the Potential-Current Density State for Cathodically Polarized Steel in Seawater  

Microsoft Academic Search

Recent emphasis in marine cathodic protection studies has focused on the slope parameter approach to polarization data representation according to the relationship: Ï{sub c} = (R{sub t} x A{sub c})i{sub c} + Ï{sub a}, whereby Ï{sub c} and i{sub c} are cathode potential and current density, respectively, Ï{sub a} is anode potential, R{sub t} is total circuit resistance, and A{sub

W. H. Hartt; S. H. Chen

2000-01-01

324

Experimental examination of intraspecific density-dependent competition during the breeding period in monarch butterflies (Danaus plexippus).  

PubMed

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

325

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

326

Density-dependent fitness benefits in quorum-sensing bacterial populations.  

PubMed

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

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

2012-05-22

327

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

PubMed Central

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

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

2012-01-01

328

Tri-trophic interactions affect density dependence of seed fate in a tropical forest palm.  

PubMed

Natural enemies, especially host-specific enemies, are hypothesised to facilitate the coexistence of plant species by disproportionately inflicting more damage at increasing host abundance. However, few studies have assessed such Janzen-Connell mechanisms on a scale relevant for coexistence and no study has evaluated potential top-down influences on the specialized pests. We quantified seed predation by specialist invertebrates and generalist vertebrates, as well as larval predation on these invertebrates, for the Neotropical palm Attalea butyracea across ten 4-ha plots spanning 20-fold variation in palm density. As palm density increased, seed attack by bruchid beetles increased, whereas seed predation by rodents held constant. But because rodent predation on bruchid larvae increased disproportionately with increasing palm density, bruchid emergence rates and total seed predation by rodents and bruchids combined were both density-independent. Our results demonstrate that top-down effects can limit the potential of host-specific insects to induce negative-density dependence in plant populations. PMID:21899693

Visser, Marco D; Muller-Landau, Helene C; Wright, S Joseph; Rutten, Gemma; Jansen, Patrick A

2011-11-01

329

Density-dependent induction of discoidin-I synthesis in exponentially growing cells of Dictyostelium discoideum.  

PubMed

The synthesis of the lectin, discoidin I, by vegetative cells of Dictyostelium discoideum (strain NC4) was monitored using immunoblot analysis and indirect immunofluorescence. Suspension cultures were used, so that the D. discoideum cell density and the concentration of bacteria could be controlled. Discoidin-I production was found to be a function of the relative densities of D. discoideum cells and food bacteria. Synthesis was initiated in exponentially growing D. discoideum cells approximately three generations before depletion of the food supply. In the growth medium of cells producing discoidin I, a soluble activity was detected that caused low-density cells to begin discoidin-I synthesis. This activity was not dialyzable and was destroyed by heat. A similar activity was produced by AX3 cells during axenic growth. Density-dependent induction of other 'early developmental' proteins was also detected in wild-type cells. These findings suggest that the expression of several 'early developmental' genes is regulated by a mechanism that measures cell density relative to food supply, not by starvation per se. PMID:3622952

Clarke, M; Kayman, S C; Riley, K

1987-01-01

330

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

331

Adapting approximate-memory potentials for time-dependent density functional theory  

NASA Astrophysics Data System (ADS)

Frequency dependent exchange-correlation kernels for time-dependent density functional theory can be used to construct approximate exchange-correlation potentials. The resulting potentials are usually not translationally covariant nor do they obey the so-called zero-force condition. These two basic symmetry requirements are essential for using the potentials in actual applications (even in the linear regime). We provide two pragmatic methods for fully imposing these conditions for both linear and nonlinear regimes. As an example, we take the Gross and Kohn frequency dependent XC functional [Phys. Rev. Lett. 55, 2850 (1985)], correct it, and numerically test it on a sodium metal cluster. Violation of the basic symmetries causes instabilities or spurious low frequency modes.

Kurzweil, Yair; Baer, Roi

2008-02-01

332

When can a single-species, density-dependent model capture the dynamics of a consumer-resource system?  

E-print Network

response can result in a nonlinear density- dependent relationship in the associated single-species modelWhen can a single-species, density-dependent model capture the dynamics of a consumer-time Functional response Handling time Logistic model Smith model a b s t r a c t Single-species population models

333

Survival Kinetics of Starving Bacteria Is Biphasic and Density-Dependent  

PubMed Central

In the lifecycle of microorganisms, prolonged starvation is prevalent and sustaining life during starvation periods is a vital task. In the literature, it is commonly assumed that survival kinetics of starving microbes follows exponential decay. This assumption, however, has not been rigorously tested. Currently, it is not clear under what circumstances this assumption is true. Also, it is not known when such survival kinetics deviates from exponential decay and if it deviates, what underlying mechanisms for the deviation are. Here, to address these issues, we quantitatively characterized dynamics of survival and death of starving E. coli cells. The results show that the assumption – starving cells die exponentially – is true only at high cell density. At low density, starving cells persevere for extended periods of time, before dying rapidly exponentially. Detailed analyses show intriguing quantitative characteristics of the density-dependent and biphasic survival kinetics, including that the period of the perseverance is inversely proportional to cell density. These characteristics further lead us to identification of key underlying processes relevant for the perseverance of starving cells. Then, using mathematical modeling, we show how these processes contribute to the density-dependent and biphasic survival kinetics observed. Importantly, our model reveals a thrifty strategy employed by bacteria, by which upon sensing impending depletion of a substrate, the limiting substrate is conserved and utilized later during starvation to delay cell death. These findings advance quantitative understanding of survival of microbes in oligotrophic environments and facilitate quantitative analysis and prediction of microbial dynamics in nature. Furthermore, they prompt revision of previous models used to analyze and predict population dynamics of microbes. PMID:25838110

Phaiboun, Andy; Zhang, Yiming; Park, Boryung; Kim, Minsu

2015-01-01

334

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

PubMed

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

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

2011-07-01

335

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

PubMed Central

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

Zarrabi, Ali A.; Lounibos, L. Philip

2012-01-01

336

A nuclear matter study using the density dependent M3Y interaction  

NASA Astrophysics Data System (ADS)

The basic nuclear matter properties are calculated within the standard Hartree-Fock scheme using a Michigan version of the in-medium NN interaction (the so-called M3Y interaction). Due to the attractive character of the M3Y interaction, the saturation requirement for the nuclear matter is not fulfilled and the nuclear matter generated with this interaction is unstable against collapse. This necessitates the introduction of an appropriate density dependence into the original M3Y interaction. Our density dependent versions of the M3Y interaction consistently reproduce the equilibrium density and binding energy of the normal nuclear matter as well as the microscopic results (obtained by Jeukenne, Lejeune and Mahaux) for the nucleon optical potential. They can be used in the folding model to calculate the nucleon-nucleus and nucleus-nucleus potentials. A possible study of the cold nuclear equation of state through heavy-ion scattering, which is based on the use of the new interaction to calculate the heavy-ion optical potential, is also discussed.

Khoa, Dao T.; von Oertzen, W.

1993-04-01

337

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

PubMed Central

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

Hone, Jim; Sibly, Richard M

2002-01-01

338

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

339

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

340

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

341

Population density-dependent regulation of the Bradyrhizobium japonicum nodulation genes.  

PubMed

The nodulation genes of Bradyrhizobium japonicum are essential for infection and establishment of a nitrogen-fixing symbiosis. Here, we demonstrate that plant-produced isoflavones induce nodulation gene expression in a population density-dependent fashion. Nodulation gene induction is highest at a low population density and significantly reduced in more dense cultures. A quorum signal molecule in the conditioned medium of B. japonicum cultures mediates this repression. Repression in response to the quorum signal results from the induction of NolA which, in turn, induces NodD2 leading to inhibition of nod gene expression. Consistent with this, nolA-lacZ and nodD2-lacZ expression increased with increasing population density. Unlike the wild type, the ability to induce nodY-lacZ expression did not decline with population density in a NolA mutant. Normally, nod gene expression is repressed in planta (i.e. within nodules). However, expression of a nodY-GUS fusion was not repressed in a NolA mutant, suggesting that quorum-sensing control may mediate in planta repression of the nod genes. Addition of conditioned medium to cultures significantly reduced nod gene expression. Treatment of inoculant cultures with conditioned medium also reduced the ability of B. japonicum to nodulate soybean plants. PMID:11679065

Loh, J T; Yuen-Tsai, J P; Stacey, M G; Lohar, D; Welborn, A; Stacey, G

2001-10-01

342

Relationship of tracheal size to maximal expiratory airflow and density dependence.  

