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1

Density-dependent Brueckner-Hartree-Fock approach in deformed nuclei  

Microsoft Academic Search

Recently a method has been developed which includes into the Brueckner-Hartree-Fock (BHF) approach in a finite nucleus those terms which produce the density dependence of the effective interaction between nucleons. This method is extended to the deformed open shell nuclei 12C and 20Ne. The results indicate that it is possible to describe these nuclei starting from a bare realistic nucleon-nucleon

H. Müther; Amand Faessler; Ram K. Tripathi

1975-01-01

2

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

USGS Publications Warehouse

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

Holland, J. Nathaniel; DeAngelis, Donald L.

2010-01-01

3

Why Density Dependent Propulsion?  

NASA Technical Reports Server (NTRS)

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

Robertson, Glen A.

2011-01-01

4

Density dependent hadron field theory  

NASA Astrophysics Data System (ADS)

A fully covariant approach to a density dependent hadron field theory is presented. The relation between in-medium NN interactions and field-theoretical meson-nucleon vertices is discussed. The medium dependence of nuclear interactions is described by a functional dependence of the meson-nucleon vertices on the baryon field operators. As a consequence, the Euler-Lagrange equations lead to baryon rearrangement self-energies which are not obtained when only a parametric dependence of the vertices on the density is assumed. It is shown that the approach is energy-momentum conserving and thermodynamically consistent. Solutions of the field equations are studied in the mean-field approximation. Descriptions of the medium dependence in terms of the baryon scalar and vector density are investigated. Applications to infinite nuclear matter and finite nuclei are discussed. Density dependent coupling constants obtained from Dirac-Brueckner calculations with the Bonn NN potentials are used. Results from Hartree calculations for energy spectra, binding energies, and charge density distributions of 16O, 40,48Ca, and 208Pb are presented. Comparisons to data strongly support the importance of rearrangement in a relativistic density dependent field theory. Most striking is the simultaneous improvement of charge radii, charge densities, and binding energies. The results indicate the appearance of a new ``Coester line'' in the nuclear matter equation of state.

Fuchs, C.; Lenske, H.; Wolter, H. H.

1995-12-01

5

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

SciTech Connect

We present a time-dependent localized Hartree-Fock density-functional linear response approach for the treatment of photoionization of atomic systems. This approach employs a spin-dependent localized Hartree-Fock exchange potential to calculate electron orbitals and kernel functions, and thus can be used to study the photoionization from atomic excited states. We have applied the approach to the calculation of photoionization cross sections of Ne ground state. The results are in agreement with available experimental data and have comparable accuracies with other ab initio theoretical results. We have also extended the approach to explore the photoionization from Ne excited states and obtained some results for the photoionization from outer-shell and inner-shell excited states.

Zhou Zhongyuan; Chu, S.-I [Department of Chemistry, University of Kansas, Lawrence, Kansas 66045 (United States)

2009-05-15

6

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

PubMed

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 N(2), CO, C(6)H(6), H(2)CO and the C(2)H(4)-C(2)F(4) 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. PMID:22510817

Zhang, Xu; Li, Zi; Lu, Gang

2012-05-23

7

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

8

The spin-flip approach within time-dependent density functional theory: Theory and applications to diradicals  

NASA Astrophysics Data System (ADS)

An extension of density functional theory to situations with significant nondynamical correlation is presented. The method is based on the spin-flip (SF) approach which is capable of describing multireference wave functions within a single reference formalism as spin-flipping, e.g., ?-->?, excitations from a high-spin (Ms=1) triplet reference state. An implementation of the spin-flip approach within the Tamm-Dancoff approximation to time-dependent density functional theory (TDDFT) is presented. The new method, SF-TDDFT/TDA or simply SF-DFT, describes target states (i.e., closed- and open-shell singlets, as well as low-spin triplets) by linear response from a reference high-spin triplet (Ms=1) Kohn-Sham state. Contrary to traditional TDDFT, the SF-DFT response equations are solved in a subspace of spin-flipping operators. The method is applied to bond-breaking (ethylene torsional potential), and equilibrium properties of eight diradicals. The results demonstrate significant improvement over traditional Kohn-Sham DFT, particularly for 50/50 hybrid functional.

Shao, Yihan; Head-Gordon, Martin; Krylov, Anna I.

2003-03-01

9

Inner-shell excitation of open-shell atoms: a spin-dependent localized Hartree Fock density-functional approach  

NASA Astrophysics Data System (ADS)

We present a spin-dependent localized Hartree-Fock (SLHF) density-functional approach for the treatment of inner-shell excited states of open-shell atomic systems. In this approach, the electron spin-orbitals and single-Slater-determinant energies of an electronic configuration are computed by solving the Kohn-Sham (KS) equation with SLHF exchange potential. The multiplet energy of an inner-shell excited state is evaluated from the single-Slater-determinant energies in terms of Slater's diagonal sum rule. Based on this procedure, we perform calculations of the total and excitation energies of inner-shell excited states of open-shell atomic systems: Li, B, Ne+, Ne2+, Ne3+ and Na. In the calculation, the electron correlation effect is taken into account via the correlation potential and energy functional of Perdew and Wang (PW) or of Lee, Yang and Parr (LYP). The calculated results are in good agreement with the available experimental and other ab initio theoretical data. In addition, new results for highly excited inner-shell states are also presented.

Zhou, Zhongyuan; Chu, Shih-I.

2007-11-01

10

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

11

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

SciTech Connect

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

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

2010-11-07

12

Investigation of intense femto-second laser ionization and dissociation of methane with time-dependent density-functional approach  

NASA Astrophysics Data System (ADS)

Three dimensional calculations of electronic dynamics of CH4 in a strong laser field are presented with time-dependent density-functional theory. Time evolution of dipole moment and electron localization function is presented. The dependence of dissociation rate on the laser characters is shown and optimal effective parameters are evaluated. The optimum field leads to 76% dissociation probability for GAUSSIAN envelope and 40 fs (FWHM) at 1016 W cm-2. The dissociation probability is calculated by optimum convolution of dual short pulses. By combining of field assisted dissociation process and Ehrenfest molecular dynamics, time variation of bond length, velocity and orientation effect are investigated.

Irani, E.; Sadighi-Bonabi, R.; Anvari, A.

2014-06-01

13

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

Microsoft Academic Search

We introduce turboTDDFT, an implementation of the Liouville–Lanczos approach to linearized time-dependent density-functional theory, designed to simulate the optical spectra of molecular systems made of up to several hundred atoms. turboTDDFT is open-source software distributed under the terms of the GPL as a component of Quantum ESPRESSO. As with other components, turboTDDFT is optimized to run on a variety of

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

2011-01-01

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

Time-dependent auxiliary density perturbation theory  

NASA Astrophysics Data System (ADS)

The recently developed auxiliary density perturbation theory is extended to time-dependent perturbations. As its static counterpart, it is based on auxiliary density functional theory in which the Coulomb and exchange-correlation potentials are expressed through one auxiliary function density. As in the case of static perturbations a noniterative alternative to the corresponding coupled perturbed Kohn-Sham method is formulated. The new methodology is validated by local and gradient corrected dynamical polarizability calculations. Comparison with experiment indicates that for low frequencies reliable dynamical polarizabilities are obtained. Our discussion also shows that the computational performance of time-dependent auxiliary density perturbation theory is similar to the previously described static approach. In order to demonstrate the potential of this new methodology, dynamic polarizabilities of C60, C180, and C240 are calculated.

Carmona-Espíndola, Javier; Flores-Moreno, Roberto; Köster, Andreas M.

2010-08-01

16

Evolution of transverse-momentum-dependent densities  

Microsoft Academic Search

We discuss different operator definitions of the transverse-momentum dependent (TMD) parton densities from the point of view of their renormalization-group (RG) properties and UV evolution. We also consider the structure of the gauge links (Wilson lines) in these operator definitions and examine the role of the soft factor in the factorization formula within the TMD approach to semi-inclusive processes.

I. O. Cherednikov; N. G. Stefanis

2011-01-01

17

Orbital-dependent correlation energy in density-functional theory based on a second-order perturbation approach: success and failure.  

PubMed

We have developed a second-order perturbation theory (PT) energy functional within density-functional theory (DFT). Based on PT with the Kohn-Sham (KS) determinant as a reference, this new ab initio exchange-correlation functional includes an exact exchange (EXX) energy in the first order and a correlation energy including all single and double excitations from the KS reference in the second order. The explicit dependence of the exchange and correlation energy on the KS orbitals in the functional fits well into our direct minimization approach for the optimized effective potential, which is a very efficient method to perform fully self-consistent calculations for any orbital-dependent functionals. To investigate the quality of the correlation functional, we have applied the method to selected atoms and molecules. For two-electron atoms and small molecules described with small basis sets, this new method provides excellent results, improving both second-order Moller-Plesset expression and any conventional DFT results significantly. For larger systems, however, it performs poorly, converging to very low unphysical total energies. The failure of PT based energy functionals is analyzed, and its origin is traced back to near degeneracy problems due to the orbital- and eigenvalue-dependent algebraic structure of the correlation functional. The failure emerges in the self-consistent approach but not in perturbative post-EXX calculations, emphasizing the crucial importance of self-consistency in testing new orbital-dependent energy functionals. PMID:16122290

Mori-Sánchez, Paula; Wu, Qin; Yang, Weitao

2005-08-01

18

Three-dimensional mixed finite element-finite volume approach for the solution of density-dependent flow in porous media  

NASA Astrophysics Data System (ADS)

The density-dependent flow and transport problem in groundwater on three-dimensional triangulations is solved numerically by means of a mixed hybrid finite element scheme for the flow equation combined with a mixed hybrid finite element-finite volume (MHFE-FV) time-splitting-based technique for the transport equation. This procedure is analyzed and shown to be an effective tool in particular when the process is advection dominated or when density variations induce the formation of instabilities in the flow field. From a computational point of view, the most effective strategy turns out to be a combination of the MHFE and a spatially variable time-splitting technique in which the FV scheme is given by a second-order linear reconstruction based on the least-squares minimization and the Barth-Jespersen limiter. The recent saltpool problem introduced as a benchmark test for density-dependent solvers is used to verify the accuracy and reliability of this approach.

Mazzia, Annamaria; Putti, Mario

2006-01-01

19

The evolution of density-dependent dispersal  

PubMed Central

Despite a large body of empirical evidence suggesting that the dispersal rates of many species depend on population density, most metapopulation models assume a density-independent rate of dispersal. Similarly, studies investigating the evolution of dispersal have concentrated almost exclusively on density-independent rates of dispersal. We develop a model that allows density-dependent dispersal strategies to evolve. Our results demonstrate that a density-dependent dispersal strategy almost always evolves and that the form of the relationship depends on reproductive rate, type of competition, size of subpopulation equilibrium densities and cost of dispersal. We suggest that future metapopulation models should account for density-dependent dispersal

Travis, J. M. J.; Murrell, D. J.; Dytham, C.

1999-01-01

20

Asymptotic correction approach to improving approximate exchange-correlation potentials: Time-dependent density-functional theory calculations of molecular excitation spectra  

NASA Astrophysics Data System (ADS)

The time-dependent density functional theory (TD-DFT) calculation of excitation spectra places certain demands on the DFT exchange-correlation potential, vxc, that are not met by the functionals normally used in molecular calculations. In particular, for high-lying excitations, it is crucial that the asymptotic behavior of vxc be correct. In a previous paper, we introduced a novel asymptotic-correction approach which we used with the local density approximation (LDA) to yield an asymptotically corrected LDA (AC-LDA) potential [Casida, Casida, and Salahub, Int. J. Quantum Chem. 70, 933 (1998)]. The present paper details the theory underlying this asymptotic correction approach, which involves a constant shift to incorporate the effect of the derivative discontinuity (DD) in the bulk region of finite systems, and a spliced asymptotic correction in the large r region. This is done without introducing any adjustable parameters. We emphasize that correcting the asymptotic behavior of vxc is not by itself sufficient to improve the overall form of the potential unless the effect of the derivative discontinuity is taken into account. The approach could be used to correct vxc from any of the commonly used gradient-corrected functionals. It is here applied to the LDA, using the asymptotically correct potential of van Leeuwen and Baerends (LB94) in the large r region. The performance of our AC-LDA vxc is assessed for the calculation of TD-DFT excitation energies for a large number of excitations, including both valence and Rydberg states, for each of four small molecules: N2, CO, CH2O, and C2H4. The results show a significant improvement over those from either the LB94 or the LDA functionals. This confirms that the DD is indeed an important element in the design of functionals. The quality of TDLDA/LB94 and TDLDA/AC-LDA oscillator strengths were also assessed in what we believe to be the first rigorous assessment of TD-DFT molecular oscillator strengths in comparison with high quality experimental and theoretical values. And a comparison has been given of TDLDA/AC-LDA excitation energies with other TD-DFT excitation energies taken from the literature, namely for the PBE0, HCTH(AC), and TDLDA/SAOP functionals. Insight into the working mechanism of TD-DFT excitation energy calculations is obtained by comparison with Hartree-Fock theory, highlighting the importance of orbital energy differences in TD-DFT.

Casida, Mark E.; Salahub, Dennis R.

2000-11-01

21

Pseudospectral time-dependent density functional theory.  

PubMed

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 in the excitation energy calculation, following earlier work employing the pseudospectral approximation for determining correlation energies in wavefunction-based methods with similar conclusions. The pseudospectral TDA-TDDFT method is shown to be up to ten times faster than a conventional algorithm for hybrid functionals without sacrificing chemical accuracy. PMID:18345873

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

2008-03-14

22

Pseudospectral time-dependent density functional theory  

NASA Astrophysics Data System (ADS)

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 in the excitation energy calculation, following earlier work employing the pseudospectral approximation for determining correlation energies in wavefunction-based methods with similar conclusions. The pseudospectral TDA-TDDFT method is shown to be up to ten times faster than a conventional algorithm for hybrid functionals without sacrificing chemical accuracy.

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

2008-03-01

23

Density Dependence of the Mass and Decay Constant of Pion in Nuclear Matter in the Dyson Schwinger Equation Approach of QCD  

Microsoft Academic Search

With the Dyson-Schwinger equation formalism at finite chemical potential, we study the density dependence of the mass and decay constant of pion in nuclear matter. The calculated results indicate that both the mass and the decay constant remain almost constant at small chemical potential. As the chemical potential gets quite large, the decay constant increases and the mass decreases with

Yu-Xin Liu; Jing-Yi Chao; Lei Chang; Wei Yuan

2005-01-01

24

Field line dependence of magnetospheric electron density  

Microsoft Academic Search

Observations of the electron density ne based on measurement of the upper hybrid resonance frequency by the Polar spacecraft Plasma Wave Instrument (PWI) are available for March, 1996 to September, 1997, during which time the Polar orbit sampled all MLT values three times. Using this data set, we assume a power law form for the electron density dependence along field

R. E. Denton; J. Goldstein; J. D. Menietti

2002-01-01

25

Three-dimensional mixed finite element-finite volume approach for the solution of density-dependent flow in porous media  

Microsoft Academic Search

The density-dependent flow and transport problem in groundwater on three-dimensional triangulations is solved numerically by means of a mixed hybrid finite element scheme for the flow equation combined with a mixed hybrid finite element-finite volume (MHFE-FV) time-splitting-based technique for the transport equation. This procedure is analyzed and shown to be an effective tool in particular when the process is advection

Annamaria Mazzia; Mario Putti

2006-01-01

26

Fragment-based time-dependent density functional theory.  

PubMed

Using the Runge-Gross theorem that establishes the foundation of time-dependent density functional theory, we prove that for a given electronic Hamiltonian, choice of initial state, and choice of fragmentation, there is a unique single-particle potential (dubbed time-dependent partition potential) which, when added to each of the preselected fragment potentials, forces the fragment densities to evolve in such a way that their sum equals the exact molecular density at all times. This uniqueness theorem suggests new ways of computing the time-dependent properties of electronic systems via fragment-time-dependent density functional theory calculations. We derive a formally exact relationship between the partition potential and the total density, and illustrate our approach on a simple model system for binary fragmentation in a laser field. PMID:23889390

Mosquera, Martín A; Jensen, Daniel; Wasserman, Adam

2013-07-12

27

Local density dependent potential for compressible mesoparticles.  

PubMed

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

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

2014-03-21

28

Partitioned density functional approach for a Lennard-Jones fluid  

NASA Astrophysics Data System (ADS)

The existing classical density functional approach for nonuniform Lennard-Jones fluid, which is based on dividing the Lennard-Jones interaction potential into a short-range, repulsive part, and a smoothly varying, long-range, attractive tail, was improved by dividing the bulk second-order direct correlation function into strongly density-depending short-range part and weakly density-depending long-range part. The latter is treated by functional perturbation expansion truncated at the lowest order whose accuracy depends on how weakly the long-range part depends on the bulk density. The former is treated by the truncated functional perturbation expansion which is rewritten in the form of the simple weighted density approximation and incorporates the omitted higher-order terms by applying Lagrangian theorem of differential calculus to the reformulated form. The two approximations are put into the density profile equation of the density functional theory formalism to predict the density distribution for Lennard-Jones fluid in contact with a hard wall or between two hard walls within the whole density range for reduced temperature T*=1.35 and a density point for reduced temperature T*=1. The present partitioned density functional theory performs much better than several previous density functional perturbation theory approaches and a recently proposed bridge density functional approximation.

Zhou, Shiqi

2003-12-01

29

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

PubMed

A long-range-corrected time-dependent density functional theory (LC-TDDFT) in combination with polarizable continuum model (PCM) have been applied to study charge transfer (CT) optical absorption and fluorescence emission energies basing on parameterized LC-BLYP xc-potential. The molecule of 4-(9-acridyl)julolidine selected for this study represents typical CT donor-acceptor dye with strongly solvent dependent optical absorption and fluorescence emission spectra. The result of calculations are compared with experimental spectra reported in the literature to derive an optimal value of the model screening parameter ?. The first absorption band appears to be quite well predictable within DFT/TDDFT/PCM with the screening parameter ? to be solvent independent (??0.245Bohr(-1)) whereas the fluorescence emission exhibits a strong dependence on the range separation with ?-value varying on a rising solvent polarity from about 0.225 to 0.151Bohr(-1). Dipolar properties of the initial state participating in the electronic transition have crucial impact on the effective screening. PMID:24682050

Kityk, A V

2014-07-15

30

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

NASA Astrophysics Data System (ADS)

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

Kityk, A. V.

