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1

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

SciTech Connect

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

Hoffman, D.K. (Department of Chemistry Ames Laboratory, Iowa State University, Ames, Iowa (USA)); Sharafeddin, O.; Judson, R.S.; Kouri, D.J. (Department of Chemistry Department of Physics, University of Houston, Houston, Texas 77204-5641 (USA))

1990-04-01

2

On angular momentum and parity dependence of nuclear level densities in a simple random sampling approach  

Microsoft Academic Search

Based on simple random sampling (SRS), we propose a Monte Carlo method for faster computation of the smoothed part of the density of nuclear states. To test the applicability of the SRS approach we study in this framework the excitation energy (E), angular momentum (J) and parity dependence of nuclear level densities for an independent particle system. As an illustrative

D. Majumdar; B. K. Agrawal; S. K. Kataria

1996-01-01

3

Skyrme-hartree-fock approach to spherical nuclei with density-dependent pairing correlations  

NASA Astrophysics Data System (ADS)

The ground-state properties of spherical nuclei over the entire periodic table are investigated by the Skyrme-Hartree-Fock approach with new force parameters SKI4 [P. G. Reinhard and H. Flocard, Nucl. Phys. A584 467 (1995)] plus a density-dependent pairing correlation. By introducing the density-dependent pairing correlation in the Skyrme-Hartree-Fock model with SKI4, both the isospin degree of freedom and the nucleon-nucleon correlation have been suitably included in the theory. The theoretical results agree very well with experimental data of binding energies, single particle energies and radii of spherical nuclei. The isotope shifts of charge radii in Ca, Ni, Sn, and Pb are also well reproduced.

Shen, Yao-Song; Ren, Zhongzhou

1996-12-01

4

A unified approach to the density-potential mapping in a family of time-dependent density functional theories  

NASA Astrophysics Data System (ADS)

It is shown that the density-potential mapping and the V-representability problems in the time-dependent current density functional theory (TDCDFT) are reduced to the solution of a certain many-body nonlinear Schrödinger equation (NLSE). The derived NLSE for TDCDFT links the earlier NLSE-based formulations of the time-dependent deformation functional theory (TDDefFT) and the time-dependent density functional theory (TDDFT). We establish a close relation between the nonlinear many-body problems which control the existence of TDCDFT, TDDFT, and TDDefFT, and thus develop a unified point of view on the whole family of the TDDFT-type theories.

Tokatly, I. V.

2011-11-01

5

Antisymmetrization and density-dependent effects within the folding model approach to alpha-nucleus scattering  

Microsoft Academic Search

The real alpha-nucleus interaction is investigated within the framework of the folding model. First the non-local alpha-nucleon interaction is calculated in the Hartree-Fock approximation and then folded in with the target matter density. The resulting alpha-nucleus potential is non-local but a local equivalent potential is obtained within the Perey-Saxon approximation. The calculations are performed both with density-independent and density-dependent forces.

F. Duggan; M. Lassaut; F. Michel; N. Vinh Mau

1981-01-01

6

Hot-electron-assisted femtochemistry at surfaces: A time-dependent density functional theory approach  

NASA Astrophysics Data System (ADS)

Using time-evolution time-dependent density functional theory (TDDFT) within the adiabatic local-density approximation, we study the interactions between single electrons and molecular resonances at surfaces. Our system is a nitrogen molecule adsorbed on a ruthenium surface. The surface is modeled at two levels of approximation, first as a simple external potential and later as a 20-atom cluster. We perform a number of calculations on an electron hitting the adsorbed molecule from inside the surface and establish a picture, where the resonance is being probed by the hot electron. This enables us to extract the position of the resonance energy through a fitting procedure. It is demonstrated that with the model we can extract several properties of the system, such as the presence of resonance peaks, the time electrons stay on the molecule before returning to the surface when hitting a molecular resonance and the lowering of the resonance energy due to an image charge effect. Finally we apply the TDDFT procedure to only consider the decay of molecular excitations and find that it agrees quite well with the width of the projected density of Kohn-Sham states.

Gavnholt, Jeppe; Rubio, Angel; Olsen, Thomas; Thygesen, Kristian S.; Schiøtz, Jakob

2009-05-01

7

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

PubMed

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

Galperin, Michael; Tretiak, Sergei

2008-03-28

8

Time-dependent density functional theory with the generalized restricted-unrestricted approach  

Microsoft Academic Search

This work presents the derivation, implementation, and first applications of the generalized restricted-unrestricted method based on the density functional Kohn-Sham formalism. By using a spin-restricted Kohn-Sham representation for the reference state the well-known spin contamination problem is avoided, while the unrestricted representation of the perturbation response retains a proper description of spin polarization. The formulation is a generalization of our

Corneliu I. Oprea; Lyudmyla Telyatnyk; Zilvinas Rinkevicius; Olav Vahtras; Hans Gren

2006-01-01

9

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

NASA Astrophysics Data System (ADS)

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

Zhang, Xu; Li, Zi; Lu, Gang

2012-05-01

10

Spatially explicit analyses unveil density dependence.  

PubMed Central

Density-dependent processes are fundamental in the understanding of species population dynamics. Whereas the benefits of considering the spatial dimension in population biology are widely acknowledged, the implications of doing so for the statistical detection of spatial density dependence have not been examined. The outcome of traditional tests may therefore differ from those that include ecologically relevant locational information on both the prey species and natural enemy. Here, we explicitly incorporate spatial information on individual counts when testing for density dependence between an insect herbivore and its parasitoids. The spatially explicit approach used identified significant density dependence more frequently and in different instances than traditional methods. The form of density dependence detected also differed between methods. These results demonstrate that the explicit consideration of patch location in density-dependence analyses is likely to significantly alter current understanding of the prevalence and form of spatial density dependence in natural populations.

Veldtman, Ruan; McGeoch, Melodie A

2004-01-01

11

Analytical approach for the excited-state Hessian in time-dependent density functional theory: Formalism, implementation, and performance  

NASA Astrophysics Data System (ADS)

The paper presents the formalism, implementation, and performance of the analytical approach for the excited-state Hessian in the time-dependent density functional theory (TDDFT) that extends our previous work [J. Liu and W. Z. Liang, J. Chem. Phys. 135, 014113 (2011)] on the analytical Hessian in TDDFT within Tamm-Dancoff approximation (TDA) to full TDDFT. In contrast to TDA-TDDFT, an appreciable advantage of full TDDFT is that it maintains the oscillator strength sum rule, and therefore yields more precise results for the oscillator strength and other related physical quantities. For the excited-state harmonic vibrational frequency calculation, however, full TDDFT does not seem to be advantageous since the numerical tests demonstrate that the accuracy of TDDFT with and without TDA are comparable to each other. As a common practice, the computed harmonic vibrational frequencies are scaled by a suitable scale factor to yield good agreement with the experimental fundamental frequencies. Here we apply both the optimized ground-state and excited-state scale factors to scale the calculated excited-state harmonic frequencies and find that the scaling decreases the root-mean-square errors. The optimized scale factors derived from the excited-state calculations are slightly smaller than those from the ground-state calculations.

Liu, Jie; Liang, Wanzhen

2011-11-01

12

Giant Monopole Transition Densities within the Local Scale ATDHF (Adiabatic Time-Dependent Hartree-Fook) Approach.  

National Technical Information Service (NTIS)

Transition densities for sup 12 C, sup 16 O, sup 28 Si, sup 32 S, sup 40 Ca, sup 48 Ca, sup 56 Ni, sup 90 Zr, sup 208 Pb even-even nuclei corresponding to nuclear giant monopole resonances obtained within a local-scale adiabatic time-dependent Hartree-Foc...

S. S. Dimitrova I. Z. Petkov M. V. Stoitsov

1986-01-01

13

Time-Dependent Density Functional Theory  

Microsoft Academic Search

Time-dependent density-functional theory (TDDFT) extends the basic ideas of ground-state density-functional theory (DFT) to\\u000a the treatment of excitations and of more general time-dependent phenomena. TDDFT can be viewed as an alternative formulation\\u000a of time-dependent quantum mechanics but, in contrast to the normal approach that relies on wave-functions and on the many-body\\u000a Schrödinger equation, its basic variable is the one-body electron

Miguel A. L. Marques; Eberhard Gross

14

Saltwater intrusion estimation in a karstified coastal system using density-dependent modelling and comparison with the sharp-interface approach  

Microsoft Academic Search

Saltwater intrusion is a naturally occurring phenomenon that is exacerbated significantly by excessive groundwater exploitation in coastal aquifers. In order to determine the extent of saltwater intrusion in a karstified coastal aquifer in Crete, Greece, a three-dimensional, density-dependent groundwater flow and transport model was developed and compared to the more traditional sharp-interface approach. The karstified medium was modelled using a

Zoi Dokou; George P. Karatzas

2012-01-01

15

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

NASA Astrophysics Data System (ADS)

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

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

2009-03-01

16

Density-dependent effective interactions  

SciTech Connect

An effective two nucleon interaction is defined by an optimal fit to select on- and half-off-of-the-energy shell {ital t} and {ital g} matrices determined by solutions of the Lippmann-Schwinger and Brueckner-Bethe-Goldstone equations with the Paris two nucleon interaction as input. As such, it reproduces the interaction on which it is based better than other commonly used, density dependent effective interactions. This new (medium modified) effective interaction, when folded with appropriate density matrices, has been used to define proton-{sup 12}C and proton-{sup 16}O optical potentials. With them elastic scattering data are well fit and the medium effects identifiable.

Dortmans, P.J.; Amos, K. [School of Physics, University of Melbourne, Parkville, 3052 Victoria (Australia)

1994-03-01

17

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

NASA Astrophysics Data System (ADS)

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

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

2011-05-01

18

Unitary group approach to reduced density matrices  

SciTech Connect

A fully spin-adapted approach to many-electron density matrices is developed in the context of the unitary group approach to many-electron systems. An explicit expression for the single-electron spin-density operator, as a polynomial of degree two in the orbital U({ital n}) generators, is derived for the case of spin-independent systems. Extensions to spin-dependent systems are also considered, leading to the appearance of total-spin transition densities, whose general properties are investigated. A corresponding formalism for the two-electron density matrix, which is capable of further generalization, is also developed. The results of this paper, together with recent developments on the matrix elements of the U(2{ital n}) generators in the electronic Gel'fand basis, afford a versatile method for the direct calculation of one- and two-body density matrices in the unitary group approach framework.

Gould, M.D. (Department of Mathematics, University of Queensland, St. Lucia, Queensland (Australia)); Paldus, J. (Department of Applied Mathematics, University of Waterloo, Waterloo, ON (Canada) Department of Chemistry, University of Waterloo, Waterloo, ON (Canada) Guelph-Waterloo Center for Graduate Work in Chemistry, University of Waterloo, Waterloo, Ontario (Canada)); Chandler, G.S. (School of Chemistry, University of Western Australia, Nedlands, Western Australia (Australia))

1990-09-15

19

Pseudospectral time-dependent density functional theory  

Microsoft Academic Search

Time-dependent density functional theory TDDFT is implemented within the Tamm-Dancoff approximation TDA using a pseudospectral approach to evaluate two-electron repulsion integrals. The pseudospectral approximation uses a split representation with both spectral basis functions and a physical space grid to achieve a reduction in the scaling behavior of electronic structure methods. We demonstrate here that exceptionally sparse grids may be used

Chaehyuk Ko; David K. Malick; Dale A. Braden; Richard A. Friesner; Todd J. Martínez

2008-01-01

20

Local exchange-correlation vector potential with memory in time-dependent density functional theory:?The generalized hydrodynamics approach  

NASA Astrophysics Data System (ADS)

Using Landau Fermi-liquid theory we derive a nonlinear nonadiabatic approximation for the exchange-correlation (xc) vector potential defined by the xc stress tensor. The stress tensor is a local nonlinear functional of two basic variables: the displacement vector and the second-rank tensor which describes the evolution of momentum in a local frame moving with Eulerian velocity. For irrotational motion and the equilibrium initial state the dependence on the tensor variable reduces to that on the metrics generated by dynamical deformation of the system.

Tokatly, I. V.; Pankratov, O.

2003-05-01

21

On the meaning of density dependence  

Microsoft Academic Search

Despite a long history, the term “density dependence” lacks a generally accepted definition. A definition is offered that seems consistent with most other definitions and general usage, that is, a density-dependent factor is any component of the environment whose intensity is correlated with population density and whose action affects survival and reproduction. This definition is used in evaluating the role

Bertram G. Murray

1982-01-01

22

Density Dependence, Regulation and Variability in Animal Populations  

Microsoft Academic Search

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

Ilkka Hanski

1990-01-01

23

Time-dependent density functional theory excited state nonadiabatic dynamics combined with quantum mechanical/molecular mechanical approach: Photodynamics of indole in water  

NASA Astrophysics Data System (ADS)

We present a combination of time-dependent density functional theory with the quantum mechanical/molecular mechanical approach which can be applied to study nonadiabatic dynamical processes in molecular systems interacting with the environment. Our method is illustrated on the example of ultrafast excited state dynamics of indole in water. We compare the mechanisms of nonradiative relaxation and the electronic state lifetimes for isolated indole, indole in a sphere of classical water, and indole + 3H2O embedded in a classical water sphere. In the case of isolated indole, the initial excitation to the S2 electronic state is followed by an ultrafast internal conversion to the S1 state with a time constant of 17 fs. The S1 state is long living (>30 ps) and deactivates to the ground state along the N-H stretching coordinate. This deactivation mechanism remains unchanged for indole in a classical water sphere. However, the lifetimes of the S2 and S1 electronic states are extended. The inclusion of three explicit water molecules opens a new relaxation channel which involves the electron transfer to the solvent, leading eventually to the formation of a solvated electron. The relaxation to the ground state takes place on a time scale of 60 fs and contributes to the lowering of the fluorescence quantum yield. Our simulations demonstrate the importance of including explicit water molecules in the theoretical treatment of solvated systems.

Wohlgemuth, Matthias; Bona?i?-Koutecký, Vlasta; Mitri?, Roland

2011-08-01

24

New insights into testing for density dependence  

Microsoft Academic Search

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

M. Holyoak

1993-01-01

25

Density-Dependent Factors in Ecology  

Microsoft Academic Search

ANDREWARTHA'S ideas on density-dependent factors1 in ecology are very misleading, and have already been criticized by Huffaker2. I wish to refer to three points which he raises in his recent communication. First, he argues that since the population models of Lotka and Volterra are based on density-dependent relationships, and these have been ``partly confirmed by laboratory experiments with such simple

G. C. Varley

1959-01-01

26

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

2012-05-16

27

Time-Dependent Current Density Functional Theory  

NASA Astrophysics Data System (ADS)

The nonlocality of the exchange-correlation (xc) potential, i.e., the fact that the xc potential at a certain position depends on the global distribution of the particles in space, is the curse of density functional theory. It is mainly because of this fact that, even after years of intensive studies, the exact form of the xc potential as a functional of the density remains unknown. Nevertheless, it is true that many accurate and useful results can be obtained from the use of an approximation - the local density approximation (LDA) - which ignores the problem altogether.

Vignale, Giovanni

28

Helminth fecundity: density dependence or statistical illusion?  

PubMed

Density-dependent constraints on fecundity or survival are critical for the regulation and stability o f all populations. Helminth parasites are no exception to this rule. In medical helminthology, it has been widely assumed that the most effective density-dependent constraints act upon parasite fecundity and are the result of intra-specific parasite competition or acquired immunity to infection. In this article, Anne Keymer and Andrew Slater advocate a more detailed examination of the evidence on which this assumption rests. PMID:15462908

Keymer, A E; Slater, A F

1987-02-01

29

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

Microsoft Academic Search

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

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

2003-01-01

30

An approach to fast fits of the unintegrated gluon density  

SciTech Connect

An approach to fast fits of the unintegrated gluon density has been developed and used to determine the unintegrated gluon density by fits to deep inelastic scatting di-jet data from HERA. The fitting method is based on the determination of the parameter dependence by help of interpolating between grid points in the parameter-observable space before the actual fit is performed.

Knutsson, Albert; Bacchetta, Alessandro; Kutak, Krzyzstof; Jung, Hannes

2009-01-01

31

Asymmetry Dependence of Nuclear Temperatures and Densities  

NASA Astrophysics Data System (ADS)

Quasi-projectile sources produced in collisions of 70Zn+70Zn, 64Zn+64Zn and 64Ni+64Ni at E/A=35MeV have been reconstructed using the charged particles and free neutrons measured in the NIMROD-ISiS 4-pi detector. Equilibrated sources were selected which have a mass A=48-52 and which are on average spherical. Caloric curves for these quasi-projectiles have been extracted with the quadrupole momentum fluctuation thermometer (QMFT) and the Albergo thermometer. The classical QMFT and the Albergo thermometer exhibit a clear dependence on the composition, (N-Z)/A, of the source. For a given excitation (E*/A), the neutron-poor sources exhibit higher temperatures. The quantum QMFT exhibits a somewhat different dependence on the composition. Since the density and temperature are correlated in the quantum QMFT, this difference may be due to a dependence of the nuclear density on the composition.

McIntosh, Alan; Bonasera, A.; Kohley, Z.; Galanopoulos, S.; Hagel, K.; May, L. W.; Marini, P.; Shetty, D. V.; Smith, W. B.; Soisson, S. N.; Souliotis, G. A.; Stein, B. C.; Tripathi, R.; Wuenschel, S.; Yennello, S. J.

2012-10-01

32

Diffusion in polymers dependence on crosslink density  

Microsoft Academic Search

Kinetics of N-methyl pyrrolidone evaporation from swollen photo-crosslinked polyacrylate was monitored thermogravimetrically at temperatures\\u000a ranging from 323 to 398 K. Crosslink density dependence of evaporation kinetics was investigated in photo-crosslinked polyacrylates\\u000a with crosslinked density ranging from ?1.2 × 102 to ?1.7 × 104 mol m?3 and number of main chain atoms between crosslinks ranging from ?70 atoms to ?6 atoms, respectively. As was shown, evaporation\\u000a kinetics was

Vadim V. Krongauz

2010-01-01

33

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

34

Density-Dependent Factors in Ecology  

Microsoft Academic Search

Solomon and Varley1 did well to direct attention to the confusion that accompanies the ecological term `density-dependent factors'. Howard and Fiske's2 original idea that the multiplication of natural populations of animals can be restricted only by some ``factor'' that exerts ``a restraining influence which is relatively more effective when other conditions favour undue increase'' has been extended in a number

H. G. Andrewartha

1959-01-01

35

Detecting Density Dependence in Recovering Seal Populations  

Microsoft Academic Search

Time series of abundance estimates are commonly used for analyses of population trends and possible shifts in growth rate.\\u000a We investigate if trends in age composition can be used as an alternative to abundance estimates for detection of decelerated\\u000a population growth. Both methods were tested under two forms of density dependence and different levels of environmental variation\\u000a in simulated time

Carl Johan Svensson; Anders Eriksson; Tero Harkonen; Karin C. Harding

2011-01-01

36

Central density dependent anisotropic compact stars  

NASA Astrophysics Data System (ADS)

Stars can be treated as a self-gravitating fluid. In this connection, we propose a model for an anisotropic star under the relativistic framework of Krori and Barua (J. Phys. A, Math. Gen. 8:508, 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.

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

2013-04-01

37

Dark matter with density-dependent interactions  

NASA Astrophysics Data System (ADS)

The decay and annihilation cross sections of dark matter particles may depend on the value of a chameleonic scalar field that both evolves cosmologically and takes different values depending on the local matter density. This possibility introduces a separation between the physics relevant for freeze-out and that responsible for dynamics and detection in the late universe. We investigate how such dark sector interactions might be implemented in a particle physics Lagrangian and consider how current and upcoming observations and experiments bound such dark matter candidates. A specific simple model allows for an increase in the annihilation cross section by a factor of 106 between freeze-out and today, while more complicated models should also allow for scattering cross sections near the astrophysical bounds.

Boddy, Kimberly K.; Carroll, Sean M.; Trodden, Mark

2012-12-01

38

Unitary group approach to reduced density matrices  

Microsoft Academic Search

Summary The structure of the algebra generated by one-electron charge and spin density operators with an appropriately defined convolution product is investigated in the context of the unitary group approach (UGA) to the many-electron correlation problem. The new idempotent density operators, defined via the unitary groupU(n) projection operators, are introduced and employed to investigate the behavior of charge and spin

J. Paldus; M. D. Gould

1993-01-01

39

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

40

Effects of multiple electronic shells on strong-field multiphoton ionization and high-order harmonic generation of diatomic molecules with arbitrary orientation: An all-electron time-dependent density-functional approach  

SciTech Connect

We present a time-dependent density-functional theory approach with proper long-range potential for an ab initio study of the effect of correlated multielectron responses on the multiphoton ionization (MPI) and high-order harmonic generation (HHG) of diatomic molecules N{sub 2} and F{sub 2} in intense short laser pulse fields with arbitrary molecular orientation. We show that the contributions of inner molecular orbitals to the total MPI probability can be sufficiently large or even dominant over the highest-occupied molecular orbital, depending on detailed electronic structure and symmetry, laser field intensity, and orientation angle. The multielectron effects in HHG are also very important. They are responsible for enhanced HHG at some orientations of the molecular axis. Even strongly bound electrons may have a significant influence on the HHG process.

Telnov, Dmitry A. [Department of Physics, St. Petersburg State University, 198504 St. Petersburg (Russian Federation); Department of Chemistry, University of Kansas, Lawrence, Kansas 66045 (United States); Chu, S.-I [Department of Chemistry, University of Kansas, Lawrence, Kansas 66045 (United States)

2009-10-15

41

Pathogens, density dependence and the coexistence of tropical trees  

Microsoft Academic Search

There is increasing interest in the role played by density-dependent mortality from natural enemies, particularly plant pathogens, in promoting the coexistence and diversity of tropical trees. Here, we review four issues in the analysis of pathogen-induced density dependence that have been overlooked or inadequately addressed. First, the methodology for detecting density dependence must be robust to potential biases. Observational studies,

Robert P. Freckleton; Owen T. Lewis

2006-01-01

42

Temporally structured density-dependence and population management  

Microsoft Academic Search

We used a simple discrete-time population model to investigate how temporally struc- tured density-dependence influences a population's response to loss due to harvesting. We assumed that reproduction is a relative discrete event in time, followed by density- dependent mortality and then harvesting, or followed by harvesting and then density- dependent mortality. Such an ordering of events in time may have

Niclas Jonzén; Per Lundberg

1999-01-01

43

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

PubMed

Time-dependent density functional theory (TD-DFT) in the adiabatic formulation exhibits known failures when applied to predicting excitation energies. One of them is the lack of the doubly excited configurations. On the other hand, the time-dependent theory based on a one-electron reduced density matrix functional (time-dependent density matrix functional theory, TD-DMFT) has proven accurate in determining single and double excitations of H(2) molecule if the exact functional is employed in the adiabatic approximation. We propose a new approach for computing excited state energies that relies on functionals of electron density and one-electron reduced density matrix, where the latter is applied in the long-range region of electron-electron interactions. A similar approach has been recently successfully employed in predicting ground state potential energy curves of diatomic molecules even in the dissociation limit, where static correlation effects are dominating. In the paper, a time-dependent functional theory based on the range-separation of electronic interaction operator is rigorously formulated. To turn the approach into a practical scheme the adiabatic approximation is proposed for the short- and long-range components of the coupling matrix present in the linear response equations. In the end, the problem of finding excitation energies is turned into an eigenproblem for a symmetric matrix. Assignment of obtained excitations is discussed and it is shown how to identify double excitations from the analysis of approximate transition density matrix elements. The proposed method used with the short-range local density approximation (srLDA) and the long-range Buijse-Baerends density matrix functional (lrBB) is applied to H(2) molecule (at equilibrium geometry and in the dissociation limit) and to Be atom. The method accounts for double excitations in the investigated systems but, unfortunately, the accuracy of some of them is poor. The quality of the other excitations is in general much better than that offered by TD-DFT-LDA or TD-DMFT-BB approximations if the range-separation parameter is properly chosen. The latter remains an open problem. PMID:22583275

Pernal, Katarzyna

2012-05-14

44

Time-dependent lunar density models  

Microsoft Academic Search

Simple models of the geochemical and geophysical evolution of the moon were constructed with the particular aim of investigating the mass density distribution within the moon as a function of time. The strongly inverted (densest material on top) density distribution resulting from fractional crystallization of the lunar magma ocean was found to rearrange itself into a highly stable stratification. Density

F. Herbert

1980-01-01

45

The scale and cause of spatial heterogeneity in strength of temporal density dependence.  

PubMed

The importance of density dependence in natural communities continues to spark much debate because it is fundamental to population regulation. We used temporal manipulations of density to explore potentially stabilizing density dependence in early survivorship among six local populations of a tropical damselfish (Dascyllus flavicaudus). Specifically, we tested the premise that spatial heterogeneity in the strength of temporal density dependence would reflect variation in density of predators, the agent of mortality. Our field manipulations revealed that mortality among successive cohorts of young fishes was density dependent at each reef, but that its strength varied by approximately 1.5 orders of magnitude. This spatial heterogeneity was well predicted by variation among the six reefs in the density of predatory fishes that consume juvenile damselfishes. Because density dependence arose from competition for enemy-free space within a shelter coral, the mortality consequence of the competition depended on the neighborhood density of predators. Thus, the scale of heterogeneity in the density dependence largely reflected attributes of the environment that shaped the local abundance of predators. These results have important implications for how ecologists explore regulatory processes in nature. Failure to account for spatial variation could frequently yield misleading conclusions regarding density dependence as a stabilizing process, obscure underlying mechanisms influencing its strength, and provide no insight into the spatial scale of the heterogeneity. Further, models of population dynamics will be improved when experimental approaches better estimate the magnitude and causes of variation in strength of stabilizing density dependence. PMID:17536410

Schmitt, Russell J; Holbrook, Sally J

2007-05-01

46

Density dependence and the stabilization of animal numbers  

Microsoft Academic Search

The claim of Varley and Gradwell that the highly density-dependent pupal predation (k5) in the population of the winter moth in Wytham Wood, England would keep density within limits (regulate) is compared in this paper with the density limits in the null model: pupal predation causes the same mean generation mortality (35%) as in the field, but is not density-dependent,

P. J. den Boer

1986-01-01

47

Density dependence in Caenorhabditis larval starvation  

PubMed Central

Availability of food is often a limiting factor in nature. Periods of food abundance are followed by times of famine, often in unpredictable patterns. Reliable information about the environment is a critical ingredient of successful survival strategy. One way to improve accuracy is to integrate information communicated by other organisms. To test whether such exchange of information may play a role in determining starvation survival strategies, we studied starvation of L1 larvae in C. elegans and other Caenorhabditis species. We found that some species in genus Caenorhabditis, including C. elegans, survive longer when starved at higher densities, while for others survival is independent of the density. The density effect is mediated by chemical signal(s) that worms release during starvation. This starvation survival signal is independent of ascarosides, a class of small molecules widely used in chemical communication of C. elegans and other nematodes.

Artyukhin, Alexander B.; Schroeder, Frank C.; Avery, Leon

2013-01-01

48

PARTICIPATORY DEPENDENCE STRUCTURE MATRIX APPROACH  

Microsoft Academic Search

Dependence Structure Matrix (DSM) has been shown to be an important and effective tool in managing complex projects through its focus on analyzing relations and dependencies. Three major outcomes of DSM analysis can be identified. The first is the design of an integrated or modular product structure based on relations among components in a product architecture or design parameters. The

Mike Danilovic; Håkan Börjesson

49

Density dependence of the depolarized light scattering spectrum of xenon  

Microsoft Academic Search

Measurements of the depolarized light scattering spectra of xenon at densities between 0.2 and 2.6 times the critical density are presented. The density dependences of the first three even spectral moments are compared with the predictions by the theory under several simplifying assumptions where use is made of data for the static structure factor from the literature. Comparison is also

J. van der Elsken; R. A. Huijts

1988-01-01

50

Factors influencing detection of density dependence in British birds  

Microsoft Academic Search

We question why density dependence has remained elusive in series of annual abundances of British birds. In particular, an earlier study reported that significant temporal trends in abundances occur in up to 74% of time series from the Common Birds Census. Several studies showed that such trends can hinder detection of density dependence. Temporal trends do not preclude the presence

Marcel Holyoak; Stephen R. Baillie

1996-01-01

51

Testing for density-dependent effects in sequential censuses  

Microsoft Academic Search

In response to Gaston and Lawton (1987), we evaluated the ability of four statistical procedures to detect density dependence. We used data from the same 16 populations as Gaston and Lawton (1987). In each population, density dependence had been previously established with techniques that use more extensive data. The major axis test (Slade 1977) was rarely (3 populations of 16)

William L. Vickery; Thomas D. Nudds

1991-01-01

52

Density dependence of the nuclear symmetry energy: A microscopic perspective  

NASA Astrophysics Data System (ADS)

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

Vidaña, Isaac; Providência, Constança; Polls, Artur; Rios, Arnau

2009-10-01

53

Covariant and gauge-invariant approach to cosmological density fluctuations  

NASA Astrophysics Data System (ADS)

It has been known for a long time that the gauge problem plagues the study of density perturbations in cosmology. The quantity ??/? (the fractional variation in density along a world line) usually determined in perturbation calculations is completely dependent on the gauge chosen. Even the fully covariant approach of Hawking (1966) is not immune. Bardeen's major paper (1980) determines a set of gauge-invariant quantities that are related to density perturbations but are not those perturbations themselves. We give a simple alternative representation of density fluctuations. This representation is both fully covariant and gauge invariant; thus it sidesteps the usual problems. The basic quantity used to represent density inhomogeneities is the comoving fractional gradient of the energy density orthogonal to the fluid flow. Our description does not make the usual assumption that this gradient is small. Exact (fully nonlinear) propagation equations are derived for this quantity. They are then linearized to give propagation equations appropriate to the case of an almost-Robertson-Walker universe. Their solutions are obtained in a simple case which can be compared with the standard theory; we recover the usual growing and decaying modes. Thus the result is standard, but its derivation is completely transparent. We give an interpretation of the Bardeen variables in terms of our formalism.

Ellis, G. F. R.; Bruni, M.

1989-09-01

54

Existence of time-dependent density-functional theory for open electronic systems: time-dependent holographic electron density theorem.  

PubMed

We present the time-dependent holographic electron density theorem (TD-HEDT), which lays the foundation of time-dependent density-functional theory (TDDFT) for open electronic systems. For any finite electronic system, the TD-HEDT formally establishes a one-to-one correspondence between the electron density inside any finite subsystem and the time-dependent external potential. As a result, any electronic property of an open system in principle can be determined uniquely by the electron density function inside the open region. Implications of the TD-HEDT on the practicality of TDDFT are also discussed. PMID:21643618

Zheng, Xiao; Yam, ChiYung; Wang, Fan; Chen, GuanHua

2011-06-06

55

Regulation of Aedes aegypti population dynamics in field systems: quantifying direct and delayed density dependence.  

PubMed

Transgenic strains of Aedes aegypti have been engineered to help control transmission of dengue virus. Although resources have been invested in developing the strains, we lack data on the ecology of mosquitoes that could impact the success of this approach. Although studies of intra-specific competition have been conducted using Ae. aegypti larvae, none of these studies examine mixed age cohorts at densities that occur in the field, with natural nutrient levels. Experiments were conducted in Mexico to determine the impact of direct and delayed density dependence on Ae. aegypti populations. Natural water, food, and larval densities were used to estimate the impacts of density dependence on larval survival, development, and adult body size. Direct and delayed density-dependent factors had a significant impact on larval survival, larval development, and adult body size. These results indicate that control methods attempting to reduce mosquito populations may be counteracted by density-dependent population regulation. PMID:23669230

Walsh, Rachael K; Aguilar, Cristobal L; Facchinelli, Luca; Valerio, Laura; Ramsey, Janine M; Scott, Thomas W; Lloyd, Alun L; Gould, Fred

2013-05-13

56

Determination of the density dependence of the nuclear incompressibility  

NASA Astrophysics Data System (ADS)

Background: The determination of the density dependence of the nuclear incompressibility can be investigated using the isoscalar giant monopole resonance.Purpose: The importance of the so-called crossing density at subsaturation density is underlined.Methods: The measurements of the isoscalar giant monopole resonance (GMR), also called the breathing mode, are analyzed with respect to their constraints on the quantity Mc, e.g., the density dependence of the nuclear incompressibility around the so-called crossing density ?c=0.1 fm-3.Results: The correlation between the centroid of the GMR, EGMR, and Mc is shown to be more accurate than the one between EGMR and the incompressibility modulus at saturation density, K?, giving rise to an improved determination on the nuclear equation of state. The relationship between Mc and K? is given as a function of the skewness parameter Q? associated with the density dependence of the equation of state. The large variation of Q? among different energy density functionals directly impacts the knowledge of K?: A better knowledge of Q? is required to deduce more accurately K?. Using the local density approximation, a simple and accurate expression relating EGMR and the quantity Mc is derived and successfully compared to the fully microscopic predictions.Conclusions: The measurement of the GMR constrains the slope of the incompressibility Mc at the crossing density rather than the incompressibility modulus at the saturation density.