PubMed

Wave-speed theory predicts that maximal expiratory flow (MEF) at high lung volumes depends strongly on size of central airways. We tested this prediction by correlating MEF and tracheal cross-section area (T-XSA) in 15 (11 males, 4 females) healthy never-smoking volunteers. T-XSA was determined by planimetric analysis of contiguous 1-cm computerized tomographic scans of the intrathoracic trachea. We found a significant correlation between T-XSA at total lung capacity (TLC) and flow at 75% of vital capacity (V75) (r = 0.88, P less than 0.001). This contrasted to the correlation found between lung volume at TLC and V75 (r = 0.60). Density dependence of airflow (percent increase in V75 in air) was 35 +/- 17% and showed a significant inverse relationship to T-XSA (r = 0.70). These results confirm predictions of wave-speed theory and demonstrate the importance of cross-sectional area of central airways in determining MEF at high lung volumes. The large variability of MEF in normal individuals partly represents variations in tracheal size. Poor correlation between lung size and airway size suggests only a loose coupling between airways and lung parenchyma consistent with dysanaptic growth. Our findings indicate that changes in density dependence of airflow are not solely determined by the status of small airways and that differences in tracheal size contribute to its variability. PMID:3949655

Dolyniuk, M V; Fahey, P J

1986-02-01

343

Constraining the high-density nuclear symmetry energy with the transverse-momentum dependent elliptic flow  

E-print Network

Within the newly updated version of the ultrarelativistic quantum molecular dynamics (UrQMD) model, the transverse-velocity dependence of the elliptic flow of free nucleons from $^{197}$Au+$^{197}$Au collisions at the incident energy 400 MeV$/$nucleon is studied within different windows of the normalized c.m. rapidity $y_0$. It is found that the elliptic flow difference $v_{2}^{n}$-$v_{2}^{p}$ and ratio $v_{2}^{n}$/$v_{2}^{p}$ of neutrons versus protons are sensitive to the density dependence of the symmetry energy, especially the ratio $v_{2}^{n}$/$v_{2}^{p}$ at small transverse velocity in the intermediate rapidity intervals $0.4hydrogen isotopes with calculations using various Skyrme interactions, all exhibiting similar values of isoscalar incompressibility but very different density dependences of the symmetry energy, a moderately soft to linear symmetry energy is extracted, in good agreement with previous UrQMD or T\\"{u}bingen QMD model calculations but contrasting results obtained with $\\pi^-/\\pi^+$ yield ratios available in the literature.

Yongjia Wang; Chenchen Guo; Qingfeng Li; Hongfei Zhang; Y. Leifels; W. Trautmann

2014-03-27

344

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

345

Effective electron displacements: A tool for time-dependent density functional theory computational spectroscopy  

SciTech Connect

We extend our previous definition of the metric ?r for electronic excitations in the framework of the time-dependent density functional theory [C. A. Guido, P. Cortona, B. Mennucci, and C. Adamo, J. Chem. Theory Comput. 9, 3118 (2013)], by including a measure of the difference of electronic position variances in passing from occupied to virtual orbitals. This new definition, called ?, permits applications in those situations where the ?r-index is not helpful: transitions in centrosymmetric systems and Rydberg excitations. The ?-metric is then extended by using the Natural Transition Orbitals, thus providing an intuitive picture of how locally the electron density changes during the electronic transitions. Furthermore, the ? values give insight about the functional performances in reproducing different type of transitions, and allow one to define a “confidence radius” for GGA and hybrid functionals.

Guido, Ciro A., E-mail: ciro.guido@ecp.fr; Cortona, Pietro [Laboratoire Structures, Propriétés et Modélisation des Solides (SPMS), CNRS UMR 8580, École Centrale Paris, Grande Voie des Vignes, F-92295 Châtenay-Malabry (France)] [Laboratoire Structures, Propriétés et Modélisation des Solides (SPMS), CNRS UMR 8580, École Centrale Paris, Grande Voie des Vignes, F-92295 Châtenay-Malabry (France); Adamo, Carlo [Laboratoire d’Électrochimie, Chimie des Interfaces et Modélisation pour l’Energie, CNRS UMR-7575, Chimie ParisTech, 11 rue P. et M. Curie, F-75231 Paris Cedex 05 (France) [Laboratoire d’Électrochimie, Chimie des Interfaces et Modélisation pour l’Energie, CNRS UMR-7575, Chimie ParisTech, 11 rue P. et M. Curie, F-75231 Paris Cedex 05 (France); Institut Universitaire de France, 103 Bd Saint-Michel, F-75005 Paris (France)

2014-03-14

346

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

347

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

PubMed Central

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

Dennis, James W.; Brewer, C. Fred

2013-01-01

348

Cosmological constraints from the redshift dependence of the Alcock-Paczynski test: galaxy density gradient field  

E-print Network

We propose a method based on the redshift dependence of the Alcock-Paczynski (AP) test to measure the expansion history of the Universe. It uses the isotropy of the galaxy density gradient field to constrain cosmological parameters. If the density parameter $\\Omega_m$ or the dark energy equation of state $w$ are incorrectly chosen, the gradient field appears to be anisotropic with the degree of anisotropy varying with redshift. We use this effect to constrain the cosmological parameters governing the expansion history of the Universe. Although redshift-space distortions (RSD) induced by galaxy peculiar velocities also produce anisotropies in the gradient field, these effects are close to uniform in magnitude over a large range of redshift. This makes the redshift variation of the gradient field anisotropy relatively insensitive to the RSD. By testing the method on mock surveys drawn from the Horizon Run 3 cosmological N-body simulations, we demonstrate that the cosmological parameters can be estimated without...

Li, Xiao-Dong; Forero-Romero, Jaime E; Kim, Juhan

2014-01-01

349

Effective electron displacements: A tool for time-dependent density functional theory computational spectroscopy  

NASA Astrophysics Data System (ADS)

We extend our previous definition of the metric ?r for electronic excitations in the framework of the time-dependent density functional theory [C. A. Guido, P. Cortona, B. Mennucci, and C. Adamo, J. Chem. Theory Comput. 9, 3118 (2013)], by including a measure of the difference of electronic position variances in passing from occupied to virtual orbitals. This new definition, called ?, permits applications in those situations where the ?r-index is not helpful: transitions in centrosymmetric systems and Rydberg excitations. The ?-metric is then extended by using the Natural Transition Orbitals, thus providing an intuitive picture of how locally the electron density changes during the electronic transitions. Furthermore, the ? values give insight about the functional performances in reproducing different type of transitions, and allow one to define a "confidence radius" for GGA and hybrid functionals.

Guido, Ciro A.; Cortona, Pietro; Adamo, Carlo

2014-03-01

350

A modified NaSch model with density-dependent randomization for traffic flow  

NASA Astrophysics Data System (ADS)

Based on the Nagel-Schreckenberg (NaSch) model of traffic flow, a modified cellular automaton (CA) traffic model with the density-dependent randomization (abbreviated as the DDR model) is proposed to simulate traffic flow. The fundamental diagram obtained by simulation shows the ability of this modified NaSch model to capture the essential features of traffic flow, e.g., synchronized flow, metastable state, hysteresis and phase separation at higher densities. Comparisons are made between this DDR model and the NaSch model, also between this DDR model and the VDR model. And the underlying mechanism is analyzed. All these results indicate that the presented model is reasonable and more realistic.

Zhu, H. B.; Ge, H. X.; Dong, L. Y.; Dai, S. Q.

2007-05-01

351

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

NASA Astrophysics Data System (ADS)

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

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

2014-12-01

352

Liquids that form due to dynamics of the molecules that depend on the local density  

E-print Network

RNA molecules in living cells form what look like liquid droplets formed by liquid/liquid phase separation. But unlike the molecules in conventional phase separating mixtures, RNA molecules are transported by molecular motors that consume energy and so are out of equilibrium. Motivated by this we consider what sort of simple rules for the dynamics of model mRNA molecules lead to liquid/liquid phase separation. We find that dynamics that slow as the local density of molecules increases, drive the formation of liquids. We also look at the analogous separation of the two blocks of a block copolymer, in which the monomers of one block have dynamics that depend on the local density of monomers of that block. We find that this block condenses and separates from the monomers of the other block. This is a simple model of the out-of-equilibrium domain formation found in the chromatin in the nucleus of cells.