2014-07-01

31

A Brief Compendium of Time-Dependent Density Functional Theory  

NASA Astrophysics Data System (ADS)

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

Ullrich, Carsten A.; Yang, Zeng-hui

2013-07-01

32

Density dependence: an ecological Tower of Babel.  

PubMed

The concept of density dependence represents the effect of changing population size on demographic rates and captures the demographic role of social and trophic mechanisms (e.g. competition, cooperation, parasitism or predation). Ecologists have coined more than 60 terms to denote different statistical and semantic properties of this concept, resulting in a formidable lexicon of synonymies and polysemies. We have examined the vocabulary of density dependence used in the modern ecological literature from the foundational lexicon developed by Smith, Allee, Haldane, Neave and Varley. A few simple rules suffice to abate terminological inconsistency and to enhance the biological meaning of this important concept. Correct citation of original references by ecologists and research journals could ameliorate terminological standards in our discipline and avoid linguistic confusion of mathematically and theoretically complex patterns. PMID:22648068

Herrando-Pérez, Salvador; Delean, Steven; Brook, Barry W; Bradshaw, Corey J A

2012-11-01

33

Density dependence of the stopping mean excitation energy  

SciTech Connect

The local-plasma model for the stopping mean excitation energy I(0) of Lindhard and Scharff is used to investigate the effects of target density. The use of the Thomas-Fermi electron density produces a universal curve for I(0)/Z, where Z is the nuclear charge number, as a function of the target density. Experimental data for I(0) are shown to be in good agreement with this predicted curve. A simple formula is derived for I(0) in the limit of high target density. This approach is known to predict a similar curve for the charge dependence of I(0)/Z for atomic ion targets. Numerical comparisons and an asymptotic formula for the large-charge limit are presented for this case.

Peek, J.M.

1987-12-01

34

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

PubMed Central

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

Poethke, Hans Joachim; Hovestadt, Thomas

2002-01-01

35

Selective fishing induces density-dependent growth.  

PubMed

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

Svedäng, Henrik; Hornborg, Sara

2014-01-01

36

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

PubMed Central

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

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

2014-01-01

37

THE DEPENDENCE OF STAR FORMATION EFFICIENCY ON GAS SURFACE DENSITY  

SciTech Connect

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

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

2013-08-10

38

The Dependence of Star Formation Efficiency on Gas Surface Density  

NASA Astrophysics Data System (ADS)

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

Burkert, Andreas; Hartmann, Lee

2013-08-01

39

Semiclassical Approaches in Density Functional Theory  

NASA Astrophysics Data System (ADS)

We discuss the use of semiclassical methods in understanding, and improving, density functional theory (DFT). It has been found[1] that semiclassical approaches can explain many features of modern exchange-correlation approximations, such as the local density approximation or generalized gradient approximations. In this work, we continue this line of inquiry, showing how semiclassical approximations may be used to construct a form for the non-interacting kinetic energy density. A semiclassical approximation to the Green's function is made[2], which, when integrated appropriately, yields leading corrections to the Thomas-Fermi result. It can be regarded as a functional of the potential. We test this approximation for various 1D systems confined within a box and present the results. This, coupled with the corresponding form of the density[2], could provide an orbital-free DFT which would allow more complex systems to be studied.[1] J. Perdew, A. Ruzsinszky, G.I. Csonka, O.A. Vydrov, G.E. Scuseria, L.A. Constantin, X. Zhou, and K. Burke, arXiv:0707.2088 (2007).[2] W. Kohn and L.J. Sham, Phys. Rev. 137, A1697 (1965).

Elliott, Peter; Cangi, Attila; Lee, Donghyung; Burke, Kieron

2008-03-01

40

Inverse density dependence and the Allee effect  

Microsoft Academic Search

The Allee effect describes a scenario in which populations at low numbers are affected by a positive relationship between population growth rate and density, which increases their likelihood of extinction. The importance of this dynamic process in ecology has been under-appreciated and recent evidence now suggests that it might have an impact on the population dynamics of many plant and

Franck Courchamp; Tim Clutton-Brock; Bryan Grenfell

1999-01-01

41

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

National Technical Information Service (NTIS)

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

M. L. Williams C. R. Weisbin

1978-01-01

42

Pathogens, density dependence and the coexistence of tropical trees.  

PubMed

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, in particular, require a careful analysis to avoid biases generated by measurement error, and existing studies could be criticized on these grounds. Experimental studies manipulating plant density and pathogen incidence will often be preferable, or should be run in parallel. Second, the form of density dependence is not well understood and, in particular, there are no data indicating whether pathogens cause compensating or overcompensating density responses. Owing to this, we argue that the potential for pathogen-induced density dependence to generate diversity-enhancing outcomes, such as the Janzen-Connell effect, remains uncertain, as coexistence is far more probable if density dependence is overcompensating. Third, there have been few studies examining the relative importance of intra- or interspecific density dependence resulting from pathogens (or, more widely, natural enemies). This is essentially equivalent to asking to what extent pathogens are host-specific. If pathogens are generalists, then mortality rates will respond to overall plant density, irrespective of plant species identity. This will weaken the intraspecific density dependence and reduce the diversity-promoting effects of pathogens. Finally, we highlight the need for studies that integrate observations and experiments on pathogens and density dependence into the whole life cycle of trees, because as yet it is not possible to be certain of the degree to which pathogens contribute to observed dynamics. PMID:17015362

Freckleton, Robert P; Lewis, Owen T

2006-12-01

43

An Infrastructureless Approach to Estimate Vehicular Density in Urban Environments  

PubMed Central

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

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

2013-01-01

44

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

NASA Astrophysics Data System (ADS)

We establish a correlation for the symmetry energy at saturation density S0, slope parameter L, and curvature parameter Ksym based on widely different mean-field interactions. With the help of this correlation and available empirical and theoretical information, the density-dependent behavior around the saturation density is determined. We compare the results obtained with the present approach with those by other analyses. With this obtained density-dependent behavior of the symmetry energy, the neutron skin thickness of 208Pb and some properties of neutron stars are investigated. In addition, it is found that the expression S(?)=S0(?/?0)? or S(?)=12.5?/?02/3+Cp?/?0? does not reproduce the density dependence of the symmetry energy as predicted by the mean-field approach around nuclear saturation density.

Dong, Jianmin; Zuo, Wei; Gu, Jianzhong; Lombardo, Umberto

2012-03-01

45

Effects of density-dependent lepton fraction on the properties of protoneutron stars  

NASA Astrophysics Data System (ADS)

The density-dependent lepton fractions approach that is used to describe the properties of matter with neutrino trapping is systematically studied. It is found that two sets of the ratio of trapped neutrinos to leptons parameters—i.e., one for a relativistic mean field parameter set with stiff equation of state and one for a parameter set with soft equation of state—yield particle composition profiles at high densities similar to those predicted by the standard fixed lepton fraction approach. However, these sets of parameters produce significantly different behavior at low densities compared to those of standard approach. The consequences of applying the density-dependent lepton fractions approach to some properties of PNS, such as the particle composition, equation of state, the onset of low-density instability, the mass-radius relation, and the mean free path of electron neutrino, are investigated. By comparing the results with those of the standard approach, we obtain the result that the PNS equation of state is not significantly influenced by the low-density behavior difference between the two approaches. However, the density-dependent lepton fractions approach yields a smaller onset of low-density instability matter and core-crust transition density but a slightly larger maximum mass as well as a larger radius of canonical mass than those predicted by the standard approach. The estimated minimum PNS mass and its radius are apparently also sensitive to the parameter used for the ratio of trapped neutrinos to leptons. For electron neutrino transport in PNSs, matter with density-dependent lepton fractions is more transparent than matter with fixed lepton fractions.

Sulaksono, A.; Satiawati, L.

2013-06-01

46

Regulation of Aedes aegypti Population Dynamics in Field Systems: Quantifying Direct and Delayed Density Dependence  

PubMed Central

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.

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

2013-01-01

47

Density dependence of reactor performance with thermal confinement scalings  

SciTech Connect

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

Stotler, D.P.

1992-03-01

48

Structural risk minimization: a robust method for density-dependence detection and model selection  

Microsoft Academic Search

Statistically distinguishing density-dependent from density-independent populations and selecting the best demographic model for a given population are problems of primary importance. Traditional approaches are PBLR (parametric bootstrapping of likelihood ratios) and Information criteria (IC), such as the Schwarz information criterion (SIC), the Akaike information criterion (AIC) or the Final prediction error (FPE). While PBLR is suitable for choosing from a

Giorgio Corani; Marino Gatto

2007-01-01

49

A Wigner Monte Carlo approach to density functional theory  

NASA Astrophysics Data System (ADS)

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

Sellier, J. M.; Dimov, I.

2014-08-01

50

Orbital-Dependent Density Functionals for Chemical Catalysis.  

National Technical Information Service (NTIS)

We developed two new orbital-dependent density functionals, M08-HX and M08-SO that have excellent performance for predicting noncovalent interactions and chemical reaction barrier heights. We carried out several applications, tests, and validations of den...

D. G. Truhlar

2011-01-01

51

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

52

Density-dependent recruitment in grassland small mammals  

Microsoft Academic Search

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

Aaron W. Reed; Norman A. Slade

2008-01-01

53

Dynamical instabilities in density-dependent hadronic relativistic models  

SciTech Connect

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

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

2008-04-15

54

Spatial variation and density-dependent dispersal in competitive coexistence.  

PubMed Central

It is well known that dispersal from localities favourable to a species' growth and reproduction (sources) can prevent competitive exclusion in unfavourable localities (sinks). What is perhaps less well known is that too much emigration can undermine the viability of sources and cause regional competitive exclusion. Here, I investigate two biological mechanisms that reduce the cost of dispersal to source communities. The first involves increasing the spatial variation in the strength of competition such that sources can withstand high rates of emigration; the second involves reducing emigration from sources via density-dependent dispersal. I compare how different forms of spatial variation and modes of dispersal influence source viability, and hence source-sink coexistence, under dominance and pre-emptive competition. A key finding is that, while spatial variation substantially reduces dispersal costs under both types of competition, density-dependent dispersal does so only under dominance competition. For instance, when spatial variation in the strength of competition is high, coexistence is possible (regardless of the type of competition) even when sources experience high emigration rates; when spatial variation is low, coexistence is restricted even under low emigration rates. Under dominance competition, density-dependent dispersal has a strong effect on coexistence. For instance, when the emigration rate increases with density at an accelerating rate (Type III density-dependent dispersal), coexistence is possible even when spatial variation is quite low; when the emigration rate increases with density at a decelerating rate (Type II density-dependent dispersal), coexistence is restricted even when spatial variation is quite high. Under pre-emptive competition, density-dependent dispersal has only a marginal effect on coexistence. Thus, the diversity-reducing effects of high dispersal rates persist under pre-emptive competition even when dispersal is density dependent, but can be significantly mitigated under dominance competition if density-dependent dispersal is Type III rather than Type II. These results lead to testable predictions about source-sink coexistence under different regimes of competition, spatial variation and dispersal. They identify situations in which density-independent dispersal provides a reasonable approximation to species' dispersal patterns, and those under which consideration of density-dependent dispersal is crucial to predicting long-term coexistence.

Amarasekare, Priyanga

2004-01-01

55

Experimental charge density matching approach to zeolite synthesis  

Microsoft Academic Search

An Experimental Charge Density Matching (ECDM) approach to zeolite synthesis is described. The method features the initial formation of a Charge Density Mismatch (CDM) aluminosilicate reaction mixture characterized by the mismatch between the charge density on the organoammonium structure directing agent (SDA) and the charge density on the potential aluminosilicate network that is expected to form. This amounts to creating

G. J. Lewis; M. A. Miller; J. G. Moscoso; B. A. Wilson; L. M. Knight; S. T. Wilson

2004-01-01

56

Cubic response functions in time-dependent density functional theory.  

PubMed

We present density-functional theory for time-dependent response functions up to and including cubic response. The working expressions are derived from an explicit exponential parametrization of the density operator and the Ehrenfest principle, alternatively, the quasienergy ansatz. While the theory retains the adiabatic approximation, implying that the time-dependency of the functional is obtained only implicitly-through the time dependence of the density itself rather than through the form of the exchange-correlation functionals-it generalizes previous time-dependent implementations in that arbitrary functionals can be chosen for the perturbed densities (energy derivatives or response functions). In particular, general density functionals beyond the local density approximation can be applied, such as hybrid functionals with exchange correlation at the generalized-gradient approximation level and fractional exact Hartree-Fock exchange. With our implementation the response of the density can always be obtained using the stated density functional, or optionally different functionals can be applied for the unperturbed and perturbed densities, even different functionals for different response order. As illustration we explore the use of various combinations of functionals for applications of nonlinear optical hyperpolarizabilities of a few centrosymmetric systems; molecular nitrogen, benzene, and the C(60) fullerene. Considering that vibrational, solvent, and local field factors effects are left out, we find in general that very good experimental agreement can be obtained for the second dynamic hyperpolarizability of these systems. It is shown that a treatment of the response of the density beyond the local density approximation gives a significant effect. The use of different functional combinations are motivated and discussed, and it is concluded that the choice of higher order kernels can be of similar importance as the choice of the potential which governs the Kohn-Sham orbitals. PMID:15740310

Jansik, Branislav; Sa?ek, Pawe?; Jonsson, Dan; Vahtras, Olav; Agren, Hans

2005-02-01

57

Electron correlation in metals: Density matrix approach  

Microsoft Academic Search

The conceptual simplicity gained by the use of density matrices to describe the ground state of an electron gas moving in\\u000a the field of a uniform background of positive charge (Sommerfeld model of a metal) is stressed. The diagonal element of the\\u000a second-order density matrix (pair function) for a low-density electron gas has been discussed, previously byMarch andYoung [1], and

N. H. March; S. Sampanthar

1962-01-01

58

Transverse momentum dependent gluon density from DIS precision data  

NASA Astrophysics Data System (ADS)

The combined measurements of proton's structure functions in deeply inelastic scattering at the HERA collider provide high-precision data capable of constraining parton density functions over a wide range of the kinematic variables. We perform fits to these data using transverse momentum dependent QCD factorization and CCFM evolution. The results of the fits to precision measurements are used to make a determination of the nonperturbative transverse momentum dependent gluon density function, including experimental and theoretical uncertainties. We present an application of this density function to vector boson + jet production processes at the LHC.

Hautmann, F.; Jung, H.

2014-06-01

59

Magnetic field dependence of superfluid density in cuprate superconductors  

NASA Astrophysics Data System (ADS)

Within the kinetic energy driven superconducting mechanism, the doping and magnetic field dependence of the superfluid density in cuprate superconductors is studied. The electromagnetic response kernel is evaluated by considering both couplings of the electron charge and electron magnetic momentum with an external magnetic field and employed to calculate the superfluid density based on the specular reflection model for a purely transverse vector potential, then the main features of the doping and magnetic field dependence of the superfluid density in cuprate superconductors are well reproduced.

Feng, Shiping; Zhao, Huaisong; Huang, Zheyu

2012-12-01

60

Dependence of polar hole density on magnetic and solar conditions  

NASA Technical Reports Server (NTRS)

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

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

1991-01-01

61

Founder takes all: density-dependent processes structure biodiversity.  

PubMed

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

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

2013-02-01

62

Transverse momentum dependent quark densities from Lattice QCD  

SciTech Connect

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

Musch, B. U. [Theory Center, Jefferson Lab, 12000 Jefferson Avenue, Newport News, VA 23606 (United States); Haegler, Ph. [Theoretische Physik T39, TU Muenchen, James-Franck-Strasse 1, 85747 Garching (Germany); Negele, J. W. [Massachusetts Institute of Technology, 77 Massachusetts Avenue, Bldg. 6-315, Cambridge, MA 02139 (United States); Schaefer, A. [Institut fuer Theoretische Physik, Universitaet Regensburg, 93040 Regensburg (Germany)

2011-10-24

63

Density-dependence as a size-independent regulatory mechanism  

Microsoft Academic Search

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

Harold P. de Vladar

2006-01-01

64

Time-dependent density functional theory based Ehrenfest dynamics  

NASA Astrophysics Data System (ADS)

Time-dependent density functional theory based Ehrenfest dynamics with atom-centered basis functions is developed in present work. The equation of motion for electrons is formulated in terms of first-order reduced density matrix and an additional term arises due to the time-dependence of basis functions through their dependence on nuclear coordinates. This time-dependence of basis functions together with the imaginary part of density matrix leads to an additional term for nuclear force. The effects of the two additional terms are examined by studying the dynamics of H2 and C2H4, and it is concluded that the inclusion of these two terms is essential for correct electronic and nuclear dynamics.

Wang, Fan; Yam, Chi Yung; Hu, Lihong; Chen, Guanhua

2011-07-01

65

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

66

Evolution of density-dependent dispersal in a structured metapopulation.  