Khan, E.; Margueron, J.

2013-09-01

57

Density-potential mapping in time-dependent density-functional theory  

SciTech Connect

The key questions of uniqueness and existence in time-dependent density-functional theory are usually formulated only for potentials and densities that are analytic in time. Simple examples, standard in quantum mechanics, lead, however, to nonanalyticities. We reformulate these questions in terms of a nonlinear Schroedinger equation with a potential that depends nonlocally on the wave function.

Maitra, N. T. [Department of Physics and Astronomy, Hunter College and the Graduate Center of the City University of New York, 695 Park Avenue, New York, New York 10065 (United States); Todorov, T. N. [School of Mathematics and Physics, Queen's University Belfast, Belfast BT7 1NN (United Kingdom); Woodward, C. [Department of Mathematics, Hill Center, Rutgers, the State University of New Jersey, 110 Frelinghuysen Road, Piscataway New Jersey 08854 (United States); Burke, K. [Department of Chemistry, 1102 Natural Sciences 2, University of California, Irvine, Irvine, California 92697 (United States)

2010-04-15

58

Parasitism in patchy environments: inverse density dependence can be stabilizing.  

PubMed

There are now many examples in the literature where the spatial distribution of per cent parasitism by insect parasitoids is either directly or inversely dependent on host density per patch. While it is well known that direct density dependent relationships can contribute markedly to the stability of a host-parasitoid interaction, inverse relationships have been more-or-less ignored. Using difference equation models, the dynamics of host-parasitoid interactions are described where parasitism per patch varies across the range from direct to inversely density dependent. These models demonstrate for a variety of host distributions that inverse relationships can also strongly promote stability. PMID:6600090

Hassell, M P

1984-01-01

59

Spatial synchrony through density-independent versus density-dependent dispersal  

Microsoft Academic Search

Many theoretical studies support the notion that strong dispersal fosters spatial synchrony. Nonetheless, the effect of conditional vs. unconditional dispersal has remained a matter of controversy. We scrutinize recent findings on a desynchronizing effect of negative density-dependent dispersal based on spatially explicit simulation models. Keeping net emigration rates equivalent, we compared density-independent and density-dependent dispersal for different types of intraspecific

Tamara Münkemüller; Karin Johst

2008-01-01

60

Seasonality, density dependence, and population cycles in Hokkaido voles  

PubMed Central

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

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

2003-01-01

61

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

62

Temperature dependence of electrical conductivity and the probability density function  

Microsoft Academic Search

The temperature dependence of electrical conductivity for many materials is shown to follow a function G(E, T) that is the derivative of the Fermi function with respect to temperature. It is noted that the Fermi function is a cumulative probability function. It then follows that the probability density function that is used in the calculation of the electron density in

P. Love

1983-01-01

63

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

64

Habitat availability mediates chironomid density-dependent oviposition  

Microsoft Academic Search

Knowledge of density-dependent processes and how they are mediated by environmental factors is critically important for understanding\\u000a population and community ecology of insects, as well as for mitigating harmful insect-borne diseases. Here, we tested whether\\u000a the oviposition of chironomids (Diptera: Chironomidae; non-biting midges), known to carry the Cholera pathogen Vibrio cholerae, is density dependent and if it is mediated by

Amit Lerner; Nir Sapir; Carynelisa Erlick; Nikolay Meltser; Meir Broza; Nadav Shashar

2011-01-01

65

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

2013-04-01

66

Factors influencing detection of density dependence in British birds  

Microsoft Academic Search

If censuses are taken at less than generation intervals, the number of successive censuses in which a given individual is recorded will depend on longevity. Repeatedly recording the same individuals could produce under-estimates of population variability and influence detection of density dependence. We investigated this possibility in 60 time series of abundances of British birds compiled from the Common Birds

Marcel Holyoak; Stephen R. Baillie

1996-01-01

67

Solar activity dependence of the thermosphere density by GRACE  

NASA Astrophysics Data System (ADS)

Thermospheric densities near 480 km from April, 2002 to December, 2009 are retrieved from accelerometer measurements of the Gravity Recovery and Climate Experiment (GRACE) satel-lite. The solar activity dependence of thermospheric density observed by GRACE is studied. The results show that thermospheric density varies significantly with solar irradiance. The density correlates remarkably well with any one of selected solar activity indices and all the correlation coefficients between them are larger than 0.95, except for solar 0.1-7 nm irradiances. The density variability can be fitted well by a quaderic function of each solar irradiance index. The relative standard deviations (RSD) and correlations between the observed densities and the fitted values for different solar indices are different. The fitted results using Mg II, Fe XVI and H I indices are better than others. The RSD for these three indices are 0.23

Chen, Guangming; Fu, Yang; Gao, Hong

68

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

69

Action formalism of time-dependent density-functional theory  

NASA Astrophysics Data System (ADS)

The Runge-Gross [E. Runge and E. K. U. Gross, Phys. Rev. Lett.PRLTAO0031-900710.1103/PhysRevLett.52.997 52, 997 (1984)] action functional of time-dependent density-functional theory leads to a well-known causality paradox; that is, a perturbation of the electronic density in the future affects the response of the system in the present. This paradox is known to be caused by an inconsistent application of the Dirac-Frenkel variational principle. In view of the recent solutions to this problem, the action functional employed by Runge and Gross in their formulation of time-dependent density-functional theory is analyzed in the context of the Keldysh contour technique. The time-dependent electronic density and the concept of causality are extended to the contour. We derive a variational equation that obeys causality and relates the exchange-correlation potential with its kernel and the functional derivative of the exchange-correlation action functional with respect to the density. It is shown that the adiabatic local-density approximation is a consistent solution of this equation and that the time-dependent optimized potential method can also be derived from it. The formalism presented here can be used to find new approximation methods for the exchange-correlation potential and to avoid the causality dilemma.

Mosquera, Martín A.

2013-08-01

70

Seasonality, density dependence, and population cycles in Hokkaido voles  

Microsoft Academic Search

Voles and lemmings show extensive variation in population dynamics regulated across and within species. In an attempt to develop and test generic hypotheses explaining these differences, we studied 84 populations of the gray-sided vole (Clethrionomys rufocanus) in Hokkaido, Japan. We show that these populations are limited by a combination of density-independent factors (such as climate) and density-dependent processes (such as

Nils C. Stenseth; Hildegunn Viljugrein; Takashi Saitoh; Thomas F. Hansen; Marte O. Kittilsen; Erik Bølviken; Fredrik Glöckner

2003-01-01

71

Density dependence and the economic efficacy of marine reserves  

Microsoft Academic Search

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

Crow White

2009-01-01

72

Water density dependence of formaldehyde reaction in supercritical water  

Microsoft Academic Search

The water density dependence of formaldehyde (HCHO) reaction in supercritical water (SCW) was studied with batch experiments. Major products from the reaction were methanol (CH3OH), formic acid (HCOOH), hydrogen (H2), carbon monoxide (CO) and carbon dioxide (CO2). It was found that the Cannizzaro reaction mechanism was the preferred reaction pathway for HCHO reaction in SCW. At higher water densities, CH3OH

Mitsumasa Osada; Masaru Watanabe; Kiwamu Sue; Tadafumi Adschiri; Kunio Arai

2004-01-01

73

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

74

Time-dependent density functional theory for quantum transport  

NASA Astrophysics Data System (ADS)

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

Kwok, Yanho; Zhang, Yu; Chen, GuanHua

2013-07-01

75

Watching excitons move: the time-dependent transition density matrix  

NASA Astrophysics Data System (ADS)

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

Ullrich, Carsten

2012-02-01

76

A density-dependent endochronic plasticity for powder compaction processes  

Microsoft Academic Search

This paper is concerned with the numerical modeling of powder cold compaction process using a density-dependent endochronic plasticity model. Endochronic plasticity theory is developed based on a large strain plasticity to describe the nonlinear behavior of powder material. The elastic response is stated in terms of hypoelastic model and endochronic plasticity constitutive equations are stated in unrotated frame of reference.

A. Bakhshiani; A. R. Khoei; M. Mofid

2004-01-01

77

Density dependent relativistic mean field theory in deformed nuclei  

Microsoft Academic Search

We perform density dependent relativistic mean field calculations for deformed nuclei in order to examine the possibility of describing nuclear deformation properties in a parameter-free framework. The results are compared with experimental data and with results from earlier phenomenological relativistic mean field calculations using nonlinear terms in the sigma field.

M. L. Cescato; P. Ring

1998-01-01

78

Time-Dependent Local Density Measurements in Unsteady Flows.  

National Technical Information Service (NTIS)

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-lon...

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

1979-01-01

79

Disease transmission models with density-dependent demographics  

Microsoft Academic Search

The models considered for the spread of an infectious disease in a population are of SIRS or SIS type with a standard incidence expression. The varying population size is described by a modification of the logistic differential equation which includes a term for disease-related deaths. The models have density-dependent restricted growth due to a decreasing birth rate and an increasing

Linda Q. Gao; Herbert W. Hethcote

1992-01-01

80

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  

NASA Astrophysics Data System (ADS)

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 density-functional theory (TDDFT) recently developed. The time-dependent exchange-correlation potential with proper short- and long- range potential is constructed by means of the time-dependent optimized effective potential (TDOEP) method and the incorporation of an explicit self-interaction-correction (SIC) term. The TDOEP-SIC equations are solved accurately and efficiently by the time-dependent generalized pseudospectral technique. In this study, all the valence electrons are treated explicitly and nonperturbatively and their partial contributions to the ionization and HHG are analyzed. The results reveal instructive and qualitatively different behavior from each subshell orbital. Moreover, we found that the HHG yields from Ne and Ar atoms are considerably larger than that of the He atom in strong fields. Three main factors are identified for accounting the observed phenomena: (a) the binding energy of the subshell valence electron, (b) the orientation of the valence electron orbital (with respect to the electric-field polarization), and (c) the effect of multiphoton resonant excitation. In particular, we found that the np0 valence electrons (in Ne and Ar) with lowest binding energies and electron orbital orientation parallel to the electric-field direction, make the dominant contributions to both ionization and HHG processes in sufficiently strong fields.

Tong, Xiao-Min; Chu, Shih-I.

2001-07-01

81

Density Dependence of Transport Coefficients from Holographic Hydrodynamics  

NASA Astrophysics Data System (ADS)

We study the transport coefficients of Quark-Gluon-Plasma in finite temperature and finite baryon density. We consider AdS/QCD of charged AdS black hole background with bulk-filling branes identifying the U(1) charge as the baryon number. Using Reissner-Nordström-AdS background, Green functions are explicitly obtained. We calculate the diffusion constant, the shear viscosity and the thermal conductivity, and plot their density and temperature dependences. Hydrodynamic relations between those are shown to hold exactly. The diffusion constant and the shear viscosity are decreasing as a function of density for fixed total energy. For fixed temperature, the fluid becomes less diffusible and more viscous for larger baryon density.

Ge, X.; Matsuo, Y.; Shu, F.; Sin, S.; Tsukioka, T.

2008-11-01

82

Estimating the functional form for the density dependence from life history data.  

PubMed

Two contrasting approaches to the analysis of population dynamics are currently popular: demographic approaches where the associations between demographic rates and statistics summarizing the population dynamics are identified; and time series approaches where the associations between population dynamics, population density, and environmental covariates are investigated. In this paper, we develop an approach to combine these methods and apply it to detailed data from Soay sheep (Ovis aries). We examine how density dependence and climate contribute to fluctuations in population size via age- and sex-specific demographic rates, and how fluctuations in demographic structure influence population dynamics. Density dependence contributes most, followed by climatic variation, age structure fluctuations and interactions between density and climate. We then simplify the density-dependent, stochastic, age-structured demographic model and derive a new phenomenological time series which captures the dynamics better than previously selected functions. The simple method we develop has potential to provide substantial insight into the relative contributions of population and individual-level processes to the dynamics of populations in stochastic environments. PMID:18589530

Coulson, T; Ezard, T H G; Pelletier, F; Tavecchia, G; Stenseth, N C; Childs, D Z; Pilkington, J G; Pemberton, J M; Kruuk, L E B; Clutton-Brock, T H; Crawley, M J

2008-06-01

83

Floater dynamics can explain positive patterns of density-dependent fecundity in animal populations.  

PubMed

After some 70 years of debate on density-dependent regulation of animal populations, there is still poor understanding of where spatial and temporal density dependence occurs. Clearly defining the portion of the population that shapes density-dependent patterns may help to solve some of the ambiguities that encircle density dependence and its patterns. In fact, individuals of the same species and population can show different dynamics and behaviors depending on their locations (e.g., breeding vs. dispersal areas). Considering this form of intrapopulation heterogeneity may improve our understanding of density dependence and population dynamics in general. We present the results of individual-based simulations on a metapopulation of the Spanish imperial eagle Aquila adalberti. Our results suggest that high rates of floater mortality within settlement areas can determine a shift in the classical relationship (from negative to positive) between the fecundity (i.e., fledglings per pair) and density (i.e., number of pairs) of the breeding population. Finally, we proved that different initial conditions affecting the breeder portion of the population can lead to the same values of fecundity. Our results can represent a starting point for new and more complex approaches studying the regulation of animal populations, where the forgotten and invisible component--the floater--is taken into account. PMID:17080366

Penteriani, Vincenzo; Otalora, Fermín; Ferrer, Miguel

2006-09-20

84

Model dependence of isospin sensitive observables at high densities  

NASA Astrophysics Data System (ADS)

Within two different frameworks of isospin-dependent transport model, i.e., Boltzmann-Uehling-Uhlenbeck (IBUU04) and Ultrarelativistic Quantum Molecular Dynamics (UrQMD) transport models, sensitive probes of nuclear symmetry energy are simulated and compared. It is shown that neutron to proton ratio of free nucleons, ?-/?+ ratio as well as isospin-sensitive transverse and elliptic flows given by the two transport models with their "best settings", all have obvious differences. Discrepancy of numerical value of isospin-sensitive n/p ratio of free nucleon from the two models mainly originates from different symmetry potentials used and discrepancies of numerical value of charged ?-/?+ ratio and isospin-sensitive flows mainly originate from different isospin-dependent nucleon-nucleon cross sections. These demonstrations call for more detailed studies on the model inputs (i.e., the density- and momentum-dependent symmetry potential and the isospin-dependent nucleon-nucleon cross section in medium) of isospin-dependent transport model used. The studies of model dependence of isospin sensitive observables can help nuclear physicists to pin down the density dependence of nuclear symmetry energy through comparison between experiments and theoretical simulations scientifically.

Guo, Wen-Mei; Yong, Gao-Chan; Wang, Yongjia; Li, Qingfeng; Zhang, Hongfei; Zuo, Wei

2013-10-01

85

Density dependence, spatial scale and patterning in sessile biota.  

PubMed

Sessile biota can compete with or facilitate each other, and the interaction of facilitation and competition at different spatial scales is key to developing spatial patchiness and patterning. We examined density and scale dependence in a patterned, soft sediment mussel bed. We followed mussel growth and density at two spatial scales separated by four orders of magnitude. In summer, competition was important at both scales. In winter, there was net facilitation at the small scale with no evidence of density dependence at the large scale. The mechanism for facilitation is probably density dependent protection from wave dislodgement. Intraspecific interactions in soft sediment mussel beds thus vary both temporally and spatially. Our data support the idea that pattern formation in ecological systems arises from competition at large scales and facilitation at smaller scales, so far only shown in vegetation systems. The data, and a simple, heuristic model, also suggest that facilitative interactions in sessile biota are mediated by physical stress, and that interactions change in strength and sign along a spatial or temporal gradient of physical stress. PMID:15968539

Gascoigne, Joanna C; Beadman, Helen A; Saurel, Camille; Kaiser, Michel J

2005-09-29

86

Density dependence of isospin observables in spinodal decomposition  

SciTech Connect

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

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

2008-06-15

87

Density dependence of isospin observables in spinodal decomposition  

NASA Astrophysics Data System (ADS)

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

Colonna, M.; Matera, F.

2008-06-01

88

Angular momentum dependence of the nuclear level density parameter  

SciTech Connect

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

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

2010-04-15

89

Multicomponent density-functional theory for time-dependent systems  

SciTech Connect

We derive the basic formalism of density functional theory for time-dependent electron-nuclear systems. The basic variables of this theory are the electron density in body-fixed frame coordinates and the diagonal of the nuclear N-body density matrix. The body-fixed frame transformation is carried out in order to achieve an electron density that reflects the internal symmetry of the system. We discuss the implications of this body-fixed frame transformation and establish a Runge-Gross-type theorem and derive Kohn-Sham equations for the electrons and nuclei. We illustrate the formalism by performing calculations on a one-dimensional diatomic molecule for which the many-body Schroedinger equation can be solved numerically. These benchmark results are then compared to the solution of the time-dependent Kohn-Sham equations in the Hartree approximation. Furthermore, we analyze the excitation energies obtained from the linear response formalism in the single pole approximation. We find that there is a clear need for improved functionals that go beyond the simple Hartree approximation.

Butriy, O.; Ebadi, H.; Boeij, P. L. de; Leeuwen, R. van; Gross, E. K. U. [Theoretical Chemistry, Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4, 9747 AG Groningen (Netherlands); Department of Physics, University of Jyvaeskylae, FI-40014, Survontie 9, Jyvaeskylae (Finland); Institut fuer Theoretische Physik, Freie Universitaet Berlin, Arnimallee 14, D-14195 Berlin (Germany)

2007-11-15

90

Quantifying density dependence in a bird population using human disturbance  

Microsoft Academic Search

Although density dependence has long been recognised as vital to population regulation, there have been relatively few studies\\u000a demonstrating it spatially in wildlife populations, often due to the confounding effects of variation in habitat quality.\\u000a We report on a study of woodlarks Lullula arborea, a species of European conservation concern, breeding on lowland heath in Dorset, England. We take the

John W. Mallord; Paul M. Dolman; Andy Brown; William J. Sutherland

2007-01-01

91

Time-dependent local density measurements in unsteady flows  

Microsoft Academic Search

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

R. L. McKenzie; D. J. Monson; R. J. Exberger

1979-01-01

92

Population cycles caused by selection by density dependent competitive interactions.  

PubMed

Several animal species have cyclic population dynamics with phase-related cycles in life history traits such as body mass, reproductive rate, and pre-reproductive period. Although many mechanisms have been proposed there is no agreement on the cause of these cycles, and no population equation that deduces both the abundance and the life history cycles from basic ecological constraints has been formulated. Here I deduce a population dynamic equation from the selection pressure of density dependent competitive interactions in order to explain the cyclic dynamics in abundance and life history traits. The model can explain cycles by evolutionary changes in the genotype or by plastic responses in the phenotype. It treats the population dynamic growth rate as an initial condition, and its density independent fundament is Fisher's (1930, The Genetical Theory of Natural Selection, Oxford: Clarendon) fundamental theorem of natural selection that predicts a hyper-geometrical increase in abundance. The predicted periods coincide with the cyclic dynamics of Lepidoptera, and the Calder hypothesis, which suggests that the period of population cycles is proportional to the 1/4 power of body mass, follows from first principles of the proposed density dependent ecology. PMID:11127516

Witting, L

2000-11-01

93

The multiconfiguration time-dependent Hartree method generalized to the propagation of density operators  

NASA Astrophysics Data System (ADS)

The multiconfiguration time-dependent Hartree (MCTDH) method is formulated for density operators and applied to their numerical propagation. We introduce two types of MCTDH density operators which are expanded in different kinds of so-called single-particle density operators. The latter may either be hermitian, or else represent ket-bra products of so-called single-particle functions. For both types of MCTDH expansions of density operators we derive equations of motion employing the Dirac-Frenkel/MacLachlan variational principle. Further an alternative set of equations of motion for the second type of density operators is proposed, which is not based on a variational principle but derived by taking partial traces. We thus obtain three sensible approaches within the framework of the MCTDH method which differ in their performance and properties. We investigate these approaches and their properties analytically and numerically. Our numerical results refer to a model of vibronic-coupling dynamics in the pyrazine molecule representing coupled electronic states with four vibrational modes and two and three electronic states respectively. We analyze the closed-system dynamics for this model with temperature-dependent initial states. The influence of temperature on state populations, on correlation functions and on absorption spectra is discussed. We assess the numerical performance of two of the three approaches and find that both can be very efficiently applied to investigate the type of systems studied here.

Raab, A.; Burghardt, I.; Meyer, H.-D.

1999-11-01

94

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

PubMed

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

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

2013-08-09

95

Angular-Momentum-Dependent Orbital-Free Density Functional Theory  

NASA Astrophysics Data System (ADS)

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

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

2013-08-01

96

Analytic density functionals with initial-state dependence and memory  

NASA Astrophysics Data System (ADS)

We analytically construct the wave function that, for a given initial state, produces a prescribed density for a quantum ring with two noninteracting particles in a singlet state. In this case the initial state is completely determined by the initial density, the initial time derivative of the density and a single integer that characterizes the (angular) momentum of the system. We then give an exact analytic expression for the exchange-correlation potential that relates two noninteracting systems with different initial states. This is used to demonstrate how the Kohn-Sham procedure predicts the density of a reference system without the need of solving the reference system's Schrödinger equation. We further numerically construct the exchange-correlation potential for an analytically solvable system of two electrons on a quantum ring with a squared cosine two-body interaction. For the same case we derive an explicit analytic expression for the exchange-correlation kernel and analyze its frequency dependence (memory) in detail. We compare the result to simple adiabatic approximations and investigate the single-pole approximation. These approximations fail to describe the doubly excited states, but perform well in describing the singly excited states.

Ruggenthaler, M.; Nielsen, S. E. B.; van Leeuwen, R.

2013-08-01

97

Experimental evidence for density-dependent reproduction in a cooperatively breeding passerine  

Microsoft Academic Search

Temporal variation in survival, fecundity, and dispersal rates is associated with density-dependent and density-independent processes. Stable natural populations are expected to be regulated by density-dependent factors. However, detecting this by investigating natural variation in density is difficult because density-dependent and independent factors affecting population dynamics may covary. Therefore, experiments are needed to assess the density dependence of demographic rates. In

Lyanne Brouwer; Joost M. Tinbergen; Rachel Bristol; David S. Richardson; Jan Komdeur

2009-01-01

98

Exploration of a modified density dependence in the Skyrme functional  

SciTech Connect

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

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

2010-10-15

99

Density-dependent dispersal and spatial population dynamics  

PubMed Central

The synchronization of the dynamics of spatially subdivided populations is of both fundamental and applied interest in population biology. Based on theoretical studies, dispersal movements have been inferred to be one of the most general causes of population synchrony, yet no empirical study has mapped distance-dependent estimates of movement rates on the actual pattern of synchrony in species that are known to exhibit population synchrony. Northern vole and lemming species are particularly well-known for their spatially synchronized population dynamics. Here, we use results from an experimental study to demonstrate that tundra vole dispersal movements did not act to synchronize population dynamics in fragmented habitats. In contrast to the constant dispersal rate assumed in earlier theoretical studies, the tundra vole, and many other species, exhibit negative density-dependent dispersal. Simulations of a simple mathematical model, parametrized on the basis of our experimental data, verify the empirical results, namely that the observed negative density-dependent dispersal did not have a significant synchronizing effect.

Ims, Rolf A; Andreassen, Harry P

2005-01-01

100

Time dependent density functional theory with DMol3  

NASA Astrophysics Data System (ADS)

Time dependent density function theory (TDDFT) has been implemented in the program DMol3, a local atomic orbital implementation of DFT. Scaling and computation times for typical TDDFT calculations are comparable to DFT-SCF calculations. The implementation is fully parallel. Three applications are presented to show what quantitative and qualitative effects can be predicted by the present implementation. These include atomic multiplets of Ti4 + , UV-vis spectra of aromatic organic molecules, and a mapping versus the reaction coordinate of the excited state potential energy surfaces of the nitroprusside ion (Fe(CN)5NO) - 2.

Delley, B.

2010-09-01

101

Stochastic seasonality and nonlinear density-dependent factors regulate population size in an African rodent  

USGS Publications Warehouse

Ecology has long been troubled by the controversy over how populations are regulated. Some ecologists focus on the role of environmental effects, whereas others argue that density-dependent feedback mechanisms are central. The relative importance of both processes is still hotly debated, but clear examples of both processes acting in the same population are rare. Keyfactor analysis (regression of population changes on possible causal factors) and time-series analysis are often used to investigate the presence of density dependence, but such approaches may be biased and provide no information on actual demographic rates. Here we report on both density-dependent and density-independent effects in a murid rodent pest species, the multimammate rat Mastomys natalensis (Smith, 1834), using statistical capture-recapture models. Both effects occur simultaneously, but we also demonstrate that they do not affect all demographic rates in the same way. We have incorporated the obtained estimates of demographic rates in a population dynamics model and show that the observed dynamics are affected by stabilizing nonlinear density-dependent components coupled with strong deterministic and stochastic seasonal components.

Leirs, H.; Stenseth, N.C.; Nichols, J.D.; Hines, J.E.; Verhagen, R.; Verheyen, W.

1997-01-01

102

The copula approach to characterizing dependence structure in neural populations.  

PubMed

The question as to the role that correlated activity plays in the coding of information in the brain continues to be one of the most important in neuroscience. One approach to understanding this role is to formally model the ensemble responses as multivariate probability distributions. We have previously introduced alternatives to linear assumptions of multivariate Gaussian dependence for spike timing in neural ensembles using the probabilistic copula approach. In probability theory the copula "couples" marginal distributions to form flexible multivariate distribution functions for characterizing ensemble behavior. The parametric copula can be factored out of the joint probability density, and as such is independent and isolated from the marginal densities. This greatly simplifies the analysis, and allows a direct examination of the shape of the dependence independent of the marginals. The shape of the copula function goes beyond describing the dependence with a single summarizing statistic. In this review, we illustrate the construction of the copula, and how it contributes to the analysis of information conveyed by populations of neurons. PMID:21793349

Jenison, Rick L

2010-12-31

103

Stochastic Time-Dependent Current-Density Functional Theory  

NASA Astrophysics Data System (ADS)

Static and dynamical density functional methods have been applied with a certain degree of success to a variety of closed quantum mechanical systems, i.e., systems that can be described via a Hamiltonian dynamics. However, the relevance of open quantum systems - those coupled to external environments, e.g., baths or reservoirs - cannot be overestimated. To investigate open quantum systems with DFT methods we have introduced a new theory, we have named Stochastic Time-Dependent Current Density Functional theory (S-TDCDFT) [1]: starting from a suitable description of the system dynamics via a stochastic Schrödinger equation [2], we have proven that given an initial quantum state and the coupling between the system and the environment, there is a one-to-one correspondence between the ensemble-averaged current density and the external vector potential applied to the system.In this talk, I will introduce the stochastic formalism needed for the description of open quantum systems, discuss in details the theorem of Stochastic TD-CDFT, and provide few examples of its applicability like the dissipative dynamics of excited systems, quantum-measurement theory and other applications relevant to charge and energy transport in nanoscale systems.[1] M. Di Ventra and R. D'Agosta, Physical Review Letters 98, 226403 (2007)[2] N.G. van Kampen, Stochastic processes in Physics and Chemistry, (North Holland, 2001), 2nd ed.

D'Agosta, Roberto

2008-03-01

104

Which conditions promote negative density dependent selection on prey aggregations?  

PubMed

Negative density dependent selection on individuals in prey aggregations (negative DDS, the preferential selection by predators of spatially isolated prey) is assumed to contribute in many cases to the evolution and maintenance of aggregation. Both positive and negative DDS on prey groups have been documented in nature but there is no existing framework to predict when each of these forms of natural selection is most likely. By exploiting the tendency of artificial neural networks to exhibit consumer-like emergent behaviours, I isolate at least two environmental factors impinging on the consumer organism that may determine which form of density dependent natural selection is shown: the distribution of prey group size attacked by the predator and the spatial conformation (dispersed or compacted) of the prey group. Numerous forms of DDS on artificial prey (positive, negative, and non-DDS) are displayed through different combinations of these factors. I discuss in detail how the predictions of the model may be tested by empiricists in order to assess the usefulness of the framework presented. I stress the importance of understanding DDS on prey groups given the recent emergence of these systems as test beds for ideas on biological self-organisation. PMID:21540037

Tosh, Colin R

2011-04-29

105

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.; Bj?rnstad, O. N.; Falck, W.

1997-01-01

106

Time-dependent approach to atomic autoionization  

NASA Astrophysics Data System (ADS)

A time-dependent approach to the study of atomic autoionization in two-electron systems is formulated. In the first step a two-dimensional (2D) model ``He'' atom is constructed by replacing the full 3D electrostatic interaction with a 1D soft-core interaction. The autoionization decay of doubly excited states constructed within the model is calculated by both standard perturbation theory and direct solution of the time-dependent Schrödinger equation. Configuration-interaction theory is invoked to obtain correlated resonance states, and strong laser fields are found to alter the decay rates. In the second step the full 6D wave function for the He atom is expanded in coupled spherical harmonics using a procedure as described by C. Bottcher, D. R. Schultz, and D. H. Madison [Phys. Rev. A 49, 1714 (1994)]. Solution of the time-dependent Schrödinger equation reduces to solving the propagator equations for the 3D expansion coefficients on a B-spline collocation lattice. Autoionizing decay rate calculations using product resonance states are found to be in qualitative agreement with the 2D model results.

Schultz, D. R.; Bottcher, C.; Madison, D. H.; Peacher, J. L.; Buffington, G.; Pindzola, M. S.; Gorczyca, T. W.; Gavras, P.; Griffin, D. C.

1994-08-01

107

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

108

Neural network approximations to posterior densities: an analytical approach  

Microsoft Academic Search

In Hoogerheide, Kaashoek and Van Dijk (2002) the class of neural networksampling methods is introduced to sample from a target (posterior)distribution that may be multi-modal or skew, or exhibit strong correlationamong the parameters. In these methods the neural network is used as animportance function in IS or as a candidate density in MH. In this note wesuggest an analytical approach

L. F. Hoogerheide; J. F. Kaashoek; H. K. van Dijk

2003-01-01

109

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

PubMed

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

Vikas

2011-05-19

110

Probing the density dependence of the symmetry energy via multifragmentation at subsaturation densities  

NASA Astrophysics Data System (ADS)

Symmetry energy for asymmetric nuclear matter at subsaturation densities was investigated in the framework of an isospin-dependent quantum molecular dynamics model. A single ratio of neutrons and protons is compared with the experimental data of Famiano [Phys. Rev. Lett.PRLTAO0031-900710.1103/PhysRevLett.97.052701 97, 052701 (2006)] We have also performed a comparison for the double ratio with experimental as well as different theoretical results of Boltzmann-Uehling-Uhlenbeck in 1997, Isospin-dependent Boltzmann-Uehling-Uhlenbeck in 2004, Boltzmann-Nordeim-Vlasov, and Improved Quantum Molecular Dynamics models. It is found that the double ratio predicts the softness of symmetry energy, which is a little underestimated in the single ratio. Furthermore, the study of the single ratio is extended for different kinds of fragments, while the double ratio is extended for different neutron-rich isotopes of Sn.

Kumar, Sanjeev; Ma, Y. G.; Zhang, G. Q.; Zhou, C. L.

2011-10-01

111

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.

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

1998-01-01

112

Modeling ultrafast solvated electronic dynamics using time-dependent density functional theory and polarizable continuum model.  