Richard P. Sear

2015-03-26

353

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 Marie

354

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

355

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

356

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

E-print Network

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

M. G. McPhie; G. Naegele

2008-11-26

357

Measuring Depth-dependent Dislocation Densities and Elastic Strains in an Indented Ni-based Superalloy  

SciTech Connect

The indentation-induced elastic-plastic zone in an IN 740 Ni-based superalloy was studied by three-dimensional (3-D) x-ray microdiffraction and electron back scattering diffraction (EBSD). Large lattice reorientations and the formation of geometrically necessary dislocations are observed in the area with a radius of {approx}75 {mu}m. A residual compression zone is found close to the indent edge. An elastic-plastic transition is observed at {approx}20 {mu}m from the indent edge. Depth dependent dislocation densities are determined at different distances from the indent edge.

Barabash, O.M.; Santella, M.; Barabash, R.I.; Ice, G.E.; Tischler, J. (ORNL)

2011-12-14

358

[A broad educational approach to insulin-dependent diabetics].  

PubMed

Self monitoring of diabetes requires technical knowledge and suitable diabetes oriented behaviour; the latter arises from the patient's psycho-affective functioning, maturity and self sufficiency. A group of 9 insulin dependent ambulatory, adult, diabetics met 15 times over 5 months. The somatic and psycho-affective aspects of the disease were approached by an original methodology. Group animation was focussed on the participants themselves. Dynamic mobilisation of the persons was sought by listening to their explicit and implicit requests, by acting out (case studies and role-playing), by restatement and questioning. Diabetes oriented and psycho-affective behaviours were assessed on the basis of questionnaires and statements. At the end of the study, the psycho-affective attitude of the participants towards their illness was modified. The aggressiveness, mockery and rejection of diabetes and diabetics, initially noted, were progressively replaced by self-awareness and acceptance. As a result, better adjustment of technical behaviour responses was noted. Without precluding any psychodynamic explanation (reunification of the person), this study suggests that the participant's reactions towards their own image can be improved with consequential educational benefit. PMID:3979647

de Poret, D; Pouliquen, A; Dejours, C; Assan, R

1985-02-01

359

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

NASA Astrophysics Data System (ADS)

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

González-Jiménez, Nicolás; Petrovich, Cristóbal; Reisenegger, Andreas

2010-08-01

360

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

Microsoft Academic Search

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

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

2009-01-01

361

Density-dependent processes in the life history of fishes: evidence from laboratory populations of zebrafish Danio rerio.  

PubMed

Population regulation is fundamental to the long-term persistence of populations and their responses to harvesting, habitat modification, and exposure to toxic chemicals. In fish and other organisms with complex life histories, regulation may involve density dependence in different life-stages and vital rates. We studied density dependence in body growth and mortality through the life-cycle of laboratory populations of zebrafish Danio rerio. When feed input was held constant at population-level (leading to resource limitation), body growth was strongly density-dependent in the late juvenile and adult phases of the life-cycle. Density dependence in mortality was strong during the early juvenile phase but declined thereafter and virtually ceased prior to maturation. Provision of feed in proportion to individual requirements (easing resource limitation) removed density dependence in growth and substantially reduced density dependence in mortality, thus indicating that 'bottom-up' effects act on growth as well as mortality, but most strongly on growth. Both growth and mortality played an important role in population regulation, with density-dependent growth having the greater impact on population biomass while mortality had the greatest impact on numbers. We demonstrate a clear ontogenic pattern of change in density-dependent processes within populations of a very small (maximum length 5 mm) fish, maintained in constant homogeneous laboratory conditions. The patterns are consistent with those distilled from studies on wild fish populations, indicating the presence of broad ontogenic patterns in density-dependent processes that are invariant to maximum body size and hold in homogeneous laboratory, as well as complex natural environments. PMID:22655056

Hazlerigg, Charles R E; Lorenzen, Kai; Thorbek, Pernille; Wheeler, James R; Tyler, Charles R

2012-01-01

362

Path integral density matrix dynamics: A method for calculating time-dependent properties in thermal adiabatic and non-adiabatic systems  

SciTech Connect

We introduce a new approach for calculating quantum time-correlation functions and time-dependent expectation values in many-body thermal systems; both electronically adiabatic and non-adiabatic cases can be treated. Our approach uses a path integral simulation to sample an initial thermal density matrix; subsequent evolution of this density matrix is equivalent to solution of the time-dependent Schrödinger equation, which we perform using a linear expansion of Gaussian wavepacket basis functions which evolve according to simple classical-like trajectories. Overall, this methodology represents a formally exact approach for calculating time-dependent quantum properties; by introducing approximations into both the imaginary-time and real-time propagations, this approach can be adapted for complex many-particle systems interacting through arbitrary potentials. We demonstrate this method for the spin Boson model, where we find good agreement with numerically exact calculations. We also discuss future directions of improvement for our approach with a view to improving accuracy and efficiency.

Habershon, Scott [Department of Chemistry and Centre for Scientific Computing, University of Warwick, Coventry CV4 7AL (United Kingdom)] [Department of Chemistry and Centre for Scientific Computing, University of Warwick, Coventry CV4 7AL (United Kingdom)

2013-09-14

363

Path integral density matrix dynamics: a method for calculating time-dependent properties in thermal adiabatic and non-adiabatic systems.  

PubMed

We introduce a new approach for calculating quantum time-correlation functions and time-dependent expectation values in many-body thermal systems; both electronically adiabatic and non-adiabatic cases can be treated. Our approach uses a path integral simulation to sample an initial thermal density matrix; subsequent evolution of this density matrix is equivalent to solution of the time-dependent Schro?dinger equation, which we perform using a linear expansion of Gaussian wavepacket basis functions which evolve according to simple classical-like trajectories. Overall, this methodology represents a formally exact approach for calculating time-dependent quantum properties; by introducing approximations into both the imaginary-time and real-time propagations, this approach can be adapted for complex many-particle systems interacting through arbitrary potentials. We demonstrate this method for the spin Boson model, where we find good agreement with numerically exact calculations. We also discuss future directions of improvement for our approach with a view to improving accuracy and efficiency. PMID:24050328

Habershon, Scott

2013-09-14

364

Zoospore density-dependent behaviors of Phytophthora nicotianae are autoregulated by extracellular products.  

PubMed

Phytophthora species are destructive fungus-like plant pathogens that use asexual single-celled flagellate zoospores for dispersal and plant infection. Many of the zoospore behaviors are density-dependent although the underlying mechanisms are poorly understood. Here, we use P. nicotianae as a model and demonstrate autoregulation of some zoospore behaviors using signal molecules that zoospores release into the environment. Specifically, zoospore aggregation, plant targeting, and infection required or were enhanced by threshold concentrations of these signal molecules. Below the threshold concentration, zoospores did not aggregate and move toward a cauline leaf of Arabidopsis thaliana (Col-0) and failed to individually attack annual vinca (Catharanthus roseus cv. Little Bright Eye). These processes were reversed when supplemented with zoospore-free fluid (ZFF) prepared from a zoospore suspension above threshold densities but not with calcium chloride at a concentration equivalent to extracellular Ca(2+) in ZFF. These results suggest that Ca(2+) is not a primary signal molecule regulating these communal behaviors. Zoospores coordinated their communal behaviors by releasing, detecting, and responding to signal molecules. This chemical communication mechanism raises the possibility that Phytophthora plant infection may not depend solely on zoospore number in the real world. Single zoospore infection may take place if it is signaled by a common molecule available in the environment which contributes to the destructiveness of these plant pathogens. PMID:20528180

Kong, Ping; Hong, Chuanxue

2010-07-01

365

Cosmological constraints from the redshift dependence of the Alcock-Paczynski test: galaxy density gradient field  

E-print Network

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

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

2014-12-11

366

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

SciTech Connect

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

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

2009-12-07

367

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

368

Constraints on neutron skin thickness in 208Pb and density-dependent symmetry energy  

NASA Astrophysics Data System (ADS)

Accurate knowledge about the neutron skin thickness ? Rn p in 208Pb has far-reaching implications for different communities of nuclear physics and astrophysics. Yet, the novel lead radius experiment (PREX) did not yield any stringent constraint on ? Rn p recently. We employ a more practicable strategy to probe the neutron skin thickness of 208Pb based on a high linear correlation between the ? Rn p and J -asym , where J and asym are the symmetry energy (coefficient) of nuclear matter at saturation density and of 208Pb. An accurate J -asym thus places a strong constraint on the ? Rn p . Compared with the parity-violating asymmetry APV in the PREX, the reliably experimental information on the J -asym is much more easily available thanks to the wealth of measured data on nuclear masses and decay energies. The density dependence of the symmetry energy is also well constrained with the J -asym . Finally, with a "tomoscan" method, we find that one just needs to measure the nucleon densities in 208Pb starting from Rm=7.61 ±0.04 fm to obtain the ? Rn p in hadron scattering experiments, regardless of its interior profile being hampered by strong absorption.