PubMed

We study the evolution of density-dependent dispersal in a structured metapopulation subject to local catastrophes that eradicate local populations. To this end we use the theory of structured metapopulation dynamics and the theory of adaptive dynamics. The set of evolutionarily possible dispersal functions (i.e., emigration rates as a function of the local population density) is derived mechanistically from an underlying resource-consumer model. The local resource dynamics is of a flow-culture type and consumers leave a local population with a constant probability per unit of time kappa when searching for resources but not when handling resources (i.e., eating and digesting). The time an individual spends searching (as opposed to handling) depends on the local resource density, which in turn depends on the local consumer density, and so the average per capita emigration rate depends on the local consumer density as well. The derived emigration rates are sigmoid functions of local consumer population density. The parameters of the local resource-consumer dynamics are subject to evolution. In particular, we find that there exists a unique evolutionarily stable and attracting dispersal rate kappa( *) for searching consumers. The kappa( *) increases with local resource productivity and decreases with resource decay rate. The kappa( *) also increases with the survival probability during dispersal, but as a function of the catastrophe rate it reaches a maximum before dropping off to zero again. PMID:19361534

Geritz, Stefan A H; Gyllenberg, Mats; Ondrácek, Petr

2009-06-01

67

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

68

Parity dependence of level densities in sup 49 V  

SciTech Connect

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

York, B.W.

1991-01-01

69

A new approach to density functional theory  

NASA Astrophysics Data System (ADS)

I will explain a new way to think about density functional theory, based on a simple principle: asymptotic exactness for large particle number. This explains many features of existing functionals [1], and makes the connection between semiclassical and density functional approximations. It underlies the restoration of the gradient expansion in PBEsol[2]. It also provides a path toward highly accurate, orbital-free, non-local functionals of the potential, for both the density itself and the non-interacting kinetic energy.[1] J.P. Perdew, L.A. Constantin, E. Sagvolden, and K. Burke, Phys. Rev. Lett. 97, 223002 (2006).[2] J.P. Perdew, A. Ruzsinszky, G.I. Csonka, O.A. Vydrov, G.E. Scuseria, L.A. Constantin, X. Zhou, and K. Burke, arXiv:0707.2088 (2007).

Burke, Kieron; Elliott, Peter; Cangi, Attila; Lee, Donghyung

2008-03-01

70

Simple preconditioning for time-dependent density functional perturbation theory  

NASA Astrophysics Data System (ADS)

By far, the most common use of time-dependent density functional theory is in the linear-reponse regime, where it provides information about electronic excitations. Ideally, the linear-response equations should be solved by a method that avoids the use of the unoccupied Kohn-Sham states -- such as the Sternheimer method -- as this reduces the complexity and increases the precision of the calculation. However, the Sternheimer equation becomes ill-conditioned near and indefinite above the first resonant frequency, seriously hindering the use of efficient iterative solution methods. To overcome this serious limitation, and to improve the general convergence properties of the iterative techniques, we propose a simple preconditioning strategy. In our method, the Sternheimer equation is solved directly as a linear equation using an iterative Krylov subspace method, i.e., no self-consistent cycle is required. Furthermore, the preconditioner uses the information of just a few unoccupied states and requires simple and minimal modifications to existing implementations. In this way, convergence can be reached faster and in a considerably wider frequency range than the traditional approach.

Lehtovaara, Lauri; Marques, Miguel A. L.

2011-07-01

71

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

PubMed

When nest predation levels are very high or very low, the absolute range of observable nest success is constrained (a floor/ceiling effect), and it may be more difficult to detect density-dependent nest predation. Density-dependent nest predation may be more detectable in years with moderate predation rates, simply because there can be a greater absolute difference in nest success between sites. To test this, we replicated a predation experiment 10 years after the original study, using both natural and artificial nests, comparing a year when overall rates of nest predation were high (2000) to a year with moderate nest predation (2010). We found no evidence for density-dependent predation on artificial nests in either year, indicating that nest predation is not density-dependent at the spatial scale of our experimental replicates (1-ha patches). Using nearest-neighbor distances as a measure of nest dispersion, we also found little evidence for "dispersion-dependent" predation on artificial nests. However, when we tested for dispersion-dependent predation using natural nests, we found that nest survival increased with shorter nearest-neighbor distances, and that neighboring nests were more likely to share the same nest fate than non-adjacent nests. Thus, at small spatial scales, density-dependence appears to operate in the opposite direction as predicted: closer nearest neighbors are more likely to be successful. We suggest that local nest dispersion, rather than larger-scale measures of nest density per se, may play a more important role in density-dependent nest predation. PMID:22179311

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

2012-07-01

72

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

USGS Publications Warehouse

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

Josh T Ackerman;Ringelman, KM;Eadie, JM

2012-01-01

73

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

74

Time-Dependent Density Functional Theory from a Practitioners Perspective  

NASA Astrophysics Data System (ADS)

We give a short overview of time-dependent density functional theory applied to dynamics of metal clusters. The actual level of treatment is the time-dependent local-density approximation (TDLDA) coupled to classical molecular dynamics for the ions (TDLDA-MD). We discuss the performance of LDA for this particular application and necessary extensions, as e.g. the self-interaction correction (SIC). Finally, a few selected examples of cluster dynamics are presented, from optical response to pump and probe experiments.

Andrae, K.; Pohl, A.; Reinhard, P.-G.; Legrand, C.; Ma, M.; Suraud, E.

2003-04-01

75

Seasonal Density Dependence in Atlantic Salmon over Varying Spatial Scales  

Microsoft Academic Search

Scale and ontogeny are important in understanding how various ecological processes structure populations. This is expected to be the case for density-dependent effects (DDE), which influence the population dynamics of many organisms. Our goal was to evaluate stage-specific DDE on the growth, movement, and survival of Atlantic salmon Salmo salar from the individual to the group level over varying spatial

Michael M. Bailey; Gregg E. Horton; Benjamin H. Letcher; Michael T. Kinnison

2010-01-01

76

A study of density and momentum dependent delta function forces  

Microsoft Academic Search

The suitability of density and momentum dependent delta function forces in nuclear calculations is studied using Hartee-Fock calculations in the 2s-1d shell. The effect of the compressibility upon deformation and rms radius is investigated. Permanent address, Department of Physics, McMaster University, Hamilton 16, Ontario, Canada.

K. R. Lassey; A. B. Volkov

1971-01-01

77

Understanding Bacteriophage Therapy as a Density-dependent Kinetic Process  

Microsoft Academic Search

Studies of bacteriophage as therapeutic agents have had mixed and unpredictable outcomes. We argue that interpretation of these apparently paradoxical results requires appreciation of various density-dependent threshold effects. We use a mathematical model to delineate different categories of outcome, including therapy by simple inundation, by active biocontrol, and by delayed active biocontrol. Counter-intuitively, there are situations in which earlier inoculation

ROBERT J. H. PAYNE; VINCENT A. A. JANSEN

2001-01-01

78

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

79

Density-dependent regulation of arbuscular mycorrhiza by collembola  

Microsoft Academic Search

In a field microcosm experiment, we tested the hypothesis that adult collembola, depending on their density, may enhance or decrease mycorrhizal development and growth of maize (Zea mays L.) or red fescue (Festuca rubra L.). Collembola decreased mycorrhizal spore number in the soil. A significant decrease in the spore number, compared to the treatment without collembola, was recorded and ranged

G. Bakonyi; K. Posta; I. Kiss; M. Fábián; P. Nagy; J. N. Nosek

2002-01-01

80

Dependence of the thermospheric density on solar and magnetic activity  

NASA Astrophysics Data System (ADS)

The thermospheric mass density is dependent on a number of external and internal forces. Here we will focus on the prominent effects of solar extreme ultra-violet (EUV) and magnetic activity. We make use of air density values derived from accelerometer measurements from the two satellites CHAMP and GRACE, thus providing readings from two altitudes. Our aim is to determine the functional relation between the forcing and the thermospheric response. The solar EUV flux is approximated by a composite quantity, P10.7 = 0.5 (F10.7 + F10.7a), where F10.7a is an average of the solar flux index F10.7 averaged over 81 days. For isolating the solar influence only quiet days (Ap < 15) have been considered. As a result we obtain an excellent linear relation between P10.7 and mass density. In a second step we normalise the density readings to a fixed solar flux level and investigate the response of thermospheric density to magnetic activity. For our investigation 30 magnetic storms have been considered. The solar wind merging electric field turned out to be a suitable controlling parameter. When taking into account some delayed response to the solar wind input we find a linear dependence of the density increase on the merging E-field. Interestingly, the solar wind input causes an additive increase of the density, on top of the quiet time background, whereas the solar flux effect causes a proportional increase. The linear scaling factor between the mass density increase and the merging E-field becomes smaller at higher pressure levels. We will propose a mechanism that may explain the thermospheric variations at the two different measurement heights.

Lühr, H.; Häusler, K.

2012-04-01

81

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

82

Angular Momentum Dependent Orbital Free Density Functional Theory  

NASA Astrophysics Data System (ADS)

We report a novel and general formalism for linear scaling, angular momentum dependent (AMD) orbital free (OF) density functional theory (DFT) to advance the accuracy and applicability of OFDFT. To introduce angular momentum dependence in OFDFT, we devise a hybrid scheme by partitioning the system into muffin-tin spheres and an interstitial region: the electron density inside the spheres is expressed by a set of Kohn-Sham (KS) DFT derived atom-centered basis functions combined with an on-site density matrix NR. A general OFDFT total energy functional is introduced with a crucial nonlocal energy term E^NL which is neglected in conventional implementations of OFDFT. E^NL corrects the errors due to the use of approximate kinetic energy density functionals and local pseudopotentials for ion-electron interactions. We approximate E^NL to include AMD contributions inside the spheres: as a first step, a linear dependence on the NR is considered with a set of AMD energies E^lR. E^lR are determined by fitting a small set of bulk properties to KSDFT. We find AMD-OFDFT offers substantial improvements over conventional OFDFT, as we show for various properties of the transition metal Ti and its alloys (TixAl1-x).

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

2013-03-01

83

Energy dependence with the number of particles: Density and reduced density matrices functionals  

NASA Astrophysics Data System (ADS)

The energy of a physical domain within a molecular system considered as a quantum open system is analyzed as a functional of the electron distribution dependence with the number of particles. Our attention is focused upon the constrained-search functionals of the electron density, the 1- and 2-reduced density matrices (1-, 2-RDMs) for grand-canonical states. It is shown that functionals of the 2-RDM depend on the number of particles if the ground state energy is not a convex function of them.

Miranda-Quintana, Ramón A.; Bochicchio, Roberto C.

2014-02-01

84

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

85

Applications of Time-Dependent Density Functional Theory  

NASA Astrophysics Data System (ADS)

The progress of experimental techniques makes more and more precise measurements available. Their interpretation requires improved theoretical tools. The time-dependent density functional theory (TDDFT) allows to study electronic excitations involved in spectroscopic experiments, possibly conÂäserving a computational effort comparable to that of ground-state density functional theory. I will present some applications of TDDFT to the calculation of absorption and electron energy loss spectra in extended and finite systems, discussing the advantages and the limits of different approximation schemes.

Botti, Silvana

86

Time-dependent density-matrix-functional theory  

NASA Astrophysics Data System (ADS)

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

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

2007-01-01

87

Orbital- and state-dependent functionals in density-functional theory.  

PubMed

Shortcomings of present density-functional methods are considered. Kohn-Sham and time-dependent density-functional methods using orbital- and state-dependent functionals for exchange-correlation energies, potentials, and kernels are discussed as possible remedy for some of these shortcomings. A view on the Kohn-Sham formalism is presented which differs somewhat from the one conventionally taken. The crucial step of constructing local multiplicative exchange-correlation potentials in Kohn-Sham methods based on orbital- and state-dependent functionals is discussed. The description of open-shell systems via a symmetrized Kohn-Sham formalism employing state-dependent exchange-correlation functionals is elucidated. The generalized adiabatic connection Kohn-Sham approach for the self-consistent treatment of excited states within a density-functional framework is considered. In the latter approach orbital- and state-dependent exchange-correlation functionals occur in a density-functional framework which is no longer based on the Hohenberg-Kohn theorem but on a more general relation between electron densities and local multiplicative potentials. PMID:16122289

Görling, Andreas

2005-08-01

88

Modelling interactions of toxicants and density dependence in wildlife populations  

USGS Publications Warehouse

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

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

2013-01-01

89

Density dependence of the ionization avalanche in ultracold Rydberg gases  

NASA Astrophysics Data System (ADS)

We report on the behavior of the ionization avalanche in an ensemble of ultracold 87Rb atoms coupled to a high-lying Rydberg state and investigate extensions to the current model by including the effects of three-body recombination and plasma expansion. To separate the two effects we study the time dependence of the plasma formation at various densities as well as for different nS and nD states. At medium densities and low n we observe the onset of the avalanche as has been reported in other experiments, as well as a subsequent turn-off of the avalanche for longer excitation times, which we associate with plasma expansion. At higher densities and for higher-lying Rydberg states we observe a disappearance of the avalanche signature, which we attribute to three-body recombination.

Siercke, M.; Oon, F. E.; Mohan, A.; Wang, Z. W.; Lim, M. J.; Dumke, R.

2014-02-01

90

The dependence of natural graphite anode performance on electrode density  

SciTech Connect

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

Shim, Joongpyo; Striebel, Kathryn A.

2003-11-01

91

Density and Temperature Dependence of X-ray Thomson Scatteringjj  

NASA Astrophysics Data System (ADS)

X-ray Thomson Scattering (XRTS) provides a diagnostic tool for state-variable determination in the Warm Dense Matter (WDM) regime. We explore the density and temperature dependence of XRTS using a finite-temperature generalization of the real-space Green's function (RSGF) method. This method treats the interaction between the ionic cores and free/valence electrons non-perturbatively using full multiple-scattering theoryfootnotetextJ. J. Rehr, et al., Comptes Rendus Physique, 10, 548 (2009). Compared to perturbative treatments, we find a significantly modified density-dependence of XRTS. This modification, which is relevant for the extraction of state-variables from WDM experiments, remains when temperature is included. The effects of an exchange-correlation potential appropriate for WDM is also discussed.

Mattern, B. A.; Seidler, G. T.; Rehr, J. J.

2013-03-01

92

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

93

Time-Dependent Local Density Approximation Calculations for Absorption Spectra of Small Carbon Clusters  

NASA Astrophysics Data System (ADS)

Absorption spectra for Cn clusters (n = 2-8) are calculated using an adiabatic time-dependent density functional formalism within the local density approximation (TDLDA). We compare the calculated spectra with those computed using a simple local density approximation approach. It is readily observed that the TDLDA spectra display a significant blue shift with respect to the Kohn-Sham eigenvalue spectra. The calculated spectra exhibit a variety of features that can be used for comparison against future experimental investigations. We also obtain a significant threshold absorption, which can distinguish different ground states of the carbon clusters.

Bai, Yu-Lin; Cheng, Xiao-Hong; Cheng, Yan; Chen, Xiang-Rong; Yang, Xiang-Dong

2004-12-01

94

Quantum Drude friction for time-dependent density functional theory.  

PubMed

Friction is a desired property in quantum dynamics as it allows for localization, prevents backscattering, and is essential in the description of multistage transfer. Practical approaches for friction generally involve memory functionals or interactions with system baths. Here, we start by requiring that a friction term will always reduce the energy of the system; we show that this is automatically true once the Hamiltonian is augmented by a term of the form integral a(q;n(0)) x [partial differential j(q,t)/partial differential t] x J(q)dq, which includes the current operator times the derivative of its expectation value with respect to time, times a local coefficient; the local coefficient will be fitted to experiment, to more sophisticated theories of electron-electron interaction and interaction with nuclear vibrations and the nuclear background, or alternately, will be artificially constructed to prevent backscattering of energy. We relate this term to previous results and to optimal control studies, and generalize it to further operators, i.e., any operator of the form integral a(q;n(0))[partial differential c(q,t)/partial differential t] x C(q)dq (or a discrete sum) will yield friction. Simulations of a small jellium cluster, both in the linear and highly nonlinear excitation regime, demonstrate that the friction always reduces energy. The energy damping is essentially double exponential; the long-time decay is almost an order of magnitude slower than the rapid short-time decay. The friction term stabilizes the propagation (split-operator propagator here), therefore increasing the time-step needed for convergence, i.e., reducing the overall computational cost. The local friction also allows the simulation of a metal cluster in a uniform jellium as the energy loss in the excitation due to the underlying corrugation is accounted for by the friction. We also relate the friction to models of coupling to damped harmonic oscillators, which can be used for a more sophisticated description of the coupling, and to memory functionals. Our results open the way to very simple finite grid description of scattering and multistage conductance using time-dependent density functional theory away from the linear regime, just as absorbing potentials and self-energies are useful for noninteracting systems and leads. PMID:19045077

Neuhauser, Daniel; Lopata, Kenneth

2008-10-01

95

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

NASA Astrophysics Data System (ADS)

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

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

2014-05-01

96

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

PubMed

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

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

2013-08-01

97

Density dependence and cooperation: theory and a test with bacteria.  

PubMed

Although cooperative systems can persist in nature despite the potential for exploitation by noncooperators, it is often observed that small changes in population demography can tip the balance of selective forces for or against cooperation. Here we consider the role of population density in the context of microbial cooperation. First, we account for conflicting results from recent studies by demonstrating theoretically that: (1) for public goods cooperation, higher densities are relatively unfavorable for cooperation; (2) in contrast, for self-restraint-type cooperation, higher densities can be either favorable or unfavorable for cooperation, depending on the details of the system. We then test our predictions concerning public goods cooperation using strains of the pathogenic bacterium Pseudomonas aeruginosa that produce variable levels of a public good-iron-scavenging siderophore molecules. As predicted, we found that the relative fitness of cheats (under-producers) was greatest at higher population densities. Furthermore, as assumed by theory, we show that this occurs because cheats are better able to exploit the cooperative siderophore production of other cells when they are physically closer to them. PMID:19453724

Ross-Gillespie, Adin; Gardner, Andy; Buckling, Angus; West, Stuart A; Griffin, Ashleigh S

2009-09-01

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

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

100

Coherent phonon generation in time-dependent density functional theory  

NASA Astrophysics Data System (ADS)

We apply the time-dependent density functional theory (TDDFT) to the generation of coherent optical phonons in Si and Sb crystals. The computations are carried out by real-time evolution of the orbital wave functions on a coordinate-space mesh. The theory reproduces the main phenomena observed experimentally: dependence on polarization, strong growth at the direct band gap, and the change in phase from below to above the band gap. Comparing with more phenomenological models, we find that the TDDFT supports the impulsively stimulated Raman mechanism at low frequencies and the qualitative aspects of the displacive mechanism at higher frequencies. We also compare with the more detailed model of displacive excitation by Stevens, Kuhl, and Merlin.