PubMed

A first-principles solvated electronic dynamics method is introduced. Solvent electronic degrees of freedom are coupled to the time-dependent electronic density of a solute molecule by means of the implicit reaction field method, and the entire electronic system is propagated in time. This real-time time-dependent approach, incorporating the polarizable continuum solvation model, is shown to be very effective in describing the dynamical solvation effect in the charge transfer process and yields a consistent absorption spectrum in comparison to the conventional linear response results in solution. PMID:22277083

Liang, Wenkel; Chapman, Craig T; Ding, Feizhi; Li, Xiaosong

2012-02-21

113

Blindness of the Exact Density Response Function to Certain Types of Electronic Excitations: Implications for Time-Dependent Density-Functional Theory  

NASA Astrophysics Data System (ADS)

Examples of electronic excitations are presented and discussed that do not contribute to the sum-over-state expression of the frequency-dependent density-density response function and thus do not lead to poles of the latter. As a consequence, these excitations principally cannot be described by time-dependent density-functional theory (TDDFT) response methods or any other method relying on the poles of the density-density response function. For these excitations, the DFT response approach commonly used in molecular physics or quantum chemistry is shown to not properly represent a DFT method and to yield differences of Kohn-Sham eigenvalues as excitation energies. The discussed examples are simple valence-valence excitations in the neon atom and the nitrogen molecule.

Heßelmann, Andreas; Görling, Andreas

2009-06-01

114

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

Microsoft Academic Search

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

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

2003-01-01

115

Density-dependent population growth in a reintroduced population of North Island saddlebacks  

Microsoft Academic Search

Summary 1. Reintroductions provide a good opportunity to study density-dependent population growth, as populations can be studied at a range of densities and the change in density is not confounded with environmental conditions. An understanding of density depend- ence is also necessary to predict dynamics of reintroduced populations under different management regimens, and assess the extent to which they can

DOUG P. ARMSTRONG; R. SCOTT DAVIDSON; JOHN K. PERROTT; JON ROYGARD; LEN BUCHANAN

2005-01-01

116

An efficient method for calculating dynamical hyperpolarizabilities using real-time time-dependent density functional theory.  

PubMed

In this paper we present a time-domain time-dependent density functional theory (TDDFT) approach to calculate frequency-dependent polarizability and hyperpolarizabilities. In this approach, the electronic degrees of freedom are propagated within the density matrix based TDDFT framework using the efficient modified midpoint and unitary transformation algorithm. We use monochromatic waves as external perturbations and apply the finite field method to extract various orders of the time-dependent dipole moment. By fitting each order of time-dependent dipole to sinusoidal waves with harmonic frequencies, one can obtain the corresponding (hyper)polarizability tensors. This approach avoids explicit Fourier transform and therefore does not require long simulation time. The method is illustrated with application to the optically active organic molecule para-nitroaniline, of which the frequency-dependent polarizability ?(-?; ?), second-harmonic generation ?(-2?; ?, ?), optical rectification ?(0; -?, ?), third-harmonic generation ?(-3?; ?, ?, ?), and degenerate four-wave mixing ?(-?; ?, ?, -?) are calculated. PMID:23425458

Ding, Feizhi; Van Kuiken, Benjamin E; Eichinger, Bruce E; Li, Xiaosong

2013-02-14

117

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

PubMed

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

Bårdsen, Bård-Jørgen; Tveraa, Torkild

2011-10-10

118

The density dependence of fluid properties and non-Newtonian flows: The Weissenberg effect  

SciTech Connect

Two approaches which describe the Weissenberg effect (height profile of a non-Newtonian fluid between rotating vertical concentric cylinders) are discussed. The first is based on an earlier calculation with rheological properties of a simple liquid obtained from nonequilibrium molecular dynamics (NEMD). The calculation is redone here using new results on the density dependence of the normal pressure differences. The NEMD calculations are restricted to Couette flow, but describe specifically, in a consistent manner, the effects of finite compressibility. The pressure, viscosity, and normal pressure differences are all found from NEMD to be sensitive functions of density, which requires that the equations of motion be solved iteratively and self-consistently, and a sample calculation is presented for the soft sphere fluid. The second approach is that of Joseph and Fosdick. Their assumptions and techniques are examined and compared with the NEMD calcula- tions.

Rainwater, J.C. [Thermophysics Division, Chemical Science and Technology Laboratory, National Institute of Standards and Technology, Boulder, Colorado 80303 (United States); Hanley, H.J.M.; Narayan, A. [Department of Chemical Engineering, University of Colorado, Boulder, Colorado 80309 (United States)]|[Thermophysics Division, Chemical Science and Technology Laboratory, National Institute Standards and Technology, Boulder, Colorado 80303 (United States)

1995-11-01

119

Estimation of a Nonlinear Density-Dependence Parameter for Wild Turkey  

Microsoft Academic Search

Although previous research and theory has suggested that wild turkey (Meleagris gallopavo) populations may be subject to some form of density dependence, there has been no effort to estimate and incorporate a density-dependence parameter into wild turkey population models. To estimate a functional relationship for density dependence in wild turkey, we analyzed a set of harvest-index time series from 11

JAY D. McGHEE; JAMES M. BERKSON

2007-01-01

120

On the methods for determining density-dependence by means of regression  

Microsoft Academic Search

The determination of density-dependence of a mortality process is attempted by taking the linear regression of the logarithm of population density (or k defined by Varley and Gradwell, 1960) against the logarithm of previous density, based on the assumption that the slope, b, of the line is smaller than unity for log density-log density relationship or larger than zero for

Yosiaki Itô

1972-01-01

121

Density approach to ballistic anomalous diffusion: An exact analytical treatment  

NASA Astrophysics Data System (ADS)

This paper addresses the problem of deriving the probability distribution density of a diffusion process generated by a nonergodic dichotomous fluctuation using the Liouville equation (density method). The velocity of the diffusing particles fluctuates from the value of 1 to the value of -1, and back, with the distribution density of time durations ? of the two states proportional to 1/?? in the asymptotic time limit. The adopted density method allows us to establish an exact analytical expression for the probability distribution density of the diffusion process generated by these fluctuations. Contrary to intuitive expectations, the central part of the diffusion distribution density is not left empty when moving from ?>2 (ergodic condition) to ?<2 (nonergodic condition). The intuitive expectation is realized for ??cr, the monomodal distribution density with a minimum at the origin is turned into a bimodal one, with a central bump whose intensity increases for ?-->2. The exact theoretical treatment applies to the asymptotic time limit, which establishes for the diffusion process the ballistic scaling value ?=1. To assess the time evolution toward this asymptotic time condition, we use a numerical approach which relates the emergence of the central bump at ?=?cr with the generation of the ordinary scaling ?=0.5, which lasts for larger and larger times for ? coming closer and closer to the critical value ?=2. We assign to the waiting time distribution density two different analytical forms: one derived from the Manneville intermittence (MI) theory and one from the Mittag-Leffler (ML) survival probability. The adoption of the ML waiting time distribution density generates an exact analytical prediction, whereas the MI method allows us to get the same asymptotic time limit as the ML one for ?<2 as a result of an approximation. The joint adoption of these two waiting time distribution densities sheds light into the critical nature of the condition ?=2 and into why this is the critical point for the MI process, representing the phase transition from the nonergodic to the ergodic regime. Our main result can be interpreted as a new derivation of Lamperti distribution.

Bologna, Mauro; Ascolani, Gianluca; Grigolini, Paolo

2010-04-01

122

Quark stars with the density-dependent quark mass model  

NASA Astrophysics Data System (ADS)

The recent observation of the pulsar PSR J1614-2230 with a mass of 1.97±0.04M? gives a strong constraint on the equation of state (EoS) of the dense matter in compact stars. In this work, we calculate the maximum mass of quark stars with the density-dependent quark mass model, and explore the parameter ranges for this model fully, by considering the constraints of absolute stability of strange quark matter and the mass of PSR J1614-2230. Without the color-superconductivity, the maximum mass of unpaired quark stars is more sensitive to the parameter C, and complies with the constraints within the range of 96MeVfm?C?130MeVfm. The largest mass can reach 2.25M? at C?96.54MeVfm and m?145MeV. For the quark stars composed of the quark matter in color-flavor locked (CFL) phase, we can obtain quite large maximum masses at a sufficiently high gap value, but the value of m is very important in deciding the maximum mass of the CFL quark stars.

Wei, Wei; Zheng, Xiao-Ping

2012-09-01

123

Business Cycle Duration Dependence: A Parametric Approach  

Microsoft Academic Search

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

Daniel E Sichel

1991-01-01

124

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

125

Density-dependent demography in a Japanese temperate broad-leaved forest  

Microsoft Academic Search

A method of detecting the density-dependent dynamics of a size-structured population is developed. The method is applied to a Japanese broad-leaved forest and the density-dependent and-independent projection matrix models are constructed based on the data of the forest. Then, the difference between the density-independent and density-dependent dynamics is compared in terms of several statistical quantities obtained from the matrices. Three

Takenori Takada; Tohru Nakashizuka

1996-01-01

126

Density-dependent effects on physical condition and reproduction in North American elk: an experimental test  

Microsoft Academic Search

Density dependence plays a key role in life-history characteristics and population ecology of large, herbivorous mammals. We designed a manipulative experiment to test hypotheses relating effects of density-dependent mechanisms on physical condition and fecundity of North American elk ( Cervus elaphus) by creating populations at low and high density. We hypothesized that if density-dependent effects were manifested principally through intraspecific

Kelley M. Stewart; R. Terry. Bowyer; Brian L. Dick; Bruce K. Johnson; John G. Kie

2005-01-01

127

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

PubMed

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

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

2010-10-29

128

Lateral entrainment rate in shallow cumuli: Dependence on dry air sources and probability density functions  

NASA Astrophysics Data System (ADS)

Entrainment processes in convective clouds often occur stochastically and entrainment rate estimates depend on the distance from the cloud from which the dry air is entrained. However, no observational studies exist on either the distance dependence or probability density function of entrainment rate, hindering understanding and the parameterization of convection. Here entrainment rate in cloud cores is estimated using a recently developed mixing fraction approach that is applied to in situ aircraft measurements of cumuli from the RACORO field program. The results are used to examine, for the first time, probability density functions of entrainment rate and their dependence on the distance from the edge of the cloud core from which the dry air is entrained. The estimated entrainment rate decreases when the dry air is entrained from increasing distance from the edge of the cloud core; this is because the air farther from the edge of the cloud core is drier than the neighboring air that is within the humid shell around the cumulus cloud core. Probability density functions of entrainment rate vary with the distance and height above the cloud-base, and all are well fitted by lognormal distributions. The implications of the results for convection parameterizations are discussed.

Lu, Chunsong; Liu, Yangang; Niu, Shengjie; Vogelmann, Andrew M.

2012-10-01

129

Inoculation-density-dependent responses and pathway shifts in Saccharomyces cerevisiae.  

PubMed

The cell-density-dependent responses of Saccharomyces cerevisiae to inoculation sizes were explored by a proteomic approach. According to their gene ontology, 100 protein spots with differential expression, corresponding to 67 proteins, were identified and classed into 17 different functional groups. Upregulation of eight heat shock, oxidative response and amino acid biosynthesis-related proteins (e.g. Hsp78p, Ssa1p, Hsp60p, Ctt1p, Sod1p, Ahp1p, Met6p and Met17p), which may jointly maintain the cell redox homeostasis, was dependant on inoculation density. Significant increases in the levels of five proteins involved in glycolysis and alcohol biosynthesis pathways (e.g. Glk1p, Fba1p, Eno1p, Pdc1p and Adh1p) might play critical roles in improving ethanol productivity of the fermentation process and shortening the fermentation time when inoculation sizes were increased. Cell-density-dependent glycolytic variations of proteins involved in trehalose, glycerol biosynthesis and pentose phosphate pathway revealed shifts among metabolic pathways during fermentation with different inoculation sizes. Upregulation of three signal transduction proteins (Bmh1p, Bmh2p and Fpr1p) indicated that adequate cell-cell contacts improved cellular communication at high inoculation sizes. These findings provide insights into yeast responses to inoculation size and optimizing the direct inoculation of active dry yeast fermentation, so as to improve the ethanol production. PMID:19743421

Cheng, Jing-Sheng; Ding, Ming-Zhu; Tian, Hong-Chi; Yuan, Ying-Jin

2009-10-01

130

Modelling of unstable density-dependent solute transport: Macroscopic vs. microscopic model applications and experimental benchmarks  

NASA Astrophysics Data System (ADS)

Density-driven flow in porous media occurs in situations of subsurface flows around salt domes, saltwater intrusion and salt infiltration both near the coast and inland, flows occurring in geothermal systems, the disposal of heavy brines in aquifers and others. For the description and prediction of these complex flow processes numerical models are employed. The non-linear nature of the coupled density flow equations makes their solution more difficult than for passive tracer transport. The flow is truly rotational and naturally unstable conditions are possible. In this contibution, we investigate numerical modelling concepts for the computation of density-dependent fingered flow. Calculations were performed with an efficient multi-grid, finite volume model (d3f). A benchmark for model validation is here available with experimental data of 3D density-driven salt fingering in a controlled laboratory set-up. Unstable modes and vertical velocities of the dominant fingers are presented for up to 5 million nodes in grid resolution. A check of numerical grid convergence for such non-linear computations is essential for judgement of the accuracy and consistency of numerical results. While strict numerical grid convergence for stable density configurations could be previously achieved, the convergence in the unstable density-dependent simulations is shown here to require discretization levels down to the pore-scale. Therefore numerical models resolving pore networks are investigated to resolve the instability phenomenon at that scale. Our intent is a cross-validation of both numerical approaches based on different physical descriptions. Compared to the continuum modelling approach, the microscopic approach directly incorporates properties of the porous medium and their influence on fluid flow. Of particular interest are the short temporal scales at the onset of density-driven flow instabilities and their representation in the continuum model equations. The employed pore-network model allows a prediction of the characteristic wavelengths for macroscopic instabilities. This numerical study is done in parallel with an analogous experimental micro-model study. Numerical as well as experimental results in direct comparison are presented. We also discuss the implications for up-scaling of important pore-scale information observed under unstable flow conditions.

Held, R.; Johannsen, K.; Kinzelbach, W.

2003-04-01

131

Time-dependent density functional theory for molecules in liquid solutions  

NASA Astrophysics Data System (ADS)

A procedure based on the polarizable continuum model (PCM) has been applied to reproduce solvent effects on electronic spectra in connection with the time-dependent density functional theory (TD-DFT). To account for solute-solvent interactions, a suitable operator has been defined, which depends on the solute electronic density and can be used to modify the TD-DFT equations for the calculation of molecular polarizabilities and of electronic transition energies. The solute-solvent operator has been derived from a PCM approach depending on solute electrostatic potential: Recently, it has been shown that such an approach also provides an excellent treatment of the solute electronic charge lying far from the nuclei, being particularly reliable for this kind of applications. The method has been tested for formaldehyde in water and in diethyl-ether, and then applied to the calculation of solvent effects on the n-->?* transition of diazabenzenes in different solvents. The computed transition energies are in fairly good agreement with experimental values.

Cossi, Maurizio; Barone, Vincenzo

2001-09-01

132

Extraction of spin-dependent parton densities and their uncertainties  

SciTech Connect

We discuss techniques and results for the extraction of the nucleon's spin-dependent parton distributions and their uncertainties from data for polarized deep-inelastic lepton-nucleon and proton-proton scattering by means of a global QCD analysis. Computational methods are described that significantly increase the speed of the required calculations to a level that allows one to perform the full analysis consistently at next-to-leading order accuracy. We examine how the various data sets help to constrain different aspects of the quark, antiquark, and gluon helicity distributions. Uncertainty estimates are performed using both the Lagrange multiplier and the Hessian approaches. We use the extracted parton distribution functions and their estimated uncertainties to predict spin asymmetries for high-transverse momentum pion and jet production in polarized proton-proton collisions at 500 GeV center-of-mass system energy at the Relativistic Heavy Ion Collider at Brookhaven National Laboratory, as well as for W boson production.

Florian, Daniel de; Sassot, Rodolfo; Stratmann, Marco; Vogelsang, Werner [Departamento de Fisica, Universidad de Buenos Aires, Ciudad Universitaria, Pabellon 1 (1428) Buenos Aires (Argentina); Institut fuer Theoretische Physik, Universitaet Regensburg, 93040 Regensburg, Germany, and Institut fuer Theoretische Physik und Astrophysik, Universitaet Wuerzburg, 97074 Wuerzburg (Germany); Physics Department, Brookhaven National Laboratory, Upton, New York 11973 (United States)

2009-08-01

133

Influence of Boundary Condition Types on Unstable Density-Dependent Flow.  

PubMed

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

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

2013-05-01

134

Density-dependent growth and reproduction of the apple snail, Pomacea canaliculata : a density manipulation experiment in a paddy field  

Microsoft Academic Search

To examine density dependence in the survival, growth, and reproduction of Pomacea canaliculata, we conducted an experiment in which snail densities were manipulated in a paddy field. We released paint-marked snails of\\u000a 15–20 mm shell height into 12 enclosures (pens) of 16 m2 at one of five densities – 8, 16, 32, 64, or 128 snails per pen. The survival

Koichi Tanaka; Tomonari Watanabe; Hiroya Higuchi; Kenji Miyamoto; Yoichi Yusa; Toru Kiyonaga; Hirotsugu Kiyota; Yoshito Suzuki; Takashi Wada

1999-01-01

135

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

136

Approaches to 100 Gbit/sq. in. recording density  

NASA Astrophysics Data System (ADS)

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

Kryder, Mark H.

1994-03-01

137

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

2011-03-02

138

Nonadiabatic couplings from time-dependent density functional theory. II. Successes and challenges of the pseudopotential approximation  

Microsoft Academic Search

We present extensive calculations of nonadiabatic couplings (NACs) between the electronically ground and excited states of molecules, using time-dependent density functional theory (TDDFT) within (modified) linear response [C. Hu et al. J. Chem. Phys. 127, 064103 (2007)]. Our approach is implemented in the pseudopotential framework, with the consideration of nonlinear core corrections. The features of either the ordinary Jahn-Teller conical

Chunping Hu; Hirotoshi Hirai; Osamu Sugino

2008-01-01

139

Dependence of Natural Graphite Anode Performance on Electrode Density.  

National Technical Information Service (NTIS)

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 natu...

J. Shim K. A. Striebel

2005-01-01

140

Density Dependent Functional Forms Drive Compensation in Populations Exposed to Stressors  

EPA Science Inventory

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

141

Algal density dependent bioconcentration factors of hydrophobic chemicals  

Microsoft Academic Search

The bioconcentration factors (BCFs) of anthracene, hexachlorobenzene, 4,4?-dichlorobiphenyl, and 2,2?5,5?-tetrachlorobiphenyl decreased with increasing density of the algae Chlorella vulgaris and Chlorella pyrenoidosa. Concomitant with algal density, the organic carbon concentration as measured by chemical oxygen demand (COD) in the aqueous phase after centrifugation of the algal suspensions, increased. It is argued that the decreased BCFs in the presence of algae

Dick T. H. M. Sijm; Joost Middelkoop; Keno Vrisekoop

1995-01-01

142

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

143

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

NASA Astrophysics Data System (ADS)

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

Petrenko, Taras; Kossmann, Simone; Neese, Frank

2011-02-01

144

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

PubMed

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

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

2011-08-24

145

Density dependence of the symmetry energy probed by ??-decay energies of odd-A nuclei  

NASA Astrophysics Data System (ADS)

The mass-dependent symmetry energy coefficients asym(A) are extracted with the ??-decay energies Q?? of heavy odd-A nuclei. The dominant position of this approach is that only the Coulomb energy survives in Q?? to determine the unknown asym(A). The obtained asym(A) is employed to analyze the density dependence of the nuclear matter symmetry energy around the saturation density. The estimated asym(A) of 208Pb is 22.1–22.7 MeV. The slope parameter of the symmetry energy is found to be 50±15 MeV, with the symmetry energy S0=32.3±1.3 MeV at saturation density obtained from the analysis of pygmy dipole resonance (PDR) [Carbone , Phys. Rev. CPRVCAN0556-281310.1103/PhysRevC.81.041301 81, 041301(R) (2010)] as input. Furthermore, the corresponding neutron skin thickness in 208Pb is estimated to be ?Rnp=0.174±0.022 fm.

Dong, Jianmin; Zhang, Haifei; Wang, Longjun; Zuo, Wei

2013-07-01

146

Liquid water simulations with the density fragment interaction approach.  

PubMed

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

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

2012-04-02

147

A factorized density-dependent interaction in the folding model for heavy-ion potentials  

Microsoft Academic Search

The real part of the optical model potential for heavy ion elastic scattering is calculated in a folding model approximation, using a density-dependent M3Y interaction. The density dependence is assumed to be of a factorized form, consisting of a projectile term and a target term, and is characterized by two parameters, the strength parameter (C) and the density-dependence parameter (beta).

A. K. Chaudhuri

1986-01-01

148

Density-dependent mortality and the latitudinal gradient in species diversity  

Microsoft Academic Search

Ecologists have long postulated that density-dependent mortality maintains high tree diversity in the tropics. If species experience greater mortality when abundant, then more rare species can persist. Agents of density-dependent mortality (such as host-specific predators, and pathogens) may be more prevalent or have stronger effects in tropical forests, because they are not limited by climatic factors. If so, decreasing density-dependent

Janneke Hille Ris Lambers; James S. Clark; Brian Beckage

2002-01-01

149

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

Microsoft Academic Search

Density-dependent dispersal occurs throughout the animal kingdom, and has been shown to occur in some taxa whose populations exhibit multi-year population cycles. However, the importance of density-dependent dispersal for the spatiotemporal dynamics of cyclic populations is unknown. We investigated the potential effects of density-dependent dispersal on the properties of periodic travelling waves predicted by two coupled reaction–diffusion models: a commonly

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

2008-01-01

150

Hyperon Density Dependence of Hyperon-Nucleon Interactions in Neutron Stars  

Microsoft Academic Search

The hyperon density dependence (YDD) of hyperon-nucleon interactions are studied in the relativistic mean field (RMF) model and their influences on the properties of neutron stars are studied. The extended RMF considered the interior quarks coordinates of hyperon and bring a hyperon density dependent factor, f(rhoY), to the meson-hyperon coupling vertexes. The hyperon density dependence of YN interaction affect the

L. Dang; P. Yue; L. Li; P. Z. Ning

2008-01-01

151

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

PubMed Central

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

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

1998-01-01

152

Load balancing and density dependent jump Markov processes  

Microsoft Academic Search

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 ?n, ?<1, at a collection of n servers. Each customer chooses some constant d servers independently and uniformly at random from the n servers,

Michael Mitzenmacher

1996-01-01

153

Hyperon Density Dependence of Hyperon-Nucleon Interactions in Neutron Stars  

NASA Astrophysics Data System (ADS)

The hyperon density dependence (YDD) of hyperon-nucleon interactions are studied in the relativistic mean field (RMF) model and their influences on the properties of neutron stars are studied. The extended RMF considered the interior quarks coordinates of hyperon and bring a hyperon density dependent factor, f(?Y), to the meson-hyperon coupling vertexes. The hyperon density dependence of YN interaction affect the properties of neutron stars only after the corresponding hyperon appears. Then, the influences of the density dependence factors are almost ignored at low densities, which are clear at high densities. The compositions and properties of neutron stars are studied with and without the YDD of YN interactions for the different ?--nucleus effective potentials, (30, 0, -30)MeV. The calculated results indicated that the YDD of YN interaction soften the equation of state of neutron stars at high densities.

Dang, L.; Yue, P.; Li, L.; Ning, P. Z.

154

Density dependence of quasifree single-nucleon knockout reactions  

SciTech Connect

We address the issue of whether quasifree single-nucleon knockout measurements carry sufficient information about the nuclear interior. To this end, we present comparisons of the reaction probability densities for A(e,e{sup '}p) and A(p,2p) in quasifree kinematics for the target nuclei {sup 4}He, {sup 12}C, {sup 56}Fe, and {sup 208}Pb. We adopt a comprehensive framework based on the impulse approximation and on a relativized extension of Glauber multiple-scattering reaction theory in which the medium effects related to short-range correlations (SRCs) are implemented. It is demonstrated that SRCs weaken the effect of attenuation. For light target nuclei, both the quasifree (p,2p) and (e,e{sup '}p) can probe average densities of the same order as nuclear saturation density {rho}{sub 0}. For heavy nuclei such as {sup 208}Pb, the probed average densities are smaller than 0.1{rho}{sub 0} and the (e,e{sup '}p) reaction is far more efficient in probing the bulk regions than (p,2p).

Ryckebusch, Jan; Cosyn, Wim; Vanhalst, Maarten [Department of Physics and Astronomy, Ghent University, Proeftuinstraat 86, B-9000 Ghent (Belgium)

2011-05-15

155

Density-dependent effects on physical condition and reproduction in ...  

Treesearch

Age-specific pregnancy rates were lower in the high-density area, although there were no differences in ... Pregnancy rates were most affected by body condition and mass, although successful ... on the role of spring and summer nutrition on population regulation in large, northern herbivores. ... Last Modified: July 21, 2013.

156

Broadcasting but not receiving: density dependence considerations for SETI signals  

Microsoft Academic Search

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. Using this basis, it estimates the 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

Reginald D. Smith

2009-01-01

157

Generalist predators and the importance of spatial density dependence  

Microsoft Academic Search

The model examples in this paper illustrate that non-random spatial distributions of predation or parasitism by polyphagous natural enemies will not necessarily promote population persistence. The test of whether such responses are stabilizing requires the spatial distribution of predation to be examined over a range of total prey densities. As shown in Fig. 2, a contribution to stability will occur

J. Latto; M. P. Hassell

1988-01-01

158

Pressure and temperature dependence of the density of liquid lubricants  

Microsoft Academic Search

A theoretical model describing the influence of temperature T and pressure p on the density of liquid lubricants is described. It is based on the molecular interaction theory expressed in the van der Waals equation of state. Theoretical results for two different kinds of liquid lubricants were obtained. They correlate well with data measured by different experimental techniques. Characteristics of

P. L. Wong; R. Wang; S. Lingard

1996-01-01

159

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

PubMed

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

Johnson, Darren W

2006-05-01

160

Approaching chemical accuracy with density functional calculations: Diatomic energy corrections  

NASA Astrophysics Data System (ADS)

Density functional theory (DFT) is widely used to predict materials properties, but the local density approximation (LDA) and generalized gradient approximation (GGA) exchange-correlation functionals are known to poorly predict the energetics of reactions involving molecular species. In this paper, we obtain corrections for the O2, H2, N2, F2, and Cl2 molecules within the Perdew-Burke-Enzerhof GGA, Perdew-Wang GGA, and Perdew-Zunger LDA exchange-correlation functionals by comparing DFT-calculated formation energies of oxides, hydrides, nitrides, fluorides, and chlorides to experimental values. We also show that the choice of compounds used to obtain the correction is significant, and we use a leave-one-out cross-validation approach to rigorously determine the proper fit set. We report confidence intervals with our correction values, which quantifies the variation caused by the choice of fit set after outlier removal. The remaining variation in the correction values is of the order of 1 kcal/mol, which indicates that chemical accuracy is a realistic goal for these systems.

Grindy, Scott; Meredig, Bryce; Kirklin, Scott; Saal, James E.; Wolverton, C.

2013-02-01

161

A general approach to charge/spin density waves electrodynamics  

NASA Astrophysics Data System (ADS)

We reconsider microscopic grounds for the electric field response and for the phase dynamics of charge/spin density waves in pure systems. We suggest transparent and free of lengthy calculations way to derive the local Lagrangian valid at any temperature 0 < T < T_MF and for arbitrary electronic spectrum provided it supports the existence of the long range DW order below the mean field transition temperature T_MF. The analysis is based on classification of normal carriers in two categories - intrinsic and extrinsic ones with respect to the DW gap vicinity, and on a proper treatment of perturbative and nonperturbative (the so-called anomalies) contributions. This approach results e.g. in a helpful relation between the “generalized condensate density" and the complex dielectric susceptibility of intrinsic carriers. On this basis we easily describe main properties of the DW's both at low T and near T_MF. Separately for CDW and SDW we discuss the spectra and the attenuation for the TO and LO modes, the low frequency relaxation and the reaction to an external voltage. Our studies cover systematically and generalize most of the previously derived results which have been used for pure systems or as preliminary steps to approaching the pinning problem. New results of a potential experimental significance describe the TO, LO and zero sound spectra interplay, the anomalous Landau damping of both LO and TO modes near T_MF, the relaxation rates for narrow gap DW's, the relation between the current and the driving electric field and between the inherent and the observed nonlinear conductivity.

Brazovskii, S.

1993-12-01

162

Density dependence across multiple life stages in a temperate old-growth forest of northeast China.  

PubMed

Recent studies on species coexistence suggest that density dependence is an important mechanism regulating plant populations. However, there have been few studies of density dependence conducted for more than one life-history stage or that control for habitat heterogeneity, which may influence spatial patterns of survival and mask density dependence. We explored the prevalence of density dependence across multiple life stages, and the effects of controlling for habitat heterogeneity, in a temperate forest in northeast China. We used generalized linear mixed-effects models to test for density-dependent mortality of seedlings and spatial point pattern analysis to detect density dependence for sapling-to-juvenile transitions. Conspecific neighbors had a negative effect on survival of plants in both life stages. At the seedling stage, we found a negative effect of conspecific seedling neighbors on survival when analyzing all species combined. However, in species-level analyses, only 2 of 11 focal species were negatively impacted by conspecific neighbors, indicating wide variation among species in the strength of density dependence. Controlling for habitat heterogeneity did not alter our findings of density dependence at the seedling stage. For the sapling-to-juvenile transition stage, 11 of 15 focal species showed patterns of local scale (<10 m) conspecific thinning, consistent with negative density dependence. The results varied depending on whether we controlled for habitat heterogeneity, indicating that a failure to account for habitat heterogeneity can obscure patterns of density dependence. We conclude that density dependence may promote tree species coexistence by acting across multiple life-history stages in this temperate forest. PMID:23053238

Piao, Tiefeng; Comita, Liza S; Jin, Guangze; Kim, Ji Hong

2012-10-02

163

The Time-Dependent Density Matrix Renormalization Group  

NASA Astrophysics Data System (ADS)

The time-dependent DMRG is a remarkable and highly flexible tool to simulate real-time dynamics of strongly correlated systems. It can be used to calculate spectral functions, and to study systems far from equilibrium. In this chapter we describe how to generalize the DMRG method to solve the time-dependent Schrödinger equation, in both, real, and imaginary time. We illustrate these ideas with different applications such as transport problems in nano-structures, quantum quenches, and the calculation of time-dependent correlations functions.

Feiguin, Adrian E.

164

An Automated Approach for Estimation of Breast Density  

PubMed Central

Breast density is a strong risk factor for breast cancer; however, no standard assessment method exists. An automated breast density method (ABDM) was modified and compared with a semi-automated user-assisted display method (CM) and the Breast Imaging Reporting and Data System (BI-RADS) four-category tissue composition measure for their ability to predict future breast cancer risk. The three estimation methods were evaluated in a matched breast cancer case (n=372) control (n=713) study at the Mayo Clinic using digitized film mammograms. Mammograms from the craniocaudal (CC) view of the noncancerous breast were acquired on average seven years before diagnosis. Two controls with no prior history of breast cancer from the screening practice were matched to each case on age, number of prior screening mammograms, final screening exam date, menopausal status at this date, interval between earliest and latest available mammograms, and residence. Both Pearson linear correlation (R) and Spearman rank correlation ( r ) coefficients were used for comparing the three methods where appropriate. Conditional logistic regression was used to estimate the risk of breast cancer (odds ratios [ORs] and 95% confidence intervals [CIs]) associated with the quartiles of percent density (ABDM, CM) or BI-RADS category. The area under the receiver operator characteristic curve (AUC) was estimated and used to compare the discriminatory capabilities of each approach. The continuous measures ABDM and CM were highly correlated with each other (R=0.70) but less with BI-RADS (r=0.49 for ABDM and r=0.57 for CM). Risk estimates associated with the lowest to highest quartiles of ABDM were greater in magnitude (ORs: 1.0[ref], 2.3, 3.0, 5.2, p-trend<0.001) than the corresponding quartiles for CM (ORs: 1.0[ref], 1.7, 2.1 and 3.8; p-trend<0.001) and BI-RADS (ORs: 1.0[ref], 1.6, 1.5, 2.6; p-trend<0.001) methods. However, all methods similarly discriminated between case and control status: AUCs were 0.64, 0.63 and 0.61 for ABDM, CM and BI-RADS, respectively. The ABDM is a viable option for quantitatively assessing breast density from digitized film mammograms.