Dong, Jianmin; Zuo, Wei; Gu, Jianzhong

2015-03-01

369

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

SciTech Connect

Time-dependent density functional theory (TD-DFT) calculations of transition energy and oscillator strength of formaldehyde, benzene, ethylene and methane molecules are performed. It was found that the LDFT transition energies tend to be smaller than experimental values by 0.1 - 1.3 eV. Inclusion of nonlocal effects made the calculated energies to be larger than the LDFT (local density functional theory) values and thus made the energies to be closer to the experimental values for the case of formaldehyde, ethylene and methane molecules. For benzene, no significant change in the calculated transition energies induced by the addition of nonlocal effects was observed. For the oscillator strength, it was found that a drastic improvement in the accuracy for the prediction from that at the CIS (configuration interaction singles) level can be achieved with the TD-DFT method. The agreement between our TD-DFT values and experimental values were excellent both at the LDFT and NLDFT (nonlocal density functional theory) level with the latter being slightly more close to experimental values than the former.

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

2001-05-24

370

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

371

Density-dependent habitat selection of spawning Chinook salmon: broad-scale evidence and implications.  

PubMed

An extensive body of theory suggests that density-dependent habitat selection drives many fundamental ecological processes. The ideal free distribution and the ideal despotic distribution make contrasting predictions about the effect of total population size on relative abundances among habitats. Empirical assessment of these habitat selection models is uncommon because data must be collected over large temporal and spatial scales. I ask whether fluctuation in Chinook salmon (Oncorhynchus tshawytscha) spawner population size through time leads to different relative densities over space. Twenty-six years of monitoring data on spawning Chinook salmon across the entire coast of Oregon, USA, were used to evaluate models that make contrasting statements about the interactions of a latent population abundance parameter with physical habitat characteristics. There is strong information-theoretic support for models that include terms that allow the spatial variation in density to change as population size changes through time. Analysis of the best model reveals nonlinear isodars, which suggests a 'despotic' or 'preemptive' distribution of individuals across habitats, indicating that dominant or early-arriving individuals exclude others from breeding sites. This finding has implications for genetic dynamics, population dynamics and conservation metrics of these highly valued fish. The novel application of modelling techniques used here to assess mechanisms of habitat selection from observational data can be used in the emerging field of eco-evolutionary dynamics. PMID:25283166

Falcy, Matthew R

2014-10-01

372

Fruit removal rate depends on neighborhood fruit density, frugivore abundance, and spatial context.  

PubMed

Fleshy-fruited plants depend fundamentally on interactions with frugivores for effective seed dispersal. Recent models of frugivory within spatially explicit networks make two general predictions regarding these interactions: rate of fruit removal increases (i.e., is facilitated) as densities of conspecific neighborhood fruits increase, and fruit removal rate varies positively with frugivore abundance. We conducted a field experiment that constitutes the first empirical and simultaneous test of these two primary predictions. We manipulated neighborhood abundances of arrowwood (Viburnum recognitum and Viburnum dentatum) fruits in southern New England's maritime shrub community and monitored removal rates by autumn-migrating birds. Focal arrowwood plants in neighborhoods with high conspecific fruit density sustained moderately decreased fruit removal rates (i.e., competition) relative to those in low-density neighborhoods, a result that agrees with most field research to date but contrasts with theoretical expectation. We suggest the spatial contexts that favor competition (i.e., high-abundance neighborhoods and highly aggregated landscapes) are considerably more common than the relatively uniform, low-aggregation fruiting landscapes that promote facilitation. Patterns of arrowwood removal by avian frugivores generally varied positively with, and apparently in response to, seasonal changes in migratory frugivore abundance. However, we suggest that dense stands of arrowwood concentrated frugivore activity at the neighborhood scale, thus counteracting geographic patterns of frugivore abundance. Our results underscore the importance of considering spatial context (e.g., fruit distribution and aggregation, frugivory hubs) in plant-avian frugivore interactions. PMID:24305861

Smith, Adam D; McWilliams, Scott R

2014-03-01

373

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

PubMed Central

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

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

2008-01-01

374

Wavelet-based linear-response time-dependent density-functional theory  

NASA Astrophysics Data System (ADS)

Linear-response time-dependent (TD) density-functional theory (DFT) has been implemented in the pseudopotential wavelet-based electronic structure program BIGDFT and results are compared against those obtained with the all-electron Gaussian-type orbital program DEMON2K for the calculation of electronic absorption spectra of N2 using the TD local density approximation (LDA). The two programs give comparable excitation energies and absorption spectra once suitably extensive basis sets are used. Convergence of LDA density orbitals and orbital energies to the basis-set limit is significantly faster for BIGDFT than for DEMON2K. However the number of virtual orbitals used in TD-DFT calculations is a parameter in BIGDFT, while all virtual orbitals are included in TD-DFT calculations in DEMON2K. As a reality check, we report the X-ray crystal structure and the measured and calculated absorption spectrum (excitation energies and oscillator strengths) of the small organic molecule N-cyclohexyl-2-(4-methoxyphenyl)imidazo[1, 2-a]pyridin-3-amine.

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

2012-06-01

375

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

376

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

377

Density  

NSDL National Science Digital Library

Targeting a middle and high school population, this web page has an introduction to the concept of density. It is an appendix of a larger site called, MathMol (Mathematics and Molecules), designed as an introduction to molecular modeling.

378

{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

379

The Impairment of Osteogenesis in Bone Sialoprotein (BSP) Knockout Calvaria Cell Cultures Is Cell Density Dependent  

PubMed Central

Bone sialoprotein (BSP) belongs to the "small integrin-binding ligand N-linked glycoprotein" (SIBLING) family, whose members interact with bone cells and bone mineral. BSP is strongly expressed in bone and we previously showed that BSP knockout (BSP-/-) mice have a higher bone mass than wild type (BSP+/+) littermates, with lower bone remodelling. Because baseline bone formation activity is constitutively lower in BSP-/- mice, we studied the impact of the absence of BSP on in vitro osteogenesis in mouse calvaria cell (MCC) cultures. MCC BSP-/- cultures exhibit fewer fibroblast (CFU-F), preosteoblast (CFU-ALP) and osteoblast colonies (bone nodules) than wild type, indicative of a lower number of osteoprogenitors. No mineralized colonies were observed in BSP-/- cultures, along with little/no expression of either osteogenic markers or SIBLING proteins MEPE or DMP1. Osteopontin (OPN) is the only SIBLING expressed in standard density BSP-/- culture, at higher levels than in wild type in early culture times. At higher plating density, the effects of the absence of BSP were partly rescued, with resumed expression of osteoblast markers and cognate SIBLING proteins, and mineralization of the mutant cultures. OPN expression and amount are further increased in high density BSP-/- cultures, while PHEX and CatB expression are differentiatlly regulated in a manner that may favor mineralization. Altogether, we found that BSP regulates mouse calvaria osteoblast cell clonogenicity, differentiation and activity in vitro in a cell density dependent manner, consistent with the effective skeletogenesis but the low levels of bone formation observed in vivo. The BSP knockout bone microenvironment may alter the proliferation/cell fate of early osteoprogenitors. PMID:25710686

Bouet, Guenaelle; Bouleftour, Wafa; Juignet, Laura; Linossier, Marie-Thérèse; Thomas, Mireille; Vanden-Bossche, Arnaud; Aubin, Jane E.; Vico, Laurence; Marchat, David; Malaval, Luc

2015-01-01

380

Reconstructive approaches to one- and two-electron density matrix theory  

NASA Astrophysics Data System (ADS)