Bertsch, George

2013-03-01

101

Unified approach for molecular dynamics and density-functional theory  

Microsoft Academic Search

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

R. Car; M. Parrinello

1985-01-01

102

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

103

Structural Risk Minimization: a robust method for density-dependence detection and model selection =Ecography, 2007 (preprint)  

Microsoft Academic Search

Statistically distinguishing density-dependent from density-independent populations and selecting the best demographic model for a given population are problems of primary importance. Traditional approaches are PBLR (parametric bootstrapping of likelihood ratios) and Information Criteria (IC), such as the Schwarz Information Criterion (SIC), the Akaike Information Criterion (AIC) or the Final Prediction Er- ror (FPE). While PBLR is suitable for choosing from

Giorgio Corani; Marino Gatto

104

Current density partitioning in time-dependent current density functional theory  

NASA Astrophysics Data System (ADS)

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

Mosquera, Martín A.; Wasserman, Adam

2014-05-01

105

Current density partitioning in time-dependent current density functional theory.  

PubMed

We adapt time-dependent current density functional theory to allow for a fragment-based solution of the many-electron problem of molecules in the presence of time-dependent electric and magnetic fields. Regarding a molecule as a set of non-interacting subsystems that individually evolve under the influence of an auxiliary external electromagnetic vector-scalar potential pair, the partition 4-potential, we show that there are one-to-one mappings between this auxiliary potential, a sharply-defined set of fragment current densities, and the total current density of the system. The partition electromagnetic (EM) 4-potential is expressed in terms of the real EM 4-potential of the system and a gluing EM 4-potential that accounts for exchange-correlation effects and mutual interaction forces between fragments that are required to yield the correct electron dynamics. We prove the zero-force theorem for the fragmented system, establish a variational formulation in terms of action functionals, and provide a simple illustration for a charged particle in a ring. PMID:24832333

Mosquera, Martín A; Wasserman, Adam

2014-05-14

106

Relativistic Hartree-Fock-Bogoliubov Theory With Density Dependent Meson Couplings in Axial Symmetry  

SciTech Connect

Most nuclei on the nuclear chart are deformed, and the development of new RIB facilities allows the study of exotic nuclei near the drip lines where a successful theoretical description requires both realistic pairing and deformation approaches. Relativistic Hartree-Fock-Bogoliubov model taking into account axial deformation and pairing correlations is introduced. Preliminary illustrative results with density dependent meson-nucleon couplings in axial symmetry will be discussed.

Ebran, J.-P.; Khan, E.; Arteaga, D. Pena; Grasso, M. [Institut de Physique Nucleaire, 15 rue Georges Clemenceau 91406 Orsay Cedex (France); Vretenar, D. [Physics Department, Faculty of Science, University of Zagreb, 10 000 Zagreb (Croatia)

2009-08-26

107

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

NASA Astrophysics Data System (ADS)

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

2012-11-01

108

The dependence of ZnO photoluminescence efficiency on excitation conditions and defect densities  

NASA Astrophysics Data System (ADS)

The quantum efficiencies of both the band edge and deep-level defect emission from annealed ZnO powders were measured as a function of excitation fluence and wavelength from a tunable sub-picosecond source. A simple model of excitonic decay reproduces the observed excitation dependence of rate constants and associated trap densities for all radiative and nonradiative processes. The analysis explores how phosphor performance deteriorates as excitation fluence and energy increase, provides an all-optical approach for estimating the number density of defects responsible for deep-level emission, and yields new insights for designing efficient ZnO-based phosphors.

Simmons, Jay G.; Foreman, John V.; Liu, Jie; Everitt, Henry O.

2013-11-01

109

Dye chemistry with time-dependent density functional theory.  

PubMed

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

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

2014-06-25

110

Temperature dependence of step density on vicinal Pb(111)  

SciTech Connect

The temperature dependence of step density on the vicinal Pb(111) surface is investigated using reflection high-energy electron diffraction. When the temperature is increased from 323 to 590 K, the average terrace width and the average string length at the step edge decrease from 85{plus_minus}25 to 37{plus_minus}16 {Angstrom} and from 220{plus_minus}33 to 25{plus_minus}8 {Angstrom}, respectively. Thermal step collapse on the Pb(111) surface near its bulk melting temperature is not observed. Above 530{plus_minus}7 K, the change in the string length at the step edge with temperature becomes small, and the intensity of the (00) beam is significantly decreased. We conclude that partial step melting at the step edge occurs at the surface above 530{plus_minus}7 K. {copyright} {ital 1998} {ital The American Physical Society}

Zhang, Z.H.; Elsayed-Ali, H.E. [Department of Electrical and Computer Engineering, Old Dominion University, Norfolk, Virginia 23529 (United States)] [Department of Electrical and Computer Engineering, Old Dominion University, Norfolk, Virginia 23529 (United States)

1998-06-01

111

Bone density in transfusion dependent thalassemia patients in Urmia, Iran  

PubMed Central

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

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

2014-01-01

112

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

113

Simultaneous extraction of charge density dependent mobility and variable contact resistance from thin film transistors  

NASA Astrophysics Data System (ADS)

A model for the extraction of the charge density dependent mobility and variable contact resistance in thin film transistors is proposed by performing a full derivation of the current-voltage characteristics both in the linear and saturation regime of operation. The calculated values are validated against the ones obtained from direct experimental methods. This approach allows unambiguous determination of gate voltage dependent contact and channel resistance from the analysis of a single device. It solves the inconsistencies in the commonly accepted mobility extraction methods and provides additional possibilities for the analysis of the injection and transport processes in semiconducting materials.

Di Pietro, Riccardo; Venkateshvaran, Deepak; Klug, Andreas; List-Kratochvil, Emil J. W.; Facchetti, Antonio; Sirringhaus, Henning; Neher, Dieter

2014-05-01

114

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

Microsoft Academic Search

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

Ozgur Erdinc; Peter Willett; Yaakov Bar-Shalom

2006-01-01

115

Development of a new variational approach for thermal density matrices.  

PubMed

A McLachlan-type variational principle is derived for thermal density matrices. In this approach, the trace of the mean square of the differences between the derivatives of the exact and model density matrices is minimized with respect to the parameters in the model Hamiltonian. Applications to model anharmonic systems in the independent particle model show that the method can provide thermodynamic state functions accurately (within 5% of the converged basis set results) and at the same level of accuracy as the results using Feynman-Gibbs-Bogoliubov variational principle at this level of approximation. PMID:21663347

Roy, Tapta Kanchan; Prasad, M Durga

2011-06-01

116

Couplings between electronic transitions in a subsystem formulation of time-dependent density functional theory  

NASA Astrophysics Data System (ADS)

A subsystem formulation of time-dependent density functional theory (TDDFT) within the frozen-density embedding (FDE) framework and its practical implementation are presented, based on the formal TDDFT generalization of the FDE approach by Casida and Wesolowski [Int. J. Quantum Chem. 96, 577 (2004)]. It is shown how couplings between electronic transitions on different subsystems can be seamlessly incorporated into the formalism to overcome some of the shortcomings of the approximate TDDFT-FDE approach in use so far, which was only applicable for local subsystem excitations. In contrast to that, the approach presented here allows to include couplings between excitations on different subsystems, which become very important in aggregates composed of several similar chromophores, e.g., in biological or biomimetic light-harvesting systems. A connection to Förster- and Dexter-type excitation energy coupling expressions is established. A hybrid approach is presented and tested, in which excitation energy couplings are selectively included between different chromophore fragments, but neglected for inactive parts of the environment. It is furthermore demonstrated that the coupled TDDFT-FDE approach can cure the inability of the uncoupled FDE approach to describe induced circular dichroism in dimeric chromophores, a feature known as a ``couplet,'' which is also related to couplings between (nearly) degenerate electronic transitions.

Neugebauer, Johannes

2007-04-01

117

Time-dependent density functional theory of extreme environments  

NASA Astrophysics Data System (ADS)

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

Magyar, Rudolph; Shulenburger, Luke; Desjarlais, Michael

2013-03-01

118

Nature of curvature coupling of amphiphysin with membranes depends on its bound density  

PubMed Central

Cells are populated by a vast array of membrane-binding proteins that execute critical functions. Functions, like signaling and intracellular transport, require the abilities to bind to highly curved membranes and to trigger membrane deformation. Among these proteins is amphiphysin 1, implicated in clathrin-mediated endocytosis. It contains a Bin-Amphiphysin-Rvs membrane-binding domain with an N-terminal amphipathic helix that senses and generates membrane curvature. However, an understanding of the parameters distinguishing these two functions is missing. By pulling a highly curved nanotube of controlled radius from a giant vesicle in a solution containing amphiphysin, we observed that the action of the protein depends directly on its density on the membrane. At low densities of protein on the nearly flat vesicle, the distribution of proteins and the mechanical effects induced are described by a model based on spontaneous curvature induction. The tube radius and force are modified by protein binding but still depend on membrane tension. In the dilute limit, when practically no proteins were present on the vesicle, no mechanical effects were detected, but strong protein enrichment proportional to curvature was seen on the tube. At high densities, the radius is independent of tension and vesicle protein density, resulting from the formation of a scaffold around the tube. As a consequence, the scaling of the force with tension is modified. For the entire density range, protein was enriched on the tube as compared to the vesicle. Our approach shows that the strength of curvature sensing and mechanical effects on the tube depends on the protein density.

Sorre, Benoit; Callan-Jones, Andrew; Manzi, John; Goud, Bruno; Prost, Jacques; Bassereau, Patricia; Roux, Aurelien

2012-01-01

119

On the role of density-dependent symmetry energy and momentum dependent interactions in multi-fragmentation  

NASA Astrophysics Data System (ADS)

We study the multi-fragmentation for the different parameterizations of the density dependent symmetry energy using an isospin-dependent quantum molecular dynamics (IQMD) model. We analyze the sensitivity of fragment production towards various forms of the density dependent symmetry energy. The inclusion of momentum dependent interactions (MDI) results in a larger variation of fragment production. We here highlighted the collective response of the MDI and symmetry energy towards the fragmentation of colliding nuclei at intermediate energies.

Vinayak, Karan Singh; Kumar, Suneel

2012-12-01

120

Automatic breast density segmentation: an integration of different approaches  

NASA Astrophysics Data System (ADS)

Mammographic breast density has been found to be a strong risk factor for breast cancer. In most studies, it is assessed with a user-assisted threshold method, which is time consuming and subjective. In this study, we develop a breast density segmentation method that is fully automatic. The method is based on pixel classification in which different approaches known in the literature to segment breast density are integrated and extended. In addition, the method incorporates the knowledge of a trained observer, by using segmentations obtained by the user-assisted threshold method as training data. The method is trained and tested using 1300 digitized film mammographic images acquired with a variety of systems. Results show a high correspondence between the automated method and the user-assisted threshold method. Pearson's correlation coefficient between our method and the user-assisted method is R = 0.911 for percent density and R = 0.895 for dense area, which is substantially higher than the best correlation found in the literature (R = 0.70, R = 0.68). The area under the receiver operating characteristic curve obtained when discriminating between fatty and dense pixels is 0.987. A combination of segmentation strategies outperforms the application of single segmentation techniques.

Kallenberg, Michiel G. J.; Lokate, Mariëtte; van Gils, Carla H.; Karssemeijer, Nico

2011-05-01

121

Automatic breast density segmentation: an integration of different approaches.  

PubMed

Mammographic breast density has been found to be a strong risk factor for breast cancer. In most studies, it is assessed with a user-assisted threshold method, which is time consuming and subjective. In this study, we develop a breast density segmentation method that is fully automatic. The method is based on pixel classification in which different approaches known in the literature to segment breast density are integrated and extended. In addition, the method incorporates the knowledge of a trained observer, by using segmentations obtained by the user-assisted threshold method as training data. The method is trained and tested using 1300 digitized film mammographic images acquired with a variety of systems. Results show a high correspondence between the automated method and the user-assisted threshold method. Pearson's correlation coefficient between our method and the user-assisted method is R = 0.911 for percent density and R = 0.895 for dense area, which is substantially higher than the best correlation found in the literature (R = 0.70, R = 0.68). The area under the receiver operating characteristic curve obtained when discriminating between fatty and dense pixels is 0.987. A combination of segmentation strategies outperforms the application of single segmentation techniques. PMID:21464531

Kallenberg, Michiel G J; Lokate, Mariëtte; van Gils, Carla H; Karssemeijer, Nico

2011-05-01

122

Density Functional Approach to Investigate the Stability of Nanobubbles  

NASA Astrophysics Data System (ADS)

Recently, some experimental results showing the existence of nanobubbles which are bubbles of tens or hundreds of nanometer diameter in water have been reported. However, it is generally considered that such tiny bubbles are inherentlyunstable because of a strong effect of surface tension. Therefore, the stability of the nanobubbles are regarded as an unresolved problem. In order to investigate the stability mechanism, our approach is based on a microscopic standpoint. Especially, we introduced the density functional theory of classical systems to take into account the microscopic information on gas-- liquid interface and to execute the stability analyses in the microscopic system. The density functional approach has many advantages in the stability analysis of the two-phase microscopic system compared with the molecular dynamics approach. First, natural external conditions such as the grand canonical ensemble can be imposed. Second, the free energy evaluation which is required in the stability analyses can be performed directly. In this study, we focused on the effect of impurities in water on the stability of the nanobubbles. In the experimental system, electrolytes or surfactants are usually used in the literature, and we consider that the effect of the impurities is crucial.

Kikugawa, Gota

2005-03-01

123

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

PubMed

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

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

2014-05-01

124

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

PubMed

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

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

2014-07-22

125

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

PubMed

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

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

2012-05-01

126

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

PubMed Central

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

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

2012-01-01

127

A density-functional approach to nucleation in micellar solutions  

NASA Astrophysics Data System (ADS)

We present a density-functional theory for binary mixtures of amphiphiles with solvent, placing particular emphasis on the structure and free energy of stable and metastable clusters of amphiphiles in dilute solution. These correspond to micelles (complexes of amphiphiles with the solvophobic groups clustered in the interior) and vesicles (spherical complexes with bilayer membranes of amphiphiles with solvent both inside and outside the membrane shell). Saddle points that connect these states are also explored to study the nucleation behavior of these systems. Our approach is applicable to both strong and weak amphiphiles, and can be readily extended to the full ternary phase equilibria characteristic of microemulsions.

Talanquer, V.; Oxtoby, D. W.

2000-10-01

128

Cell density-dependent oligopeptide production in cyanobacterial strains.  

PubMed

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

Pereira, Daniel A; Giani, Alessandra

2014-04-01

129

Density dependence and population dynamics of black rhinos (Diceros bicornis michaeli) in Kenya's rhino sanctuaries  

Microsoft Academic Search

Density-dependent feedback mechanisms provide insights into the population dynamics and interactions of large herbivores with their ecosystem. Sex ratio also has particularly important implications for growth rates of many large mammal populations through its influence on reproductive potential. Therefore, the interrelationships between density-dependent factors, comprising density, sex ratio and underlying growth rates (r) were examined for the Eastern black rhino

B. Okita-Ouma; R. Amin; Langevelde van F; N. Leader-Williams

2010-01-01

130

Pressure dependence of the density of n-alkanes  

SciTech Connect

Accurate density data for n-alkanes are essential for the measurement of interfacial tension of liquid-liquid systems as a function of pressure. The variation of density with pressure for three n-alkanes, n-hexane, n-heptane, and n-decane, was measured at 21.2[degrees]C and pressures ranging from 0.1 to 35 MPa with a digital density meter. The Tait equation of the form ([rho] [minus] [rho][sub 0])/[rho] = C log[(B + P)/(B + P[sub 0])] was used to represent the experimental data. 20 refs., 2 figs., 4 tabs.

Susnar, S.S.; Budziak, C.J.; Neumann, A.W. (Univ. of Toronto, Ontario (Canada)); Hamza, H.A. (CANMET, Devon, Alberta (Canada))

1992-05-01

131

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

132

Loss of density-dependence and incomplete control by dominant breeders in a territorial species with density outbreaks  

Microsoft Academic Search

Background  A territory as a prerequisite for breeding limits the maximum number of breeders in a given area, and thus lowers the proportion\\u000a of breeders if population size increases. However, some territorially breeding animals can have dramatic density fluctuations\\u000a and little is known about the change from density-dependent processes to density-independence of breeding during a population\\u000a increase or an outbreak. We

Jana A Eccard; Ilmari Jokinen; Hannu Ylönen

2011-01-01

133

Loss of density-dependence and incomplete control by dominant breeders in a territorial species with density outbreaks  

PubMed Central

Background A territory as a prerequisite for breeding limits the maximum number of breeders in a given area, and thus lowers the proportion of breeders if population size increases. However, some territorially breeding animals can have dramatic density fluctuations and little is known about the change from density-dependent processes to density-independence of breeding during a population increase or an outbreak. We suggest that territoriality, breeding suppression and its break-down can be understood with an incomplete-control model, developed for social breeders and social suppression. Results We studied density dependence in an arvicoline species, the bank vole, known as a territorial breeder with cyclic and non-cyclic density fluctuations and periodically high densities in different parts of its range. Our long-term data base from 38 experimental populations in large enclosures in boreal grassland confirms that breeding rates are density-regulated at moderate densities, probably by social suppression of subordinate potential breeders. We conducted an experiment, were we doubled and tripled this moderate density under otherwise the same conditions and measured space use, mortality, reproduction and faecal stress hormone levels (FGM) of adult females. We found that mortality did not differ among the densities, but the regulation of the breeding rate broke down: at double and triple densities all females were breeding, while at the low density the breeding rate was regulated as observed before. Spatial overlap among females increased with density, while a minimum territory size was maintained. Mean stress hormone levels were higher in double and triple densities than at moderate density. Conclusions At low and moderate densities, breeding suppression by the dominant breeders, But above a density-threshold (similar to a competition point), the dominance of breeders could not be sustained (incomplete control). In our experiment, this point was reached after territories could not shrink any further, while the number of intruders continued to increase with increasing density. Probably suppression becomes too costly for the dominants, and increasing number of other breeders reduces the effectiveness of threats. In wild populations, crossing this threshold would allow for a rapid density increase or population outbreaks, enabling territorial species to escape density-dependency.