Heine, John J.; Carston, Michael J.; Scott, Christopher G.; Brandt, Kathleen R.; Wu, Fang-Fang; Pankratz, V. Shane; Sellers, Thomas A.; Vachon, Celine M.

2009-01-01

165

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

Microsoft Academic Search

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

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

2008-01-01

166

Broadcasting but not receiving: density dependence considerations for SETI signals  

Microsoft Academic Search

This paper develops a detailed quantitative model which uses the Drake\\u000aequation and an assumption of an average maximum radio broadcasting distance by\\u000aan communicative civilization to derive a minimum civilization density for\\u000acontact between two civilizations to be probable in a given volume of space\\u000aunder certain conditions, the amount of time it would take for a first contact,

Reginald D. Smith

2009-01-01

167

Broadcasting but not receiving: density dependence considerations for SETI signals  

NASA Astrophysics Data System (ADS)

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. Using this basis, it estimates the 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 the question of whether reciprocal contact is possible.

Smith, Reginald D.

2009-04-01

168

Energy dependence of the exchange-correlation kernel of time-dependent density functional theory: A simple model for solids  

NASA Astrophysics Data System (ADS)

Time-dependent density functional theory faces an important problem when it comes to extended systems: The long-range component of the exchange-correlation kernel fxc is completely absent from local density or generalized gradient approximations, but it is believed to be present in the “exact” fxc . Several attempts have been made to solve this issue, the simplest of them being the use of a model static long-range kernel of the form -?static/q2 . In this paper, we propose and motivate a dynamical extension of this model of the form -(?+??2)/q2 . The dynamical model is then used to calculate the dielectric function of a large variety of semiconductors and insulators. The absorption spectra of large gap insulators are remarkably improved with respect to calculations where the kernel is taken to be static. This approach is valid also for energies in the range of plasmons, and hence it yields, e.g., good electron energy loss spectra. Finally, we present some simple theoretical arguments that relate the parameters of the model to physical quantities, like the dielectric constant and the plasmon frequency.

Botti, Silvana; Fourreau, Armel; Nguyen, François; Renault, Yves-Olivier; Sottile, Francesco; Reining, Lucia

2005-09-01

169

From patterns to processes and back: analysing density-dependent responses to an abiotic stressor by statistical and mechanistic modelling  

PubMed Central

Our knowledge about population-level effects of abiotic stressors is limited, largely due to lack of appropriate time-series data. To analyse interactions between an abiotic stressor and density-dependent processes, we used experimental time-series data for stage-structured populations (the blowfly Lucilia sericata) exposed to the toxicant cadmium through 20 generations. Resource limitation results in competition both in the larval and the adult stages. The toxicant has only negative effects at the organism level, but nevertheless, there were positive population-level effects. These are necessarily indirect, and indicate overcompensatory density-dependent responses. A non-parametric model (generalized additive model) was used to investigate the density-dependent structures of the demographic rates, without making assumptions about the functional forms. The estimated structures were used to develop a parametric model, with which we analysed effects of the toxicant on density-dependent and density-independent components of the stage-specific demographic rates. The parameter estimates identified both synergistic and antagonistic density–toxicant interactions. It is noteworthy that the synergistic interaction occurred together with a net positive effect of the toxicant. Hence, the effects of such interactions should be considered together with the capacity for compensatory responses. The combination of the two modelling approaches provided new insight into mechanisms for compensatory responses to abiotic stressors.

Jannicke Moe, S; Kristoffersen, Anja B; Smith, Robert H; Stenseth, Nils Chr

2005-01-01

170

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

PubMed Central

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

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

1998-01-01

171

Density dependence of symmetry energy and collective transverse in-plane flow  

Microsoft Academic Search

We study the sensitivity of the collective transverse in-plane flow to the symmetry energy and its density dependence at Fermi energies and higher incident energies. We find that collective transverse in-plane flow is sensitive to the symmetry energy and its density dependence at Fermi energies whereas it shows insensitivity at higher incident energies.

Sakshi Gautam; Aman D. Sood; Rajeev K. Puri; J. Aichelin

2011-01-01

172

Modeling density dependent flow in the sedimentary basin of Thuringia  

NASA Astrophysics Data System (ADS)

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

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

2012-04-01

173

Hybrid density functional theory meets quasiparticle calculations: A consistent electronic structure approach  

NASA Astrophysics Data System (ADS)

We propose a scheme to obtain a system-dependent fraction of exact exchange (?) within the framework of hybrid density functional theory (DFT) that is consistent with the G0W0 approach, where G0 is the noninteracting Green function of the system and W0 the screened Coulomb interaction. We exploit the formally exact condition of exact DFT that the energy of the highest occupied molecular orbital corresponds to the ionization potential of a finite system. We identify the optimal ? value for which this statement is obeyed as closely as possible and thereby remove the starting point dependence from the G0W0 method. This combined approach is essential for describing electron transfer (as exemplified by the TTF/TCNQ dimer) and yields the vertical ionization potentials of the G2 benchmark set with a mean absolute percentage error of only ?3%.

Atalla, Viktor; Yoon, Mina; Caruso, Fabio; Rinke, Patrick; Scheffler, Matthias

2013-10-01

174

Relative importance of density-dependent regulation and environmental stochasticity for butterfly population dynamics  

Microsoft Academic Search

The relative contribution of density-dependent regulation and environmental stochasticity to the temporal dynamics of animal\\u000a populations is one of the central issues of ecology. In insects, the primary role of the latter factor, typically represented\\u000a by weather patterns, is widely accepted. We have evaluated the impact of density dependence as well as density-independent\\u000a factors, including weather and mowing regime, on

Piotr Nowicki; Simona Bonelli; Francesca Barbero; Emilio Balletto

2009-01-01

175

Calculating state-to-state transition probabilities within time-dependent density-functional theory  

SciTech Connect

The determination of the elements of the S matrix within the framework of time-dependent density-functional theory (TDDFT) has remained a widely open question. We explore two different methods to calculate state-to-state transition probabilities. The first method closely follows the extraction of the S matrix from the time-dependent Hartree-Fock approximation. This method suffers from cross-channel correlations resulting in oscillating transition probabilities in the asymptotic channels. An alternative method is proposed, which corresponds to an implicit functional of the time-dependent density. Evaluated with the exact time-dependent density it gives rise to stable and accurate transition probabilities. However, the functional shows an extreme sensitivity with respect to errors in the time-dependent density when evaluated using an approximate density from an actual TDDFT calculation. Two exactly solvable two-electron systems serve as a benchmark for a quantitative test.

Rohringer, Nina; Peter, Simone; Burgdoerfer, Joachim [Institute for Theoretical Physics, Vienna University of Technology, A-1040 Vienna (Austria)

2006-10-15

176

Nonlinear absorption and density-dependent dephasing in Rydberg electromagnetically-induced-transparency media  

NASA Astrophysics Data System (ADS)

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

Gärttner, Martin; Evers, Jörg

2013-09-01

177

Density-dependent growth responses in two clonal herbs: regulation of shoot density  

Microsoft Academic Search

It has been shown in clonal perennial herbs that shoot natality decreases, and shoot mortality increases, in stands of increasing density. In a two-year garden experiment, we have tested Hutchings' (1979) hypothesis that these responses are the result of physiological integration, i.e. the exchange of resources and growth substances between shoots of a single clone. Dense monocultures of two rhizomatous

Hans De Kroon; René Kwant

1991-01-01

178

Interactions between density-dependent processes, population dynamics and control of an invasive plant species, Tripleurospermum perforatum (scentless chamomile)  

Microsoft Academic Search

Tripleurospermum perforatum is an invasive weedy species which exhibits strong over- compensating density dependence. Interactions between density-dependent survival, probability of flowering and fecundity were modelled and their impact on the population dynamics were examined. When only fecundity was density-dependent, the dynamics were similar to those observed in the model containing all three density-dependent terms. Density-dependent survival was a stabilizing process

Yvonne M. Buckley; Hariet L. Hinz; Diethart Matthies; Mark Rees

2001-01-01

179

A novel approach to evaluating breast density using ultrasound tomography  

Microsoft Academic Search

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

Carri K. Glide

2007-01-01

180

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

181

Efficient optimal effective potential approach for pe- riodic plane-wave density functional theory  

NASA Astrophysics Data System (ADS)

Kohn-Sham (KS) density functional theory (DFT) formulates equations for non-interacting electrons subject to a mean-field KS potential. The exchange and correlation (XC) between electrons are accounted for by density-based XC-functionals. The introduction of orbital-dependent functionals allows for a more accurate treatment of exchange and correlation, a prominent example being the exact treatment of Hartree-Fock exchange. Such a construction, however, is not straightforward in KS DFT, as all Kohn Sham orbitals fulfill the same KS equation. For a given orbital-dependent functional, direct solutions to find the corresponding KS potential are numerically cumbersome or even unstable. By extending and combining previous approaches [Phys. Rev. B 62, 15521 (2000), Phys. Rev. B 84, 165122 (2011)], we introduce a momentum-space based formulation that allows for an efficient treatment of orbital-dependent functionals. We include the full spin degrees of freedom, as well as periodic boundary conditions and k-point sampling. We show that for the spin-free case, our formulation becomes similar to the orbital-shift approach [Phys. Rev. B 68, 035103 (2003)] but numerically better suited for implementation in plane-wave DFT codes. Finally, we discuss practical applications.

Libisch, Florian; Dieterich, Johannes M.; Huang, Chen; Carter, Emily A.

2013-03-01

182

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

PubMed

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

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

2012-08-22

183

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

184

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

SciTech Connect

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

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

2011-05-15

185

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

SciTech Connect

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

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

2006-03-17

186

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.

Loe, Leif Egil; Mysterud, Atle; Veiberg, Vebj?rn; Langvatn, Rolf

2009-01-01

187

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

NASA Astrophysics Data System (ADS)

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

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

2012-06-01

188

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

PubMed

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

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

2012-05-04

189

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

2006-06-01

190

Resonance Raman spectra of ortho-nitrophenol calculated by real-time time-dependent density functional theory.  

PubMed

A new approach for the calculation of resonance Raman spectra is presented. The new method is based on dynamic polarizabilities from real-time time-dependent density functional theory, and its estimations are compared to two established techniques for the prediction of resonance Raman spectra. These established methods either use dynamic polarizabilities from linear-response time-dependent density functional theory or employ excited-state gradients. The three different ways to calculate resonance Raman spectra are investigated using the example of ortho-nitrophenol. The three methods give very similar results, respectively, for the four different exchange-correlation functionals applied. Thus, the new approach is validated for the calculation of resonance Raman intensities and advantages as well as disadvantages are discussed. PMID:23387562

Thomas, Martin; Latorre, Federico; Marquetand, Philipp

2013-01-28

191

Combined Stochastic and Deterministic Approach for Classification Using Generalized Mixture Densities.  

National Technical Information Service (NTIS)

This work investigates a combined stochastic and deterministic optimization approach for multivariate mixture density estimation. Mixture probability density models are selected and optimized by combining the optimization characteristics of a multiagent s...

D. E. Waagen J. R. McDonnell

1995-01-01

192

A feed-back control approach for global simulation of high density plasma discharges  

NASA Astrophysics Data System (ADS)

Spatially averaged global simulations have been widely used to study discharge characteristics of high density plasma discharge, because the global insight on the dependence of quantities such as densities and temperatures can be obtained. Above all, the global model can quickly predict the plasma parameters on the external or the chamber parameters comparing with multi-dimensional modeling. However, the global model can be expensive to compute the plasma parameters for complex or mixed gas discharges. Therefore, in this work, we applied a feed-back approach to solve a set of spatially averaged fluid equations for charged particles, neutrals, and radicals. The results were shown that the developed model could efficiently enhance convergence of simulations.

Kwon, Deuk-Chul; Song, Mi-Young; Yoon, Jung-Sik; Yoon, Nam-Sik

2013-10-01

193

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

PubMed

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

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

2013-05-03

194

Effects of spinor distortion and density-dependent form factors upon quasifree 16O(e-->,e'p-->)  

Microsoft Academic Search

We propose an effective current operator for nucleon electromagnetic knockout that incorporates spinor distortion and density-dependent nucleon form factors using an effective momentum approximation. This method can be used in a coordinate-space approach with either relativistic or nonrelativistic optical potentials and overlap functions. We studied these effects for the 16O(e-->,e'p-->) reaction at Q2=0.8 (GeV\\/c)2. Spinor distortion substantially enhances the left-right

James J. Kelly

1999-01-01

195

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

Microsoft Academic Search

Photoionization cross sections and angular distributions of silicon tetrafluoride have been calculated in the framework of the time-dependent density-functional theory approach. Both valence and inner shell ionizations have been considered in an extended photon energy range. Calculations have been carried out by using two different exchange-correlation (xc) potentials characterized by the correct asymptotic behavior. Theoretical results obtained with both the

D. Toffoli; M. Stener; P. Decleva

2006-01-01

196

Effects of chemical reactions on density-dependent fluid flow: on the numerical formulation and the development of instabilities  

Microsoft Academic Search

A three-dimensional, reactive numerical flow model is developed that couples chemical reactions with density-dependent mass transport and fluid flow. The model includes equilibrium reactions for the aqueous species, kinetic reactions between the solid and aqueous phases, and full coupling of porosity and permeability changes that result from precipitation and dissolution reactions in porous media. A one-step, global implicit approach is

Vicky L. Freedman; Motomu Ibaraki

2002-01-01

197

Remarks on the use of projected densities in the density-dependent part of Skyrme or Gogny functionals  

NASA Astrophysics Data System (ADS)

I discuss the inadequacy of the 'projected density' prescription to be used in density-dependent forces/functionals when calculations beyond mean field are pursued. The case of calculations aimed at the symmetry restoration of mean fields obtained with effective realistic forces of the Skyrme or Gogny type is considered in detail. It is shown that, at least for the restoration of spatial symmetries like rotations, translations or parity, the above prescription yields catastrophic results for the energy that drive the intrinsic wave-function to configurations with infinite deformation, thereby preventing its use both in projection after and before variation.

Robledo, L. M.

2010-06-01

198

Toward an integrative approach in the analysis of dependency problems.  

PubMed

This article takes some preliminary steps towards an integrated analysis of dependency problems e.g., long-term tranquillizer use, alcohol dependence, problematic use of narcotics. It argues for the need to outline important theoretical, epistemological, and methodological prerequisites in the analysis of the complex dynamic developmental processes involved in dependency problems. The dynamic process leading to dependence can be studied by the aid of an artificial science neural network approach in combination with a mixed method strategy including a clarification of a combination of different epistemological positions. It is intended that the empirical output of this complex strategy will provide a starting point for a new theoretical analysis which, in turn, will lead to new and more relevant input variables in the neural network approach that will help us to extend our knowledge of the dynamic processes leading to dependency. PMID:11592475

Larsson, S; Lilja, J; Borg, S; Buscema, M; Hamilton, D

199

Sex-based differences in density-dependent sociality: an experiment with a gregarious ungulate.  

PubMed

For animals living in natural or semi-natural settings, empirical data on how sociality changes in response to increasing population density are few, especially with respect to true conspecific density and not group size. However, insight into this line of research may be far-reaching--from understanding density dependence in sexual selection to improving models of disease transmission. Using elk (Cervus elaphus Linnaeus) held in enclosures, we conducted sex-stratified experiments to test how the frequency of dyadic pairings (interaction rate) and their quality (duration) responded to manipulations in exposure to density. Using proximity-logging radio collars we recorded when and for how long individuals shared a space within 1.4 m of each other. As predicted, males increased their interaction rate as density increased. Female interaction rates, however, increased initially as density increased but soon declined to become indistinguishable from rates at low density. Females interacted for longer periods at medium densities, whereas male interaction length clearly decreased as density increased. We highlight a sexually dichotomous, density-dependent response in sociality that has yet to be reported. In addition to furthering our understanding of sociobiology (e.g., implications of time constraints presented by density on dyadic interactions), our results have implications for managing communicable disease in gregarious species of livestock and wildlife. PMID:22486100

Vander Wal, E; Yip, H; McLoughlin, P D

2012-01-01

200

The Density Matrix in the de Broglie--Bohm Approach  

Microsoft Academic Search

If the density matrix is treated as an objective description of individual systems, it may become possible to attribute the same objective significance to statistical mechanical properties, such as entropy or temperature, as to properties such as mass or energy. It is shown that the de Broglie--Bohm interpretation of quantum theory can be consistently applied to density matrices as a

O. J. E. Maroney

2005-01-01

201

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...

202

Temporal changes in the strength of density-dependent mortality and growth in intertidal barnacles.  

PubMed

1. In demographically open marine systems, the extent to which density-dependent processes in the benthic adult phase are required for population persistence is unclear. At one extreme, represented by the recruitment limitation hypothesis, larval supply may be insufficient for the total population size to reach a carrying capacity and density-independent mortality predominates. At the opposite extreme, populations are saturated and density-dependent mortality is sufficiently strong to reshape patterns established at settlement. 2. We examined temporal variation in the way density-independent and density-dependent mortality interact in a typical sessile marine benthic invertebrate, the acorn barnacle Semibalanus balanoides (L.), over a 2-year period. 3. Recruitment was manipulated at two high recruitment sites in north Wales, UK to produce recruit densities covering the range naturally found in this species. Following manipulation, fixed quadrats were monitored using digital photography and temporal changes in mortality and growth rate were examined. 4. Over a 2-year period there was a clear, spatially consistent, over-compensatory relationship between the density of recruits and adult abundance indicating strong density-dependent mortality. The strength of density dependence intensified with increasing recruitment. 5. Density-dependent mortality did not operate consistently over the study period. It only operated in the early part of the benthic phase, but the pattern of adult abundance generated was maintained throughout the whole 2-year period. Thus, early life-history processes dictated adult population abundance and dynamics. 6. Examination of the natural recruitment regime in the area of study indicated that both positive and negative effects of recruitment will occur over scales varying from kilometres to metres. PMID:18284479

Jenkins, Stuart R; Murua, Jefferson; Burrows, Michael T

2008-02-13

203

Time-dependent gas density and temperature measurements in pulsed helicon discharges in argon  

Microsoft Academic Search

Time-dependent measurements of the temperature and density of the Arast(3P0) metastable atoms in two high-density pulsed helicon discharges, deduced from the absorption profile of the 772.42 nm argon line, are presented. A single-mode tuneable diode laser is used for recording these profiles, and temperatures up to 1000 K are obtained from their Doppler width. As in high-density plasmas the metastable

B. Clarenbach; B. Lorenz; M. Krämer; N. Sadeghi

2003-01-01

204

Experimental evidence for density-dependent responses to mortality of snake-necked turtles  

Microsoft Academic Search

Density-dependent compensation has rarely been demonstrated in long-lived vertebrates in highly vari- able environments, such as the wet-dry tropics, where com- plex factors impact on vital rates. We used an experimental manipulation of population density in six replicated wild populations of the northern snake-necked turtle (Chelodina rugosa). We show that this species can rebound rapidly fol- lowing reductions in density,

Damien A. Fordham; Arthur Georges; Barry W. Brook

2008-01-01

205

Experimental evidence for density-dependent responses to mortality of snake-necked turtles  

Microsoft Academic Search

Density-dependent compensation has rarely been demonstrated in long-lived vertebrates in highly variable environments, such\\u000a as the wet–dry tropics, where complex factors impact on vital rates. We used an experimental manipulation of population density\\u000a in six replicated wild populations of the northern snake-necked turtle (Chelodina rugosa). We show that this species can rebound rapidly following reductions in density, and so is

Damien A. Fordham; Arthur Georges; Barry W. Brook

2009-01-01

206

Dependence of Capacitance of Metal-Molten Salt Interface on Local Density Profiles Near Electrode.  

National Technical Information Service (NTIS)

The mean spherical approximation applied to a metal-molten salt interface is generalized to take further account of the local density profiles near the electrode. The temperature dependence of the differential capacitance is shown to arise, in large measu...

P. Ballone G. Pastore M. P. Tosi K. R. Painter P. J. Grout

1983-01-01

207

Development and Verification of a Three Dimensional Density Dependent Solute Transport Model for Seawater Intrusion  

Microsoft Academic Search

Seawater intrusion is identified as a critical issue that needs to be addressed in coastal hydrology, due its irreversible effect on the water quality of coastal freshwater aquifers. Due to the density differences in seawater and freshwater, the density dependent flow phenomenon is dominant seawater intrusion. Three dimensional numerical model is necessary to address the complex hydro-geological formations. Therefore, it

Edangodage Duminda; Pradeep PERERA; Kenji JINNO; Atsushi TSUTSUMI; Yoshinari HIROSHIRO

2008-01-01

208

Density-dependent foraging behaviour of sheep on alpine pastures: effects of scale  

Microsoft Academic Search

Foraging patterns of large herbivores may give important clues as to why their life history varies depending on population density. In this landscape-scale experi- ment, domestic sheep Ovis aries were kept at high (80 sheep km? 2) and low (25 sheep km? 2) population densities during summer in high mountain pastures in Hol, Norway. We predicted an increasing use of

K. Kausrud; A. Mysterud; Y. Rekdal; O. Holand; G. Austrheim

2006-01-01

209

Temporal changes in the strength of density-dependent mortality and growth in intertidal barnacles  

Microsoft Academic Search

Summary 1. In demographically open marine systems, the extent to which density-dependent processes in the benthic adult phase are required for population persistence is unclear. At one extreme, represented by the recruitment limitation hypothesis, larval supply may be insufficient for the total population size to reach a carrying capacity and density-independent mortality predominates. At the opposite extreme, populations are saturated

Stuart R. Jenkins; Jefferson Murua; Michael T. Burrows

2008-01-01

210

Time-Dependent Density of Viscous Modified Chaplygin Gas with Arbitrary ?  

NASA Astrophysics Data System (ADS)

In this paper we study modified Chaplygin gas which has viscosity for the case of arbitrary ? instead of ?=0.5. We obtain behavior of the energy density of modified Chaplygin gas with respect to the constant and time-dependent bulk viscosity. We find also, numerically, effect of ? on the energy density.

Saadat, H.

2013-11-01

211

An Experimental Field Study of Delayed Density Dependence in Natural Populations of Aedes albopictus  

PubMed Central

Aedes albopictus, a species known to transmit dengue and chikungunya viruses, is primarily a container-inhabiting mosquito. The potential for pathogen transmission by Ae. albopictus has increased our need to understand its ecology and population dynamics. Two parameters that we know little about are the impact of direct density-dependence and delayed density-dependence in the larval stage. The present study uses a manipulative experimental design, under field conditions, to understand the impact of delayed density dependence in a natural population of Ae. albopictus in Raleigh, North Carolina. Twenty liter buckets, divided in half prior to experimentation, placed in the field accumulated rainwater and detritus, providing oviposition and larval production sites for natural populations of Ae. albopictus. Two treatments, a larvae present and larvae absent treatment, were produced in each bucket. After five weeks all larvae were removed from both treatments and the buckets were covered with fine mesh cloth. Equal numbers of first instars were added to both treatments in every bucket. Pupae were collected daily and adults were frozen as they emerged. We found a significant impact of delayed density-dependence on larval survival, development time and adult body size in containers with high larval densities. Our results indicate that delayed density-dependence will have negative impacts on the mosquito population when larval densities are high enough to deplete accessible nutrients faster than the rate of natural food accumulation.

Walsh, Rachael K.; Bradley, Caitlin; Apperson, Charles S.; Gould, Fred

2012-01-01

212

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

213

Density dependence in the terrestrial life history stage of two anurans.  

PubMed

Populations of species with complex life cycles have the potential to be regulated at multiple life history stages. However, research tends to focus on single stage density-dependence, which can lead to inaccurate conclusions about population regulation and subsequently hinder conservation efforts. In amphibians, many studies have demonstrated strong effects of larval density and have often assumed that populations are regulated at this life history stage. However, studies examining density regulation in the terrestrial stages are rare, and the functional relationships between terrestrial density and vital rates in amphibians are unknown. We determined the effects of population density on survival, growth and reproductive development in the terrestrial stage of two amphibians by raising juvenile wood frogs (Rana sylvatica) and American toads (Bufo americanus) at six densities in terrestrial enclosures. Density had strong negative effects on survival, growth and reproductive development in both species. We fitted a priori recruitment functions to describe the relationship between initial density and the density of survivors after one year, and determined the functional relationship between initial density and mass after one year. Animals raised at the lowest densities experienced growth and survival rates that were over twice as great as those raised at the highest density. All female wood frogs in the lowest density treatment showed signs of reproductive development, compared to only 6% in the highest density treatment. Female American toads reached minimum reproductive size only at low densities, and male wood frogs and American toads reached maturity only in the three lowest density treatments. Our results demonstrate that in the complex life cycle of amphibians, density in the terrestrial stage can reduce growth, survival and reproductive development and may play an important role in amphibian population regulation. We discuss the implications of these results for population regulation in complex life cycles and for amphibian conservation. PMID:17622562

Harper, Elizabeth B; Semlitsch, Raymond D

2007-07-11

214

Comparative population dynamics of Peromyscus leucopus in North America: influences of climate, food, and density dependence  

Microsoft Academic Search

Temporal variation in population size is regulated by both exogenous forces and density-dependent feedbacks. Furthermore,\\u000a accumulating evidence indicates that temporal and spatial variation in climate and resources can modify the strength of density\\u000a dependence in animal populations. We analyzed six long-term time series estimates of Peromyscus leucopus (white-footed mouse) abundance from Kansas, Ohio, Pennsylvania, Virginia, Vermont, and Maine, USA, using

Guiming Wang; Jerry O. Wolff; Stephen H. Vessey; Norman A. Slade; Jack W. Witham; Joseph F. Merritt; Malcolm L. Hunter; Susan P. Elias

2009-01-01

215

The role of density-dependent individual growth in the persistence of freshwater salmonid populations  

Microsoft Academic Search

Theoretical and empirical models of populations dynamics have paid little attention to the implications of density-dependent\\u000a individual growth on the persistence and regulation of small freshwater salmonid populations. We have therefore designed a\\u000a study aimed at testing our hypothesis that density-dependent individual growth is a process that enhances population recovery\\u000a and reduces extinction risk in salmonid populations in a variable

Simone Vincenzi; Alain J. Crivelli; Dusan Jesensek; Giulio A. De Leo

2008-01-01

216

On the variational inequalities related to viscous density-dependent incompressible fluids  

Microsoft Academic Search

We consider some variational inequality formulations related to density-dependent incompressible fluids. Firstly, we state\\u000a the density-dependent micropolar model, which let us to introduce a generic (vectorial) differential inequality formulation.\\u000a Then, two relaxations of this differential inequality will be considered, driving to concepts of weak and generalized solutions\\u000a (observing that the weak solutions are generalized solutions but the contrary is not

Francisco Guillén-González; Mariano Poblete-Cantellano; Marko A. Rojas-Medar

2010-01-01

217

Density-dependent outcomes in a digestive mutualism between carnivorous Roridula plants and their associated hemipterans  

Microsoft Academic Search

Recent studies have shown that mutualisms often have variable outcomes in space and time. In particular, the outcomes may\\u000a be dependent on the density of the partners with unimodal or saturating outcomes providing stability to the mutualism. We\\u000a examine density-dependent outcomes of an obligate, species-specific mutualism between a South African carnivorous plant (Roridula dentata) and a hemipteran (Pameridea) that facilitates

Bruce Anderson; Jeremy J. Midgley

2007-01-01

218

Mechanisms of density- and number-dependent population regulation of a coral-reef fish  

Microsoft Academic Search

Density-dependent post-settlement losses are a common feature of many reef fish populations, and resent observations suggest that losses may also scale with population size independent of density (i.e. 'number-dependence'). Despite the potential importance of these two forms of compensatory loss, there have been relatively few attempts to determine the mechanisms underlying these demographic patterns. A factorial experiment was conducted to

Jeffrey S. Shima

2002-01-01

219

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

NASA Astrophysics Data System (ADS)

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 ?d and ?b according to ?dmpropto?d? and ?bpropto?b?, while the effectively constant dark energy density is dependent upon a random variable phiQ according to ?QproptophiQn. The ratio of the baryon density to the dark energy density at initial collapse, rQ, 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 rQ and r and show that the observed values of rQ 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 phiQ4 potential, the range of rQ and r which corresponds to the observed Universe is a quite natural, with a probability which is broadly similar to other ranges of rQ and r.

McDonald, John

2013-05-01

220

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

PubMed Central

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

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

2010-01-01

221

New Approach of Designing Superalloys for Low Density.  

National Technical Information Service (NTIS)

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 we...

J. L. Smialek M. V. Nathal R. A. MacKay T. P. Gabb

2010-01-01

222

A novel approach to evaluating breast density using ultrasound tomography  

NASA Astrophysics Data System (ADS)

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

Glide, Carri K.

223

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

PubMed

Density-dependence is hypothesized as the major mechanism of population regulation. However, the lack of long-term demographic data has hampered the use of density-dependent models in nonhuman primates. In this study, we make use of the long-term demographic data from Cayo Santiago's rhesus macaques to parameterize and analyze both a density-independent and a density-dependent population matrix model, and compare their projections with the observed population changes. We also employ a retrospective analysis to determine how variance in vital rates, and covariance among them, contributed to the observed variation in long-term fitness across different levels of population density. The population exhibited negative density-dependence in fertility and the model incorporating this relationship accounted for 98% of the observed population dynamics. Variation in survival and fertility of sexually active individuals contributed the most to the variation in long-term fitness, while vital rates displaying high temporal variability exhibited lower sensitivities. Our findings are novel in describing density-dependent dynamics in a provisioned primate population, and in suggesting that selection is acting to lower the variance in the population growth rate by minimizing the variation in adult survival at high density. Because density-dependent mechanisms may become stronger in wild primate populations due to increasing habitat loss and food scarcity, our study demonstrates that it is important to incorporate variation in population size, as well as demographic variability into population viability analyses for a better understanding of the mechanisms regulating the growth of primate populations. Am. J. Primatol. 75:1152-1164, 2013. © 2013 Wiley Periodicals, Inc. PMID:23847126

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

2013-07-11

224

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, Jr. , J. F.

2005-01-01

225

Dependence of the switching current density on the junction sizes in spin transfer torque  

NASA Astrophysics Data System (ADS)

We investigate the dependence of switching current density on the junction sizes in the in-plane spin transfer torque nanopillar structures by using micromagnetic simulations. While the macrospin model predicts weak dependence of switching current density on the junction sizes, we find that the switching current density is a sensitive function of the junction sizes. It can be explained with the complicated spin configurations and dynamics during the switching process. The detail spin configurations and dynamics are determined by spin wave excitation with the finite wave vector, which is related with the exchange coupling energy and junction shape.

You, Chun-Yeol; Jung, Myung-Hwa

2013-02-01

226

Conditional Dependencies: A Principled Approach to Improving Data Quality  

Microsoft Academic Search

Real-life data is often dirty and costs billions of pounds to businesses worldwide each year. This paper presents a promising\\u000a approach to improving data quality. It effectively detects and fixes inconsistencies in real-life data based on conditional\\u000a dependencies, an extension of database dependencies by enforcing bindings of semantically related data values. It accurately\\u000a identifies records from unreliable data sources by

Wenfei Fan; Floris Geerts; Xibei Jia

2009-01-01

227

Density-dependent outcomes in a digestive mutualism between carnivorous Roridula plants and their associated hemipterans.  

PubMed

Recent studies have shown that mutualisms often have variable outcomes in space and time. In particular, the outcomes may be dependent on the density of the partners with unimodal or saturating outcomes providing stability to the mutualism. We examine density-dependent outcomes of an obligate, species-specific mutualism between a South African carnivorous plant (Roridula dentata) and a hemipteran (Pameridea) that facilitates prey digestion, but also sucks plant sap. Plants occur in sandy, leached, nitrogen-poor soils and have no digestive enzymes to digest prey. Instead they rely on obligately dependent hemipterans to supply nitrogen by digesting prey for them and defecating on their leaves. We documented the densities of Pameridea on Roridula in the field. In the greenhouse, we manipulated the hemipteran densities on Roridula and measured the mean relative growth rates of plants with differing hemipteran densities. Plants exhibited a unimodal response to the density of their mutualist partners. Those with no hemipterans had negative growth rates, suggesting that hemipterans are important in facilitating nitrogen absorption. Plants with intermediate hemipteran densities had positive growth rates but growth rates were negative under very high hemipteran densities. Our research provides support for variable and unimodal outcomes in mutualism. Unimodal outcomes may be particularly important in obligate mutualisms and this is one of the few studied outside of pollinating seed parasite mutualisms. In this system, extrinsic factors such as other predators may affect the mutualism by altering the numbers of hemipterans. PMID:17221256

Anderson, Bruce; Midgley, Jeremy J

2007-01-13

228

An exactly solvable model of population dynamics with density-dependent migrations and the Allee effect.  