Novel computational methods for electronic structure theory are explored, in which the fundamental variable is either the one- or the two-electron reduced density matrix (1- or 2-RDM), rather than the electronic wavefunction. A unifying theme among these methods is density matrix reconstruction, that is, decoupling approximations that express higher-order density matrices as functionals of lower-order ones. On the 2-RDM side, a connected (extensive) version of the Contracted Schrodinger Equation (CSE) is developed, in which the basic unknowns are the RDM cumulants through order four. Reconstruction functionals that neglect the 3- and 4-RDM cumulants are examined and revealed to be significantly less accurate than suggested by previous minimal-basis results. Exact 3-RDM cumulants for some four-electron systems are calculated and found to be comparable in importance to unconnected products of lower-order cumulants. Decoupling approximations for the 3- and 4-RDM cumulants are developed based upon a renormalized, diagrammatic perturbation theory for the three- and four-particle Green's functions, in which the effective, pairwise interaction is extracted from the two-particle cumulant. Diagram rules suitable for both the time-dependent and time-independent versions of this perturbation theory are derived. Reconstructive approaches to natural orbital (1-RDM) functional theory are also examined, wherein the 2-RDM is parametrized in terms of the natural orbitals and their (generally fractional) occupancies. It is demonstrated, at the theorem level, that proposed "corrected Hartree" and "corrected Hartree-Fock" natural orbital functionals necessarily violate positivity of the 2-RDM, which is closely related to their failure to respect antisymmetry. Calculations demonstrate that negative eigenvalues of the 2-RDM are associated with a large, stabilizing (but ultimately spurious) contribution to the energy. Nevertheless, a partially self-interaction-corrected version of the corrected Hartree functional is shown to yield high-quality potential energy curves---comparable to multireference configuration interaction---for the strongly-correlated diatomic molecules HF and C2. A new class of functionals that preserve both anti-symmetry and positivity are proposed and subjected to some preliminary tests.

Herbert, John Michael

381

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

PubMed Central

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

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

2014-01-01

382

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

383

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

NASA Astrophysics Data System (ADS)

We 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

384

Phase behavior of ionic fluids in slitlike pores: A density functional approach for the restricted primitive model  

NASA Astrophysics Data System (ADS)

We present a density functional theory of nonuniform ionic fluids. This theory is based on the application of the electrostatic contribution to the free energy functional arising from mean spherical approximation for a bulk restricted primitive model and from the energy route bulk equation of state. In order to employ this functional we define a reference fluid and additional averaged densities, according to the approach introduced by Gillespie, Nonner and Eisenberg [J. Phys.: Condens. Matter 14, 12129 (2002)]. In the case of bulk systems the proposed theory reduces to the mean spherical approximation equation of state, arising from the energy route and thus it predicts the first-order phase transition. We use this theory to investigate the effects of confinement on the liquid-vapor equilibria. Two cases are considered, namely an electrolyte confined to the pore with uncharged walls and with charged walls. The dependence of the capillary evaporation diagrams on the pore width and on the electrostatic potential is determined.

Pizio, O.; Patrykiejew, A.; Soko?owski, S.

2004-12-01

385

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

386

Analytical Blowup Solutions to the 4-dimensional Pressureless Navier-Stokes-Poisson Equations with Density-dependent Viscosity  

E-print Network

We study the 4-dimensional pressureless Navier--Stokes-Poisson equations with density-dependent viscosity. The analytical solutions with arbitrary time blowup, in radial symmetry, are constructed in this paper.

Manwai Yuen

2008-11-09

387

Reconstruction of Layer Densities in a Multilayer Snowpack using a Bayesian Approach to Inverse Modeling  

NASA Astrophysics Data System (ADS)

Estimation of snowpack parameters such as depth, density and grain structure is a central focus of hydrology in seasonally snow-covered lands. These parameters are directly estimated by field observations, indirectly estimated from other parameters using statistical correlations, or simulated with a model. Difficulty in sampling thin layers and uncertainty in the transition between layers can cause significant uncertainty in measurements of these parameters. Snow density is one of the most important parameters to measure because it is strictly related with snow water content, an important component of the global water balance. We develop a mathematical framework to estimate snow density from measurements of temperature and thickness of snowpack layers over a particular time period, in conjunction with a physics-based model of snowpack evolution. We formulate a Bayesian approach to estimate the snowpack density profile, using a full range of possible simulations that incorporate key sources of uncertainty to build in prior snowpack knowledge. The posterior probability density function of the snow density, conditioned on snowpack temperature measurements, is computed by multiplying the likelihoods and assumed prior distribution function. Random sampling is used to generate a range of densities with same probability when prior uniform probability function is assumed. A posterior probability density function calculated directly via Bayes' theorem is used to calculate the probability of every sample generated. The forward model is a 1D, multilayer snow energy and mass balance model, which solves for snow temperature, density, and liquid water content on a finite element mesh. The surface and ground temperature data of snowpack (boundary conditions), are provided by the Center for Snow and Avalanche Studies (CSAS), Silverton CO, from snow pits made at Swamp Angel and Senator Beck study plot sites. Standard errors between field observations and results computed denote the quality of the estimations and facilitate further arrangements of this approach.

Aguayo, M.; Marshall, H.; McNamara, J. P.; Mead, J.; Flores, A. N.

2013-12-01

388

An obstructive sleep apnea detection approach using kernel density classification based on single-lead electrocardiogram.  

PubMed

Obstructive sleep apnea (OSA) is a common sleep disorder that often remains undiagnosed, leading to an increased risk of developing cardiovascular diseases. Polysomnogram (PSG) is currently used as a golden standard for screening OSA. However, because it is time consuming, expensive and causes discomfort, alternative techniques based on a reduced set of physiological signals are proposed to solve this problem. This study proposes a convenient non-parametric kernel density-based approach for detection of OSA using single-lead electrocardiogram (ECG) recordings. Selected physiologically interpretable features are extracted from segmented RR intervals, which are obtained from ECG signals. These features are fed into the kernel density classifier to detect apnea event and bandwidths for density of each class (normal or apnea) are automatically chosen through an iterative bandwidth selection algorithm. To validate the proposed approach, RR intervals are extracted from ECG signals of 35 subjects obtained from a sleep apnea database ( http://physionet.org/cgi-bin/atm/ATM ). The results indicate that the kernel density classifier, with two features for apnea event detection, achieves a mean accuracy of 82.07 %, with mean sensitivity of 83.23 % and mean specificity of 80.24 %. Compared with other existing methods, the proposed kernel density approach achieves a comparably good performance but by using fewer features without significantly losing discriminant power, which indicates that it could be widely used for home-based screening or diagnosis of OSA. PMID:25732075

Chen, Lili; Zhang, Xi; Wang, Hui

2015-05-01

389

A comparison of density-functional-theory and coupled-cluster frequency-dependent polarizabilities and hyperpolarizabilities  

Microsoft Academic Search

The frequency-dependent polarizabilities and hyperpolarizabilities of HF, CO, H2O and para-nitroaniline calculated by density-functional theory are compared with accurate coupled-cluster results. Whereas the local-density approximation and the generalized gradient approximation (BLYP) perform very similarly and overestimate polarizabilities and, in particular, the hyperpolarizabilities, hybrid density-functional theory (B3LYP) performs better and produces results similar to those obtained by coupled-cluster singles-and-doubles theory. Comparisons

Pawel Salek; Trygve Helgaker; Olav Vahtras; Hans Ågren; Jürgen Gauss

2005-01-01

390

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

391

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

E-print Network

Approach for control of high-density plasma reactors through optimal pulse shaping* Tyrone L of which is the operation of plasma reactors in the pulsed mode where the power input to the reactor for pulsed operation of plasma reactors where the power input is modulated using pulse shapes

Raja, Laxminarayan L.