2011-01-01

134

New dependability approach for implanted medical devices  

Microsoft Academic Search

Functional electrical stimulation (FES) is an attractive solution to restore some lost or failing physiological functions. Obviously, the FES system may be hazardous for patient and the reliability and dependability of the system must be maximal. Unfortunately, the present context, where the associated systems are more and more complex and their development needs very cross-disciplinary experts, is not favorable to

Fabien Soulier; Fanny Le Floch; Serge Bernard; Guy Cathébras

2009-01-01

135

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

136

Reduced density matrix hybrid approach: Application to electronic energy transfer  

NASA Astrophysics Data System (ADS)

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.; Markland, Thomas E.; Reichman, David R.

2012-02-01

137

Approaches to 100 Gbit/sq. in. recording density  

NASA Technical Reports Server (NTRS)

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

Kryder, Mark H.

1994-01-01

138

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

139

The Time-Dependent Approach to Inverse Scattering  

Microsoft Academic Search

In these lectures I give an introduction to the time-dependent approach to inverse scattering, that has been developed recently. The aim of this approach is to solve various inverse scattering problems with time-dependent methods that closely follow the physical (and geometrical) intuition of the scattering phenomena. This method has been applied to many linear and nonlinear scattering problems.

Ricardo Weder

2002-01-01

140

A hypoelasto-plastic finite strain simulation of powder compaction processes with density-dependent endochronic model  

Microsoft Academic Search

In this paper, a new approach is developed based on an endochronic density-dependent plasticity model for describing the isothermal deformation behavior of metal powder at low homologous temperature. As large deformations are observed in powder compaction processes, the endochronic constitutive model is presented based on large strain plasticity and an integration scheme is established for the rate constitutive equations. Endochronic

A. R. Khoei; A. Bakhshiani

2004-01-01

141

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...

142

Charge-transfer excitation energies with a time-dependent density-functional method suitable for orbital-dependent exchange-correlation kernels  

SciTech Connect

A time-dependent density-functional (TDDFT) response equation for the effective Kohn-Sham potential instead of the electron densities is presented that enables the use of orbital-dependent exchange-correlation kernels. In combination with the frequency-dependent exact-exchange kernel the present approach describes long-range charge-transfer excitations qualitatively correct in contrast to standard TDDFT methods, as shown by formal analysis and applications to molecular systems. Even charge-transfer excitations obtained with the adiabatic frequency-independent exact-exchange kernel exhibit a long-range dependence with the distance of the charge transfer, which, however, is too weak by a factor of 2. This indicates that it is not the frequency dependence of the kernel alone that leads to a correct description of charge-transfer excitations.

Hesselmann, Andreas; Ipatov, Andrey; Goerling, Andreas [Lehrstuhl fuer Theoretische Chemie, Universitaet Erlangen- Nuernberg, Egerlandstr. 3, D-91058 Erlangen (Germany)

2009-07-15

143

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

PubMed

The treatment of dispersion interactions is ubiquitous but computationally demanding for seamless ab initio approaches. A highly popular and simple remedy consists in correcting for the missing interactions a posteriori by adding an attractive energy term summed over all atom pairs to standard density functional approximations. These corrections were originally based on atom pairwise parameters and, hence, had a strong touch of empiricism. To overcome such limitations, we recently proposed a robust system-dependent dispersion correction, dDsC, that is computed from the electron density and that provides a balanced description of both weak inter- and intramolecular interactions. From the theoretical point of view and for the sake of increasing reliability, we here verify if the self-consistent implementation of dDsC impacts ground-state properties such as interaction energies, electron density, dipole moments, geometries, and harmonic frequencies. In addition, we investigate the suitability of the a posteriori scheme for molecular dynamics simulations, for which the analysis of the energy conservation constitutes a challenging tests. Our study demonstrates that the post-SCF approach in an excellent approximation. PMID:24832324

Brémond, Eric; Golubev, Nikolay; Steinmann, Stephan N; Corminboeuf, Clémence

2014-05-14

144

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

PubMed

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

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

2014-06-01

145

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

NASA Astrophysics Data System (ADS)

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

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

2014-05-01

146

Frequency dependence of the plasma density for helicon plasmas  

SciTech Connect

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

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

1997-05-01

147

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

148

Average excitation energies from time-dependent density functional response theory  

NASA Astrophysics Data System (ADS)

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

Hu, Chunping; Sugino, Osamu

2007-02-01

149

Fibroblast PER2 Circadian Rhythmicity Depends on Cell Density  

PubMed Central

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

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

2013-01-01

150

Dependence of Type Ia Supernovae on Progenitor Central Density  

NASA Astrophysics Data System (ADS)

Recent observational studies of Type Ia supernovae (SNeIa) suggest correlations between the brightness of an event and properties of the host galaxy that may involve the age of the progenitor population. One way to influence the explosion systematically is through the ignition density, which is determined by both the mass of the white dwarf before the onset of accretion and the subsequent accretion history, and is therefore related to the age of the progenitor. We present the results of a suite of two-dimensional SNeIa simulations investigating how the central density of the progenitor white dwarf at flame ignition influences the production of Fe-group material, particularly the production of radioactive Ni-56 that powers the light curve. The simulation suite constitutes a theoretical population of events allowing a statistically well-controlled investigation into systematic trends (Townsley et al. 2009). The simulations indicate that with increased central density, more Fe-group material is produced but with less total Ni-56, which we attribute to a higher rate of neutronization occurring at higher density. Additionally, we compare our results with those of Roepke et al. (2006). 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.

Krueger, Brendan K.; Calder, A. C.; Townsley, D. M.; Jackson, A. P.; Brown, E. F.

2010-01-01

151

Dependence patterns across financial markets: a mixed copula approach  

Microsoft Academic Search

This paper studies the modelling and estimation of dependence across international financial markets, with a focus on the structure of dependence. A new approach is proposed based on a mixed copula model and the model is constructed so that it can capture various patterns of dependence structures. The marginal distribution of asset returns in each market is estimated non-parametrically and

Ling Hu

2006-01-01

152

Momentum, density, and isospin dependence of symmetric and asymmetric nuclear matter properties  

SciTech Connect

Properties of symmetric and asymmetric nuclear matter have been investigated in the relativistic Dirac-Brueckner-Hartree-Fock approach based on projection techniques using the Bonn A potential. The momentum, density, and isospin dependence of the optical potentials and nucleon effective masses are studied. It turns out that the isovector optical potential depends sensitively on density and momentum, but is almost insensitive to the isospin asymmetry. Furthermore, the Dirac mass m{sub D}* and the nonrelativistic mass m{sub NR}* which parametrizes the energy dependence of the single particle spectrum, are both determined from relativistic Dirac-Brueckner-Hartree-Fock calculations. The nonrelativistic mass shows a characteristic peak structure at momenta slightly above the Fermi momentum k{sub F}. The relativistic Dirac mass shows a proton-neutron mass splitting of m{sub D,n}*m{sub NR,p}* which is in agreement with the results from nonrelativistic calculations.

Dalen, E.N.E. van; Fuchs, C.; Faessler, Amand [Institut fuer Theoretische Physik, Universitaet Tuebingen, Auf der Morgenstelle 14, D-72076 Tuebingen (Germany)

2005-12-15

153

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

NASA Astrophysics Data System (ADS)

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

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

2014-05-01

154

Spin density waves in the Hubbard model: A DMFT approach  

NASA Astrophysics Data System (ADS)

We analyze spin density waves (SDWs) in the Hubbard model on a square lattice within the framework of inhomogeneous dynamical mean field theory (IDMFT). Doping the half-filled Hubbard model results in a change of the antiferromagnetic Néel state, which exists exactly at half filling, to a phase of incommensurate SDWs. Previous studies of this phase mainly rely on static mean field calculations. In this paper, we will use large-scale IDMFT calculations to study properties of SDWs in the Hubbard model. A great advantage of IDMFT over static mean field approaches is the inclusion of local screening effects and the easy access to dynamical correlation functions. Furthermore, this technique is not restricted to the Hubbard model, but can be easily used to study incommensurate phases in various strongly correlated materials.

Peters, Robert; Kawakami, Norio

2014-04-01

155

Density-dependent effects in experimental larval populations of Anopheles arabiensis (Diptera: Culicidae) can be negative, neutral, or overcompensatory depending on density and diet levels.  

PubMed

Anopheles arabiensis Patton (Diptera: Culicidae) larvae were reared from hatching to the adult stage in the laboratory under a range of diet and larval concentrations using a factorial design. The range circumscribed most of the larval densities and diet concentrations that would allow larval growth and survival using the particular diet formulation and water volume we tested. We determined how these variables affected three outcomes, as follows: larval development rate, survival, and wing length. As has been reported previously, negative density dependence of survival as a function of increased larval density was the prevalent effect on all outcomes when diet was limiting. When diet was not limiting, density dependence was not observed, and three cases of overcompensatory survival were seen. We discuss these results in the context of diet and larval densities for mass rearing and the effect of larval competition on control strategies. PMID:21485365

Gilles, J R L; Lees, R S; Soliban, S M; Benedict, M Q

2011-03-01

156

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

157

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

NASA Astrophysics Data System (ADS)

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

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

2002-03-01

158

Liquid Water Simulations with Density Fragment Interaction Approach  

PubMed Central

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

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

2013-01-01

159

Density-dependent tree mortality in pinyon-juniper woodlands  

Microsoft Academic Search

Pathogens and bark beetles are integral disturbance agents in many forest and woodland ecosystems. Over the past decade, pinyon Ips (Ips confusus) has interacted with regional drought to cause widespread mortality in singleleaf pinyon (Pinus monophylla) and Colorado pinyon (Pinus edulis) woodlands of western North America. We used mixed-effects and logistic regression modeling approaches to explore biotic and abiotic influences

David L. Greenwood; Peter J. Weisberg

2008-01-01

160

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

161

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

PubMed Central

Density dependence can have marked effects on social behaviors such as group size. We tested whether changes in population density of a large herbivore (elk, Cervus canadensis) affected sex-specific group size and whether the response was density- or frequency-dependent. We quantified the probability and strength of changes in group sizes and dispersion as population density changed for each sex. We used group size data from a population of elk in Manitoba, Canada, that was experimentally reduced from 1.20 to 0.67 elk/km2 between 2002 and 2009. Our results indicated that functional responses of group size to population density are sex-specific. Females showed a positive density-dependent response in group size at population densities ?0.70 elk/km2 and we found evidence for a minimum group size at population density ?0.70 elk/km2. Changes in male group size were also density-dependent; however, the strength of the relationship was lower than for females. Density dependence in male group size was predominantly a result of fusion of solitary males into larger groups, rather than fusion among existing groups. Our study revealed that density affects group size of a large herbivore differently between males and females, which has important implications for the benefits e.g., alleviating predation risk, and costs of social behaviors e.g., competition for resources and mates, and intra-specific pathogen transmission.

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

2013-01-01

162

Temperature and electron density dependence of spin relaxation in GaAs/AlGaAs quantum well  

PubMed Central

Temperature and carrier density-dependent spin dynamics for GaAs/AlGaAs quantum wells (QWs) with different structural symmetries have been studied by using time-resolved Kerr rotation technique. The spin relaxation time is measured to be much longer for the symmetrically designed GaAs QW comparing with the asymmetrical one, indicating the strong influence of Rashba spin-orbit coupling on spin relaxation. D'yakonov-Perel' mechanism has been revealed to be the dominant contribution for spin relaxation in GaAs/AlGaAs QWs. The spin relaxation time exhibits non-monotonic-dependent behavior on both temperature and photo-excited carrier density, revealing the important role of non-monotonic temperature and density dependence of electron-electron Coulomb scattering. Our experimental observations demonstrate good agreement with recently developed spin relaxation theory based on microscopic kinetic spin Bloch equation approach.

2011-01-01

163

Reaction of massive clusters to gas expulsion - The cluster density dependence  

NASA Astrophysics Data System (ADS)

Context. The expulsion of the unconverted gas at the end of the star formation process potentially leads to the expansion of the just formed stellar cluster and membership loss. The degree of expansion and mass loss depends largely on the star formation efficiency and scales with the mass and size of the stellar group when stellar interactions can be neglected. Aims: We investigate the circumstances in which stellar interactions between cluster members become so important that the fraction of bound stars after gas expulsion is significantly altered. Methods: The Nbody6 code is used to simulate the cluster dynamics after gas expulsion for different star formation efficiences. Concentrating on the most massive clusters observed in the Milky Way, we test to what extent the results depend on the model, i.e. stellar mass distribution, stellar density profile etc., and the cluster parameters, such as cluster density and size. Results: We find that stellar interactions leading to ejections are responsible for up to 20% mass loss in the most compact massive clusters in the Milky Way. Therefore, ejections are the prime mass loss process in these systems. Even in the loosely bound OB associations, stellar interactions are responsible for at least ~5% mass loss. The main reason why the importance of encounters for massive clusters has been largely overlooked is because of the often used approach of a single-mass representation instead of a realistic distribution for the stellar masses. The density dependence on the encounter-induced mass loss is shallower than expected because of the increasing importance of few-body interactions in dense clusters compared to sparse clusters where 2-body encounters dominate.

Pfalzner, S.; Kaczmarek, T.

2013-07-01

164

Test of the variable nature of density-dependent mortality in fish populations  

Microsoft Academic Search

Veracity of past analyses on the impact of impingement and entrainment of fish in power-plant cooling systems was questioned. Density-dependent dynamics were hypothesized as variable, not constant, making previous analyses based on constant density-dependent or density-independent mortality in error. This hypothesis was tested with a simulation model based on complex biological and behavioral mechanisms for individual fish, thereby foregoing the

1978-01-01

165

Analytical excited state forces for the time-dependent density-functional tight-binding method.  

PubMed

An analytical formulation for the geometrical derivatives of excitation energies within the time-dependent density-functional tight-binding (TD-DFTB) method is presented. The derivation is based on the auxiliary functional approach proposed in [Furche and Ahlrichs, J Chem Phys 2002, 117, 7433]. To validate the quality of the potential energy surfaces provided by the method, adiabatic excitation energies, excited state geometries, and harmonic vibrational frequencies were calculated for a test set of molecules in excited states of different symmetry and multiplicity. According to the results, the TD-DFTB scheme surpasses the performance of configuration interaction singles and the random phase approximation but has a lower quality than ab initio time-dependent density-functional theory. As a consequence of the special form of the approximations made in TD-DFTB, the scaling exponent of the method can be reduced to three, similar to the ground state. The low scaling prefactor and the satisfactory accuracy of the method makes TD-DFTB especially suitable for molecular dynamics simulations of dozens of atoms as well as for the computation of luminescence spectra of systems containing hundreds of atoms. PMID:17568436

Heringer, D; Niehaus, T A; Wanko, M; Frauenheim, Th

2007-12-01

166

Time-dependent density functional theory based upon the fragment molecular orbital method  

NASA Astrophysics Data System (ADS)

Time-dependent density functional theory (TDDFT) was combined with the two-body fragment molecular orbital method (FMO2). In this FMO2-TDDFT scheme, the system is divided into fragments, and the electron density for fragments is determined self-consistently. Consequently, only one main fragment of interest and several fragment pairs including it are calculated by TDDFT. To demonstrate the accuracy of FMO2-TDDFT, we computed several low-lying singlet and triplet excited states of solvated phenol and polyalanine using our method and the standard TDDFT for the full system. The BLYP functional with the long-range correction (LC-BLYP) was employed with the 6-31G* basis set (some tests were also performed with 6-311G*, as well as with B3LYP and time-dependent Hartree-Fock). Typically, FMO2-TDDFT reproduced the full TDDFT excitation energies within 0.1 eV, and for one excited state the error was about 0.2 eV. Beside the accurate reproduction of the TDDFT excitation energies, we also automatically get an excitation energy decomposition analysis, which provides the contributions of individual fragments. Finally, the efficiency of our approach was exemplified on the LC-BLYP/6-31G* calculation of the lowest singlet excitation of the photoactive yellow protein which consists of 1931 atoms, and the obtained value of 3.1 eV is in agreement with the experimental value of 2.8 eV.

Chiba, Mahito; Fedorov, Dmitri G.; Kitaura, Kazuo

2007-09-01

167

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

168

Charge density dependent two-channel conduction in organic electric double layer transistors (EDLTs).  

PubMed

A transport model based on hole-density-dependent trapping is proposed to explain the two unusual conductivity peaks at surface hole densities above 10(13) cm(-2) in rubrene electric double layer transistors (EDLTs). Hole transport in rubrene is described to occur via multiple percolation pathways, where conduction is dominated by transport in the free-site channel at low hole density, and in the trap-site channel at larger hole density. PMID:24496822

Xie, Wei; Liu, Feilong; Shi, Sha; Ruden, P Paul; Frisbie, C Daniel

2014-04-23

169

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

170

Time-dependent density-functional theory for trapped strongly interacting fermionic atoms  

SciTech Connect

The dynamics of strongly interacting trapped dilute two-component Fermi gases (dilute in the sense that the range of interatomic potential is small compared with interparticle spacing) is investigated in a single-equation approach to the time-dependent density-functional theory. For the ground-state energy per particle of the system in the homogeneous phase, we have constructed an Pade parametrization based on Monte Carlo data and asymptotic behavior. Our numerical results for collective frequencies in the BCS-BEC crossover regime are in good agreement with recent experimental data obtained by the Duke University group. In addition, we show that the calculated corrections to the hydrodynamic approximation may be important, even for systems with a rather large number of atoms.