PubMed

We consider a single-species model of population dynamics allowing for migrations and the Allee effect. Two types of migration are taken into account: one caused by environmental factors (e.g., a passive transport with the wind or water current) and the other associated with biological mechanisms. While the first type is apparently density-independent, the speed of migration in the second one can depend on the population density. Mathematically, this model consists of a non-linear partial differential equation of advection-diffusion-reaction type. Using an appropriate change of variables, we obtain an exact solution of the equation describing propagation of travelling population fronts. We show that, depending on parameter values and thus on the relative intensity of density-dependent and density-independent factors, the direction of the propagation can be different thus describing either species invasion or species retreat. PMID:14527748

Petrovskii, Sergei; Li, Bai Lian

2003-11-01

229

Density-dependent effects on hatching success of the olive ridley turtle, Lepidochelys olivacea.  

PubMed

Historically, the olive ridley arribada at Playa Nancite, Costa Rica, was one of the largest olive ridley arribadas in the eastern Pacific with 70,000 nesting females in a year. Recently the Nancite arribada drastically declined. We hypothesized that the population decline at Playa Nancite could have been due to low hatching success as a result of the high density of nests on the beach, such that recruitment to the population was insufficient to balance losses. To test this hypothesis, we examined density-dependent effects on hatching success and their underlying mechanisms by experimentally manipulating nest densities in experimental plots on the nesting beach. We set up four nest-density treatments in five experimental blocks. We measured effects of density on hatching success, CO(2) and O(2) concentrations and temperature both within nests and in sand adjacent to nests frequently during incubation. Experimental nest densities affected hatching success with the highest density having the lowest hatching success. Higher nest density led to lower O(2) levels and higher CO(2) levels in the nest with greater changes in the latter part of the incubation. Highest temperatures occurred in high-density areas. Temperatures were lower in sand surrounding the nest than in the nest. Effects of density on temperature, CO(2) and O(2) were confirmed at a naturally high-density nesting beach, Playa La Flor, Nicaragua. Long-term failure in production of hatchlings due to historic high densities may have contributed to the decline of arribadas on Playa Nancite. Thus, density-dependent population control would have operated at the embryonic life stage in this population of olive ridley turtles. PMID:18481091

Honarvar, Shaya; O'Connor, Michael P; Spotila, James R

2008-05-15

230

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

PubMed

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

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

2008-06-17

231

Structural Implications of Reciprocal Exchange: A Power-Dependence Approach  

ERIC Educational Resources Information Center

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

Bonacich, Phillip; Bienenstock, Elisa Jayne

2009-01-01

232

Assessing UAE's oil dependence: an optimal control approach  

Microsoft Academic Search

Purpose – The main purpose of this paper is to understand the impact on the United Arab Emirates (UAE) economy of the objective of reducing its dependence on oil, trying to achieve the Gulf Cooperation Council (GCC) fiscal convergence criterion and the inevitable depletion of oil resources. Design\\/methodology\\/approach – An 18 equation compact macro-econometric model is constructed and is evaluated

Cedwyn Fernandes; Ajit Karnik

2009-01-01

233

Nonperturbative time-dependent approach to breakup of halo nuclei  

Microsoft Academic Search

The time-dependent mesh method is proposed as an efficient tool for a quantitative analysis of the Coulomb breakup of halo nuclei. The approach allows a treatment of breakup reactions in the nonperturbative regime. It avoids any multipole expansion for the Coulomb interaction between the projectile and the target. Moreover, it permits using more general trajectories allowing an estimation of postacceleration

V. S. Melezhik; D. Baye

1999-01-01

234

Using the Narrative Approach in Groups for Chemically Dependent Mothers  

Microsoft Academic Search

The powerful and healing effects of the narrative approach with the mothers at Family House, a residential treatment program for chemically dependent women, support the use of narratives in groups. Creative clinical techniques that facilitate the telling of stories are detailed. Eight themes were constructed from a retrospective review of group process notes, graduation ceremonies, and one author's written stories

M. Carlean Gilbert; Anne E. Beidler

2002-01-01

235

A simple weighted-density-functional approach to the structure of inhomogeneous fluids  

NASA Astrophysics Data System (ADS)

A simple weighted-density-functional approach is developed for inhomogeneous fluids by approximating the excess free energy density functional by that of the corresponding uniform system evaluated at an effective density. This effective weighted density is obtained by a suitable local averaging procedure of the actual density distribution using a weight function which is evaluated only once at the bulk density, thus being decoupled from the weighted density. This approach does not involve any power series expansion or solution of any nonlinear differential equation at every point and each iteration, as is required in some of the analogous schemes. The new theory is applied to predict the structure of simple classical fluids, interacting with hard sphere, Coulombic as well as Yukawa potentials, under confinement in several geometries. The calculated results on the density profiles are shown to compare quite well with available simulation data.

Patra, Chandra N.; Ghosh, Swapan K.

2002-05-01

236

Adsorption of fluids on colloidal particles: a density functional approach  

Microsoft Academic Search

Density functional results are presented for adsorption of simple fluids on colloidal particles. Two models of colloidal particles are considered: first a study of adsorption on a large hard sphere, and second the application of the theory to describe adsorption on a large particle interacting via Lennard-Jones type attractive-repulsive forces. In both cases the theoretical predictions are compared with Monte

Douglas Henderson; Stefan Sokolowski; Andzej Patrykiejew

1995-01-01

237

Momentum-Dependent Variational Approach to Correlated Electron System  

NASA Astrophysics Data System (ADS)

We present here the momentum dependent local-ansatz wavefunction approach (MLA) with the best choice of the self-consistent variational parameters to describe the ground-state properties of the correlated electron systems in solids. With use of the self-consistent variational scheme, we performed the numerical calculations for the half-filled band as well as non-half-filled band Hubbard model on the hypercubic lattice in infinite dimensions. The ground-state energy in the MLA is lower than those of the local-ansatz approach (LA) and the Gutzwiller approach (GA) in weak and intermediate Coulomb interaction regimes. The double occupation number is suppressed as compared with the LA. We observe the distinct momentum dependence of the momentum distribution functions which is qualitatively different from those of the LA and the GA.

Patoary, M. A. R.; Kakehashi, Y.

2012-12-01

238

Simple assumptions on age composition lead to erroneous conclusions on the nature of density dependence in age-structured populations  

Microsoft Academic Search

Ecologists have debated the nature of density dependence in natural populations for decades, and efforts to detect density dependence from time series of abundance data have paralleled these debates. Yet due to the correlative nature of time series data, these undertakings have been statistically problematic. Most analyses of density dependence have focused on simple population models (i.e., non-overlapping generations), but

Richard W. Zabel; Phillip S. Levin

2002-01-01

239

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

240

Evolution of foraging behavior in Drosophila by density-dependent selection  

PubMed Central

One of the rare examples of a single major gene underlying a naturally occurring behavioral polymorphism is the foraging locus of Drosophila melanogaster. Larvae with the rover allele, forR, have significantly longer foraging path lengths on a yeast paste than do those homozygous for the sitter allele, fors. These variants do not differ in general activity in the absence of food. The evolutionary significance of this polymorphism is not as yet understood. Here we examine the effect of high and low animal rearing densities on the larval foraging path-length phenotype and show that density-dependent natural selection produces changes in this trait. In three unrelated base populations the long path (rover) phenotype was selected for under high-density rearing conditions, whereas the short path (sitter) phenotype was selected for under low-density conditions. Genetic crosses suggested that these changes resulted from alterations in the frequency of the fors allele in the low-density-selected lines. Further experiments showed that density-dependent selection during the larval stage rather than the adult stage of development was sufficient to explain these results. Density-dependent mechanisms may be sufficient to maintain variation in rover and sitter behavior in laboratory populations.

Sokolowski, Marla B.; Pereira, H. Sofia; Hughes, Kimberley

1997-01-01

241

Leaf damage and density-dependent effects on six Inga species in a neotropical forest.  

PubMed

Many models have been proposed to explain the possible role of pests in the coexistence of a high diversity of plant species in tropical forests. Prominent among them is the Janzen-Connell model. This model suggests that specialized herbivores and pathogens limit tree recruitment as a function of their density or proximity to conspecifics. A large number of studies have tested the predictions of this model with respect to patterns of recruitment and mortality at different life stages, yet only a few have directly linked those density- or distance-dependent effects to pest attack. If pest-attack is an important factor in density- or distance-dependent mortality, there should be spatial heterogeneity in pest pressure. I studied the spatial distribution of leaf damage in saplings of six common Inga species (Fabaceae: Mimosoideae) in the 50ha forest dynamic plot of Barro Colorado Island, Panama. The percent leaf damage of Inga saplings was not heterogeneous in space, and the density of conspecific, congener or confamilial neighbors was uncorrelated with the observed damage levels in focal plants. One of the focal species did suffer density-dependent mortality, suggesting that spatial variation in plant performance in these species is not directly driven by leaf damaging agents. While multiple studies suggest that density-dependent effects on performance are common in tropical plant communities, our understanding of the mechanisms that drive those effects is still incomplete and the underlying assumption that these patterns result from differential herbivore attack deserves more scrutiny. PMID:23342505

Brenes-Arguedas, Tania

2012-12-01

242

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

243

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.

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

2008-01-01

244

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

245

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

Bates, Jefferson E.; Furche, Filipp

2012-10-01

246

Electron density building block approach for metal organic frameworks  

NASA Astrophysics Data System (ADS)

A general introduction to the state of the art in modeling metal organic materials using transferable atomic multipoles is provided. The method is based on the building block partitioning of the electron density, which is illustrated with some examples of potential applications and with detailed discussions of the advantages and pitfalls. The interactions taking place between building blocks are summarized and are used to discuss the properties that can be calculated.

Chimpri, Abita S.; Macchi, Piero

2013-04-01

247

Plot the Dot: A Graphical Approach to Density  

NSDL National Science Digital Library

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

Rathjen, Don

2009-01-01

248

Charge carrier dynamics in phonon-induced fluctuation systems from time-dependent wavepacket diffusion approach.  

PubMed

A time-dependent wavepacket diffusion method is proposed to deal with charge transport in organic crystals. The electron-phonon interactions in both site energies and electronic couplings are incorporated by the time-dependent fluctuations which are generated from the corresponding spectral density functions. The numerical demonstrations reveal that the present approach predicts the consistent charge carrier dynamics with the rigorous quantum approaches. In addition, the diffusion coefficients obtained from the Marcus formula are well reproduced at the weak electronic coupling and high temperature limits. It is also found that the charge mobility feature of the crossover from the band-like to the hopping-type cannot be predicted from the fluctuations induced by the linear electron-phonon interactions with an Ohmic spectral density; however, it indeed appears as the electronic coupling fluctuation exponentially depends on the nuclear coordinates. Finally, it should be noted that although the present approach neglects the imaginary fluctuation, it essentially incorporates the coherent motion of the charge carrier and quantum effect of the phonon motion with a broad regime of the fluctuations for symmetric systems. Besides, the approach can easily be applied to systems having thousands of sites, which allows one to investigate charge transport in nanoscale organic crystals. PMID:21992285

Zhong, Xinxin; Zhao, Yi

2011-10-01

249

Dynamical exchange interaction from time-dependent spin density functional theory  

NASA Astrophysics Data System (ADS)

We report on ab initio time-dependent spin-dynamics simulations for a two-center magnetic molecular complex within the framework of the time-dependent noncollinear spin-density functional theory. In particular, we discuss how the dynamical behavior of the ab initio spin-density in the time domain can be mapped onto a model Hamiltonian based on the classical Heisenberg spin-spin interaction JS1·S2. By analyzing individual localized-spin trajectories, extracted from the spin-density evolution, we demonstrate a practical method for evaluating the effective Heisenberg exchange coupling constant J from first-principles simulations. We find that J, extracted in such a dynamical way, agrees quantitatively with that calculated by the standard density functional theory broken-symmetry scheme.

Stamenova, Maria; Sanvito, Stefano

2013-09-01

250

Delayed density-dependent season length alone can lead to rodent population cycles.  

PubMed

Studies of cyclic microtine populations (voles and lemmings) have suggested a relationship between the previous year's population density and the subsequent timing of the onset of reproduction by overwintered breeding females. No studies have explored the importance of this relationship in the generation of population cycles. Here we mathematically examine the implications of variation in reproductive season length caused by delayed density-dependent changes in its start date. We demonstrate that when reproductive season length is a function of past population densities, it is possible to get realistic population cycles without invoking any changes in birth rates or survival. When parameterized for field voles (Microtus agrestis) in Kielder Forest (northern England), our most realistic model predicts population cycles of similar periodicity to the Kielder populations. Our study highlights the potential importance of density-dependent reproductive timing in microtine population cycles and calls for investigations into the mechanism(s) underlying this phenomenon. PMID:16671013

Smith, Matthew J; White, Andrew; Lambin, Xavier; Sherratt, Jonathan A; Begon, Michael

2006-03-20

251

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

NASA Astrophysics Data System (ADS)

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

Ma, Haibo; Ma, Jing

2011-08-01

252

Density-dependent regulation in populations of potato-colonizing aphids  

Microsoft Academic Search

Scarcity of long-term (over 30 years) data series represents a major challenge for an accurate estimation of the role of density-dependent processes in population regulation. We analyzed population densities of the wingless parthenogenic morphs of buckthorn aphid (BA), Aphis nasturtii Kaltenbach, potato aphid (PA), Macrosiphum euphorbiae (Thomas), and green peach aphid (GPA), Myzus persicae (Sulzer) from 1949 to 2003 for signs

Andrei Alyokhin; Francis A. Drummond; Gary Sewell

2005-01-01

253

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

254

Evidence for positive density-dependent effects in recovering Diadema antillarum populations  

Microsoft Academic Search

Recovering populations may experience positive density-dependent feedbacks that contribute to population increases. Diadema antillarum, a keystone herbivore on Caribbean coral reefs, suffered a well-documented mass mortality in 1983–84. High densities of adults of this long-spined urchin could provide effective refuge from predation for juveniles under a spine canopy, as has been suggested for other urchin species. We evaluated the effect

Robert J. Miller; Aaron J. Adams; John P. Ebersole; Elisse Ruiz

2007-01-01

255

Density-dependent migration and stability in a system of linked populations  

Microsoft Academic Search

The effect of adding density-dependent migration between nearest neighbour populations of a single discrete-generation species in a chain of habitat fragments is investigated. The larger the population on a particular habitat fragment, the greater the fraction of inhabitants who migrate before reproducing. It has previously been shown for similar models with density-independent migration that coupling populations in this way has

Graeme D. Ruxton

1996-01-01

256

Current-density functional theory of time-dependent linear response in quantal fluids: recent progress  

Microsoft Academic Search

Vignale and Kohn have recently formulated a local density approximation to\\u000athe time-dependent linear response of an inhomogeneous electron system in terms\\u000aof a vector potential for exchange and correlation. The vector potential\\u000adepends on the induced current density through spectral kernels to be evaluated\\u000aon the homogeneous electron-gas. After a brief review of their theory, the case\\u000aof inhomogeneous

M. P. Tosi; M. L. Chiofalo; A. Minguzzi; R. NIFOSÌ

1998-01-01

257

Concentration-dependent variations in the density of C60 fullerene solutions in aromatic solvents  

NASA Astrophysics Data System (ADS)

The densities of C60 fullerene solutions in benzene, toluene, and p-xylene have been determined by pyknometry and studied as a function of the fullerene concentration in solution. All dependences exhibit a non-monotonic character, whereby the density initially decreases, passes through a minimum, and then grows with increasing fullerene concentration. The position of the minimum shifts toward greater C60 concentrations on the passage from benzene to toluene and p-xylene.

Ginzburg, B. M.; Tuichiev, Sh.; Tabarov, S. Kh.

2007-08-01

258

Protocol Dependence in Jammed Particulate Media: Statistics of the Density Landscape  

NASA Astrophysics Data System (ADS)

The density at which hard-sphere fluids jam into amorphous solids depends strongly on the compression protocol. Extremely fast quenching protocols bring each initial point in configuration space to the closest basin-maximum on the density landscape. In contrast, slower quench protocols allow the system to relax and explore configuration space. The protocol-dependence of the density, other structural quantities, and mechanical properties depends strongly on statistical features of the landscape. In this talk, I describe calculations of the the basin volumes associated with jammed hard sphere packings, and the critical quench rate &*circ; above which the probabilities for obtaining jammed packings are determined by their basin volumes. Basin volumes are exponentially distributed; thus, for ?> &*circ;, so are jammed packing probabilities. We discuss the implications of this result on the statistical mechanics of jammed systems.

Sampangiraj, Ashwin S.

2011-03-01

259

Calculation of matrix fracture flow in different scales with density-dependent processes  

NASA Astrophysics Data System (ADS)

In solid rock aquifers low matrix permeability is often accompanied with fractures and faults of different scales. Therefore, numerical modelling of such coupled systems is very complex. Density dependent flow of groundwater and mass transport processes are coupled in fractured systems. Therefore, numeric representation of such coupled systems is highly complex. Flow and density dependent flow in fractures have to be modeled along discrete surfaces joint to the common matrix representation. This leads to high demands upon the quality of finite element mesh generation. This paper presents a strategy to calculate three dimensional finite element meshes, which allows the modelling of density dependent groundwater flow and contaminant transport in a coupled matrix fracture aquifer system.

König, Christoph; Seidel, Torsten; Larue, Jürgen

2013-04-01

260

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.

Kokko, Hanna; Rankin, Daniel J

2006-01-01

261

Prediction of Iron K-Edge Absorption Spectra Using Time-Dependent Density Functional Theory  

SciTech Connect

Iron K-edge X-ray absorption pre-edge features have been calculated using a time-dependent density functional approach. The influence of functional, solvation, and relativistic effects on the calculated energies and intensities has been examined by correlation of the calculated parameters to experimental data on a series of 10 iron model complexes, which span a range of high-spin and low-spin ferrous and ferric complexes in O{sub h} to T{sub d} geometries. Both quadrupole and dipole contributions to the spectra have been calculated. We find that good agreement between theory and experiment is obtained by using the BP86 functional with the CP(PPP) basis set on the Fe and TZVP one of the remaining atoms. Inclusion of solvation yields a small improvement in the calculated energies. However, the inclusion of scalar relativistic effects did not yield any improved correlation with experiment. The use of these methods to uniquely assign individual spectral transitions and to examine experimental contributions to backbonding is discussed.

George, S.DeBeer; Petrenko, T.; Neese, F.

2009-05-14

262

Linear-scaling time-dependent density-functional theory in the linear response formalism.  

PubMed

We present an implementation of time-dependent density-functional theory (TDDFT) in the linear response formalism enabling the calculation of low energy optical absorption spectra for large molecules and nanostructures. The method avoids any explicit reference to canonical representations of either occupied or virtual Kohn-Sham states and thus achieves linear-scaling computational effort with system size. In contrast to conventional localised orbital formulations, where a single set of localised functions is used to span the occupied and unoccupied state manifold, we make use of two sets of in situ optimised localised orbitals, one for the occupied and one for the unoccupied space. This double representation approach avoids known problems of spanning the space of unoccupied Kohn-Sham states with a minimal set of localised orbitals optimised for the occupied space, while the in situ optimisation procedure allows for efficient calculations with a minimal number of functions. The method is applied to a number of medium sized organic molecules and a good agreement with traditional TDDFT methods is observed. Furthermore, linear scaling of computational cost with system size is demonstrated on (10,0) carbon nanotubes of different lengths. PMID:23947840

Zuehlsdorff, T J; Hine, N D M; Spencer, J S; Harrison, N M; Riley, D J; Haynes, P D

2013-08-14

263

Spatial and temporal features of density-dependent contaminant transport: experimental investigation and numerical modeling.  

PubMed

We investigate the spatial and temporal features of variable-density contaminant plumes migration in porous materials. Our analysis is supported by novel experimental results concerning concentration profiles inside a vertical column setup that has been conceived at CEA to this aim. The experimental method relies on X-ray spectrometry, which allows determining solute profiles as a function of time at several positions along the column. The salient outcomes of the measurements are elucidated, with focus on miscible fluids in homogeneous saturated media. The role of the injected solution molarity is evidenced. As molarity increases, the solutes plume transport progressively deviates from the usual Fickian behavior, and pollutants distribution becomes skewed in the direction dictated by gravity. By resorting to a finite elements approach, we numerically solve the nonlinear equations that rule the pollutants migration: a good agreement is found between the simulated profiles and the experimental data. At high molarity, a strong dependence on initial conditions is found. Finally, we qualitatively explore the (unstable) interfacial dynamics between the dense contaminant plume and the lighter resident fluid that saturates the column, and detail its evolution for finite-duration contaminant injections. PMID:19717206

Zoia, Andrea; Latrille, Christelle; Beccantini, Alberto; Cartadale, Alain

2009-07-28

264

Postnatal development of depth-dependent collagen density in ovine articular cartilage  

PubMed Central

Background Articular cartilage (AC) is the layer of tissue that covers the articulating ends of the bones in diarthrodial joints. Adult AC is characterised by a depth-dependent composition and structure of the extracellular matrix that results in depth-dependent mechanical properties, important for the functions of adult AC. Collagen is the most abundant solid component and it affects the mechanical behaviour of AC. The current objective is to quantify the postnatal development of depth-dependent collagen density in sheep (Ovis aries) AC between birth and maturity. We use Fourier transform infra-red micro-spectroscopy to investigate collagen density in 48 sheep divided over ten sample points between birth (stillborn) and maturity (72 weeks). In each animal, we investigate six anatomical sites (caudal, distal and rostral locations at the medial and lateral side of the joint) in the distal metacarpus of a fore leg and a hind leg. Results Collagen density increases from birth to maturity up to our last sample point (72 weeks). Collagen density increases at the articular surface from 0.23 g/ml ± 0.06 g/ml (mean ± s.d., n = 48) at 0 weeks to 0.51 g/ml ± 0.10 g/ml (n = 46) at 72 weeks. Maximum collagen density in the deeper cartilage increases from 0.39 g/ml ± 0.08 g/ml (n = 48) at 0 weeks to 0.91 g/ml ± 0.13 g/ml (n = 46) at 72 weeks. Most collagen density profiles at 0 weeks (85%) show a valley, indicating a minimum, in collagen density near the articular surface. At 72 weeks, only 17% of the collagen density profiles show a valley in collagen density near the articular surface. The fraction of profiles with this valley stabilises at 36 weeks. Conclusions Collagen density in articular cartilage increases in postnatal life with depth-dependent variation, and does not stabilize up to 72 weeks, the last sample point in our study. We find strong evidence for a valley in collagen densities near the articular surface that is present in the youngest animals, but that has disappeared in the oldest animals. We discuss that the retardance valley (as seen with polarised light microscopy) in perinatal animals reflects a decrease in collagen density, as well as a decrease in collagen fibril anisotropy.

2010-01-01

265

Modeling the effect of density-dependent chemical interference upon seed germination.  

PubMed

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

Sinkkonen, Aki

2006-08-08

266

Modeling the effect of density-dependent chemical interference upon seed germination.  

PubMed

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

Sinkkonen, Aki

2005-04-01

267

Nucleation in n-alkanes: A density-functional approach  

NASA Astrophysics Data System (ADS)

A density-functional theory for a polyatomic system is applied to gas-liquid nucleation in n-butane and n-heptane, employing an interaction site model and the rotational isomeric state approximation. Effects of chain length and flexibility on equilibrium properties and nucleation are discussed. It is shown that the n-alkane systems cannot be well approximated by a system with the spherically symmetric Lennard-Jones potential, giving a nucleation rate scaled by the classical rate smaller by six orders of magnitude.

Seok, Chaok; Oxtoby, David W.

1998-11-01

268

Scale-dependent shifts in the species composition of flower visitors with changing floral density.  

PubMed

Responses of flower-visiting animals to floral density can alter interactions between plants, influencing a variety of biological processes, including plant population dynamics and the evolution of flowering phenology. Many studies have found effects of floral or plant density on pollinator visitation rates at patch scales, but little is known about responses of flower visitors to floral densities at larger scales. Here, I present data from an observational field study in which I measured the effects of floral density on visitation to the annual composite Holocarpha virgata at both patch (4 m(2)) and site (12.6 ha) spatial scales. The species composition of flower visitors changed with floral density, and did so in different ways at the two scales. At the site scale, average floral density within patches of H. virgata or within patches of all summer-flowering species combined had a significant positive effect on per-flowerhead visitation by the long-horned bee Melissodes lupina and no significant effects on visitation by any other taxa. At the patch scale, per-flowerhead visitation by honeybees significantly increased whereas visitation by M. lupina often decreased with increasing floral density. For both species, responses to patch-scale floral density were strongest when site-scale floral density was high. The scale-dependence of flower visitor responses to floral density and the interactions between site- and patch-scale effects of floral density observed in this study underscore the importance of improving our understanding of pollinators' responses to floral density at population scales. PMID:22752187

Essenberg, Carla J

2012-06-30

269

Population Dynamic and Genetic Consequences of Spatial Density-Dependent Dispersal in Patchy Populations.  

PubMed

Predictions about sex-specific, spatial density-dependent dispersal and their demographic and genetic consequences were tested in experimental populations of root voles (Microtus oeconomus). Each population consisted of two demes inhabiting equal-sized habitat patches imbedded in a barren matrix area. We used a neutral two-allele allozyme marker to monitor gene flow. Initially, the two demes were genetically distinct and had different densities so that the size of a high-density deme (genotype bb) was four times larger than that of a low-density deme (genotype aa). The sex-specific dispersal pattern was in accordance with our prediction. Male dispersal was unconditional on deme-specific densities, and the majority of the first-generation males became dispersed from both demes, whereas female dispersal was strongly density dependent, so that dispersal took place exclusively from the high-density to the low-density deme. The demographic implication of this dispersal pattern was that the initial density difference between the demes was quickly canceled out. We built a mathematical model that predicted that the initially rare allele (a) would increase in frequency given the dispersal pattern, and this was supported by our experimental data. This result relies mostly on the density-independent male-dispersal strategy, which presumably stems from inbreeding avoidance. Our study highlights the importance of incorporating sex-specific dispersal strategies in population genetic models. Sex-biased dispersal may act as a deterministic force counteracting the tendency for stochastic loss of alleles in small and fragmented populations. PMID:10686164

Aars; Ims

2000-02-01

270

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

SciTech Connect

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

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

2010-04-15

271

Density dependence on tree survival in an old-growth temperate forest in northeastern China  

Microsoft Academic Search

– \\u000a \\u000a • Density dependence is a major mechanism for shaping plant communities. However, its role in regulating diverse, mixed natural\\u000a tree communities is less certain.\\u000a \\u000a \\u000a \\u000a \\u000a – \\u000a \\u000a • In this study we investigated density-dependent effects in a large-scale (25 ha) old-growth temperate forest in northeastern\\u000a China. Spatial patterns of neighborhood distribution in the plot were analyzed using various methods for inferring

Jian Zhang; Zhanqing Hao; I.-Fang Sun; Bo Song; Ji Ye; Buhang Li; Xugao Wang

2009-01-01

272

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

PubMed

The determination of state population probability within the framework of time-dependent density functional theory (TDDFT) has remained a widely open question. The aim of this study is to find out whether and how this probability can be extracted from time-dependent density, which has been used as the basic variable within TDDFT. We propose an effective method to calculate state population probabilities, which has been well validated in benchmark case studies on nonresonant (detuned) Rabi oscillations of a Na atom, Na2 dimer, and Na4 cluster irradiated by a monochromatic laser. PMID:24028103

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

2013-09-01

273

A novel combinatorial approach to high-density peptide arrays.  

PubMed

Combinatorial synthesis of peptides on solid supports (1), either as spots on cellulose membranes (2) or with split-pool-libraries on polymer beads (3), substantially forwarded research in the field of peptide-protein interactions. Admittedly, these concepts have specific limitations, on one hand the number of synthesizable peptide sequences per area, on the other hand elaborate decoding/encoding strategies, false-positive results and sequence limitations. We recently established a method to produce high-density peptide arrays on microelectronic chips (4). Solid amino acid microparticles were charged by friction and transferred to defined pixel electrodes onto the chip's surface, where they couple to a functional polymer coating simply upon melting (Fig. 16.1 A-D,F). By applying standard Fmoc chemistry according to Merrifield, peptide array densities of up to 40,000 spots per square centimetre were achieved (Fig. 16.1G). The term "Merrifield synthesis" describes the consecutive linear coupling and deprotecting of L-amino acids modified with base-labile fluorenylmethoxy (Fmoc) groups at the N-terminus and different acid-sensitive protecting groups at their side chains. Removing side chain protecting groups takes place only once at the very end of each synthesis and generates the natural peptide sequence thereby. PMID:19649602

Beyer, Mario; Block, Ines; König, Kai; Nesterov, Alexander; Fernandez, Simon; Felgenhauer, Thomas; Schirwitz, Christopher; Leibe, Klaus; Bischoff, Ralf F; Breitling, Frank; Stadler, Volker

2009-01-01

274

Time-Dependent Density Functional Theory for Open Quantum Systems and Quantum Computation  

NASA Astrophysics Data System (ADS)

First-principles electronic structure theory explains properties of atoms, molecules and solids from underlying physical principles without input from empirical parameters. Time-dependent density functional theory (TDDFT) has emerged as arguably the most widely used first-principles method for describing the time-dependent quantum mechanics of many-electron systems. In this thesis, we will show how the fundamental principles of TDDFT can be extended and applied in two novel directions: The theory of open quantum systems (OQS) and quantum computation (QC). In the first part of this thesis, we prove theorems that establish the foundations of TDDFT for open quantum systems (OQS-TDDFT). OQS-TDDFT allows for a first-principles description of non-equilibrium systems, in which the electronic degrees of freedom undergo relaxation and decoherence due to coupling with a thermal environment, such as a vibrational or photon bath. We then discuss properties of functionals in OQS-TDDFT and investigate how these differ from functionals in conventional TDDFT using an exactly solvable model system. Next, we formulate OQS-TDDFT in the linear-response regime, which gives access to environmentally broadened excitation spectra. Lastly, we present a hybrid approach in which TDDFT can be used to construct master equations from first-principles for describing energy transfer in condensed phase systems. In the second part of this thesis, we prove that the theorems of TDDFT can be extended to a class of qubit Hamiltonians that are universal for quantum computation. TDDFT applied to universal Hamiltonians implies that single-qubit expectation values can be used as the basic variables in quantum computation and information theory, rather than wavefunctions. This offers the possibility of simplifying computations by using the principles of TDDFT similar to how it is applied in electronic structure theory. Lastly, we discuss a related result; the computational complexity of TDDFT.

Tempel, David Gabriel

275

Nonadiabatic dynamics and simulation of time resolved photoelectron spectra within time-dependent density functional theory: Ultrafast photoswitching in benzylideneaniline  

SciTech Connect

We present a theoretical approach for the nonadiabatic dynamics 'on the fly' based on the combination of the time-dependent density functional theory (TDDFT) with Tully's stochastic surface hopping method. Our formulation is based on localized Gaussian basis sets and is suitable for the simulation of ultrafast processes in complex molecular systems including all degrees of freedom. Our approach is used for the simulation of time resolved photoelectron spectra in the framework of the Wigner distribution approach. In order to illustrate the scope of the method, we study the ultrafast photoswitching dynamics of the prototype Schiff base benzylideneaniline (BAN). The nonradiative lifetime of the S{sub 1} state of BAN is determined to be {approx}200 fs. The mechanism of the photoisomerization has been investigated and a connection between the time resolved photoelectron signal and the underlying nonadiabatic processes has been established.