392

A new approach for filtering noise from high-density oligonucleotide microarray datasets  

Microsoft Academic Search

Although DNA microarrays are powerful tools for profiling gene expression, the dynamic range and the sheer number of signals produced require efficient procedures for distinguishing false positive results (noise) from changes in expression that are 'real' (independently reproducible). We have developed an approach to filter noise from datasets generated when high density oligonucleotide-based micro- arrays are used to compare two

Jason C. Mills; Jeffrey I. Gordon

2001-01-01

393

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

394

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

395

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

396

Redshift space correlations and scale-dependent stochastic biasing of density peaks  

SciTech Connect

We calculate the redshift space correlation function and the power spectrum of density peaks of a Gaussian random field. Our derivation, which is valid on linear scales k < or approx. 0.1 hMpc{sup -1}, is based on the peak biasing relation given by Desjacques [Phys. Rev. D, 78, 103503 (2008)]. In linear theory, the redshift space power spectrum is P{sub pk}{sup s}(k,{mu})=exp(-f{sup 2{sigma}}{sub vel}{sup 2}k{sup 2{mu}2})[b{sub pk}(k)+b{sub vel}(k)f{mu}{sup 2}]{sup 2}P{sub {delta}(k)}, where {mu} is the angle with respect to the line of sight, {sigma}{sub vel} is the one-dimensional velocity dispersion, f is the growth rate, and b{sub pk}(k) and b{sub vel}(k) are k-dependent linear spatial and velocity bias factors. For peaks, the value of {sigma}{sub vel} depends upon the functional form of b{sub vel}. When the k dependence is absent from the square brackets and b{sub vel} is set to unity, the resulting expression is assumed to describe models where the bias is linear and deterministic, but the velocities are unbiased. The peak model is remarkable because it has unbiased velocities in this same sense--peak motions are driven by dark matter flows--but, in order to achieve this, b{sub vel} must be k dependent. We speculate that this is true in general: k dependence of the spatial bias will lead to k dependence of b{sub vel} even if the biased tracers flow with the dark matter. Because of the k dependence of the linear bias parameters, standard manipulations applied to the peak model will lead to k-dependent estimates of the growth factor that could erroneously be interpreted as a signature of modified dark energy or gravity. We use the Fisher formalism to show that the constraint on the growth rate f is degraded by a factor of 2 if one allows for a k-dependent velocity bias of the peak type. Our analysis also demonstrates that the Gaussian smoothing term is part and parcel of linear theory. We discuss a simple estimate of nonlinear evolution and illustrate the effect of the peak bias on the redshift space multipoles. For k < or approx. 0.1 hMpc{sup -1}, the peak bias is deterministic but k dependent, so the configuration-space bias is stochastic and scale dependent, both in real and redshift space. We provide expressions for this stochasticity and its evolution.

Desjacques, Vincent [Institute for Theoretical Physics, University of Zuerich, Winterthurerstrasse 190, CH-8057 Zuerich (Switzerland); Sheth, Ravi K. [Center for Particle Cosmology, University of Pennsylvania, 209 South 33rd Street, Philadelphia, Pennsylvania 19104 (United States)

2010-01-15

397

Global stability of a delayed SIR epidemic model with density dependent birth and death rates  

NASA Astrophysics Data System (ADS)

An SIR epidemic model with density dependent birth and death rates is formulated. In our model it is assumed that the total number of the population is governed by logistic equation. The transmission of infection is assumed to be of the standard form, namely proportional to I(t-h)/N(t-h) where N(t) is the total (variable) population size, I(t) is the size of the infective population and a time delay h is a fixed time during which the infectious agents develop in the vector. We consider transmission dynamics for the model. Stability of an endemic equilibrium is investigated. The stability result is stated in terms of a threshold parameter, that is, a basic reproduction number R0.

Yoshida, Naoki; Hara, Tadayuki

2007-04-01

398

Correlation in time-dependent density functional theory studies of antiproton-helium collisions  

NASA Astrophysics Data System (ADS)

Correlation effects are examined in the context of time-dependent density functional theory (TDDFT) calculations of antiproton helium collisions. An approximation for the correlation potential as well as two models for the correlation integral (Ic) are explored. While one of these models (frozen correlation (FCM)) is entirely new the other is appropriated from the world of laser-induced ionization (Wilken and Bauer (WB)). Total cross sections for both single and double ionization in the range 1-2000 keV are presented. These calculations make use of the basis generator method (BGM) and incorporate microscopic response. While the FCM results provide little improvement over an independent electron model description the WB model agrees quite well with experimental results for both single and double ionization. Our results also lend credence to the belief that an appropriate approximation of Ic is more important in reproducing correlation effects than the correlation potential.

Baxter, Matthew

399

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

SciTech Connect

The improved quark mass density-dependent model (IQMDD) based on soliton bag model is studied at finite temperature. Applying 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{sub C}{approx_equal}110 MeV. We also calculate the soliton solutions of the IQMDD model at finite temperature. It turns out that when TT{sub C} the bag constant B(T)=0 and there is no soliton solution, therefore, the confinement of quarks are removed quickly.

Mao Hong [CCAST (World Laboratory), P.O. Box 8730, Beijing 100080 (China); Su Rukeng [CCAST (World Laboratory), P.O. Box 8730, Beijing 100080 (China); Department of Physics, Fudan University, Shanghai 200433 (China); Zhao Weiqin [CCAST (World Laboratory), P.O. Box 8730, Beijing 100080 (China); Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049 (China)

2006-11-15

400

Thickness dependence of the charge-density-wave transition temperature in VSe{sub 2}  

SciTech Connect

A set of three-dimensional charge-density-wave (3D CDW) VSe{sub 2} nano-flakes with different thicknesses were obtained by the scotch tape-based micro-mechanical exfoliation method. Resistivity measurements showed that the 3D CDW transition temperature T{sub p} decreases systematically from 105?K in bulk to 81.8?K in the 11.6?nm thick flake. The Hall resistivity ?{sub xy} of all the flakes showed a linear dependent behavior against the magnetic field with a residual electron concentration of the order of ?10{sup 21}?cm{sup ?3} at 5?K. The electron concentration n increases slightly as the thickness d decreases, possibly due to the CDW gap is reduced with the decrease of the thickness.

Yang, Jiyong; Liu, Yan; Du, Haifeng; Ning, Wei; Zheng, Guolin; Jin, Chiming; Han, Yuyan; Wang, Ning; Tian, Mingliang, E-mail: tianml@hmfl.ac.cn; Zhang, Yuheng [High Magnetic Field Laboratory, Chinese Academy of Sciences, Hefei 230031, Anhui (China); Wang, Weike; Yang, Zhaorong [Institute of Solid State Physics, Chinese Academy of Sciences, Heifei 230031, Anhui (China)

2014-08-11

401

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

402

Optimized Effective Potential for Quantum Electrodynamical Time-Dependent Density Functional Theory  

E-print Network

We propose a practical approximation to the exchange-correlation functional of (time-dependent) density functional theory for many-electron systems coupled to photons. The (time non-local) optimized effective potential (OEP) equation for the electron- photon system is derived. We test the new approximation in the Rabi model from weak to strong coupling regimes. It is shown that the OEP (i) improves the classical description, (ii) reproduces the quantitative behavior of the exact ground-state properties and (iii) accurately captures the dynamics entering the ultra-strong coupling regime. The present formalism opens the path to a first-principles description of correlated electron-photon systems, bridging the gap between electronic structure methods and quantum optics for real material applications.

Camilla Pellegrini; Johannes Flick; Ilya V. Tokatly; Heiko Appel; Angel Rubio

2014-12-15

403

Density dependence of the nuclear symmetry energy from measurements of neutron radii in nuclei  

SciTech Connect

We study the density dependence of the nuclear symmetry energy, characterized by its slope parameter L, by means of the information provided by the neutron radius and the neutron skin thickness in finite nuclei. These quantities are extracted from the analysis of data obtained in antiprotonic atoms, from the parity-violating asymmetry at low-momentum transfer in polarized electron scattering in {sup 208}Pb, and from the electric dipole polarizability obtained via polarized proton inelastic scattering at forward angles in {sup 208}Pb. All these experiments provide different constraints on the slope L of the symmetry energy but the corresponding values have a considerable overlap in a range around 50 MeV ? L ? 70 MeV, in a reasonable agreement with other estimates that use different observables and methods to extract L.