Kim, Yeong E.; Zubarev, Alexander L. [Purdue Nuclear and Many-Body Theory Group (PNMBTG), Department of Physics, Purdue University, West Lafayette, Indiana 47907 (United States)

2004-09-01

171

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

172

Determining diode parameters of illuminated current density-voltage curves considering the voltage-dependent photo current density  

NASA Astrophysics Data System (ADS)

Suitable procedures for the determination of diode parameters of illuminated and dark current density-voltage (j-V) curves of solar cells are investigated. Within these procedures we avoid misinterpretations by the commonly used shifting approximation which assumes a voltage-independent photo carrier collection. To this end, the voltage-dependent collection efficiency is determined by j-V measurements under incremental illumination. After that, illuminated curves can be converted into dark curves taking the voltage-dependent photo current density into account. The converted dark curve next is consigned into a program that numerically solves the implicit equation of the current density and derives the diode parameters by means of a weighted least-squares minimization. Only if fluctuations of the illumination are noticeably high this procedure cannot be used, since the converted dark curve also reveals a high noise level. Then, the determination of diode parameters is performed stepwise with the help of commonly used methods.

Obereigner, F.; Scheer, R.

2014-02-01

173

Assessment of dispersion-corrected density functional approaches for extended systems  

NASA Astrophysics Data System (ADS)

Standard density functional (DFT) methods are known to fail in describing the long range van der Waals interactions, and currently, there is a great interest in incorporating dispersion corrections in density functionals. Recently, Tkatchenko and Scheffler introduced a new scheme where dispersion corrections are included by a summation of damped interatomic C6/R^6 terms. However, contrary to the DFT-D2 approach of Grimme, the C6 coefficients depend on the electron density through a Hirshfeld atom-in-a-molecule decomposition scheme. We have implemented the vdW-TS approach in VASP and applied it to the study of a series of prototype dispersion-dominated systems including layered materials, noble-gas solids and molecular crystals. Full optimization of all degrees of freedom is possible in our implementation because dispersion corrections are computed for the forces acting on the atoms, and also the stresses on the unitcell. Our results show that the vdW-TS method yield good structure, bulk moduli, and cohesive energies of weakly bonded systems in much better agreement with experiment than those obtained with standard DFT approaches.

Al-Saidi, Wissam; Voora, Vamsee; Jordan, Ken

2011-03-01

174

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

175

Time-dependent density-functional theory of exciton-exciton correlations in the nonlinear optical response  

NASA Astrophysics Data System (ADS)

We analyze possible nonlinear exciton-exciton correlation effects in the optical response of semiconductors by using a time-dependent density-functional theory (TDDFT) approach. For this purpose we derive the nonlinear (third-order) TDDFT equation for the excitonic polarization. In this equation the nonlinear time-dependent effects are described by the time-dependent (nonadiabatic) part of the effective exciton-exciton interaction, which depends on the exchange-correlation (XC) kernel. We apply the approach to study the nonlinear optical response of a GaAs quantum well. In particular, we calculate the two-dimensional Fourier spectrum (2DFS) of the system and compare it with experimental data. We find that the memory effects play a crucial role in this response, and in particular that it is necessary to use a nonadiabatic XC kernel to describe excitonic bound states, biexcitons, which are formed due to the retarded TDDFT exciton-exciton interaction.

Turkowski, Volodymyr; Leuenberger, Michael N.

2014-02-01

176

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

177

Influence of density dependence on predator-prey seabird interactions at large spatio-temporal scales  

PubMed Central

Theoretical investigations of competitive dynamics have noted that numbers of predator and prey influence each other. However, few empirical studies have demonstrated how a life-history trait of the prey (such as fecundity) can be affected simultaneously by its own density and the density of predators. For instance, density dependence can reduce fecundity with increasing number of prey, while inverse density dependence or Allee effects may occur especially when the prey is a social organism. Here we analysed an intraguild predator–prey system of two seabird species at a large spatio-temporal scale. As expected, we found that fecundity of prey was negatively affected by predator density. Nevertheless, fecundity of prey also increased nonlinearly with its own density and strikingly with the prey–predator ratio. Small groups of prey were probably not able to defend their nests especially against large number of predators. At the highest prey densities (i.e. when anti-predator strategies should be most efficient), prey fecundity also lowered, suggesting the appearance of density dependence mediated by food competition. Allee effects and density dependence occurred across a broad range of population sizes of both the prey and the predator at several local populations facing different ecological environments.

Oro, Daniel; Martinez-Abrain, Alejandro; Paracuellos, Mariano; Nevado, Juan Carlos; Genovart, Meritxell

2005-01-01

178

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

NASA Astrophysics Data System (ADS)

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

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

2014-05-01

179

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

NASA Astrophysics Data System (ADS)

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

Canbula, B.; Bulur, R.; Canbula, D.; Babacan, H.

2014-09-01

180

Photoabsorption In (CdS)n Clusters On The Basis of Time-dependent Density Functional Theory  

NASA Astrophysics Data System (ADS)

We present the study of the optical properties of the small size (CdS) n = 1 to 4 clusters, using the plane wave based density functional formalism. Photoabsorption spectrum have been computed in a lower energy range using the time dependent Density Functional Theory, that uses time propagation method. The computed results are in good agreement with those obtained from another approach developed earlier known as Casida's formalism. Our results are reported for frequency range in which photoabsorption is prominent. The study can be useful for the future experimental investigations.

Dhuvad, P.; Sharma, A. C.

2009-06-01

181

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

NASA Astrophysics Data System (ADS)

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

Ullrich, Carsten A.; D'Amico, Irene

2013-03-01

182

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

NASA Astrophysics Data System (ADS)

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

Yoshida, K.

2014-09-01

183

Self-interaction-corrected time-dependent density-functional-theory calculations of x-ray-absorption spectra  

SciTech Connect

We outline an approach within time-dependent density functional theory that predicts x-ray spectra on an absolute scale. The approach rests on a recent formulation of the resonant-convergent first-order polarization propagator [P. Norman et al., J. Chem. Phys. 123, 194103 (2005)] and corrects for the self-interaction energy of the core orbital. This polarization propagator approach makes it possible to directly calculate the x-ray absorption cross section at a particular frequency without explicitly addressing the excited-state spectrum. The self-interaction correction for the employed density functional accounts for an energy shift of the spectrum, and fully correlated absolute-scale x-ray spectra are thereby obtained based solely on optimization of the electronic ground state. The procedure is benchmarked against experimental spectra of a set of small organic molecules at the carbon, nitrogen, and oxygen K edges.

Tu, Guangde; Rinkevicius, Zilvinas; Vahtras, Olav; Aagren, Hans; Ekstroem, Ulf; Norman, Patrick; Carravetta, Vincenzo [Laboratory of Theoretical Chemistry, Royal Institute of Technology, SE-106 91 Stockholm (Sweden); Department of Physics, Chemistry and Biology, Linkoeping University, SE-581 83 Linkoeping (Sweden); Institute of Chemical-Physical Processes, C.N.R., via G. Moruzzi 1, I-56124 Pisa (Italy)

2007-08-15

184

Hard core Yukawa fluid with temperature and density dependent interaction: Phase diagram of the AOT/water/decane microemulsion  

NASA Astrophysics Data System (ADS)

A fluid with an interaction potential consisting of a hard core and an attractive Yukawa tail is considered. The strength of the attraction is taken to depend both on density and temperature in order to take into account the state dependence of the effective interaction suggested by the analysis of structural data of several colloidal suspensions, in particular microemulsions of AOT reverse micelles. The thermodynamics of this fluid is investigated starting from the inverse temperature expansion of the free energy in the mean spherical approximation. The temperature and density dependence of the interaction is incorporated in consistent expressions for the pressure, isothermal compressibility, and chemical potential. The phase diagram predicted by this method is in agreement with experimental data on AOT reverse micelles analyzed in the effective one component approach.

Bouaskarne, M.; Amokrane, S.; Regnaut, C.

1999-08-01

185

Structure of spherical electric double layers: A density functional approach  

Microsoft Academic Search

A density functional theory is presented for the structure of spherical electric double layers within the restricted primitive model, where the macroion is considered as a hard sphere having uniform surface charge density, the small ions as charged hard spheres, and the solvent is taken as a dielectric continuum. The theory is partially perturbative as the hard-sphere contribution to the

Teena Goel; Chandra N. Patra

2007-01-01

186

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

187

A density-dependent effective interaction for relativistic Hartree-Fock calculations  

Microsoft Academic Search

We present a general method to derive density-dependent effective interactions to be used in the relativistic Hartree-Fock approximation. In the non-relativistic limit (i.e. at low density), this method is identical to the usual variational ansatz with a two-body correlation function whose range is used as a variational parameter. At normal nuclear matter density, it provides a useful prescription to fit

S. Marcos; J.-F. Mathiot; M. López-Quelle; R. Niembro; P. Bernardos

1996-01-01

188

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

189

Influence of field dependent critical current density on flux profiles in high Tc superconductors  

NASA Technical Reports Server (NTRS)

The field distribution for superconducting cylinders and slabs with field dependent critical current densities in combined DC and AC magnetic fields and the corresponding magnetic fluxes are calculated. It is shown that all features of experimental magnetic-field profile measurements can be explained in the framework of field dependent critical current density. Even the quantitative agreement between the experimental and theoretical results using Kim's model is very good.

Takacs, S.

1990-01-01

190

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

Microsoft Academic Search

The incorporation of semiconductor quantum dots into different heterostructures for applications in nanoscale lasing and amplification has been an active area of research in recent years. Here, we use ultrafast differential transmission spectroscopy to temporally and spectrally resolve density-dependent carrier dynamics in a quantum dots-in-a-well (DWELL) heterostructure. We observe excitation-dependent shifts of the quantum dot energy levels at low densities,

R. P. Prasankumar; W. W. Chow; J. Urayama; R. S. Attaluri; R. V. Shenoi; S. Krishna; A. J. Taylor

2010-01-01

191

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

192

Density and size-dependent mortality of a settling coral-reef damselfish ( Pomacentrus moluccensis Bleeker)  

Microsoft Academic Search

Density-dependent mortality may regulate many populations, but due to an offshore larval phase in benthic marine organisms, it is often difficult to quantify the effects of mortality of arriving individuals. We added approximately 600 recruit-sized individuals onto coral reef that parallels patterns in naturally settling fish. Strong, positive density-dependent mortality occurred 3 days, 1–2 weeks, and 4 months after release. Since our study

Benjamin J. Brunton; David J. Booth

2003-01-01

193

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

194

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

USGS Publications Warehouse

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

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

2009-01-01

195

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

196

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

PubMed

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

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

2010-08-01

197

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

PubMed Central

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

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

2008-01-01

198

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

2007-10-01

199

Sampling-Based Approaches to Calculating Marginal Densities  

Microsoft Academic Search

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

Alan E. Gelfand; Adrian F. M. Smith

1990-01-01

200

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

NASA Astrophysics Data System (ADS)

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

Vikas, Hash(0x125f4490)

2011-02-01

201

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. 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

2013-12-01

202

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

PubMed

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

Aquino, Fredy W; Schatz, George C

2014-01-16

203

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.

204

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

NASA Astrophysics Data System (ADS)

The incorporation of semiconductor quantum dots into different heterostructures for applications in nanoscale lasing and amplification has been an active area of research in recent years. Here, we use ultrafast differential transmission spectroscopy to temporally and spectrally resolve density-dependent carrier dynamics in a quantum dots-in-a-well (DWELL) heterostructure. We observe excitation-dependent shifts of the quantum dot energy levels at low densities, while at high densities we observe an anomalous induced absorption at the quantum dot excited state that is correlated with quantum well population dynamics. These studies reveal unique Coulomb interaction-induced phenomena with important implications for DWELL-based lasers and amplifiers.

Prasankumar, R. P.; Chow, W. W.; Urayama, J.; Attaluri, R. S.; Shenoi, R. V.; Krishna, S.; Taylor, A. J.

2010-01-01

205

Functional and inclusion dependencies a graph theoretic approach  

Microsoft Academic Search

We present a new graph theoretic approach to the implication problem for functional (FD) and inclusion (IND) dependencies. Using this methodology we prove decidability for the case of typed IND's and acyclic FD's. We provide new lower bounds for the implication of typed, acyclic IND's and FD's --- NP-hardness and PSPACE-hardness for bounded domains. Finally, we show that there is

Stavros S. Cosmadakis; Paris C. Kanellakis

1984-01-01

206

Data Dependent Systems approach to solar energy simulation inputs  

Microsoft Academic Search

The paper proposes and illustrates an application of a recently developed methodology called Data Dependent Systems (DDS) to modeling and analysis of solar insolation data. Such an approach is shown to be capable of combining the advantages of deterministic as well as stochastic models. Major dynamic patterns are successfully reproduced by the models. The model characteristics reveal the relation of

S. M. Pandit; K. P. Rajurkar

1983-01-01

207

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

NASA Astrophysics Data System (ADS)

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

Loman, Jon; Lardner, Björn

2009-11-01

208

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

PubMed

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

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

2013-12-01

209

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

210

Density dependence of the /s-wave repulsion in pionic atoms  

NASA Astrophysics Data System (ADS)

Several mechanisms of density dependence of the s-wave repulsion in pionic atoms, beyond the conventional model, are tested by parameter fits to a large (106 points) set of data from 16O to 238U, including 'deeply bound' states in 205Pb. Special attention is paid to the proper choice of nuclear density distributions. A density-dependent isovector scattering amplitude suggested recently by Weise to result from a density dependence of the pion decay constant is introduced and found to account for most of the so-called anomalous repulsion. The presence of such an effect might indicate partial chiral symmetry restoration in dense matter. The anomalous repulsion is fully accounted for when an additional relativistic impulse approximation term is included in the potential.

Friedman, E.

2002-11-01

211

Equations of motion for a time-dependent open system: An algebraic approach  

NASA Astrophysics Data System (ADS)

An algebraic approach based on the concept of local densities is introduced in order to provide an alternative derivation of several equations central to the hydrodynamical formulation of quantum mechanics. The origin of this work lays in an algebraic equation which is built based on the concept of quantum densities. This enables us to derive the regional and local forms of several significant quantum laws and equations, namely Heisenberg equation of motion, hypervirial theory and quantum Navier-Stokes equation. In particular, atomic force law and local virial theorem for a time-dependent open system are extracted without referring to rigorous Schwinger's principle of stationary action.

Nasertayoob, Payam; Sabbaghan, Masoud

2013-02-01

212

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

PubMed

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

Marino, Andrea; Pascual, Miguel; Baldi, Ricardo

2014-08-01

213

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

PubMed

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

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

2008-09-01

214

An information theory approach to the density of the earth  

NASA Technical Reports Server (NTRS)

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

Graber, M. A.

1977-01-01

215

Nanodroplets on a planar solid surface: temperature, pressure, and size dependence of their density and contact angles.  

PubMed

The shape and the density of a liquid drop on a planar solid surface, embedded in an inert gas at constant temperature and pressure, were examined on the basis of a microscopic density functional approach that accounts for the entropic (temperature-dependent) and energetic contributions to the free energy of the system. Integro-differential equations describing the profile and the density of a cylindrical (2D) drop were derived by the variational minimization of the Gibbs free energy with respect to both the drop profile and density under the assumption of uniform density. The equations were solved numerically using the constraint of a constant number of molecules N(l) per unit length of the drop. It was shown that for temperatures lower than a certain temperature Tw the free energy against density has generally two minima, representing a stable equilibrium state and a metastable one. One of those minima is located at a density corresponding to the density of a normal liquid, whereas the other one is located at a density comparable to the density of the surrounding inert gas. For this reason, the latter state of the drop cannot be stable. For T > Tw, the minimum corresponding to the liquid state disappears, and no drop can be formed on the surface. The temperature Tw depends on N(l) and the external pressure p and increases when N(l) and p increase. The true wetting angle theta0 that the drop profile makes with the solid surface depends on the parameters characterizing the microscopic interactions, the density, and the surface densities. If in the thermodynamically stable state absolute value(cos theta0) > 1, then no drop is formed on the surface. If in that state absolute value(cos theta0) < 1, then at any pressure the true contact angle decreases when the temperature increases and approaches Tw. However, theta0 does not reach a zero value for T < or = Tw but has for T = Tw a discontinuity from a finite to a zero value. The true contact angle also depends on the number of molecules in the drop and on the external pressure. For all considered values of N(l), p, and microscopic parameters of the intermolecular interactions, the density of the drop decreases with increasing temperature. The rate of decrease is constant for temperatures sufficiently far from Tw and increases when T approaches Tw. At a given temperature and pressure, the density of the drop decreases with decreasing N(l). For relatively large drops (N(l) approximately = 10(14)-10(20)), the rate of decrease is very small, whereas for small droplets (N(l) approximately = 10(12)) it becomes much larger. PMID:16430266

Berim, Gersh O; Ruckenstein, Eli

2006-01-31

216

Universal time dependence of nighttime F region densities at high latitudes  

NASA Technical Reports Server (NTRS)

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

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

1985-01-01

217

The probability density function (pdf) approach to reacting turbulent flows  

Microsoft Academic Search

This chapter deals only with the probability density function (pdf) method and within that constraint is limited entirely to the equation of evolution of the pdf as obtained from the conservation laws of a reactive flow system. The subject is new and currently being developed, hindered mainly by a lack of solutions to the turbulent transport problem, and it is

E. E. O'Brien

218

Disclosure Control of Business Microdata: A Density-Based Approach  

Microsoft Academic Search

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

Daniela Ichim

2009-01-01

219

New semiclassical approach for the spatial density of nuclear systems.  