Mitric, Roland; Werner, Ute; Bonacic-Koutecky, Vlasta [Institut fuer Chemie, Humboldt-Universitaet zu Berlin, Brook-Taylor-Strasse 2, D-12489 Berlin (Germany)

2008-10-28

276

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

NASA Astrophysics Data System (ADS)

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

Shen, G.; Horowitz, C. J.; Teige, S.

2010-07-01

277

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

SciTech Connect

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

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

2010-07-15

278

Time-dependent quasirelativistic density-functional theory based on the zeroth-order regular approximation.  

PubMed

A time-dependent quasirelativistic density-functional theory for excitation energies of systems containing heavy elements is developed, which is based on the zeroth-order regular approximation (ZORA) for the relativistic Hamiltonian and a noncollinear form for the adiabatic exchange-correlation kernel. To avoid the gauge dependence of the ZORA Hamiltonian a model atomic potential, instead of the full molecular potential, is used to construct the ZORA kinetic operator in ground-state calculations. As such, the ZORA kinetic operator no longer responds to changes in the density in response calculations. In addition, it is shown that, for closed-shell ground states, time-reversal symmetry can be employed to simplify the eigenvalue equation into an approximate form that is similar to that of time-dependent nonrelativistic density-functional theory. This is achieved by invoking an independent-particle approximation for the induced density matrix. The resulting theory is applied to investigate the global potential-energy curves of low-lying LambdaS- and omega omega-coupled electronic states of the AuH molecule. The derived spectroscopic parameters, including the adiabatic and vertical excitation energies, equilibrium bond lengths, harmonic and anharmonic vibrational constants, fundamental frequencies, and dissociation energies, are in good agreement with those of time-dependent four-component relativistic density-functional theory and ab initio multireference second-order perturbation theory. Nonetheless, this two-component relativistic version of time-dependent density-functional theory is only moderately advantageous over the four-component one as far as computational efforts are concerned. PMID:16238368

Peng, Daoling; Zou, Wenli; Liu, Wenjian

2005-10-01

279

Density-dependent habitat selection and the modeling of sperm whale ( Physeter macrocephalus ) exploitation  

Microsoft Academic Search

The monitoring and management of sperm whale (Physeter macrocephalus) populations have proved prob- lematic. Studies of living animals indicate that movements are largely determined by resource availability, thus suggest- ing that density-dependent habitat selection may be a realistic framework within which to study sperm whale populations. A model, in which animals migrate betwee n2×2 °squares at rates that depend on

Hal Whitehead

2000-01-01

280

The role of density-dependent dispersal in source–sink dynamics  

Microsoft Academic Search

I investigate two aspects of source–sink theory that have hitherto received little attention: density-dependent dispersal and the cost of dispersal to sources. The cost arises because emigration reduces the per capita growth rate of sources, thus predisposing them to extinction. I show that source–sink persistence depends critically on the interplay between these two factors. When the emigration rate increases with

Priyanga Amarasekare

2004-01-01

281

Nuclear matter symmetry energy from generalized polarizabilities: Dependences on momentum, isospin, density, and temperature  

Microsoft Academic Search

Symmetry energy terms from macroscopic mass formulas are investigated as generalized polarizabilities of nuclear matter. Besides the neutron-proton (n-p) symmetry energy, the spin-dependent symmetry energies and a scalar one are also defined. They depend on the nuclear densities (rho), neutron-proton asymmetry (b), temperature (T), and exchanged energy and momentum (q). Based on a standard expression for the generalized polarizabilities, a

Fábio L. Braghin

2005-01-01

282

Nuclear matter symmetry energy from generalized polarizabilities: Dependences on momentum, isospin, density, and temperature  

Microsoft Academic Search

Symmetry energy terms from macroscopic mass formulas are investigated as generalized polarizabilities of nuclear matter. Besides the neutron-proton (n-p) symmetry energy, the spin-dependent symmetry energies and a scalar one are also defined. They depend on the nuclear densities (ρ), neutron-proton asymmetry (b), temperature (T), and exchanged energy and momentum (q). Based on a standard expression for the generalized polarizabilities, a

Fábio L. Braghin; Fabio L

2005-01-01

283

Data-Driven Bandwidth Choice for Density Estimation Based on Dependent Data  

Microsoft Academic Search

The bandwidth selection problem in kernel density estimation is investigated in situations where the observed data are dependent. The classical leave-out technique is extended, and thereby a class of cross-validated bandwidths is defined. These bandwidths are shown to be asymptotically optimal under a strong mixing condition. The leave-one out, or ordinary, form of cross-validation remains asymptotically optimal under the dependence

Jeffrey D. Hart; Philippe Vieu

1990-01-01

284

Time-dependent density-functional calculation of the stopping power for protons and antiprotons in metals  

NASA Astrophysics Data System (ADS)

Time-dependent density-functional theory is used to calculate the energy loss of antiprotons and protons traversing metal clusters of variable size. We find that the effective energy loss per unit path length inside the cluster shows no significant cluster size effects over the wide range of projectile velocities studied. This allows us to compare the calculated stopping power with the experimental values for a solid metal target. Excellent agreement between the theoretical results and recent experimental data is found for velocities below the inner-shell excitation threshold. We thus present a nonperturbative quantum-mechanical approach to obtain the energy loss of charges in solids.

Quijada, M.; Borisov, A. G.; Nagy, I.; Muiño, R. Díez; Echenique, P. M.

2007-04-01

285

Effects of spinor distortion and density-dependent form factors upon $^{16}O(\\\\vec{e},e'\\\\vec{p})$  

Microsoft Academic Search

We propose an effective current operator for nucleon electromagnetic knockout\\u000athat incorporates spinor distortion and density-dependent nucleon form factors\\u000ausing an effective momentum approximation. This method can be used in a\\u000acoordinate-space approach with either relativistic or nonrelativistic optical\\u000apotentials and overlap functions. We studied these effects for the\\u000a$^{16}O(\\\\vec{e},e' \\\\vec{p})$ reaction at Q^2 = 0.8 (GeV\\/c)^2. Spinor distortion\\u000asubstantially

James J. Kelly

1999-01-01

286

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.

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

2013-01-01

287

Native birds and alien insects: spatial density dependence in songbird predation of invading oak gallwasps.  

PubMed

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

2013-01-14

288

Density-dependent sorting of physiologically different cells of Vibrio parahaemolyticus.  

PubMed

A pure bacterial culture is composed of clonal cells in different physiological states. Separation of those subpopulations is critical for further characterization and for understanding various processes in the cultured cells. We used density-dependent cell sorting with Percoll to separate subpopulations from cultures of a marine bacterium, Vibrio parahaemolyticus. Cells from cultures in the exponential and stationary phases were fractionated according to their buoyant density, and their culturability and ability to maintain culturability under low-temperature and low-nutrient stress (stress resistance) were determined. The buoyant density of the major portion of the cells decreased with culture age. The culturability of stationary-phase cells increased with increasing buoyant density, but that of exponential-phase cells did not. Stress resistance decreased with increasing buoyant density regardless of the growth phase. The results indicate that density-dependent cell sorting is useful for separating subpopulations of different culturabilities and stress resistances. We expect that this method will be a powerful tool for analyzing cells in various physiological states, such as the viable but nonculturable state. PMID:12788764

Nishino, Tomohiko; Nayak, Binaya B; Kogure, Kazuhiro

2003-06-01

289

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

290

Density-Gradient Theory: A Macroscopic Approach to Quantum Confinement and Tunneling in Semiconductor Devices.  

National Technical Information Service (NTIS)

Density-gradient theory provides a macroscopic approach to modeling quantum transport that is particularly well adapted to semiconductor device analysis and engineering. After some introductory observations, the basis of the theory in macroscopic and micr...

M. G. Ancona

2011-01-01

291

The Nuclear Energy Density Functionals with Modified Radial Dependence of the Isoscalar Effective Mass  

NASA Astrophysics Data System (ADS)

Calculations for infinite nuclear matter with realistic nucleon-nucleon interactions suggest that the isoscalar effective mass (IEM) of a nucleon at the saturation density equals m*/m 0.8 ± 0.1, at variance with empirical data on the nuclear level density in finite nuclei which are consistent with m*/m ? 1. This contradicting results might be reconciled by enriching the radial dependence of IEM. In this work four new terms are introduced into the Skyrme-force inspired local energy-density functional: ?(??)2, ? {{d? }}/{{dr}}, ?2 and ???. The aim is to investigate how they influence the radial dependence of IEM and, in turn, the single-particle spectra.

Zalewski, M.; Olbratowski, P.; Satu?a, W.

292

Magneto-optical rotation of nonmonochromatic fields and its nonlinear dependence on optical density  

SciTech Connect

We calculate magneto-optical rotation of nonmonochromatic fields in an optically thick cold atomic medium. We show that the nonmonochromatic nature leads to a nonlinear dependence of the rotation angle on optical density. Using our calculations, we provide a quantitative analysis of the recent experimental results of Labeyrie et al. [Phys. Rev. A 64, 033402 (2001)] using cold {sup 85}Rb atoms.

Agarwal, G. S.; Dasgupta, Shubhrangshu [Physical Research Laboratory, Navrangpura, Ahmedabad-380 009, (India)

2003-06-01

293

Density-dependent energy use contributes to the self-thinning relationship of cohorts.  

PubMed

In resource-limited populations, an increase in average body size can occur only with a decline in abundance. This is known as self-thinning, and the decline in abundance in food-limited populations is considered proportional to the scaling of metabolism with body mass. This popular hypothesis may be inaccurate, because self-thinning populations can also experience density-dependent competition, which could alter their energy use beyond the predictions of metabolic scaling. This study tested whether density-dependent competition has an energetic role in self-thinning, by manipulating the abundance of the fish Macquaria novemaculeata and tank size to partition the effects of competition from metabolic scaling. We found that self-thinning can be density dependent and that changes in intraspecific competition may be more influential than metabolic scaling on self-thinning relationships. The energetic mechanism we propose is that density-dependent competition causes variation in the allocation of energy to growth, which alters the energetic efficiency of self-thinning cohorts. The implication is that food-limited cohorts and populations with competitive strategies that encourage fast-growing individuals will have less body mass at equilibrium and higher mortality rates. This finding sheds light on the processes structuring populations and can be used to explain inconsistencies in the mass-abundance scaling of assemblages and communities (the energetic-equivalence rule). PMID:23448883

Smith, James A; Baumgartner, Lee J; Suthers, Iain M; Fielder, D Stewart; Taylor, Matthew D

2013-01-18

294

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

295

Phase-Dependent Superconductive Current Density Superconductor - Normal Metal - Superconductor Junction at T = T Sub Cn.  

National Technical Information Service (NTIS)

A general equation for the phase-dependent supercurrent density across the weak-link is obtained, from which one finds that J max at T = T sub CN decreases very slowly with d sub n when compared with J max at T sub CS. (Author)

R. H. T. Yeh H. Mechetti

1968-01-01

296

Adaptation, density dependence and the responses of trophic level abundances to mortality  

Microsoft Academic Search

We use simple models to examine how the abundances of three trophic levels change in response to mortality imposed on each one of the levels. The models contain two factors whose joint effects have not been incorporated into previous analyses: direct density dependence ('self-damping') at trophic levels above the bottom level, and adaptive change on the middle trophic level. The

P. A. Abrams; M. Vos

2003-01-01

297

Stochastic seasonality and nonlinear density-dependent factors regulate population size in an African rodent  

Microsoft Academic Search

Ecology has long been troubled by the controversy over how populations are regulated,. Some ecologists focus on the role of environmental effects, whereas others argue that density-dependent feedback mechanisms are central. The relative importance of both processes is still hotly debated, but clear examples of both processes acting in the same population are rare,. Key-factor analysis (regression of population changes

Herwig Leirs; Nils Chr. Stenseth; James D. Nichols; James E. Hines; Ron Verhagen; Walter Verheyen

1997-01-01

298

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

Microsoft Academic Search

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

K. J. Gaston; J. H. Lawton

1987-01-01

299

Herbivore-specific, density-dependent induction of plant volatiles: Honest or “cry wolf” signals?  

Microsoft Academic Search

Plants release volatile chemicals upon attack by herbivorous arthropods. They do so commonly in a dose-dependent manner: the more herbivores, the more volatiles released. The volatiles attract predatory arthropods and the amount determines the probability of predator response. We show that seedlings of a cabbage variety (Brassica oleracea var. capitata, cv Shikidori) also show such a response to the density

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

2010-01-01

300

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

301

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

302

Environmentally-controlled, density-dependent secondary dispersal in a local estuarine crab population  

Microsoft Academic Search

The mechanisms driving the pelagic secondary dispersal of aquatic organisms following initial settlement to benthic habitats are poorly characterized. We examined the physical environmental (wind, diel cycle, tidal phase) and biological (ontogenetic, density-dependent) factors that contribute to the secondary dispersal of a benthic marine invertebrate, the blue crab ( Callinectes sapidus) in Pamlico Sound, NC, USA. Field studies conducted in

Nathalie B. Reyns; David B. Eggleston

2004-01-01

303

Analytical solutions to the Navier-Stokes equations with density-dependent viscosity and with pressure  

NASA Astrophysics Data System (ADS)

This article is the continued version of the analytical solutions for the pressureless Navier-Stokes equations with density-dependent viscosity [Yuen, M. W., ``Analytical solutions to the Navier-Stokes equations,'' J. Math. Phys. 49, 113102 (2008)]. We are able to extend the similar solution structure to the case with pressure under some restriction to the constants ? and ?.

Yeung, Ling Hei; Manwai, Yuen

2009-08-01

304

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

Microsoft Academic Search

We calculate the equation of state (EOS) of dense matter using a relativistic mean field (RMF) model with a density dependent coupling that is a slightly modified form of the original NL3 interaction. For nonuniform nuclear matter we approximate the unit lattice as a spherical Wigner-Seitz cell, wherein the meson mean fields and nucleon Dirac wave functions are solved fully

G. Shen; C. J. Horowitz; S. Teige

2010-01-01

305

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...

306

Density dependence of trace tritium transport in H-mode Joint European Torus plasma  

NASA Astrophysics Data System (ADS)

Tritium transport in edge localized mode (ELM) high confinement (H-mode) plasmas is analyzed here as a function of density for discharges from the recent trace tritium experimental campaign performed on Joint European Torus. In this campaign small amounts of tritium have been puffed or injected (with neutral beam injectors) into deuterium plasmas [K.-D. Zastrow, J. M. Adams, Yu. Baranov et al., Plasma Phys. Controlled Fusion 46, B255 (2004)]. Information about the tritium has been obtained from the evolution of the profiles of neutron emission simulated via the TRANSP [R. J. Goldston, D. C. McCune, H. H. Towner, S. L. Davis, R. J. Hawryluk, and G. L. Schmidt, J. Comput. Phys. 43, 61 (1981)] and SANCO (L. Lauro-Taroni, B. Alper, R. Giannella, K. Lawson, F. Marcus, M. Mattioli, P. Smeulders, and M. Von Hellermann, Proceedings of the 21st European Conference on Controlled Fusion and Plasma Physics, Montpelier, France, 1994) codes. A strong inverse correlation of tritium transport with plasma density is found in this analysis. The low tritium transport at high density is close to neoclassical values while the transport becomes strongly anomalous in low density plasmas. The thermal transport does not exhibit such a strong density dependence, leading to a varying ratio of thermal to tritium transport in these discharges. An interpretation of the density effects on the trace tritium transport, partially based on the test particle simulations in plasmas with stochastic magnetic field, is proposed. A simple model for the tritium diffusion coefficient and convective velocity, which includes the modification of the neoclassical particle diffusion in presence of electromagnetic turbulence [A. I. Smolyakov and P. N. Yushmanov, Nucl. Fusion 35, 383 (1993)] completed with an empirical density dependence, is developed. This model has positive ? dependence in agreement with the results of the similarity experiments performed for trace tritium transport.

Voitsekhovitch, I.; Garbet, X.; McDonald, D. C.; Zastrow, K.-D.; Adams, M.; Baranov, Yu.; Belo, P.; Bertalot, L.; Budny, R.; Conroy, S.; Cordey, J. G.; Garzotti, L.; Mantica, P.; McCune, D.; Ongena, J.; Parail, V.; Popovichev, S.; Stork, D.; Whiteford, A. D.

2005-05-01

307

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

2007-03-14

308

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.

Schroder, Arne

2013-01-01

309

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

PubMed

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

2013-10-04

310

Diameter-dependent surface photovoltage and surface state density in single semiconductor nanowires.  

PubMed

Based on single-nanowire surface photovoltage measurements and finite-element electrostatic simulations, we determine the surface state density, N(s), in individual n-type ZnO nanowires as a function of nanowire diameter. In general, N(s) increases as the diameter decreases. This identifies an important origin of the recently reported diameter dependence of the surface recombination velocity, which has been commonly considered to be independent of the diameter. Furthermore, through the determination of the surface carrier lifetime, we suggest that the diameter dependence of the surface state density accounts for the rather abrupt transition from bulk-limited to surface-limited carrier transport over a narrow nanowire diameter regime (~30-40 nm). These findings are supported by the comparison between bulk-limited and surface-dependent minority carrier diffusion lengths measured at various diameters. PMID:22985208

Soudi, Afsoon; Hsu, Cheng-Han; Gu, Yi

2012-09-20

311

Vertebrate herbivory impacts seedling recruitment more than niche partitioning or density-dependent mortality.  

PubMed

In tropical forests, resource-based niches and density-dependent mortality are mutually compatible mechanisms that can act simultaneously to limit seedling populations. Differences in the strengths of these mechanisms will determine their roles in maintaining species coexistence. In the first assessment of these mechanisms in a Congo Basin forest, we quantified their relative strengths and tested the extent to which density-dependent mortality is driven by the distance-dependent behavior of seed and seedling predators predicted by the Janzen-Connell hypothesis. We conducted a large-scale seed addition experiment for five randomly selected tropical tree species, caging a subset of seed addition quadrats against vertebrate predators. We then developed models to assess the mechanisms that determine seedling emergence (three months after seed addition) and survival (two years after seed addition). As predicted, both niche differentiation and density-dependent mortality limited seedling recruitment, but predation had the strongest effects on seedling emergence and survival. Seedling species responded differently to naturally occurring environmental variation among sites, including variation in light levels and soil characteristics, supporting predictions of niche-based theories of tropical tree species coexistence. The addition of higher densities of seeds into quadrats initially led to greater seedling emergence, but survival to two years decreased with seed density. Seed and seedling predation reduced recruitment below levels maintained by density-dependent mortality, an indication that predators largely determine the population size of tree seedlings. Seedling recruitment was unrelated to the distance to or density of conspecific adult trees, suggesting that recruitment patterns are generated by generalist vertebrate herbivores rather than the specialized predators predicted by the Janzen-Connell hypothesis. If the role of seed and seedling predation in limiting seedling recruitment is a general phenomenon, then the relative abundances of tree species might largely depend on species-specific adaptations to avoid, survive, and recover from damage induced by vertebrate herbivores. Likewise, population declines of herbivorous vertebrate species (many of which are large and hunted) may trigger shifts in species composition of tropical forests. PMID:22624210

Clark, C J; Poulsen, J R; Levey, D J

2012-03-01

312

Augmenting basis set for time-dependent density functional theory calculation of excitation energies: Slater-type orbitals for hydrogen to krypton  

Microsoft Academic Search

Attention is given to the dual nature of the time-dependent density functional theory approach for predicting excitation spectra, namely its theoretical basis in dynamic polarizability and its practical implementation in computer programs as a single configuration interaction. A procedure for generating diffuse functions to be added to standard Slater-type orbital basis sets for H to Kr is proposed and tested

D. P. Chong

2005-01-01

313

Photopyroelectric Characterization of Industrial Polymers: Temperature Dependent Thermal Diffusivity of Packaging Foils Manufactured from Low Density Polyethylene (LDPE) and Polyvinyl Chloride (PVC)  

Microsoft Academic Search

The temperature dependence of thermal diffusivity of the low density polyethylene (LDPE) and polyvinyl chloride (PVC) packaging foils was determined using the photopyroelectric (PPE) approach in the standard configuration. Glassy phase transitions observed in foils within the -30°C to 70°C range cause a 7 to 20% increase in values of thermal diffusivity for these test materials. The results obtained by

Jan Paul Favier; Dorin Dadarlat; Dane Bicanic; Klaas Jan Riezebos; Cornelius Van Den Berg; Edo Gerkema

1999-01-01

314

Native defects in oxide semiconductors: a density functional approach.  

PubMed

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

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

2010-09-07

315

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

NASA Astrophysics Data System (ADS)

We explore the effects of the deflagration to detonation transition (DDT) density on the production of 56Ni 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 56Ni 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 56Ni and NSE material synthesized for a suite of simulated explosions in which the transition density is varied in the range (1-3) ×107 g cm-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 sun for a 1 Z sun increase in metallicity evaluated about solar metallicity. For the same change in metallicity, this result translates to a 0.067 ± 0.004 M sun decrease in the 56Ni 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.; Townsley, Dean M.; Chamulak, David A.; Brown, Edward F.; Timmes, F. X.

2010-09-01

316

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

317

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

318

Benzodiazepines: Dependence and a Therapeutic Approach to Gradual Withdrawal  

PubMed Central

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

Blais, Diane; Petit, Louise

1990-01-01

319

Time-dependent, lattice approach to atomic collisions  

SciTech Connect

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

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

1995-12-31

320

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

321

Quantum Well Width Dependence of Threshold Current Density in InGaN Lasers  

SciTech Connect

The quantum confined Stark effect was found to result in a strong quantum well width dependence of threshold current density in strained group-III nitride quantum well lasers. For an In{sub 0.2}Ga{sub 0.8}N/GaN structure with quantum well width in the neighborhood of 3.5nm, our analysis shows that the reduction in spontaneous emission loss by the electron-hole spatial separation outweighs the corresponding reduction in gain to produce a threshold current density minimum.

Amano, H.; Chow, W.W.; Han, J.; Takeuchi, T.

1999-03-16

322

Density-Dependent Reorientation and Rehybridization of Chemisorbed Conjugated Molecules for Controlling Interface Electronic Structure  

NASA Astrophysics Data System (ADS)

The adsorption of the molecular acceptor hexaazatriphenylene-hexacarbonitrile on Ag(111) was investigated as function of layer density. We find that the orientation of the first molecular layer changes from a face-on to an edge-on conformation depending on layer density, facilitated through specific interactions of the peripheral molecular cyano groups with the metal. This is accompanied by a rehybridization of molecular and metal electronic states, which significantly modifies the interface and surface electronic properties, as rationalized by theoretical modeling.

Bröker, B.; Hofmann, O. T.; Rangger, G. M.; Frank, P.; Blum, R.-P.; Rieger, R.; Venema, L.; Vollmer, A.; Müllen, K.; Rabe, J. P.; Winkler, A.; Rudolf, P.; Zojer, E.; Koch, N.

2010-06-01

323

Density dependence of quark masses and stability of color-flavor locked phases  

SciTech Connect

Considering the density dependence of quark masses, we investigate the color-flavor-locked matter and its stability relative to (unpaired) strange quark matter. We find that, when the current mass of strange quark m{sub s} is small, the strange quark matter remains stable for moderate baryon densities. When m{sub s} is large, the gapless phase of the color-flavor-locked matter is found to be difficult to be stable. A schematic phase diagram of three-flavor quark matter is presented, in which the color-flavor-locked phase region is suppressed in comparison with the previous results.

Zhang Xiaobing; Li Xueqian [Department of Physics, Nankai University, Tianjin 300071 (China)

2005-09-01

324

Reexamining the neutron skin thickness within a density dependent hadronic model  

NASA Astrophysics Data System (ADS)

In the present work, we calculate the Pb208 neutron skin thickness, binding energy, surface energy, and density profiles within the Dirac solution of a density dependent hadronic model. The same calculation is performed with the NL3 parametrization of the nonlinear Walecka model. The asymmetry of a polarized electron scattered from a heavy target is also obtained within a partial wave expansion calculation. The theoretical results are then ready to be compared with the experimental results expected to be available very soon at the Thomas Jefferson National Accelerator Facility. For completeness, other nuclei such as Ca40, Ca48, Ni66, and Zr90 are also investigated.

Avancini, S. S.; Marinelli, J. R.; Menezes, D. P.; Moraes, M. M. W.; Schneider, A. S.

2007-12-01

325

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

326

A New Approach for Analyzing Bird Densities from Variable Circular-Plot Counts  

Microsoft Academic Search

2 ABSTRACT: An approach for calculating bird densities from variable circular -plot counts is described. The approach differs from previous methods in that data from several surveys are pooled and detection distances are adjusted as if all distances were recorded by a single observer under a given set of field conditions. Adjustments for covariates that affect detection distances such as

STEVEN G. FANCY

1997-01-01

327

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

Microsoft Academic Search

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

Jeffrey S. Shima

2001-01-01

328

Dynamical quantum-electrodynamics embedding: Combining time-dependent density functional theory and the near-field method  

NASA Astrophysics Data System (ADS)

We develop an approach for dynamical (? > 0) embedding of mixed quantum mechanical (QM)/classical (or more precisely QM/electrodynamics) systems with a quantum sub-region, described by time-dependent density functional theory (TDDFT), within a classical sub-region, modeled here by the recently proposed near-field (NF) method. Both sub-systems are propagated simultaneously and are coupled through a common Coulomb potential. As a first step we implement the method to study the plasmonic response of a metal film which is half jellium-like QM and half classical. The resulting response is in good agreement with both full-scale TDDFT and the purely classical NF method. The embedding method is able to describe the optical response of the whole system while capturing quantum mechanical effects, so it is a promising approach for studying electrodynamics in hybrid molecules-metals nanostructures.

Gao, Yi; Neuhauser, Daniel

2012-08-01

329

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

PubMed

A combined linear-response-frozen electron-density model has been implemented in a molecular-dynamics scheme derived from an extended Lagrangian formalism. This approach is based on a partition of the electronic charge distribution into a frozen region described by Kim-Gordon theory [J. Chem. Phys. 56, 3122 (1972); J. Chem. Phys. 60, 1842 (1974)] and a response contribution determined by the instantaneous ionic configuration of the system. The method is free from empirical pair potentials and the parametrization protocol involves only calculations on properly chosen subsystems. We apply this method to a series of alkali halides in different physical phases and are able to reproduce experimental structural and thermodynamic properties with an accuracy comparable to Kohn-Sham density-functional calculations. PMID:16229560

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

2005-08-15

330

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

NASA Astrophysics Data System (ADS)

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

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

2009-08-01

331

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

NASA Astrophysics Data System (ADS)

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 (?/?? >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.

2012-05-01

332

PTPN14 is required for the density-dependent control of YAP1.  

PubMed

Through an shRNA-mediated loss-of-function screen, we identified PTPN14 as a potential tumor suppressor. PTPN14 interacts with yes-associated protein 1 (YAP1), a member of the hippo signaling pathway. We showed that PTPN14 promotes the nucleus-to-cytoplasm translocation of YAP1 during contact inhibition and thus inhibits YAP1 transactivation activity. Interestingly, PTPN14 protein stability was positively controlled by cell density. We identified the CRL2(LRR1) (cullin2 RING ubiquitin ligase complex/leucine-rich repeat protein 1) complex as the E3 ligase that targets PTPN14 for degradation at low cell density. Collectively, these data suggest that PTPN14 acts to suppress cell proliferation by promoting cell density-dependent cytoplasmic translocation of YAP1. PMID:22948661

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

2012-09-01

333

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

334

Degradation of Diethyl Phthalate by Sono-Fenton Process and its Dependence on the Power Density  

NASA Astrophysics Data System (ADS)

The first-order rate constants of diethyl phthalate (DEP) degradation by sonochemical, Fenton and sono-Fenton processes were compared, and the synergistic effect of the combined sono-Fenton process and the dependence of the degradation rate on the power density (80, 180, and 330 W L-1) were investigated. The rate constants for the individual sonochemical and Fenton processes were 10-3 to 10-2 and 1.02 × 10-2 min-1, respectively. The synergy from the combined process increased significantly as the applied power density increased, and it was 1.97 with the input of ultrasonic energy of 330 W L-1. This increase was attributed to the higher efficiency of hydroxyl radical production from several pathways. The relationship between the degradation rate constant and power density was linear because the cavitation rate increased with increasing power level of the applied ultrasound.

Hwang, Anna; Na, Seungmin; Ha, Jeongsook; Khim, Jeehyeong

2011-07-01

335

Remarks on time-dependent [current]-density functional theory for open quantum systems.  

PubMed

Time-dependent [current]-density functional theory for open quantum systems (OQS) has emerged as a formalism that can incorporate dissipative effects in the dynamics of many-body quantum systems. Here, we review and clarify some formal aspects of these theories that have been recently questioned in the literature. In particular, we provide theoretical support for the following conclusions: (1) contrary to what we and others had stated before, within the master equation framework, there is in fact a one-to-one mapping between vector potentials and current densities for fixed initial state, particle-particle interaction, and memory kernel; (2) regardless of the first conclusion, all of our recently suggested Kohn-Sham (KS) schemes to reproduce the current and particle densities of the original OQS, and in particular, the use of a KS closed driven system, remains formally valid; (3) the Lindblad master equation maintains the positivity of the density matrix regardless of the time-dependence of the Hamiltonian or the dissipation operators; (4) within the stochastic Schrödinger equation picture, a one-to-one mapping from stochastic vector potential to stochastic current density for individual trajectories has not been proven so far, except in the case where the vector potential is the same for every member of the ensemble, in which case, it reduces to the Lindblad master equation picture; (5) master equations may violate certain desired properties of the density matrix, such as positivity, but they remain as one of the most useful constructs to study OQS when the environment is not easily incorporated explicitly in the calculation. The conclusions support our previous work as formally rigorous, offer new insights into it, and provide a common ground to discuss related theories. PMID:23787804

Yuen-Zhou, Joel; Aspuru-Guzik, Alán

2013-08-14

336

Postcatastrophe population dynamics and density dependence of an endemic island duck  

USGS Publications Warehouse

Laysan ducks (Anas laysanensis) are restricted to approximately 9 km2 in the Northwestern Hawaiian Islands, USA. To evaluate the importance of density dependence for Laysan ducks, we conducted a Bayesian analysis to estimate the parameters of a Gompertz model and the magnitude of process variation and observation error based on the fluctuations in Laysan duck abundance on Laysan Island from 1994 to 2007. This model described a stationary distribution for the population at carrying capacity that fluctuates around a long-term mean of 456 ducks and is between 316 to 636 ducks 95% of the time. This range of expected variability can be used to identify changes in population size that warn of catastrophic events. Density-dependent population dynamics may explain the recovery of Laysan duck from catastrophic declines and allow managers to identify population monitoring thresholds.

Seavy, N.E.; Reynolds, M.H.; Link, W. A.; Hatfield, J.S.

2009-01-01

337

Density  

NSDL National Science Digital Library

Density is a property of materials included in the National Science Education Standards Physical Science Content Standard B. It is a property by which mixtures can be separated but has much more profound applications outside the classroom such as rock formation, severe weather and living systems. But none of these concepts are fully comprehensible without a fundamental conceptual understanding of density.The resources here provide examples designed to help you facilitate student acquisition of a conceptual understanding of density.

University, Staff A.

2008-03-07

338

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.

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

2010-01-01

339

Dependence of the LR-115 radon detector calibration factor on track density.  

PubMed

The reliability and accuracy of the methodology based on using LR-115 track detectors for radon measurements have been studied by determining the dependence of their calibration factors on radon exposure at levels reaching 13 MBq m(-3) h. This factor results not constant and demonstrated a decreasing exponential trend vs. exposure that has been explained in terms of the saturation effect and verified using a numerical simulation. This dependence does not affect the parameter that normalizes track density vs. film thickness. This parameter results constant and equal to - 0.30±0.02 cm(-2)/µm in the 300-8000 kBq h m(-3) exposure range. PMID:23694685

De Cicco, F; Pugliese, M; Roca, V; Sabbarese, C

2013-04-25

340

Multi-configuration time-dependent density-functional theory based on range separation  

NASA Astrophysics Data System (ADS)

Multi-configuration range-separated density-functional theory is extended to the time-dependent regime. An exact variational formulation is derived. The approximation, which consists in combining a long-range Multi-Configuration-Self-Consistent Field (MCSCF) treatment with an adiabatic short-range density-functional (DFT) description, is then considered. The resulting time-dependent multi-configuration short-range DFT (TD-MC-srDFT) model is applied to the calculation of singlet excitation energies in H2, Be, and ferrocene, considering both short-range local density (srLDA) and generalized gradient (srGGA) approximations. As expected, when modeling long-range interactions with the MCSCF model instead of the adiabatic Buijse-Baerends density-matrix functional as recently proposed by Pernal [J. Chem. Phys. 136, 184105 (2012)], the description of both the 11D doubly-excited state in Be and the 11?u+ state in the stretched H2 molecule are improved, although the latter is still significantly underestimated. Exploratory TD-MC-srDFT/GGA calculations for ferrocene yield in general excitation energies at least as good as TD-DFT using the Coulomb attenuated method based on the three-parameter Becke-Lee-Yang-Parr functional (TD-DFT/CAM-B3LYP), and superior to wave-function (TD-MCSCF, symmetry adapted cluster-configuration interaction) and TD-DFT results based on LDA, GGA, and hybrid functionals.