Viñas, X.; Centelles, M. [Departament d'Estructura i Constituents de la Matèria and Institut de Ciències del Cosmos, Facultat de Física, Universitat de Barcelona, Martí i Franquès 1, 08028 Barcelona (Spain); Roca-Maza, X. [Dipartimento di Fisica, Università degli Studi di Milano and INFN, Sezione di Milano, I-20133 Milano (Italy); Warda, M. [Katedra Fizyki Teoretycznej, Uniwersytet Marii Curie–Sk?odowskiej ul. Radziszewskiego 10, 20-031 Lublin (Poland)

2014-07-23

404

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

NASA Astrophysics Data System (ADS)

Optical properties of nanoscale structures have attracted much attention experimentally and theoretically. It is not appropriate to apply the conventional density-functional theory (DFT) to investigation of the optical properties, because the excited states, which are not adequately represented by the DFT, play essential roles in these phenomena. To go beyond the DFT, we adopt time-dependent DFT (TDDFT) calculations with the linear- response theory, which is a powerful computational tool for calculating the excited states of nanostructures properly. In this study, we report the results of excitation energies and photo-absorption spectra in aromatic molecules, such as naphthalene and anthracene. A high spectral intensity in a low frequency region occurs in the spectra for these molecules. This characteristic spectrum is shifted to lower energy with increasing molecular size. We also discuss the details of the electronic excitations utilizing the TDDFT calculations in a real-time scheme.

Noguchi, Tomoyuki; Araidai, Masaaki; Watanabe, Kazuyuki

2005-03-01

405

Inelastic collisions and density-dependent excitation suppression in a {sup 87}Sr optical lattice clock  

SciTech Connect

We observe two-body loss of {sup 3} P{sub 0} {sup 87}Sr atoms trapped in a one-dimensional optical lattice. We measure loss rate coefficients for atomic samples between 1 and 6 {mu}K that are prepared either in a single nuclear-spin sublevel or with equal populations in two sublevels. The measured temperature and nuclear-spin preparation dependence of rate coefficients agree well with calculations and reveal that rate coefficients for distinguishable atoms are only slightly enhanced over those of indistinguishable atoms. We further observe a suppression of excitation and losses during interrogation of the {sup 1} S{sub 0}-{sup 3} P{sub 0} transition as density increases and Rabi frequency decreases, which suggests the presence of strong interactions in our dynamically driven many-body system.

Bishof, M.; Martin, M. J.; Swallows, M. D.; Benko, C.; Lin, Y.; Quemener, G.; Rey, A. M.; Ye, J. [JILA, NIST and University of Colorado, Boulder, Colorado 80309-0440 (United States); Department of Physics, University of Colorado, Boulder, Colorado 80309-0390 (United States)

2011-11-15

406

Vibration induced airflow through granular beds and density-dependent segregation  

NASA Astrophysics Data System (ADS)

We describe a quantitative model for air-driven fluidization in a vertically vibrated granular system and use experimental results and simulations to evaluate it. The model involves a mechanism for vibrationally induced interstitial gas flow [L.I. Reyes, I. Sánchez, G. Gutiérrez, Physica A, cond-mat/0502376, in press] that can generate fluid-like states that produce density-dependent segregation of large objects, through a buoyancy force. A criterion for the onset of fluidization analogous to that of a gas-fluidized static bed is used. We calculate the vertical displacement of a large sphere occurring within one period of oscillation, in specific parts of the cycle where the granular system behaves like a regular fluid.

Gutiérrez, G.; Reyes, L. I.; Sánchez, I.; Rodríguez, K.; Idler, V.; Paredes V, R.

2005-10-01

407

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

408

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

409

Modeling fast electron dynamics with real-time time-dependent density functional theory: application to small molecules and chromophores  

SciTech Connect

The response of matter to external fields forms the basis for a vast wealth of fundamental physical processes ranging from light harvesting to nanoscale electron transport. Accurately modeling ultrafast electron dynamics in excited systems thus o_ers unparalleled insight, but requires an inherently non-linear time-resolved approach. To this end, an e_cient and massively parallel real-time real-space time-dependent density functional theory (RT-TDDFT) implementation in NWChem is presented. The implementation is first validated against linearresponse TDDFT and experimental results for a series of molecules subjected to small electric field perturbations. Second, non-linear excitation of green fluorescent protein is studied, which shows a blue-shift in the spectrum with increasing perturbation, as well as a saturation in absorption. Next, the charge dynamics of optically excited zinc porphyrin is presented in real-time and real-space, with relevance to charge injection in photovoltaic devices. Finally, intermolecular excitation in an adenine-thymine base pair is studied using the BNL range separated functional [Baer, R.; Neuhauser, D. Phys. Rev. Lett. 2005, 94, 043002], demonstrating the utility of a real-time approach in capturing charge transfer processes.

Lopata, Kenneth A.; Govind, Niranjan

2011-05-10

410

Charge Density-Dependent Modifications of Hydration Shell Waters by Hofmeister Ions  

PubMed Central

Gadolinium (Gd3+) vibronic side band luminescence spectroscopy (GVSBLS) is used to probe, as a function of added Hofmeister series salts, changes in the OH stretching frequency derived from first shell waters of aqueous Gd3+ and of Gd3+ coordinated to three different types of molecules: i) a chelate (EDTA), ii) structured peptides (mSE3/SE2) of the lanthanide-binding tags (LBTs) family with a single high affinity binding site; and iii) a calcium binding protein (calmodulin) with four binding sites. The vibronic side band (VSB) corresponding to the OH stretching mode of waters coordinated to Gd3+, whose frequency is inversely correlated with the strength of the hydrogen bonding to neighboring waters, exhibits an increase in frequency when Gd3+ becomes coordinated to either EDTA, calmodulin or mSE3 peptide. In all of these cases, the addition of cation chloride or acetate salts to the solution increases the frequency of the vibronic band originating from the OH stretching mode of the coordinated waters in a cation and concentration-dependent fashion. The cation dependence of the frequency increase scales with charge density of the cations giving rise to an ordering consistent with the Hofmeister ordering. On the other hand water Raman shows no significant change upon addition of these salts. Additionally, it is shown that the cation effect is modulated by the specific anion used. The results indicate a mechanism of action for Hofmeister series ions in which hydrogen bonding among hydration shell waters is modulated by several factors. High charge density cations sequester waters in a configuration that precludes strong hydrogen bonding to neighboring waters. Under such conditions anion effects emerge as anions compete for hydrogen bonding sites with the remaining free waters on the surface of the hydration shell. The magnitude of the anion effect is both cation and Gd3+-binding site specific. PMID:19603752

Guo, Feng

2009-01-01

411

Functionalization density dependent toxicity of oxidized multiwalled carbon nanotubes in a murine macrophage cell line.  

PubMed

The present study investigates the effect of functionalization density on the toxicity and cellular uptake of oxidized multiwalled carbon nanotubes (f-MWCNTs) in vitro. The toxicity of f-MWCNTs at varying degrees of carboxylation was assessed in a murine macrophage RAW 264.7 cell line, a model for liver Kupffer cells. In vitro cytotoxicity of oxidized MWCNTs was directly proportional to their functionalization density. The increased cytotoxicity was associated with a concurrent increase in the number of apoptotic cells and production of reactive nitrogen species (RNS). In contrast, reactive oxygen species (ROS) generation was the highest in the case of pristine MWCNTs and decreased with increased functionalization density. Quantitative cellular uptake studies indicated that endogenous ROS production was independent of the concentration of CNTs internalized by a specific cell population and was directly proportional to their surface hydrophobicity. Mechanistic studies suggested that cellular uptake of CNTs was critically charge-dependent and mediated through scavenger receptors, albeit the involvement of nonscavenger receptor mechanisms at low CNT concentrations and their saturation at the experimental concentration cannot be ruled out. A mathematical model was established to correlate between the cellular uptake of CNTs with their length and zeta potential. In an attempt to correlate the results of in vitro toxicity experiments with those of the in vivo toxicity in the mouse model, we found that the toxicity trends in vitro and in vivo are rather opposing. The apparent anomaly was explained on the basis of different experimental conditions and doses associated with cells under in vivo and in vitro culture conditions. PMID:22994501

Singh, Raman Preet; Das, Manasmita; Thakare, Vivek; Jain, Sanyog

2012-10-15

412

Heliocentric distance and temporal dependence of the interplanetary density-magnetic field magnitude correlation  

NASA Technical Reports Server (NTRS)

The Helios, IMP 8, ISEE 3, ad Voyager 2 spacecraft are used to examine the solar cycle and heliocentric distance dependence of the correlation between density n and magnetic field magnitude B in the solar wind. Previous work had suggested that this correlation becomes progressively more negative with heliocentric distance out to 9.5 AU. Here it is shown that this evolution is not a solar cycle effect, and that the correlations become even more strongly negative at heliocentric distance larger than 9.5 AU. There is considerable variability in the distributions of the correlations at a given heliocentric distance, but this is not simply related to the solar cycle. Examination of the evolution of correlations between density and speed suggest that most of the structures responsible for evolution in the anticorrelation between n and B are not slow-mode waves, but rather pressure balance structures. The latter consist of both coherent structures such as tangential discontinuities and the more generally pervasive 'pseudosound' which may include the coherent structures as a subset.