National Technical Information Service (NTIS)

A new decomposition of the many-body Wigner function in the squeezed states basis set of the quantum phase space-associated to bound states-allows one to separate the spatial density of the many body system in a semiclassical part plus its quantum complem...

D. Galetti S. Pawel

1990-01-01

220

Exciton and trions binding energies in single-layer MoS2: applications of the density-matrix time dependent density  

NASA Astrophysics Data System (ADS)

Exciton and trion binding energies of a single layer of MoS2 are studied using a time- dependent density-functional theory formalism. Kohn-Sham orbitals of the initial state were obtained using ab initio electronic structure calculations based on density functional theory. Several types of exchange-correlation (XC) kernels are implemented in our code to compare their performance. As expected our results depend crucially on the XC kernels used. In particular, the exchange-only adiabatic local density approximation kernel results in the binding energy about 0.1 eV, which is smaller than those obtained using the GW theory approximation (˜ 0.9 eV) [1]. We have generalized the approach on the case of trion excitations, which gives the trion binding energy ˜ 0.3eV when one used the LDA approximation. On the other hand, we demonstrate that the results for the experimental binding energies can be reproduced by using phenomenological local and long-range XC kernels. [1] T. Cheiwchanchamnangij and W. R. L. Lambrecht, Phys. Rev. B 85, 205302 (2012).

Ramírez-Torres, Alfredo; Turkowski, Volodymyr; Rahman, Talat S.

2013-03-01

221

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

PubMed

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

Steele, Mark A; Forrester, Graham E

2005-09-20

222

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

PubMed Central

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

Steele, Mark A.; Forrester, Graham E.

2005-01-01

223

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

USGS Publications Warehouse

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

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

2014-01-01

224

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

PubMed

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

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

2014-08-01

225

Density!  

NSDL National Science Digital Library

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

Witcher, Miss

2011-10-06

226

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

227

Density dependence vs. independence, and irregular population dynamics of a swallow-wort fruit fly.  

PubMed

Although most long-term studies of consumer-resource (e.g., predator-prey) interactions select species showing cyclic population dynamics, strong consumer-resource interactions can also produce irregular, noncyclic dynamics. Here, we present a case in which a seed predator, the tephritid fruit fly Euphranta connexa, shows fluctuations in density of more than two orders of magnitude over a 22-year period. To explain these fluctuations, we analyzed a stage-specific data set to quantify the density-dependent and density-independent components of larval survivorship and realized fecundity. Both larval survivorship and realized fecundity were strongly density dependent. Larval survivorship dropped from 0.62 at low larval density to 0.081 at high larval density, whereas fecundity dropped from 84.3 to 0.32 eggs per individual, more than a 100-fold decrease. We divided density-independent variation in E. connexa population dynamics into components for variability in (1) larval survivorship, (2) realized fecundity, and (3) annual fruit abundance. Of these components, 96% of the density-independent variance in per capita population growth rates was caused by fluctuations in fruit abundance. This highlights the importance of the strong consumer-resource interactions in driving fluctuations in E. connexa abundance. It also demonstrates that E. connexa dynamics are remarkably simple, and aside from the 4% of unexplained variance in per capita population growth rates, our understanding of E. connexa dynamics is remarkably complete. PMID:17601139

Solbreck, Christer; Ives, Anthony R

2007-06-01

228

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

PubMed

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

Bates, Jefferson E; Furche, Filipp

2012-10-28

229

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

NASA Astrophysics Data System (ADS)

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

Casida, Mark E.

2005-02-01

230

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

231

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

PubMed

In this work we have calculated excitation energies and photoionization cross sections of Be and Ne in the exact-exchange (EXX) approximation of time-dependent density functional theory (TDDFT). The main focus has been on the frequency dependence of the EXX kernel and on how it affects the spectrum as compared to the corresponding adiabatic approximation. We show that for some discrete excitation energies the frequency dependence is essential to reproduce the results of time-dependent Hartree-Fock theory. Unfortunately, we have found that the EXX approximation breaks down completely at higher energies, producing a response function with the wrong analytic structure and making inner-shell excitations disappear from the calculated spectra. We have traced this failure to the existence of vanishing eigenvalues of the Kohn-Sham non-interacting response function. Based on the adiabatic TDDFT formalism we propose a new way of deriving the Fano parameters of autoionizing resonances. PMID:19655840

Hellgren, Maria; von Barth, Ulf

2009-07-28

232

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

NASA Astrophysics Data System (ADS)

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

Hellgren, Maria; von Barth, Ulf

2009-07-01

233

Hartree-Fock calculations of light deformed nuclei using density-dependent effective interactions  

Microsoft Academic Search

The Hartree-Fock states of the doubly even N = Z nuclei from 12C to 40Ca have been calculated using the density-dependent effective interaction of Negele. The results are compared with those obtained with the density-independent Bl interaction of Brink and Boeker as well as with those of calculations using other interactions. A detailed comparison of quadrupole moments, radii, separation and

J. Zofka; G. Ripka

1971-01-01

234

Renormalized Brueckner-Hartree-Fock and density dependent Hartree-Fock theories of finite nuclei  

Microsoft Academic Search

Renormalized Brueckner-Hartree-Fock and density dependent Hartree-Fock calculations in the literature have been difficult to compare because they involve both different physical approximations and also technical computational differences. Hence, results obtained in both calculations for 40Ca are corrected for technical differences and compared in detail. It is shown that comparable Brueckner-Hartree-Fock calculations using an oscillator basis and using the local density

K. T. Davies; R. J. McCarthy; J. W. Negele; P. U. Sauer

1974-01-01

235

Tight-binding approach to strain-dependent DNA electronics  

NASA Astrophysics Data System (ADS)

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

Malakooti, Sadeq; Hedin, Eric; Joe, Yong

2013-07-01

236

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

NASA Astrophysics Data System (ADS)

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.

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

2013-08-01

237

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

238

A Concept for Airborne Precision Spacing for Dependent Parallel Approaches  

NASA Technical Reports Server (NTRS)

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

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

2012-01-01

239

Temperature and density dependence of XANES spectra in warm dense aluminum plasmas  

SciTech Connect

Using ab initio molecular-dynamics simulations combined with linear-response theory, we calculate the density and temperature dependence of the x-ray absorption near-edge structure (XANES) of a dense aluminum plasma. At solid density and for temperatures increasing up to 6 eV, we see that the XANES spectrum loses its well-known room-temperature structure, first due to melting and second due to loss of correlation in the liquid. Similarly, as the density decreases and the system evolves from a liquid to a plasma, the XANES spectrum becomes less structured. As the density is further lowered and the system turns into an atomic fluid, a pre-edge forms as the 3p state becomes bound. We suggest that direct measurements of the XANES spectra in this density region is a unique opportunity to validate pressure ionization models routinely used in plasma physics modeling.

Recoules, V.; Mazevet, S. [CEA, DAM, DIF, F-91297 Arpajon (France)

2009-08-01

240

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

PubMed

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

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

2013-08-28

241

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

NASA Astrophysics Data System (ADS)

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

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

2013-08-01

242

Time-Dependent Density-Functional Theory and Strongly Correlated Systems: Insight from Numerical Studies  

NASA Astrophysics Data System (ADS)

We illustrate the scope of time-dependent density-functional theory for strongly correlated (lattice) models out of equilibrium. Using the exact many-body time evolution, we reverse engineer the exact exchange correlation (xc) potential vxc for small Hubbard chains exposed to time-dependent fields. We introduce an adiabatic local density approximation to vxc for the 1D Hubbard model and compare it to exact results, to gain insight about approximate xc potentials. Finally, we provide some remarks on the v-representability for the 1D Hubbard model.

Verdozzi, Claudio

2008-10-01

243

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

244

Density-dependent nucleon-nucleon interaction from Urbana UIX three-nucleon force  

NASA Astrophysics Data System (ADS)

A density-dependent two-nucleon potential has been derived within the formalism of correlated basis function. The effects of three-particle interactions has been included by integrating out the degrees of freedom of the third nucleon. The potential can be easily employed in nuclear matter calculations. It yields results in agreement with those obtained from the underlying three-body potential. Using the density dependent potential, we have carried out a study of the effects of three-nucleon interactions in symmetric nuclear matter within the Auxiliary Field Diffusion Monte Carlo (AFDMC) computational scheme.

Lovato, Alessandro; Benhar, Omar; Fantoni, Stefano; Illarionov, Alexey Yu; Schmidt, Kevin E.

2011-12-01

245

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

246

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

2013-01-01

247

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

PubMed Central

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

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

2002-01-01

248

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

PubMed

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

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

2012-05-01

249

A new approach to mass spectrometer measurements of thermospheric density  

NASA Technical Reports Server (NTRS)

The gas sampling problem in satellite and high velocity probes was investigated by applying the theory of a drifting Maxwellian gas. A lens system using a free stream ion source was developed and experimentally evaluated over the pressure range of 0.00001 to 0.01 N/m sq (approx. 10 to the minus 7th power to 0.0001 torr). The source has high beam transparency, which minimizes gas-surface collisions within, or near, the ionization volume. It is shown that for high ion energy (60 eV), the extracted ion beam has an on-axis energy spread of less than 4 eV, and that 90 percent of the ions are within 2.5 deg of the beam axis. It is concluded that the molecular beam mass spectrometer concept, developed for gas density measurements in the upper atmosphere, substantially reduces gas-surface scattering and gas-surface reactions in the sample, and preserves the integrity of the gas sample during the analysis process. Studies show that both the Scout and Delta launch vehicles have adequate volume, control, velocity, and data acquisition capability necessary to obtain thermospheric number density in real time.

Melfi, L. T., Jr.; Brock, F. J.; Brown, C. A., Jr.

1974-01-01

250

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

251

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

NASA Astrophysics Data System (ADS)

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 S1 state of BAN is determined to be ~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.

Mitri?, Roland; Werner, Ute; Bona?i?-Koutecký, Vlasta

2008-10-01

252

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

SciTech Connect

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

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

2005-12-15

253

Simulating Nanofluids via the Weighted Density Lattice Boltzmann Approach  

NASA Astrophysics Data System (ADS)

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

Melchionna, Simone; Marconi, Umberto Marini Bettolo

2013-12-01

254

Linear-response time-dependent density-functional theory with pairing fields  

NASA Astrophysics Data System (ADS)

Recent development in particle-particle random phase approximation (pp-RPA) broadens the perspective on ground state correlation energies [H. van Aggelen, Y. Yang, and W. Yang, Phys. Rev. A 88, 030501 (2013), Y. Yang, H. van Aggelen, S. N. Steinmann, D. Peng, and W. Yang, J. Chem. Phys. 139, 174110 (2013); D. Peng, S. N. Steinmann, H. van Aggelen, and W. Yang, J. Chem. Phys. 139, 104112 (2013)] and N ± 2 excitation energies [Y. Yang, H. van Aggelen, and W. Yang, J. Chem. Phys. 139, 224105 (2013)]. So far Hartree-Fock and approximated density-functional orbitals have been utilized to evaluate the pp-RPA equation. In this paper, to further explore the fundamentals and the potential use of pairing matrix dependent functionals, we present the linear-response time-dependent density-functional theory with pairing fields with both adiabatic and frequency-dependent kernels. This theory is related to the density-functional theory and time-dependent density-functional theory for superconductors, but is applied to normal non-superconducting systems for our purpose. Due to the lack of the proof of the one-to-one mapping between the pairing matrix and the pairing field for time-dependent systems, the linear-response theory is established based on the representability assumption of the pairing matrix. The linear response theory justifies the use of approximated density-functionals in the pp-RPA equation. This work sets the fundamentals for future density-functional development to enhance the description of ground state correlation energies and N ± 2 excitation energies.

Peng, Degao; van Aggelen, Helen; Yang, Yang; Yang, Weitao

2014-05-01

255

Linear-response time-dependent density-functional theory with pairing fields.  

PubMed

Recent development in particle-particle random phase approximation (pp-RPA) broadens the perspective on ground state correlation energies [H. van Aggelen, Y. Yang, and W. Yang, Phys. Rev. A 88, 030501 (2013), Y. Yang, H. van Aggelen, S. N. Steinmann, D. Peng, and W. Yang, J. Chem. Phys. 139, 174110 (2013); D. Peng, S. N. Steinmann, H. van Aggelen, and W. Yang, J. Chem. Phys. 139, 104112 (2013)] and N ± 2 excitation energies [Y. Yang, H. van Aggelen, and W. Yang, J. Chem. Phys. 139, 224105 (2013)]. So far Hartree-Fock and approximated density-functional orbitals have been utilized to evaluate the pp-RPA equation. In this paper, to further explore the fundamentals and the potential use of pairing matrix dependent functionals, we present the linear-response time-dependent density-functional theory with pairing fields with both adiabatic and frequency-dependent kernels. This theory is related to the density-functional theory and time-dependent density-functional theory for superconductors, but is applied to normal non-superconducting systems for our purpose. Due to the lack of the proof of the one-to-one mapping between the pairing matrix and the pairing field for time-dependent systems, the linear-response theory is established based on the representability assumption of the pairing matrix. The linear response theory justifies the use of approximated density-functionals in the pp-RPA equation. This work sets the fundamentals for future density-functional development to enhance the description of ground state correlation energies and N ± 2 excitation energies. PMID:24832330

Peng, Degao; van Aggelen, Helen; Yang, Yang; Yang, Weitao

2014-05-14

256

Optical properties of gold metal clusters: A time-dependent local-density-approximation investigation  

Microsoft Academic Search

:   The optical response of free and matrix-embedded gold metal clusters AuN is investigated in the framework of the time-dependent local-density-approximation (TDLDA). The characteristics of the surface\\u000a plasmon resonance are carefully analyzed as a function of the model parameters and the particle radius. The strong influence\\u000a of the frequency-dependence of the 5d core-electron dielectric function in the vicinity of the

J. Lermé; B. Palpant; B. Prével; E. Cottancin; M. Pellarin; M. Treilleux; J. L. Vialle; A. Perez; M. Broyer

1998-01-01

257

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

Microsoft Academic Search

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

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

1981-01-01

258

Density  

NSDL National Science Digital Library

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

Hansen, Mr.

2010-10-26

259

Population-level consequences of heterospecific density-dependent movements in predator-prey systems.  

PubMed

In this paper we elucidate how small-scale movements, such as those associated with searching for food and avoiding predators, affect the stability of predator-prey dynamics. We investigate an individual-based Lotka-Volterra model with density-dependent movement, in which the predator and prey populations live in a very large number of coupled patches. The rates at which individuals leave patches depend on the local densities of heterospecifics, giving rise to one reaction norm for each of the two species. Movement rates are assumed to be much faster than demographics rates. A spatial structure of predators and prey emerges which affects the global population dynamics. We derive a criterion which reveals how demographic stability depends on the relationships between the per capita covariance and densities of predators and prey. Specifically, we establish that a positive relationship with prey density and a negative relationship with predator density tend to be stabilizing. On a more mechanistic level we show how these relationships are linked to the movement reaction norms of predators and prey. Numerical results show that these findings hold both for local and global movements, i.e., both when migration is biased towards neighbouring patches and when all patches are reached with equal probability. PMID:24060621

Sjödin, Henrik; Brännström, Ke; Söderquist, Mårten; Englund, Göran

2014-02-01

260

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

NASA Astrophysics Data System (ADS)

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

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

2014-07-01

261

Nonadiabatic electron dynamics in time-dependent density-functional theory  

NASA Astrophysics Data System (ADS)

Time-dependent density-functional theory (TDDFT) treats dynamical exchange and correlation (xc) via a single-particle potential, Vxc(r,t) , defined as a nonlocal functional of the density n(r',t') . The popular adiabatic local-density approximation (ALDA) for Vxc(r,t) uses only densities at the same space-time point (r,t) . To go beyond the ALDA, two local approximations have been proposed based on quantum hydrodynamics and elasticity theory: (a) using the current as the basic variable (C-TDDFT) [G. Vignale, C. A. Ullrich, and S. Conti, Phys. Rev. Lett. 79, 4847 (1997)], (b) working in a comoving Lagrangian reference frame (L-TDDFT) [I. V. Tokatly, Phys. Rev. B 71, 165105 (2005)]. In this paper we illustrate, compare, and analyze both nonadiabatic theories for simple time-dependent model densities in the linear and nonlinear regime, for a broad range of time and frequency scales. C- and L-TDDFT are identical in certain limits, but, in general, exhibit qualitative and quantitative differences in their respective treatment of elastic and dissipative electron dynamics. In situations where the electronic density rapidly undergoes large deformations, it is found that nonadiabatic effects can become significant, causing the ALDA to break down.

Ullrich, C. A.; Tokatly, I. V.