Fromager, Emmanuel; Knecht, Stefan; Jensen, Hans Jørgen Aa.

2013-02-01

341

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

2012-09-12

342

Parametric dependence of the density of specularly reflected ions at quasi-perpendicular collisionless shocks  

Microsoft Academic Search

Two simple analytical models, based on the Rankine-Hugoniot conservation laws are presented and used to investigate the dependence of the number density of ions specularly reflected at quasi-perpendicular shocks upon four different upstream parameters. The fraction of incident ions reflected is found to increase with the angle between the direction of the upstream magnetic field and the shock normal, theta-Bn,

W. P. Wilkinson; S. J. Schwartz

1990-01-01

343

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

344

Small low-density lipoprotein particles and endothelium-dependent vasodilation in postmenopausal women  

Microsoft Academic Search

Low concentrations of estrogen may decrease endothelial function in postmenopausal women. Elevated plasma triglycerides after menopause are frequently associated with a small, dense low-density lipoprotein (LDL) phenotype. Small LDL particles that are more susceptible to oxidation can also inhibit endothelium-dependent vasodilation. The purpose of the present study was to investigate whether hypertriglyceridemia-induced small LDL particles are associated with endothelial dysfunction

Akihiko Wakatsuki; Nobuo Ikenoue; Koichi Shinohara; Kazushi Watanabe; Takao Fukaya

2004-01-01

345

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

Microsoft Academic Search

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

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

2000-01-01

346

Time-dependent density functional theory study of squaraine dye-sensitized solar cells  

Microsoft Academic Search

We employ time-dependent density functional theory to study the optical properties of a squaraine sensitized TiO2 system, as a model for the corresponding dye-sensitized solar cell. The all-organic squaraine dye is particularly promising for light absorption in the red spectral region. We compute the photoabsorption spectrum of a periodic TiO2 slab exposing anatase (101) surfaces, both for the clean slab

Dario Rocca; Ralph Gebauer; Filippo De Angelis; Mohammad K. Nazeeruddin; Stefano Baroni

2009-01-01

347

Population density-dependent metal tolerance: One possible basis and its ecological implications  

Microsoft Academic Search

A population density-dependent copper (Cu) resistance mechanism in a gram-negative soil bacterium, strain TDCd1, was shown to be inducible and was accompanied by changes in the protein composition of the outer membrane of the cell envelope. Characteristically, following inoculation of TDCd1 into Cu-supplemented growth media, there was a period of growth inhibition during which the number of individuals tolerant to

Trevor DtLxbury; Roslyn McIntyre

1989-01-01

348

Measuring Time Dependence of Dark Energy Density from Type IA Supernova Data  

Microsoft Academic Search

Observations of high-redshift supernovae imply an accelerating universe that can only be explained by an unusual energy component such as vacuum energy or quintessence. To assess the ability of current and future supernova data to constrain the properties of the dark energy, we allow its density to have arbitrary time dependence, rhoX(z). This leads to an equation of state for

Yun Wang; Peter M. Garnavich

2001-01-01

349

Retinol induces density-dependent growth inhibition and changes in glycolipids and LETS  

Microsoft Academic Search

CHEMICAL carcinogenesis and viral transformation are prevented or inhibited by vitamin A or its analogues1,2. Vitamin A may also play a part in membrane glycosylation as a lipid intermediate3,4. Recently, effects of retinoic acid on proliferation and density-dependent growth of mouse L cells5 and many other untransformed and transformed cells in vitro have been described6. In view of these observations,

Leonard M. Patt; Koichi Itaya; Sen-Itiroh Hakomori

1978-01-01

350

Theoretical Calculations on ?-DECAY Half-Lives by the Density-Dependent Cluster Model  

NASA Astrophysics Data System (ADS)

The theoretical calculations of ?-decay half-lives for three kinds of ?-emitters (even-even, even-odd, odd-odd nuclei) are systematically reviewed within the framework of the density-dependent cluster model (DDCM). The half-lives of three typical ?-emitters (8Be, 212Po, and 270Ds) are discussed in detail. The good agreement between experimental and theoretical results shows that DDCM is applicable in the whole mass table from very light nuclei to heavy and superheavy ones.

Ren, Zhongzhou; Xu, Chang

351

Filler-content Dependence of Dielectric Properties of Low-Density Polyethylene\\/MgO Nanocomposites  

Microsoft Academic Search

This paper reports measurement results of instrumental analyses, permittivity, conduction current, and space charge distribution profiles observed in low-density polyethylene\\/MgO nanocomposites with various filler contents, done with the intention to clarify the filler-content dependence of dielectric properties. The permittivity shows the lowest value in the sample with 1-phr fillers and then increases monotonically with an increase in filler content. The

Toshiaki Kikuma; Norikazu Fuse; Toshikatsu Tanaka; Yoshinao Murata; Yoshimichi Ohki

2006-01-01

352

Density-dependent eÄects on tree survival in an old- growth Douglas fir forest  

Microsoft Academic Search

Summary 1 We mapped the locations of live and dead trees in a large forest plot dominated by pioneer Douglas fir (Pseudotsuga menziesii) with an understorey of the invading late-successional species western hemlock (Tsuga heterophylla) and western red cedar (Thuja plicata) on Vancouver Island, British Columbia, Canada, to test for intra- and interspecific density-dependent eÄects on tree survival. 2 We

FANGLIANG HE; RICHARD P. DUNCAN

353

Population dynamics of two marine polychaetes: the relative role of density dependence, predation, and winter conditions  

Microsoft Academic Search

Van der Meer, J., Beukema, J. J., and Dekker, R. 2000. Population dynamics of two marine polychaetes: the relative role of density dependence, predation, and winter conditions. - ICES Journal of Marine Science, 57: 1488-1494. We modelled the population dynamics of two polychaete species, a prey (Scoloplos armiger) and its predator (Nephtys hombergii) by simple linear and non-linear time-series models

J VANDERMEER; Jan J. Beukema; Rob Dekker

2000-01-01

354

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

355

Geographical variation in the influence of density dependence and climate on the recruitment of Norwegian moose  

Microsoft Academic Search

The effects of variation in climate on population dynamics are likely to differ within the distributional range of a species,\\u000a yet the consequences of such regional variation on demography and population dynamics are rarely considered. Here we examine\\u000a how density dependence and different climate variables affect spatio-temporal variation in recruitment rates of Norwegian\\u000a moose using data collected over a large

Vidar Grøtan; Bernt-Erik Sæther; Magnar Lillegård; Erling J. Solberg; Steinar Engen

2009-01-01

356

Green sea turtles: a discrete simulation of density-dependent population regulation.  

PubMed

Field data on the nesting of the green sea turtle were used to construct a stochastic model. This model was simulated by use of a digital computer language simscript (Monte Carlo) to determine the relation between the percentage of nests destroyed and the size of the turtle population. Nest destruction is dependent on population density and provides a mechanism to regulate population size. PMID:17737318

Bustard, H R; Tognetti, K P

1969-02-28

357

Relativistic mean-field theory and a density-dependent spin-orbit Skyrme force  

SciTech Connect

We generalize the Skyrme force by making its spin-orbit component density dependent. One of the two extra parameters thereby introduced is fixed by performing Hartree-Fock calculations on semi-infinite nuclear matter, and comparing with relativistic Hartree calculations performed on the same system in the nonlinear [sigma]-[omega] model. In this way an essential feature of the relativistic theory is incorporated into the nonrelativistic framework of Skyrme forces.

Pearson, J.M. (Laboratoire de Physique Nucleaire, Universite de Montreal, Montreal, Quebec, H3C 3J7 (Canada) Sektion Physik, Ludwig-Maximilians-Universitaet Muenchen, D-85748 Garching (Germany)); Farine, M. (Ecole Navale, Lanveoc-Poulmic, 29249 Brest Naval (France) Laboratoire de Physique Nucleaire, Universite de Nantes, 44072 Nantes (France))

1994-07-01

358

Coupled hydromechanical paleoclimate analyses of density-dependant groundwater flow in discretely fractured crystalline rock settings  

Microsoft Academic Search

A high resolution sub-regional scale (84 km2) density-dependent, fracture zone network groundwater flow model with hydromechanical coupling and pseudo-permafrost, was developed from a larger 5734 km2 regional scale groundwater flow model of a Canadian Shield setting in fractured crystalline rock. The objective of the work is to illustrate aspects of regional and sub-regional groundwater flow that are relevant to the

S. D. Normani; J. F. Sykes; M. R. Jensen

2009-01-01

359

On the numerical simulation of population dynamics with density-dependent migrations and the Allee effects  

NASA Astrophysics Data System (ADS)

In this paper, the variational iteration method (VIM) and the Adomian decomposition method (ADM) are presented for the numerical simulation of the population dynamics model with density-dependent migrations and the Allee effects. The convergence of ADM is proved for the model problem. The results obtained by these methods are compared to the exact solution. It is found that these methods are always converges to the right solutions with high accuracy. Furthermore, VIM needs relative less computational work than ADM.

Sweilam, H. N.; Khader, M. M.; Al-Bar, F. R.

2008-02-01

360

Time-dependent approach to electron pumping in open quantum systems  

NASA Astrophysics Data System (ADS)

We use a recently proposed time-dependent approach to investigate the motion of electrons in quantum pump device configurations. The occupied one-particle states are propagated in real time and employed to calculate the local electron density and current. The approach can also be embedded in the framework of time-dependent density functional theory to include electron-electron interactions. An advantage of the present computational scheme is that the same computational effort is required to simulate monochromatic, polychromatic, and nonperiodic drivings. Furthermore, initial-state dependence and history effects are naturally accounted for. We present results for one-dimensional devices exposed to a traveling potential wave. (i) We show that for pumping across a single potential barrier, electrons are transported in pockets and the transport mechanism resembles pumping of water with the Archimedean screw; (ii) we propose a simple model to study pumping through semiconductor nanostructures and we address the phenomenon of the current flowing in the opposite direction to the field propagation; (iii) we present the first numerical evidence of long-lived superimposed oscillations as induced by the presence of bound states and discuss the dependence of their lifetime on the frequency and amplitude of the driving field. By combining Floquet theory with nonequilibrium Green’s functions, we also obtain a general expression for the pumped current in terms of inelastic transmission probabilities. This latter result is used for benchmarking our propagation scheme in the long-time limit. Finally, we discuss the limitations of Floquet-based algorithms and suggest our approach as a possible way to go beyond them.

Stefanucci, G.; Kurth, S.; Rubio, A.; Gross, E. K. U.

2008-02-01

361

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

2012-05-07

362

Density-dependent expression of keratins in transformed rat liver cell lines.  

PubMed

Immunomorphological examination of the distribution of three keratins in cultured rat liver-derived epithelial cell lines of the IAR series was performed in order to find out the effects of neoplastic evolution on the expression of these epithelium-specific markers. Specific monoclonal antibodies were used to reveal various intermediate filament proteins: keratins with molecular masses of 55, 49 or 40 kD (K55, K49 or K40), and vimentin. The expression of keratins was negligible in sparse and dense cultures of non-transformed lines, which had typical epithelial morphology. The examined keratins were also absent in the sparse cultures of transformed lines, which have lost partially or completely the morphological features of epithelia. However, cells in dense cultures of most transformed lines contained numerous keratin filaments. It is suggested that the paradoxical increase of keratin expression after transformation is due to increased saturation density of transformed cultures; this high density favours the expression. As shown by the experiments with culture wounding, the effects of density are local and reversible. While K55 was present in all the cells of dense cultures, the expression of the other two keratins was dependent on the cell position within these cultures. It is suggested that the expression of the latter two keratins, besides high cell density, also requires the presence (K40) or the absence (K49) of cell-substratum contacts. Possible mechanisms of the effects of cell density on the expression of keratins are discussed. PMID:2423254

Troyanovsky, S M; Bannikov, G A; Montesano, R; Vasiliev, J M

1986-04-01

363

Separation of active and inactive fractions from starved culture of Vibrio parahaemolyticus by density dependent cell sorting  

Microsoft Academic Search

The co-existence of physiologically different cells in bacterial cultures is a general phenomenon. We have examined the applicability of the density dependent cell sorting (DDCS) method to separate subpopulations from a long-term starvation culture of Vibrio parahaemolyticus. The cells were subjected to Percoll density gradient and separated into 12 fractions of different buoyant densities, followed by measuring the cell numbers,

Binaya Bhusan Nayak; Eriko Kamiya; Tomohiko Nishino; Minoru Wada; Masahiko Nishimura; Kazuhiro Kogure

2005-01-01

364

Modeling electricity spot and futures price dependence: A multifrequency approach  

NASA Astrophysics Data System (ADS)

Electricity prices are known to exhibit multifractal properties. We accommodate this finding by investigating multifractal models for electricity prices. In this paper we propose a flexible Copula-MSM (Markov Switching Multifractal) approach for modeling spot and weekly futures price dynamics. By using a conditional copula function, the framework allows us to separately model the dependence structure, while enabling use of multifractal stochastic volatility models to characterize fluctuations in marginal returns. An empirical experiment is carried out using data from Nord Pool. A study of volatility forecasting performance for electricity spot prices reveals that multifractal techniques are a competitive alternative to GARCH models. We also demonstrate how the Copula-MSM model can be employed for finding optimal portfolios, which minimizes the Conditional Value-at-Risk.

Malo, Pekka

2009-11-01

365

Chiral multicritical points driven by isospin density in the Ginzburg-Landau approach  

NASA Astrophysics Data System (ADS)

We study how a chiral tricritical point (TCP) on QCD phase diagram is affected by the imbalance of up and down quark densities (isospin density), using the generalized Ginzburg-Landau (GL) approach. The resulting phase diagram near TCP shows a rich fine structure which includes inhomogeneities of both the chiral and the charged pion condensations. It turns out that the TCP splits into multicritical points.

Iwata, Yuhei; Abuki, Hiroaki; Suzuki, Katsuhiko

2012-10-01

366

Compound nucleus evaporative decay as a probe for the isospin dependence of the level density  

NASA Astrophysics Data System (ADS)

The evaporative decay of the compound nucleus 139Eu produced by the 180MeV 32S + 107Ag reaction was studied with the aim to test the empirical isospin expressions of the level density, recently appeared in the literature. We measured light charged particle spectra and angular correlations in coincidence with the evaporation residues and the invariant velocity distribution of the evaporation residues. In addition, an independent experiment was performed on the reaction 32S + 109Ag at the same incident energy. Evaporation residue angular distribution was measured and the fusion-evaporation cross-section was determined. All the measured quantities are compared with the predictions of different level density prescriptions: a) isospin independence, b) a dependence from N - Z and c) a dependence from Z-Z0 as proposed by Al-Quraishi et al. Results show that the predictions of the Z-Z0 dependence are far off the experimental data for all the measured observables. Regarding the isospin independent prescription and the N - Z dependence, although no great differences appear between their predictions the N - Z prescription seems to better describe the experimental data.

Moro, R.; Brondi, A.; Gelli, N.; Barbui, M.; Boiano, A.; Cinausero, M.; Di Nitto, A.; Fabris, D.; Fioretto, E.; La Rana, G.; Lucarelli, F.; Lunardon, M.; Montagnoli, G.; Ordine, A.; Prete, G.; Rizzi, V.; Trotta, M.; Vardaci, E.

2012-11-01

367

Trade-Off between Toxicity and Signal Detection Orchestrated by Frequency- and Density-Dependent Genes  

PubMed Central

Behaviors in insects are partly highly efficient Bayesian processes that fulfill exploratory tasks ending with the colonization of new ecological niches. The foraging (for) gene in Drosophila encodes a cGMP-dependent protein kinase (PKG). It has been extensively described as a frequency-dependent gene and its transcripts are differentially expressed between individuals, reflecting the population density context. Some for transcripts, when expressed in a population at high density for many generations, concomitantly trigger strong dispersive behavior associated with foraging activity. Moreover, genotype-by-environment interaction (GEI) analysis has highlighted a dormant role of for in energetic metabolism in a food deprivation context. In our current report, we show that alleles of for encoding different cGMP-dependent kinase isoforms influence the oxidation of aldehyde groups of aromatic molecules emitted by plants via Aldh-III and a phosphorylatable adaptor. The enhanced efficiency of oxidation of aldehyde odorants into carboxyl groups by the action of for lessens their action and toxicity, which should facilitate exploration and guidance in a complex odor environment. Our present data provide evidence that optimal foraging performance requires the fast metabolism of volatile compounds emitted by plants to avoid neurosensory saturation and that the frequency-dependent genes that trigger dispersion influence these processes.

Arthaud, Laury; Rokia-Mille, Selim Ben; Raad, Hussein; Dombrovsky, Aviv; Prevost, Nicolas

2011-01-01

368

Improving the worthiness of the Henry problem as a benchmark for density-dependent groundwater flow models  

Microsoft Academic Search

This study considers the worthiness of the Henry saltwater intrusion problem as a test case to benchmark density-dependent groundwater flow models. Previously published results from a coupled versus uncoupled analysis indicate that the flow patterns associated with the standard Henry problem are largely dictated by the boundary forcing and not necessarily a result of density-dependent effects [Simpson and Clement, 2003].

Matthew J. Simpson; T. Prabhakar Clement

2004-01-01

369

Consequences of potential density-dependent mechanisms on recovery of ocean-type chinook salmon ( Oncorhynchus tshawytscha )  

Microsoft Academic Search

Restoring salmon populations depends on our ability to predict the consequences of improving aquatic habi- tats used by salmon. Using a Leslie matrix model for chinook salmon (Oncorhynchus tshawytscha) that specifies transi- tions among spawning nests (redds), streams, tidal deltas, nearshore habitats, and the ocean, we compared the relative importance of different habitats under three density-dependent scenarios: juvenile density independence,

Correigh M. Greene; Timothy J. Beechie

2004-01-01

370

Density-dependent growth as a key mechanism in the regulation of fish populations: evidence from among-population comparisons  

Microsoft Academic Search

It is generally assumed that ® sh populations are regulated primarily in the juvenile (pre-recruit) phase of the life cycle, although density dependence in growth and reproductive parameters within the recruited phase has been widely reported. Here we present evidence to suggest that density-dependent growth in the recruited phase is a key process in the regulation of many ® sh

Kai Lorenzen; Katja Enberg

2002-01-01

371

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

372

View-dependent approach to MIP for very large data  

NASA Astrophysics Data System (ADS)

A simple and yet useful approach to visualize a variety of structures from sampled data is the Maximum Intensity Projection (MIP). Higher valued structures of interest pass in the projection over occluding structures. This can make MIP images difficult to interpret due to the loss of depth information. Animating about the data is one key way to try to decipher such ambiguities. The challenge is that MIP is inherently expensive and thus high frame rates are difficult to achieve. Variations to the original MIP algorithm and classification can help to further alleviate ambiguities and also provide improved image quality and very different visualizations. But they make the technique even more expensive. In addition, they require much parameter searching and tweaking. As today's data sizes are increasingly getting larger, current methods only allow very limited interaction. We explore a view-dependent approach using concepts from image-based rendering. A novel multi-layered image representation storing scalar information is computed at a view sample and then warped to the user's view. We present algorithms using OpenGL to quickly compute MIP and its variations using commodity off-the-shelf graphics hardware to achieve near interactive rates.

Shareef, Naeem; Crawfis, Roger

2002-03-01

373

Improving the accuracy of density-functional theory calculation: The genetic algorithm and neural network approach  

NASA Astrophysics Data System (ADS)

The combination of genetic algorithm and neural network approach (GANN) has been developed to improve the calculation accuracy of density functional theory. As a demonstration, this combined quantum mechanical calculation and GANN correction approach has been applied to evaluate the optical absorption energies of 150 organic molecules. The neural network approach reduces the root-mean-square (rms) deviation of the calculated absorption energies of 150 organic molecules from 0.47 to 0.22 eV for the TDDFT/B3LYP/6-31G(d) calculation, and the newly developed GANN correction approach reduces the rms deviation to 0.16 eV.

Li, Hui; Shi, Lili; Zhang, Min; Su, Zhongmin; Wang, Xiujun; Hu, Lihong; Chen, Guanhua

2007-04-01

374

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

375

Dependence calibration in conditional copulas: a nonparametric approach.  

PubMed

The study of dependence between random variables is a mainstay in statistics. In many cases, the strength of dependence between two or more random variables varies according to the values of a measured covariate. We propose inference for this type of variation using a conditional copula model where the copula function belongs to a parametric copula family and the copula parameter varies with the covariate. In order to estimate the functional relationship between the copula parameter and the covariate, we propose a nonparametric approach based on local likelihood. Of importance is also the choice of the copula family that best represents a given set of data. The proposed framework naturally leads to a novel copula selection method based on cross-validated prediction errors. We derive the asymptotic bias and variance of the resulting local polynomial estimator, and outline how to construct pointwise confidence intervals. The finite-sample performance of our method is investigated using simulation studies and is illustrated using a subset of the Matched Multiple Birth data. PMID:20731648

Acar, Elif F; Craiu, Radu V; Yao, Fang

2010-08-23

376

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

NASA Astrophysics Data System (ADS)

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

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

2010-01-01

377

BMP-2 Dependent Increase of Soft Tissue Density in Arthrofibrotic TKA  

PubMed Central

Arthrofibrosis after total knee arthroplasty (TKA) is difficult to treat, as its aetiology remains unclear. In a previous study, we established a connection between the BMP-2 concentration in the synovial fluid and arthrofibrosis after TKA. The hypothesis of the present study was, therefore, that the limited range of motion in arthrofibrosis is caused by BMP-2 induced heterotopic ossifications, the quantity of which is dependent on the BMP-2 concentration in the synovial fluid. Eight patients with arthrofibrosis after TKA were included. The concentration of BMP-2 in the synovial fluid from each patient was determined by ELISA. Radiologically, digital radiographs were evaluated and the grey scale values were determined as a measure of the tissue density of defined areas. Apart from air, cutis, subcutis and muscle, the soft-tissue density in the area of the capsule of the suprapatellar pouch was determined. The connection between the BMP-2 concentration and the soft-tissue density was then investigated. The average BMP-2 concentration in the synovial fluid was 24.3 ± 6.9 pg/ml. The density of the anterior knee capsule was on average 136 ± 35 grey scale values. A linear correlation was shown between the BMP-2 concentration in the synovial fluid and the radiological density of the anterior joint capsule (R=0.84, p = 0.009). We were able to show that there is a connection between BMP-2 concentration and soft-tissue density in arthrofibrosis after TKA. This opens up the possibility of conducting a prophylaxis against arthrofibrosis in risk patients by influencing the BMP-2 pathway.

Pfitzner, Tilman; Rohner, Eric; Krenn, Veit; Perka, Carsten; Matziolis, Georg

2012-01-01

378

A variational approach to spatially dependent non-rigid registration  

NASA Astrophysics Data System (ADS)

In this paper we propose a new method for non-rigid registration of PET/CT datasets incorporating prior knowledge about the rigidity of regions within the PET volumes into the matching process. State-of-the-art medical image registration approaches usually assume that the whole image domain is associated with a homogeneous deformation property, thus bone structure and soft tissue have the same stiffness, for instance. This assumption, however, is invalid in the majority of cases. In many applications the deformation properties can be estimated automatically by a segmentation step, beforehand. The presented non-rigid registration method integrates knowledge about the tissue directly into the deformation field computation. For this reason, no additional post-processing steps, like filtering of the deformation field, are required. To integrate the tissue constraints the regularizer is replaced by a novel spatially dependent smoother. Dependent on the location within the image, the smoother is able to explicitly adjust the rigidity. Thus, different tissue classes can be treated in the registration process. To pass the stiffness coefficients to the algorithm an additional mask image is used. The registration results are illustrated on synthetic data first to give a good intuition about the effectiveness of the proposed method. Finally, we illustrate the improvement of the registration using real clinical data. It is shown that the mono-modal registration of PET images yields more reasonable results using a spatially dependent regularizer constraining the deformations of regions with high tracer concentration than using a normal curvature regularizer. Furthermore, the method is evaluated on multi-modal PET/CT registration problems.

Jäger, Florian; Han, Jingfeng; Hornegger, Joachim; Kuwert, Torsten

2006-03-01

379

On the density dependence of the self-diffusion coefficient of interacting Brownian particles  

NASA Astrophysics Data System (ADS)

A brief summary is given of our theoretical approach to the statistical description of interacting Brownian particles. For the discussion of particle transport processes on the Smoluchowski level a hierarchy of coupled diffusion-like equations in configuration space for the reduced particle density functions and the corresponding particle flows is derived. Non-equilibrium processes are taken into account by means of linear perturbation theory. Applying the direct correlation force formalism, the hierarchy is cut off by replacing the three-particle density function nabc(r1, r2, r3, t). Here closure relations between the direct correlation force and nab(r1, r2, t) have to be added. Specializing the brownons to hard spheres and neglecting the hydrodynamic interaction, the two-particle density function is evaluated from the well-known equilibrium PY approximation and (as a first step) from the low-density limit of the non-equilibrium closure. Using a linear response theory of particle transport, the self-diffusion coefficient in a one-component hard sphere system of Brownian particles is calculated through a formal non-equilibrium picture based on tracer diffusion.

Sonnenburg, Jörn; Kremp, Dietrich; Sändig, Rainer

380

Temperature-dependent combinatorial level densities with the D1M Gogny force  

NASA Astrophysics Data System (ADS)

The combinatorial model of nuclear level densities has now reached a level of accuracy comparable to that of the best global analytical expressions without suffering from the limits imposed by the statistical hypothesis on which the latter expressions rely. In particular, it provides naturally, non-Gaussian spin distribution as well as nonequipartition of parities which are known to have a significant impact on cross section predictions at low energies. Our previous global model [S. Goriely, S. Hilaire, and A. J. Koning, Phys. Rev. CPRVCAN0556-281310.1103/PhysRevC.78.064307 78, 064307 (2008)] suffered from deficiencies, in particular in the way the collective effects—both vibrational and rotational—were treated. We have recently improved the level density calculations using simultaneously the single-particle levels and collective properties predicted by a newly derived Gogny interaction [S. Goriely, S. Hilaire, M. Girod, and S. Péru, Phys. Rev. Lett.PRLTAO0031-900710.1103/PhysRevLett.102.242501 102, 242501 (2009)], therefore enabling a microscopic description of energy-dependent shell, pairing, and deformation effects. In addition, for deformed nuclei, the transition to sphericity is coherently taken into account on the basis of a temperature-dependent Hartree-Fock calculation which provides at each temperature the structure properties needed to build the level densities. This new method is described and shown to give reasonable results with respect to available experimental data.

Hilaire, S.; Girod, M.; Goriely, S.; Koning, A. J.

2012-12-01

381

Simulating Regional Groundwater Flow Patterns in South Florida Using Density-Dependent Numerical Models  

NASA Astrophysics Data System (ADS)

Aquifer Storage and Recovery (ASR) is the storage of fresh water in an aquifer via injection during times when water is available, and recovery of the water from the same aquifer via pumping during times when it is needed. ASR is one of the proposed alternatives recommended by the Comprehensive Everglades Restoration Plan (CERP) to help with water supply, storage, and distribution of water in South Florida. To evaluate the numerous design considerations and the variation in aquifer response resulting from CERP ASR, regional density- dependent numerical modeling of the Floridan Aquifer System (FAS) in the southern half of Florida is in progress. This modeling incorporates use of two density-dependent numerical codes, SEAWAT and WASH123D, and the synthesis of regional knowledge of the FAS in terms of geologic parameters, groundwater flow patterns, and salinity influences. Several challenges have been discovered in replicating the existing regional groundwater flow patterns, most notably, that in south-central Florida the simulated heads are considerably lower than observed values. Recent model studies of several factors that could affect south Florida regional flow patterns indicate that the inclusion of preferential flow as well as the effects of temperature on groundwater density yield results that are more consistent with observed values. Future work will focus on the investigation of field data to support the application of preferential flow and the addition of groundwater injection and withdrawal including existing and proposed ASR projects.

England, S. M.; Stevens, G. T.

2008-05-01

382

Cell density-dependent mitogenic effect and -independent cellular handling of epidermal growth factor in primary cultured rat hepatocytes  

Microsoft Academic Search

Aims: Mitogenic effect and cellular handling of epidermal growth factor (EGF) were analyzed in primary cultured rat hepatocytes at several cell densities.Methods\\/Results: DNA synthesis, assessed by the incorporation of 125I-deoxyuridine, was accelerated by EGF at a low cell density while that stimulated by EGF was relatively low at the highest cell density, suggesting a cell density-dependent regulation of mitrogenic response

Teruki Hamada; Yukio Kato; Tetsuya Terasaki; Yuichi Sugiyama

1997-01-01

383

Analytic energy gradient in combined time-dependent density functional theory and polarizable force field calculation  

NASA Astrophysics Data System (ADS)

Formulas for evaluating analytic energy gradient are derived for combined time-dependent density functional theory (TDDFT) and polarizable force field methods that incorporate dipole polarizability tensors and linearly induced point dipoles. The Z-vector method for determining relaxed one-particle difference density matrix in regular TDDFT methods is extended to include induced dipoles. The analytic gradient of the mutual polarization energy of the force field and the TDDFT excited state can be formulated by using the TDDFT difference density-induced dipoles and the transition state density-induced dipoles. All the forces and torques involving induced dipoles can be efficiently evaluated using standard electrostatic formulas as if the induced dipoles were permanent dipoles. The formulas are given in the most general form and are applicable to various flavors of polarizable force fields. Implementation and tests with a polarizable five-point water model show that the formulas are rigorous. The carbonyl vibration modes and infrared spectrum intensities of a cluster formed by acetone and two water molecules are studied.

Si, Dejun; Li, Hui

2010-10-01

384

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

SciTech Connect

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

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

2006-04-15

385

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

NASA Astrophysics Data System (ADS)

Photoionization cross sections and angular distributions of silicon tetrafluoride have been calculated in the framework of the time-dependent density-functional theory approach. Both valence and inner shell ionizations have been considered in an extended photon energy range. Calculations have been carried out by using two different exchange-correlation (xc) potentials characterized by the correct asymptotic behavior. Theoretical results obtained with both the van Leeuwen-Baerends and statistical average of orbital potentials (SAOP) xc potentials are compared with photoabsorption, photoionization, and electron-scattering experiments as well as with previous theoretical calculations. It is suggested that even if both xc potentials provide a reasonably good description of the photoionization dynamics, correlation effects are phenomenologically better accounted for by the SAOP xc potential. Overall, the good accuracy attained with the linear combination of atomic orbitals-time dependent density-functional theory method in reproducing the experimental findings for SiF4 makes it a promising and powerful method for the characterization of the photoionization dynamics from medium and large-size molecules.

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

2006-04-01

386

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

SciTech Connect

We discuss the origin of pathological behaviors that have been recently identified in particle-number-restoration calculations performed within the nuclear energy density functional framework. A regularization method that removes the problematic terms from the multi reference energy density functional and that applies (i) to any symmetry-restoration- and/or generator-coordinate-method-based configuration mixing calculation and (ii) to energy density functionals depending only on integer powers of the density matrices was proposed in [D. Lacroix, T. Duguet, and M. Bender, Phys. Rev. C 79, 044318 (2009)] and implemented for particle-number-restoration calculations in [M. Bender, T. Duguet, and D. Lacroix, Phys. Rev. C 79, 044319 (2009)]. In the present article, we address the viability of noninteger powers of the density matrices in the nuclear energy density functional. Our discussion builds on the analysis already carried out in [J. Dobaczewski et al., Phys. Rev. C 76, 054315 (2007)]. First, we propose to reduce the pathological nature of terms depending on a noninteger power of the density matrices by regularizing the fraction that relates to the integer part of the exponent using the method proposed in [D. Lacroix, T. Duguet, and M. Bender, Phys. Rev. C 79, 044318 (2009)]. Then, we discuss the spurious features brought about by the remaining fractional power. Finally, we conclude that noninteger powers of the density matrices are not viable and should be avoided in the first place when constructing nuclear energy density functionals that are eventually meant to be used in multi reference calculations.