Roberts, D. A.

1990-01-01

413

Density-dependent reproductive and vegetative allocation in the aquatic plant Pistia stratiotes (Araceae).  

PubMed

Pistia stratiotes is an aquatic macrophyte that grows in temporary-ponds in the southern Pantanal, Brazil. It reproduces both sexually and asexually and is usually observed forming dense mats on the water surface, a condition favored by the plant's vegetative reproduction coupled with an ability for rapid growth. In this study we examined the effect of densely crowded conditions on the production of reproductive and vegetative structures. In addition, we verified whether there is a trade-off between clonal growth and investment in sexual reproductive structures, and whether there is an allocation pattern with plant size. Individual plant biomass and the number of the rosettes producing sexual reproductive structures and vegetative growth structures both increased with density. Increase in plant size resulted in increased proportional allocation to sexual reproductive structures and vegetative growth structures. Allocation of biomass to reproduction did not occur at the expense of clonal growth. Thus, the density response appears as a increase of rosettes producing sexual reproductive structures and vegetative growth structures. Therefore, long leaves and stolons may be adaptive under densely crowded conditions where competition for light is intense. An important aspect in the study of trade-offs is the size-dependency of the allocation patterns .Usually, larger plants produce more biomass. Therefore, larger plants can allocate more biomass to both vegetative and sexual reproduction than smaller plants and thus show a positive correlation between both traits rather than the expected negative one. PMID:17354448

Coelho, Flávia Freitas; Deboni, Liene; Lopes, Frederico Santos

2005-01-01

414

Transgenerational effects modulate density-dependent prophylactic resistance to viral infection in a lepidopteran pest.  

PubMed

There is an increasing appreciation of the importance of transgenerational effects on offspring fitness, including in relation to immune function and disease resistance. Here, we assess the impact of parental rearing density on offspring resistance to viral challenge in an insect species expressing density-dependent prophylaxis (DDP); i.e. the adaptive increase in resistance or tolerance to pathogen infection in response to crowding. We quantified survival rates in larvae of the cotton leafworm (Spodoptera littoralis) from either gregarious- or solitary-reared parents following challenge with the baculovirus S. littoralis nucleopolyhedrovirus. Larvae from both the parental and offspring generations exhibited DDP, with gregarious-reared larvae having higher survival rates post-challenge than solitary-reared larvae. Within each of these categories, however, survival following infection was lower in those larvae from gregarious-reared parents than those from solitary-reared, consistent with a transgenerational cost of DDP immune upregulation. This observation demonstrates that crowding influences lepidopteran disease resistance over multiple generations, with potential implications for the dynamics of host-pathogen interactions. PMID:25808002

Wilson, Kenneth; Graham, Robert I

2015-03-01

415

Density-Dependent Relations among Properties of Hadronic Matter and Applications to Hadron-Quark Stars  

SciTech Connect

Density-dependent relations among the saturation properties of symmetric nuclear matter and hyperonic matter, and properties of hadron-(strange) quark stars are shown by applying the conserving nonlinear {sigma}-{omega}-{rho} hadronic mean-field theory. Nonlinear interactions are renormalized self-consistently as effective coupling constants, effective masses, and sources of equations of motion by maintaining thermodynamic consistency to the mean-field approximation. Effective masses and coupling constants at the saturation point of symmetric nuclear matter simultaneously determine the binding energy and saturation properties of hyperonic matter. The coupling constants expected from the hadronic mean-field model and SU(6) quark model for the vector coupling constants are compared by calculating masses of hadron-quark neutron stars. The nonlinear {sigma}-{omega}-{rho} mean-field approximation with vacuum fluctuation corrections and strange quark matter defined by the MIT-bag model were employed to examine properties of hadron-(strange) quark stars. We found that hadron-(strange) quark stars become more stable at high densities compared to pure hadronic and strange quark stars.

Uechi, Hiroshi [Department of Distributions and Communication Sciences, Osaka Gakuin University, Osaka (Japan); Uechi, Schun T. [Research Center for Nuclear Physics (RCNP), Osaka University, Osaka (Japan)

2011-05-06

416

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

417

Constraints on neutron skin thickness in 208Pb and density-dependent symmetry energy  

E-print Network

Accurate knowledge about the neutron skin thickness $\\Delta R_{np}$ in $^{208}$Pb has far-reaching implications for different communities of nuclear physics and astrophysics. Yet, the novel Lead Radius Experiment (PREX) did not yield stringent constraint on the $\\Delta R_{np}$ recently. We employ a more practicable strategy currently to probe the neutron skin thickness of $^{208}$Pb based on a high linear correlation between the $\\Delta R_{np}$ and $J-a_{\\text{sym}}$, where $J$ and $a_{\\text{sym}}$ are the symmetry energy (coefficient) of nuclear matter at saturation density and of $^{208}$Pb. An accurate $J-a_{\\text{sym}}$ thus places a strong constraint on the $\\Delta R_{np}$. Compared with the parity-violating asymmetry $A_{\\text{PV}}$ in the PREX, the reliably experimental information on the $J-a_{\\text{sym}}$ is much more easily available attributed to a wealth of measured data on nuclear masses and on decay energies. The density dependence of the symmetry energy is also well constrained with the $J-a_{\\...

Dong, Jianmin; Gu, Jianzhong

2015-01-01

418

Theoretical approach for optical response in electrochemical systems: Application to electrode potential dependence of surface-enhanced Raman scattering  

SciTech Connect

We propose a theoretical approach for optical response in electrochemical systems. The fundamental equation to be solved is based on a time-dependent density functional theory in real-time and real-space in combination with its finite temperature formula treating an electrode potential. Solvation effects are evaluated by a dielectric continuum theory. The approach allows us to treat optical response in electrochemical systems at the atomistic level of theory. We have applied the method to surface-enhanced Raman scattering (SERS) of 4-mercaptopyridine on an Ag electrode surface. It is shown that the SERS intensity has a peak as a function of the electrode potential. Furthermore, the real-space computational approach facilitates visualization of variation of the SERS intensity depending on an electrode potential.

Iida, Kenji; Noda, Masashi; Nobusada, Katsuyuki, E-mail: nobusada@ims.ac.jp [Department of Theoretical and Computational Molecular Science, Institute for Molecular Science, Okazaki 444-8585 (Japan)

2014-09-28

419

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

420

Density-Dependent Regulation of Brook Trout Population Dynamics along a Core-Periphery Distribution Gradient in a Central Appalachian Watershed  

PubMed Central

Spatial population models predict strong density-dependence and relatively stable population dynamics near the core of a species' distribution with increasing variance and importance of density-independent processes operating towards the population periphery. Using a 10-year data set and an information-theoretic approach, we tested a series of candidate models considering density-dependent and density-independent controls on brook trout population dynamics across a core-periphery distribution gradient within a central Appalachian watershed. We sampled seven sub-populations with study sites ranging in drainage area from 1.3–60 km2 and long-term average densities ranging from 0.335–0.006 trout/m. Modeled response variables included per capita population growth rate of young-of-the-year, adult, and total brook trout. We also quantified a stock-recruitment relationship for the headwater population and coefficients of variability in mean trout density for all sub-populations over time. Density-dependent regulation was prevalent throughout the study area regardless of stream size. However, density-independent temperature models carried substantial weight and likely reflect the effect of year-to-year variability in water temperature on trout dispersal between cold tributaries and warm main stems. Estimated adult carrying capacities decreased exponentially with increasing stream size from 0.24 trout/m in headwaters to 0.005 trout/m in the main stem. Finally, temporal variance in brook trout population size was lowest in the high-density headwater population, tended to peak in mid-sized streams and declined slightly in the largest streams with the lowest densities. Our results provide support for the hypothesis that local density-dependent processes have a strong control on brook trout dynamics across the entire distribution gradient. However, the mechanisms of regulation likely shift from competition for limited food and space in headwater streams to competition for thermal refugia in larger main stems. It also is likely that source-sink dynamics and dispersal from small headwater habitats may partially influence brook trout population dynamics in the main stem. PMID:24618602

Huntsman, Brock M.; Petty, J. Todd

2014-01-01