Duguet, T. [National Superconducting Cyclotron Laboratory, 1 Cyclotron Laboratory, East Lansing, Michigan 48824 (United States); Department of Physics and Astronomy, Michigan State University, East Lansing, Michigan 48824 (United States); CEA, Irfu, SPhN, Centre de Saclay, F-911191 Gif-sur-Yvette (France); Bender, M. [Universite Bordeaux, Centre d'Etudes Nucleaires de Bordeaux Gradignan, UMR5797, F-33175 Gradignan (France); CNRS/IN2P3, Centre d'Etudes Nucleaires de Bordeaux Gradignan, UMR5797, F-33175 Gradignan (France); Bennaceur, K. [CEA, Irfu, SPhN, Centre de Saclay, F-911191 Gif-sur-Yvette (France) and Universite de Lyon, F-69003 Lyon (France) and Universite Lyon 1, F-69622 Villeurbanne, France; CNRS/IN2P3, UMR 5822 Institut de Physique Nucleaire de Lyon (France); Lacroix, D. [GANIL, CEA et IN2P3, BP 5027, 14076 Caen Cedex (France); Lesinski, T. [Universite de Lyon, F-69003 Lyon (France) and Universite Lyon 1, F-69622 Villeurbanne, France; CNRS/IN2P3, UMR 5822 Institut de Physique Nucleaire de Lyon (France)

2009-04-15

387

Density-dependent recruitment of the bloater (Coregonus hoyi) in Lake Michigan  

USGS Publications Warehouse

Density-dependent recruitment of the bloater (Coregonus hoyi) in Lake Michigan during and after recovery of the population in about 1977-1983 was best reflected in the fit of the Beverton-Holt recruitment function to age -1 and -2 recruits and estimated eggs of parents surveyed with trawls. A lower growth rate and lower lipid content of bloaters at higher population densities and no evidence of cannibalism supported the conclusion that recruitment is resource limited when alewife (Alosa pseudoharengus) abundance is low. Predation on larvae by alewives was indicated in earlier studies as the probable cause of depressed recruitment of bloaters before their recovery, which coincided with declining alewife abundance. This negative interaction masked any bloater stock-recruitment relation in the earlier period.

Brown, Edward H., Jr.; Eck, Gary W.

1992-01-01

388

Electron density and temperature of gas-temperature-dependent cryoplasma jet  

SciTech Connect

A microsize cryoplasma jet was developed and analyzed at plasma gas temperatures ranging from room temperature down to 5 K. Experimental results obtained from optical emission spectroscopy and current-voltage measurements indicate that the average electron density and electron temperature of the cryoplasma jet depend on the gas temperature. In particular, the electron temperature in the cryoplasma starts to decrease rapidly near 60 K from about 13 eV at 60 K to 2 eV at 5 K, while the electron density increases from about 10{sup 9} to approximately 10{sup 12} cm{sup -3} from room temperature to 5 K. This phenomenon induces an increase in the Coulomb interaction between electrons, which can be explained by the virial equation of state.

Noma, Yuri; Hyuk Choi, Jai; Muneoka, Hitoshi; Terashima, Kazuo [Department of Advanced Materials Science, Graduate School of Frontier Sciences, University of Tokyo, 5-1-5, Kashiwanoha, Kashiwa, Chiba 277-8561 (Japan)

2011-03-01

389

Angular momentum dependence of the nuclear level-density parameter around Z{approx}50  

SciTech Connect

{alpha}-particle evaporation spectra and {gamma}-ray multiplicities were measured for various target-projectile systems corresponding to residual nuclei in the range of Z{sub R}=48-55 and with excitation energy in the range of 30-40 MeV. The high-energy part of the evaporation spectra were analyzed using the statistical model code PACE2 to derive values of the inverse level-density parameter (K). The K values were found to be in the range of 9.0-10.5 for all systems. Angular momentum dependence of the inverse level-density parameter was investigated using the {gamma}-ray multiplicity data. It is seen that there are strong variations in K as a function of angular momentum for many systems. Present results provide important input information for a systematic understanding of the statistical properties of nuclei at moderate excitation energies and angular momenta.

Gupta, Y. K.; John, Bency; Biswas, D. C.; Nayak, B. K.; Saxena, A.; Choudhury, R. K. [Nuclear Physics Division, Bhabha Atomic Research Centre, Mumbai 400 085 (India)

2008-11-15

390

Quantum-mechanical interpretation of time-dependent density-functional theory  

NASA Astrophysics Data System (ADS)

In this paper we derive the differential virial theorem for both time-dependent (TD) Schrödinger theory and Kohn-Sham (KS) density-functional theory. As such we obtain an exact integral expression for the TD electron-interaction potential of KS theory that is independent of the choice of action. The expression, valid for general TD phenomena other than the ionization or scattering process, is afforded the physical interpretation at each instant of time as being the work done to move an electron in a conservative field. The field is a sum of four component fields representative of Pauli and Coulomb correlations, correlation-kinetic effects, and as discovered here, correlations due to the difference in current densities of the KS and Schrödinger systems. The interpretation further reduces to that for the corresponding electron-interaction potential of stationary-state KS theory for time-independent external potentials.

Qian, Zhixin; Sahni, Viraht

1998-10-01

391

Density-dependent cooperation as a mechanism for persistence and coexistence.  

PubMed

To overcome stress, such as resource limitation, an organism often needs to successfully mediate competition with other members of its own species. This may favor the evolution of defective traits that are harmful to the species population as a whole, and that may lead to its dilution or even to its extinction (the tragedy of the commons). Here, we show that this phenomenon can be circumvented by cooperation plasticity, in which an individual decides, based on environmental conditions, whether to cooperate or to defect. Specifically, we analyze the evolution of density-dependent cooperation. In our model, the population is spatially subdivided, periodically remixed, and comprises several species. We find that evolution pushes individuals to be more cooperative when their own species is at lower densities, and we show that not only could this cooperation prevent the tragedy of the commons, but it could also facilitate coexistence between many species that compete for the same resource. PMID:21967418

Lampert, Adam; Tlusty, Tsvi

2011-06-27

392

A Performant Algorithm to Calculate Spin- and Parity-Dependent Nuclear Level Densities  

NASA Astrophysics Data System (ADS)

A new algorithm for calculating the spin- and parity-dependent shell model nuclear level densities using the moments method in the proton-neutron formalism will be presented. A new, parallelized code based on this algorithm was developed and tested using up to 4,000 cores on FRANKLIN/NERSC, for a set of nuclei from the sd-, pf-, and pf+g9/2- model spaces. By comparing the low excitation energy nuclear level densities for a given nucleus calculated in two model spaces, such as pf and pf+g9/2, we could estimated its ground state energy in the larger model space, which is not accessible to direct shell model calculations because of the unmanageable dimension. Examples for the ground state energies of for ^64Ge and ^68Se in the pf+g9/2 model space will be presented. )

Senkov, Roman; Horoi, Mihai

2010-02-01

393

Nonlinear phenomena in time-dependent density-functional theory: What Rabi oscillations can teach us  

NASA Astrophysics Data System (ADS)

Through the exact solution of a two-electron singlet system interacting with a monochromatic laser we prove that all adiabatic density functionals within time-dependent density-functional theory are not able to discern between resonant and nonresonant (detuned) Rabi oscillations. This is rationalized in terms of a fictitious dynamical exchange-correlation (xc) detuning of the resonance while the laser is acting. The nonlinear dynamics of the Kohn-Sham system shows the characteristic features of detuned Rabi oscillations even if the exact resonant frequency is used. We identify the source of this error in a contribution from the xc functional to the set of nonlinear equations that describes the electron dynamics in an effective two-level system. The constraint of preventing the detuning introduces a new strong condition to be satisfied by approximate xc functionals.

Fuks, J. I.; Helbig, N.; Tokatly, I. V.; Rubio, A.

2011-08-01

394

Density dependent hadronic models and the relation between neutron stars and neutron skin thickness  

NASA Astrophysics Data System (ADS)

In the present work, we investigate the main differences in the lead neutron skin thickness, binding energy, surface energy, and density profiles obtained with two different density dependent hadron models. Our results are calculated within the Thomas-Fermi approximation with two different numerical prescriptions and compared with results obtained with a common parametrization of the nonlinear Walecka model. The neutron skin thickness is a reflex of the equation of state properties. Hence, a direct correlation is found between the neutron skin thickness and the slope of the symmetry energy. We show that within the present approximations, the asymmetry parameter for low momentum transfer polarized electron scattering is not sensitive to the model differences.

Avancini, S. S.; Marinelli, J. R.; Menezes, D. P.; Moraes, M. M. W.; Providência, C.

2007-05-01

395

Dependence of Plasma Density on Air Gap Length of Resonant Cavity with Bottom Plate Replaced by High-Density Plasma  

Microsoft Academic Search

To excite selectively transverse magnetic (TM) waves and to maximize E-field of a microwave oriented parallel to the plasma density gradient at a plasma surface, a resonant cavity was formed using an air gap between an upper conduction plate with a slot antenna (part of a waveguide) and a dielectric window of a microwave plasma source, in which a high-density

Sumio Kogoshi; Yu Yoshioka; Noboru Katayama; Yusuke Kudo

2009-01-01

396

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

SciTech Connect

We investigate the pressure dependence of the optical properties of CeTe{sub 3}, which exhibits an incommensurate charge-density-wave (CDW) state already at 300 K. Our data are collected in the mid-infrared spectral range at room temperature and at pressures between 0 and 9 GPa. The energy for the single particle excitation across the CDW gap decreases upon increasing the applied pressure, similarly to the chemical pressure by rare-earth substitution. The broadening of the bands upon lattice compression removes the perfect nesting condition of the Fermi surface and therefore diminishes the impact of the CDW transition on the electronic properties of RTe{sub 3}.

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

2009-12-14

397

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

SciTech Connect

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

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

2011-08-15

398

Time-Dependent Density Functional Theory for Open Quantum Systems with Unitary Propagation  

NASA Astrophysics Data System (ADS)

We extend the Runge-Gross theorem for a very general class of open quantum systems under weak assumptions about the nature of the bath and its coupling to the system. We show that for Kohn-Sham (KS) time-dependent density functional theory, it is possible to rigorously include the effects of the environment within a bath functional in the KS potential. A Markovian bath functional inspired by the theory of nonlinear Schrödinger equations is suggested, which can be readily implemented in currently existing real-time codes. Finally, calculations on a helium model system are presented.

Yuen-Zhou, Joel; Tempel, David G.; Rodríguez-Rosario, César A.; Aspuru-Guzik, Alán

2010-01-01

399

Measuring depth-dependent dislocation densities and Elastic strains in an indented Ni-based Superalloy  

NASA Astrophysics Data System (ADS)

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 ˜75 ?m. A residual compression zone is found close to the indent edge. An elastic-plastic transition is observed at ˜20 ?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.

2010-12-01

400

Time-dependent density functional theory for open quantum systems with unitary propagation.  

PubMed

We extend the Runge-Gross theorem for a very general class of open quantum systems under weak assumptions about the nature of the bath and its coupling to the system. We show that for Kohn-Sham (KS) time-dependent density functional theory, it is possible to rigorously include the effects of the environment within a bath functional in the KS potential. A Markovian bath functional inspired by the theory of nonlinear Schrödinger equations is suggested, which can be readily implemented in currently existing real-time codes. Finally, calculations on a helium model system are presented. PMID:20366703

Yuen-Zhou, Joel; Tempel, David G; Rodríguez-Rosario, César A; Aspuru-Guzik, Alán

2010-01-25

401

Fatigue life prediction of welded cruciform joints using strain energy density factor approach  

Microsoft Academic Search

The influences of two welding processes, namely, shielded metal arc welding (SMAW) and flux cored arc welding (FCAW), on fatigue life of cruciform joints containing lack of penetration (LOP) defects have been analyzed by using the strain energy density factor (SEDF) approach. Load carrying cruciform joints were fabricated from ASTM 517 ‘F’ grade steel. Fatigue crack growth experiments were carried

V Balasubramanian; B Guha

2000-01-01

402

Density-dependent growth as a key mechanism in the regulation of fish populations: evidence from among-population comparisons.  

PubMed Central

It is generally assumed that fish populations are regulated primarily in the juvenile (pre-recruit) phase of the life cycle, although density dependence in growth and reproductive parameters within the recruited phase has been widely reported. Here we present evidence to suggest that density-dependent growth in the recruited phase is a key process in the regulation of many fish populations. We analyse 16 fish populations with long-term records of size-at-age and biomass data, and detect significant density-dependent growth in nine. Among-population comparisons show a close, inverse relationship between the estimated decline in asymptotic length per unit biomass density, and the long-term average biomass density of populations. A simple population model demonstrates that regulation by density-dependent growth alone is sufficient to generate the observed relationship. Density-dependent growth should be accounted for in fisheries' assessments, and the empirical relationship established here can provide indicative estimates of the density-dependent growth parameter where population-specific data are lacking.

Lorenzen, Kai; Enberg, Katja

2002-01-01

403

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

SciTech Connect

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

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

2010-08-04

404

Collision Density Approach of Radiation Damage in a Multi-Species Medium.  

National Technical Information Service (NTIS)

Space-energy dependent foward type equations for the collision densities of energetic atoms in multi-species semi-infinite homogeneous medium are formulated. The introduction of the one-dimensional isotropic forward-backward model of Fermi for the scatter...

I. Lux I. Pazsit

1981-01-01

405

Time-Dependent Approach to Electron--Atom Scattering.  

NASA Astrophysics Data System (ADS)

The B-spline method [1] was used to represent the Hamiltonian describing the interaction of a projectile and a target on a discrete lattice. The time-dependent Schrödinger equation was then solved by explicit integration using different time propagation schemes, such as Taylor expansion and the ``leap-frog'' method [2]. In order to assess the accuracy and efficiency of the different approaches, the method was tested for several one-dimensional model problems, such as scattering of a Gaussian wavepacket from various arrangements of steps and barriers. The program is currently being applied to the two-dimensional Temkin-Poet model of electron scattering from atomic hydrogen [3]. The most recent results for excitation and ionization cross sections will be presented at the conference and compared with other theoretical work. 1. A.S. Umar, J. Wu, M.R. Strayer and C. Bottcher, J. Comp. Phys. 93, 1. 426 (1991) 2. W.H. Press, S.A. Teucholsky, V.T. Vetterling, and B.P. Flannery, 2. Numerical Recipes (Cambridge University Press, New York 1992) 3. A. Temkin, Phys. Rev. A 126, 130 (1962)

Krilov, Goran

1997-04-01

406

Density-Dependent Processes in the Life History of Fishes: Evidence from Laboratory Populations of Zebrafish Danio rerio  

PubMed Central

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.

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

2012-01-01

407

Verifying the Reif model of MOT loading:Trap depth and density dependence  

NASA Astrophysics Data System (ADS)

We have studied the loading of rubidium atoms into a magneto-optical trap (MOT) with the aim of verifying a long-standing conjecture referred to as the Reif model. This model predicts that the loading rate should be proportional to the escape velocity of atoms from the trap to the fourth power, or equivalently, to the trap depth squared, and is directly proportional to the background rubidium atom density. The first prediction was confirmed by comparing the MOT loading rates to trap depths deduced from optical excitation of trapped atoms to a repulsive molecular potential. The rubidium density dependence was demonstrated by comparing the elastic collision-induced loss rate of atoms from a magnetic trap (MT) and the loading rate of a MOT: since the MT loss rate is proportional to the background density, the linear correlation to the MOT loading rate verified the Reif model. As a consequence of these findings, i) we have shown that the loading rates of different MOTs can be used as a convenient measure of their relative trap depths, and ii) we have experimentally determined the relationship between the capture and escape velocities in the MOTs studied (ranging in depth from 0.5 K to 1.8 K) to be vc = 1.29(0.12)ve.

Booth, James; Haw, Magnus; Evetts, Nathan; Gunton, Will; van Dongen, Janelle; Madison, Kirk

2012-06-01

408

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.

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

2008-01-01

409

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.

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

2013-01-01

410

The diffusional properties of dendrites depend on the density of dendritic spines  

PubMed Central

We combined computational modeling and experimental measurements to determine the influence of dendritic structure on diffusion of intracellular chemical signals in mouse cerebellar Purkinje cells and hippocamal CA1 pyramidal cells. Modeling predicts that molecular trapping by dendritic spines causes diffusion along spiny dendrites to be anomalous and that the value of the anomalous exponent (dw) is proportional to spine density in both cell types. To test these predictions we combined local photorelease of an inert dye, rhodamine-dextran, with two-photon fluorescence imaging to track diffusion along dendrites. Our results show that anomalous diffusion is present in spiny dendrites of both cell types. Further, the anomalous exponent is linearly related to the density of spines in pyramidal cells and dw in Purkinje cells is consistent with such a relationship. We conclude that anomalous diffusion occurs in the dendrites of multiple types of neurons. Because spine density is dynamic and depends on neuronal activity, the degree of anomalous diffusion induced by spines can dynamically regulate the movement of molecules along dendrites.

Santamaria, Fidel; Wils, Stefan; De Schutter, Erik; Augustine, George J.

2011-01-01

411

Density-dependent predation influences the evolution and behavior of masquerading prey  

PubMed Central

Predation is a fundamental process in the interaction between species, and exerts strong selection pressure. Hence, anti-predatory traits have been intensively studied. Although it has long been speculated that individuals of some species gain protection from predators by sometimes almost-uncanny resemblances to uninteresting objects in the local environment (such as twigs or stones), demonstration of antipredatory benefits to such “masquerade” have only very recently been demonstrated, and the fundamental workings of this defensive strategy remain unclear. Here we use laboratory experiments with avian predators and twig-mimicking caterpillars as masqueraders to investigate (i) the evolutionary dynamics of masquerade; and (ii) the behavioral adaptations associated with masquerade. We show that the benefit of masquerade declines as the local density of masqueraders relative to their models (twigs, in our system) increases. This occurs through two separate mechanisms: increasing model density both decreased predators’ motivation to search for masqueraders, and made masqueraders more difficult to detect. We further demonstrated that masquerading organisms have evolved complex microhabitat selection strategies that allow them to best exploit the density-dependent properties of masquerade. Our results strongly suggest the existence of opportunity costs associated with masquerade. Careful evaluation of such costs will be vital to the development of a fuller understanding of both the distribution of masquerade across taxa and ecosystems, and the evolution of the life history strategies of masquerading prey.

Skelhorn, John; Rowland, Hannah M.; Delf, Jon; Speed, Michael P.; Ruxton, Graeme D.

2011-01-01

412

Geographical variation in the influence of density dependence and climate on the recruitment of Norwegian moose.  

PubMed

The effects of variation in climate on population dynamics are likely to differ within the distributional range of a species, yet the consequences of such regional variation on demography and population dynamics are rarely considered. Here we examine how density dependence and different climate variables affect spatio-temporal variation in recruitment rates of Norwegian moose using data collected over a large geographical area during the hunting season. After accounting for observation error by a Bayesian Markov chain Monte Carlo technique, temporal variation in recruitment rates was relatively independent of fluctuations in local population size. In fact, a positive relationship was as common as a density-dependent decrease in fecundity rates. In general, high recruitment rates were found during autumn 1 year after years with a warm February, and after a warm May or cold June in year t - 1 or in year t. Large regional variation was also found in the effects of some of the weather variables, especially during spring. These patterns demonstrate both direct and delayed effects of weather on the recruitment of moose that possibly operate through an effect of body mass on the proportion of the females that sexually mature as 1.5 or 2.5 years old. PMID:19657678

Grøtan, Vidar; Saether, Bernt-Erik; Lillegård, Magnar; Solberg, Erling J; Engen, Steinar

2009-08-06

413

Magnetization of a Current-Carrying Superconducting Disk with B-Dependent Critical Current Density  

NASA Astrophysics Data System (ADS)

In the frame work of the critical state model (CSM), the magnetic response of a thin type-II superconducting disk that carries a radial transport current and is subjected to an applied magnetic field have been studied. To this end, we have studied the process of the magnetic flux-penetration. For a disk initially containing no magnetic flux but carrying a radial current, when a perpendicular magnetic field is applied, magnetic flux-penetration occurs in three stages: (1) the magnetic flux gradually penetrates from the edges of the disk until an instability occurs, (2) there is a rapid inflow of magnetic flux into the disk’s central region, which becomes resistive, and (3) magnetic flux continues to enter the disk, while persistent azimuthal currents flow in an outer annular region where the net current density is equal to J c . Also the behavior of a current-carrying disk subjected to an AC magnetic field is calculated. The magnetic flux, the current profiles and the magnetization hysteresis loops are calculated for several commonly used J c ( B) dependences. Finally, the results of the applications of the local field-dependent of the critical current density J c ( B) are compared with those obtained from the Bean model.

Sohrabi, Mahdi; Babaei-Brojeny, Ali A.

2010-11-01

414

Chaotic attractors and physical measures for some density dependent Leslie population models  

NASA Astrophysics Data System (ADS)

Following ecologists' discoveries, mathematicians have begun studying extensions of the ubiquitous age structured Leslie population model that allow some survival probabilities and/or fertility rates to depend on population densities. These nonlinear extensions commonly exhibit very complicated dynamics: through computer studies, some authors have discovered robust Hénon-like strange attractors in several families. Population biologists and demographers frequently wish to average a function over many generations and conclude that the average is independent of the initial population distribution. This type of 'ergodicity' seems to be a fundamental tenet in population biology. In this paper we develop the first rigorous ergodic theoretic framework for density dependent Leslie population models. We study two generation models with Ricker and Hassell (recruitment type) fertility terms. We prove that for some parameter regions these models admit a chaotic (ergodic) attractor which supports a unique physical probability measure. This physical measure, having full Lebesgue measure basin, satisfies in the strongest possible sense the population biologist's requirement for ergodicity in their population models. We use the celebrated work of Wang and Young 2001 Commun. Math. Phys. 218 1-97, and our results are the first applications of their method to biology, ecology or demography.

Ugarcovici, Ilie; Weiss, Howard

2007-12-01

415

Efficacy of immunoliposomes on cancer models in a cell-surface-antigen-density-dependent manner.  

PubMed

We have recently established a cancer-reactive human monoclonal antibody, GAH, with a positive ratio of over 90% against stomach cancer. GAH was formulated as polyethyleneglycol (PEG)-modified immunoliposomal doxorubicin (DXR) (ILD) and its efficacy was examined against gastrointestinal human cancers. In in vitro studies, a comparison of ILD with PEG-modified liposomal DXR (LD) demonstrated that ILD had dose-dependent cytotoxicity for GAH-reactive B37 cancer cells, but not LD. In concordance with this result, microscopic observations showed that ILD was bound to and GAH-dependently internalised by B37 cells. In in vivo studies, ILD exhibited significantly greater antitumour activity on cancer xenograft models than LD or free DXR. The relation between efficacy and antigen density was examined on 10 xenograft models bearing cancer cells with varying GAH reactivity. Immunoliposomal doxorubicin therapeutic activity correlated with the antigen density, with a minimum number being required. Also, ILD revealed strong antitumour activity on cancers with low sensitivity to DXR or LD, suggesting that ILD overcame the DXR resistance of antigen-positive cancer cells. Thus, these results show that GAH endows liposomes with targeting activity, resulting in strong efficacy against gastrointestinal cancers. PMID:14562030

Hosokawa, S; Tagawa, T; Niki, H; Hirakawa, Y; Nohga, K; Nagaike, K

2003-10-20

416

Availability of Glutamate and Arginine during Acid Challenge Determines Cell Density-Dependent Survival Phenotype of Escherichia coli Strains  

PubMed Central

The cell density-dependent acid sensitivity phenotypes of Escherichia coli strains K-12 and O157:H7 were examined with reference to three possible mechanisms of acid resistance. There was no evidence of any diffusible substance released from dead cells which could influence the cell density-dependent acid survival phenotype. Instead, cell density-dependent acid survival phenotype was associated with induction of glutamate- and arginine-decarboxylase acid survival pathways and concomitant availability of glutamate and arginine during acid challenge.

Cui, Shenghui; Meng, Jianghong; Bhagwat, Arvind A.

2001-01-01

417

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

PubMed Central

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

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

2010-01-01

418

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

NASA Astrophysics Data System (ADS)

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

2013-09-01

419

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

420

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

NASA Astrophysics Data System (ADS)

Spin-flip time-dependent density functional theory (SF-TD-DFT) with the full noncollinear hybrid exchange-correlation kernel and its approximate variants are critically assessed, both formally and numerically. As demonstrated by the ethylene torsion and the C2v ring-opening of oxirane, SF-TD-DFT is very useful for describing nearly degenerate situations. However, it may occasionally yield unphysical results. This stems from the noncollinear form of the generalized gradient approximation, which becomes numerically instable in the presence of spin-flip excitations from the closed- to vacant-shell orbitals of an open-shell reference. To cure this defect, a simple modification, dubbed as ALDA0, is proposed in the spirit of adiabatic local density approximation (ALDA). It is applicable to all kinds of density functionals and yields stable results without too much loss of accuracy. In particular, the combination of ALDA0 with the Tamm-Dancoff approximation is a promising tool for studying global potential energy surfaces. In addition to the kernel problem, SF-TD-DFT is also rather sensitive to the choice of reference states, as demonstrated by the spin multiplet states of closed-shell molecules of H2O, CH2O, and C2H4. Surprisingly, SF-TD-DFT with pure density functionals may also fail for valance excitations with large orbital overlaps, at variance with the spin-conserving counterpart (SC-TD-DFT). In this case, the inclusion of a large amount of Hartree-Fock exchange is mandatory for quantitative results. Nonetheless, for spatially degenerate cases such as CF, CH, and NH+, SF-TD-DFT is more advantageous than SC-TD-DFT, unless the latter is also space adapted. These findings are very instructive for future development and applications of TD-DFT.

Li, Zhendong; Liu, Wenjian

2012-01-01

421

Skyrme energy-density functional approach to collective modes of excitation in exotic nuclei  

NASA Astrophysics Data System (ADS)

Low-frequency collective modes of excitation in neutron-rich nuclei are investigated in the framework of the nuclear energy-density functional method. It is shown 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. The inertial functions for large-amplitude vibration and rotation are evaluated by the local normal modes along the axial quadrupole collective coordinate. The time-odd components of the mean fields are fully included in the derived masses.

Yoshida, K.

2013-07-01

422

Static and Time Dependent Density Functional Theory with Internal Degrees of Freedom: Merits and Limitations Demonstrated for the Potts Model  

Microsoft Academic Search

We present an extension of the density-functional theory (DFT) formalism for lattice gases to systems with internal degrees of freedom. In order to test approximations commonly used in DFT approaches, we investigate the statics and dynamics of occupation (density) profiles in the one-dimensional Potts model. In particular, by taking the exact functional for this model we can directly evaluate the

S. Heinrichs; W. Dieterich; P. Maass; H. L. Frisch

2004-01-01

423

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

EPA Science Inventory

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

424

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

NASA Astrophysics Data System (ADS)

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

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

2013-10-01

425

Mixed convection and density-dependent seawater circulation in coastal aquifers  

NASA Astrophysics Data System (ADS)

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

Smith, Anthony J.

2004-08-01

426

Inflammation triggers emergency granulopoiesis through a density-dependent feedback mechanism.  

PubMed

Normally, neutrophil pools are maintained by homeostatic mechanisms that require the transcription factor C/EBP?. Inflammation, however, induces neutrophilia through a distinct pathway of "emergency" granulopoiesis that is dependent on C/EBP?. Here, we show in mice that alum triggers emergency granulopoiesis through the IL-1RI-dependent induction of G-CSF. G-CSF/G-CSF-R neutralization impairs proliferative responses of hematopoietic stem and progenitor cells (HSPC) to alum, but also abrogates the acute mobilization of BM neutrophils, raising the possibility that HSPC responses to inflammation are an indirect result of the exhaustion of BM neutrophil stores. The induction of neutropenia, via depletion with Gr-1 mAb or myeloid-specific ablation of Mcl-1, elicits G-CSF via an IL-1RI-independent pathway, stimulating granulopoietic responses indistinguishable from those induced by adjuvant. Notably, C/EBP?, thought to be necessary for enhanced generative capacity of BM, is dispensable for increased proliferation of HSPC to alum or neutropenia, but plays a role in terminal neutrophil differentiation during granulopoietic recovery. We conclude that alum elicits a transient increase in G-CSF production via IL-1RI for the mobilization of BM neutrophils, but density-dependent feedback sustains G-CSF for accelerated granulopoiesis. PMID:21655273

Cain, Derek W; Snowden, Pilar B; Sempowski, Gregory D; Kelsoe, Garnett

2011-05-31

427

Inflammation Triggers Emergency Granulopoiesis through a Density-Dependent Feedback Mechanism  

PubMed Central

Normally, neutrophil pools are maintained by homeostatic mechanisms that require the transcription factor C/EBP?. Inflammation, however, induces neutrophilia through a distinct pathway of “emergency” granulopoiesis that is dependent on C/EBP?. Here, we show in mice that alum triggers emergency granulopoiesis through the IL-1RI-dependent induction of G-CSF. G-CSF/G-CSF-R neutralization impairs proliferative responses of hematopoietic stem and progenitor cells (HSPC) to alum, but also abrogates the acute mobilization of BM neutrophils, raising the possibility that HSPC responses to inflammation are an indirect result of the exhaustion of BM neutrophil stores. The induction of neutropenia, via depletion with Gr-1 mAb or myeloid-specific ablation of Mcl-1, elicits G-CSF via an IL-1RI-independent pathway, stimulating granulopoietic responses indistinguishable from those induced by adjuvant. Notably, C/EBP?, thought to be necessary for enhanced generative capacity of BM, is dispensable for increased proliferation of HSPC to alum or neutropenia, but plays a role in terminal neutrophil differentiation during granulopoietic recovery. We conclude that alum elicits a transient increase in G-CSF production via IL-1RI for the mobilization of BM neutrophils, but density-dependent feedback sustains G-CSF for accelerated granulopoiesis.

Cain, Derek W.; Snowden, Pilar B.; Sempowski, Gregory D.; Kelsoe, Garnett

2011-01-01

428

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

429

Density dependent interactions and the consistency of folding estimates of nucleon-nucleus and nucleus-nucleus potentials  

Microsoft Academic Search

It is shown that density dependence in the interaction does effect the consistency of the results obtained in folding calculations of nucleon-nucleus and nucleus-nucleus potentials. A modified density dependent version of the new G matrix interaction of Bertsch et al. gives reasonable results for both. Present address: Bettis Atomic Power Laboratory, Westinghouse Electric Corporation, P.O. Box 79, West Mifflin, Pennsylvania

F. Petrovich; J. J. Bevelacqua

1977-01-01

430

Excitation energies of dissociating H2: A problematic case for the adiabatic approximation of time-dependent density functional theory  

NASA Astrophysics Data System (ADS)

Time-dependent density functional theory (TDDFT) is applied for calculation of the excitation energies of the dissociating H2 molecule. The standard TDDFT method of adiabatic local density approximation (ALDA) totally fails to reproduce the potential curve for the lowest excited singlet 1?u+ state of H2. Analysis of the eigenvalue problem for the excitation energies as well as direct derivation of the exchange-correlation (xc) kernel fxc(r,r',?) shows that ALDA fails due to breakdown of its simple spatially local approximation for the kernel. The analysis indicates a complex structure of the function fxc(r,r',?), which is revealed in a different behavior of the various matrix elements K1c,1cxc (between the highest occupied Kohn-Sham molecular orbital ?1 and virtual MOs ?c) as a function of the bond distance R(H-H). The effect of nonlocality of fxc(r,r') is modeled by using different expressions for the corresponding matrix elements of different orbitals. Asymptotically corrected ALDA (ALDA-AC) expressions for the matrix elements K12,12xc(??) are proposed, while for other matrix elements the standard ALDA expressions are retained. This approach provides substantial improvement over the standard ALDA. In particular, the ALDA-