2006-06-01

262

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; Andy Hector

2010-01-01

263

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

NASA Astrophysics Data System (ADS)

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

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

2014-07-01

264

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

PubMed Central

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

2012-01-01

265

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

266

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

267

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

Microsoft Academic Search

The evolution of population dynamics in a stochastic environment is analysed under a general form of density-dependence with genetic variation in r and K, the intrinsic rate of increase and carrying capacity in the average environment, and in se 2 , the environmental variance of population growth rate. The continuous-time model assumes a large population size and a stationary distribution

Russell Lande; Steinar Engen; B.-E. Saether

2009-01-01

268

Temperature Dependence of Infrared Reflectivity of Chromium Realization of Soliton Lattice Structure of Spin Density Wave  

Microsoft Academic Search

The temperature dependence of reflectivity of chromium is measured at energies below 1 eV. A systematic change of the reflectivity dips is observed below the Néel temperature (TN{=}312 K), indicating the two energy gap structure associated with the incommensurate spin density wave state in Cr. This salient feature of the reflectivity data is successfully accounted for in terms of the

Kazushige Machida; M. A. Lind; J. L. Stanford

1984-01-01

269

Calculation of excitation energies within time-dependent density functional theory using auxiliary basis set expansions  

Microsoft Academic Search

We demonstrate the use of an auxiliary basis set expansion for the treatment of Coulomb type matrix elements in time-dependent density functional (TDDFT) calculations. The TDDFT scheme is used for the calculation of excitation energies within the adiabatic approximation. Comparison between results obtained by approximate and exact treatments of the Coulomb matrix elements shows that the errors for the excitation

Rüdiger Bauernschmitt; Marco Häser; Oliver Treutler; Reinhart Ahlrichs

1997-01-01

270

Mixed convection and density-dependent seawater circulation in coastal aquifers  

Microsoft Academic Search

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

Anthony J. Smith

2004-01-01

271

Temperature and carrier-density dependence of Auger and radiative recombination in nitride optoelectronic devices  

NASA Astrophysics Data System (ADS)

Nitride light-emitting diodes are a promising solution for efficient solid-state lighting, but their performance at high power is affected by the efficiency-droop problem. Previous experimental and theoretical work has identified Auger recombination, a three-particle nonradiative carrier recombination mechanism, as the likely cause of the droop. In this work, we use first-principles calculations to elucidate the dependence of the radiative and Auger recombination rates on temperature, carrier density and quantum-well confinement. Our calculated data for the temperature dependence of the recombination coefficients are in good agreement with experiment and provide further validation on the role of Auger recombination in the efficiency reduction. Polarization fields and phase-space filling negatively impact device efficiency because they increase the operating carrier density at a given current density and increase the fraction of carriers lost to Auger recombination.

Kioupakis, Emmanouil; Yan, Qimin; Steiauf, Daniel; Van de Walle, Chris G.

2013-12-01

272

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

SciTech Connect

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

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

2010-08-15

273

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

PubMed

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

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

2013-08-29

274

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

NASA Astrophysics Data System (ADS)

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

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

2007-12-01

275

Time-dependent resonant scattering: an analytical approach.  

PubMed

A time-dependent description is given of a scattering process involving a single resonance embedded in a set of flat continua. An analytical approach is presented which starts from an incident free particle wave packet and yields the Breit-Wigner cross-section formula at infinite times. We show that at intermediate times the so-called Wigner-Weisskopf approximation is equivalent to a scattering process involving a contact potential. Applications in cold-atom scattering and resonance enhanced desorption of molecules are discussed. PMID:24182008

Lecomte, J M; Kirrander, Adam; Jungen, Ch

2013-10-28

276

Density functional approach to the dynamic response at metal surfaces: van der Waals reference plane position and excitation of electron-hole pairs  

Microsoft Academic Search

The time-dependent density functional approach is used to calculate the response of a metal surface to uniform external fields that vary exponentially in time. The frequency dependence of the dynamic image plane is evaluated for several jellium surfaces. This quantity is of interest since it directly enters the expression for the reference plane position of the atom-metal van der Waals

A. Liebsch

1986-01-01

277

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

278

Density  

NSDL National Science Digital Library

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

Carpi, Anthony

2003-01-01

279

Polymer density functional approach to efficient evaluation of path integrals.  

PubMed

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

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

2005-10-01

280

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

NASA Astrophysics Data System (ADS)

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

Faghei, Kazem

2014-06-01

281

Hybrid large-eddy simulation\\/Lagrangian filtered-density-function approach for simulating turbulent combustion  

Microsoft Academic Search

A consistent hybrid large-eddy simulation\\/filtered-density-function approach (LES-FDF) is formulated for variable-density low-Mach-number flows. The LES-FDF approach has been proposed as a suitable method for finite-rate-chemistry-based predictive modeling of turbulent reactive flows. Due to the large computational grid associated with LES, use of Lagrangian schemes is numerically expensive. In this work, a highly efficient parallel Lagrangian implementation is used for the

Venkatramanan Raman; Heinz Pitsch; Rodney O. Fox

2005-01-01

282

An iterative learning approach for density control of freeway traffic flow via ramp metering  

Microsoft Academic Search

In this work, we apply the iterative learning control approach to address the traffic density control problem in a macroscopic level freeway environment with ramp metering. The traffic density control problem is first formulated into an output tracking and disturbance rejection problem. Through rigorous analysis, it is shown that the iterative learning control method can effectively deal with this class

Zhongsheng Hou; Jian-Xin Xu; Jingwen Yan

2008-01-01

283

Native defects in oxide semiconductors: a density functional approach  

NASA Astrophysics Data System (ADS)

We report a semilocal and hybrid Hartree-Fock density functional study of native defects in three oxide semiconductors: ZnO, SrTiO3, 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 SrTiO3, 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 SrTiO3. 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 SnO2.

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

2010-09-01

284

Native defects in oxide semiconductors: a density functional approach.  

PubMed

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

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

2010-09-29

285

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

NASA Astrophysics Data System (ADS)

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

Huenerbein, Robert; Grimme, Stefan

2008-01-01

286

Carry-over effects, sequential density dependence and the dynamics of populations in a seasonal environment  

PubMed Central

Most animal populations have distinct breeding and non-breeding periods, yet the implications of seasonality on population dynamics are not well understood. Here, we introduce an experimental model system to study the population dynamics of two important consequences of seasonality: sequential density dependence and carry-over effects (COEs). Using a replicated seasonal population of Drosophila, we placed individuals at four densities in the non-breeding season and then, among those that survived, placed them to breed at three different densities. We show that COEs arising from variation in non-breeding density negatively impacts individual performance by reducing per capita breeding output by 29–77%, implying that non-lethal COEs can have a strong influence on population abundance. We then parametrized a bi-seasonal population model from the experimental results, and show that both sequential density dependence and COEs can stabilize long-term population dynamics and that COEs can reduce population size at low intrinsic rates of growth. Our results have important implications for predicting the successful colonization of new habitats, and for understanding the long-term persistence of seasonal populations in a wide range of taxa, including migratory organisms.

Betini, Gustavo S.; Griswold, Cortland K.; Norris, D. Ryan

2013-01-01

287

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

SciTech Connect

By assuming that the isospin- and momentum-dependent MDI interaction has a form similar to the Gogny-like effective two-body interaction with a Yukawa finite-range term and the momentum dependence originates only from the finite-range exchange interaction, we determine its parameters by comparing the predicted potential energy density functional in uniform nuclear matter with what has been usually given and used extensively in transport models for studying isospin effects in intermediate-energy heavy-ion collisions as well as in investigating the properties of hot asymmetric nuclear matter and neutron star matter. We then use the density matrix expansion to derive from the resulting finite-range exchange interaction an effective Skyrme-like zero-range interaction with density-dependent parameters. As an application, we study the transition density and pressure at the inner edge of neutron star crusts using the stability conditions derived from the linearized Vlasov equation for the neutron star matter.

Xu Jun [Cyclotron Institute, Texas A and M University, College Station, Texas 77843-3366 (United States); Ko Che Ming [Cyclotron Institute and Department of Physics and Astronomy, Texas A and M University, College Station, Texas 77843-3366 (United States)

2010-10-15

288

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

PubMed

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

Gui, Yue; Grant, Alastair

2008-06-01

289

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

NASA Astrophysics Data System (ADS)

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

Nazarov, Vladimir; Vignale, Giovanni

2012-02-01

290

Space-dependent color gamut mapping: a variational approach.  

PubMed

Gamut mapping deals with the need to adjust a color image to fit into the constrained color gamut of a given rendering medium. A typical use for this tool is the reproduction of a color image prior to its printing, such that it exploits best the given printer/medium color gamut, namely the colors the printer can produce on the given medium. Most of the classical gamut mapping methods involve a pixel-by-pixel mapping and ignore the spatial color configuration. Recently proposed spatial-dependent approaches for gamut mapping are either based on heuristic assumptions or involve a high computational cost. In this paper, we present a new variational approach for space-dependent gamut mapping. Our treatment starts with the presentation of a new measure for the problem, closely related to a recent measure proposed for Retinex. We also link our method to recent measures that attempt to couple spectral and spatial perceptual measures. It is shown that the gamut mapping problem leads to a quadratic programming formulation, guaranteed to have a unique solution if the gamut of the target device is convex. An efficient numerical solution is proposed with promising results. PMID:15971778

Kimmel, Ron; Shaked, Doron; Elad, Michael; Sobel, Irwin

2005-06-01

291

Local Energy-Density Functional Approach to Many-Body Nuclear Systems with S-Wave Pairing  

NASA Astrophysics Data System (ADS)

The ground-state properties of superfluid nuclear systems with 1S0 pairing are studied within a local energy-density functional (LEDF) approach. A new form of the LEDF is proposed with a volume part which fits the Friedman-Pandharipande and Wiringa-Fiks-Fabrocini equation of state at low and moderate densities and allows an extrapolation to higher densities which preserves causality. For inhomogeneous systems, a surface term is added, with two free parameters, which has a fractional form like a Padé approximant containing the square of the density gradient in both the numerator and denominator. In addition to the direct and exchange Coulomb interaction energy, an effective density-dependent Coulomb-nuclear correlation term is included with one more free parameter. A three-parameter fit to the masses and radii of about 100 spherical nuclei has shown that the latter term gives a contribution of the same order of magnitude as the Nolen-Schiffer anomaly in the Coulomb displacement energy. The root-mean-square deviations from experimental masses and radii with the proposed LEDF come out about a factor of two smaller than those obtained with the conventional functionals based on the Skyrme or finite-range Gogny force, or on relativistic mean-field theory. The generalized variational principle is formulated leading to the self-consistent Gor'kov equations which are sovled exactly, with physical boundary conditions both for the bound and scattering states. The method is used to calculate the differential observables such as odd-even mass differences and staggering in charge radii. With a zero-range density-dependent cutoff pairing interaction incorporating a density-gradient term, the evolution of these observables is reproduced reasonably well, including the kinks at magic neutron numbers and the sizes of the associated staggering. An extrapolation from the pairing properties of finite nuclei to pairing in infinite nuclear matter is discussed. A "reference" value of the pairing gap ?F? 3.3 MeV is found for subsaturated nuclear matter at about 0.65 of the equilibrium density. With the formulated LEDF approach, we study also the dilute limit in both the weak and strong coupling regimes. Within the sum rules approach it is shown that the density-dependent pairing may also induce sizeable staggering and kinks in the evolution of the mean energies of multipole excitations.

Fayans, S. A.; Zawischa, D.

292

Compositional dependence of trap density and origin in thin silicon oxynitride film investigated using spin dependent Poole-Frenkel current  

NASA Astrophysics Data System (ADS)

The compositional dependence of trap density and origin in thin silicon oxynitride (SiOxNy) films deposited by the low pressure chemical vapor deposition method was investigated using spin dependent Poole-Frenkel (SDPF) current technique. SDPF detected two kinds of traps, K-center (N3trpbndSi., where means a dangling bond) and K'-center (N2OtrpbndSi.). With increasing oxygen concentration, the amount of K-center decreases. On the other hand, K'-center increases up to O/O+N=0.25 and then it decreases. We propose the model that the change in the film strain by oxygen atoms induces these phenomena.

Yonamoto, Yoshiki; Inaba, Yutaka; Akamatsu, Naotoshi

2011-06-01

293

Application of Time-Dependent Density-Functional Theory to C6  

NASA Astrophysics Data System (ADS)

We employ a real-space pseudopotential method to determine the ground state structure of the carbon cluster C6 via simulated annealing and the corresponding optical absorption spectra from the adiabatic time-dependent density-functional theory (TDDFT) and the local density approximation (TDLDA). It is found that the ground state structure of the carbon cluster C6 belongs to a monocyclic D3h structure and the calculated spectra exhibit a variety of features that can be used for comparison against future experimental investigations.

Zhou, Xiao-Lin; Bai, Yu-Lin; Chen, Xiang-Rong; Yang, Xiang-Dong

2004-02-01

294

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

295

Elevation dependence of bedform wavelength on Tharsis Montes, Mars: Atmospheric density as a controlling parameter  

NASA Astrophysics Data System (ADS)

We measure the wavelength of aeolian bedforms on the surface of martian volcanoes, spanning a 23 km range in elevation, or nearly an order of magnitude in atmospheric pressure and density. We find that the bedform wavelength (˜1-5 m) varies as the reciprocal of density. The observed wavelengths and wavelength dependence are compared with three recent models of bedform spacing. The results are consistent with the expected increase in threshold friction speed and saturation drag length with elevation. The observations lend support to the expectation that such bedforms on Venus or Titan are too small to be observed in present data.

Lorenz, Ralph D.; Bridges, Nathan T.; Rosenthal, Alex A.; Donkor, Elise

2014-02-01

296

The Deposition Angle-Dependent Density of Amorphous Solid Water Films  

SciTech Connect

The index of refraction and thickness of amorphous solid water (ASW) films are determined using laser optical interferometry. From the film thickness, the density of ASW can be calculated directly since the molecular beam flux and the H2O condensation coefficient are both known. From the index of refraction the ASW density can also be determined using the Lorentz-Lorenz relationship. The densities determined via both methods agree within experimental uncertainty. For films deposited at 22 K using a collimated molecular beam, the index of refraction and density decrease monotonically as the deposition angle is varied from normal to oblique incidence. At normal incidence the films have an index of refraction of 1.285 and are fully dense (0.94 g/cm3). At glancing incidence (86?) the film has a refractive index of 1.05 and a density of 0.16 g/cm3, indicating a porosity exceeding 80%. The angle dependent film density is in good quantitative agreement with the results of ballistic deposition simulations of ASW film growth.

Dohnalek, Zdenek; Kimmel, Greg A.; Ayotte, Patrick; Smith, R. Scott; Kay, Bruce D.

2003-01-01

297

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

PubMed

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

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

2013-01-01

298

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

299

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

PubMed Central

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

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

2013-01-01

300

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

PubMed

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

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

2013-10-01

301

Numerical solution of continuity equation with a flux non-linearly depending on the density gradient  

NASA Astrophysics Data System (ADS)

An approach to integrate transport equations with fluxes being complex non-linear functions of physical parameters and their gradients, as it is predicted by theoretical models for micro-instabilities in plasma, is proposed. This approach operates without any splitting of the flux on diffusive and convective components often involved in transport calculations. As an example, computations of the density profile in a stationary state and during dynamic evolution are done with the Weiland's transport model. The results obtained by the proposed method and a conventional one with a flux splitting are compared.

Tokar, Mikhail Z.

2006-12-01

302

PTPN14 is required for the density-dependent control of YAP1  

PubMed Central

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

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

2012-01-01

303

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

304

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

305

Time-dependent density-functional molecular-dynamics study of the isotope effect in chemicurrents  

NASA Astrophysics Data System (ADS)

The energy dissipation into electron-hole pairs has been simulated ab initio within time-dependent density-functional theory for spin-unpolarized hydrogen atoms interacting with the Al on-top site at the Al(1 1 1) surface. The electron-hole pair excitation spectra are characterized by an approximately exponentially decaying tail of the electron energy distribution. It is shown that both the energy dissipated into electron-hole pairs and the excitation spectra, and hence the chemicurrent yield, show an isotope dependence identical to what expected from the linear friction ansatz and the forced oscillator model.

Lindenblatt, M.; Pehlke, E.

2006-12-01

306

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

2013-08-01

307

Terrestrial habitat selection and strong density-dependent mortality in recently metamorphosed amphibians.  

PubMed

To predict the effects of terrestrial habitat change on amphibian populations, we need to know how amphibians respond to habitat heterogeneity, and whether habitat choice remains consistent throughout the life-history cycle. We conducted four experiments to evaluate how the spatial distribution of juvenile wood frogs, Rana sylvatica (including both overall abundance and localized density), was influenced by habitat choice and habitat structure, and how this relationship changed with spatial scale and behavioral phase. The four experiments included (1) habitat manipulation on replicated 10-ha landscapes surrounding breeding pools; (2) short-term experiments with individual frogs emigrating through a manipulated landscape of 1 m wide hexagonal patches; and habitat manipulations in (3) small (4-m2); and (4) large (100-m2) enclosures with multiple individuals to compare behavior both during and following emigration. The spatial distribution of juvenile wood frogs following emigration resulted from differences in the scale at which juvenile amphibians responded to habitat heterogeneity during active vs. settled behavioral phases. During emigration, juvenile wood frogs responded to coarse-scale variation in habitat (selection between 2.2-ha forest treatments) but not to fine-scale variation. After settling, however, animals showed habitat selection at much smaller scales (2-4 m2). This resulted in high densities of animals in small patches of suitable habitat where they experienced rapid mortality. No evidence of density-dependent habitat selection was seen, with juveniles typically choosing to remain at extremely high densities in high-quality habitat, rather than occupying low-quality habitat. These experiments demonstrate how prediction of the terrestrial distribution of juvenile amphibians requires understanding of the complex behavioral responses to habitat heterogeneity. Understanding these patterns is important, given that human alterations to amphibian habitats may generate extremely high densities of animals, resulting in high density-dependent mortality. PMID:18831177

Patrick, David A; Harper, Elizabeth B; Hunter, Malcolm L; Calhoun, Aram J K

2008-09-01

308

Potential dependence of polaron and bipolaron densities in conducting polymers: theoretical description beyond the Nernst equations  

NASA Astrophysics Data System (ADS)

The charged species in most conducting polymers are polarons (P) and bipolarons (BP) (eventually transverse or interchain bipolarons). The electrochemical behaviour of such systems is determined (apart from kinetics) by the dependence of the P and BP densities on the electrochemical potential and on the temperature. Until now a statistics for the high density-limit (needed for high degrees of oxidation) was not known. We present here a novel approximation for finite temperatures which contains exactly both the low-density (non-degenerate) limit and the zero temperature limit. Our result can be expressed in terms of conventional Nernst equations describing oxidation by two one-electron steps, followed by thermodynamic equilibrium. This equivalence yields the connection between the two standard potentials and the formation energies and degeneracy factors of P's and BP's. For high densities we approximate the dependence of the P and BP formation energies on the densities by few-parameter interpolations based on numerical results of the discrete Brazovskii-Kirova model for polymers with a non-degenerate ground state. The theory yields a consistent description for typical experimental observations which are in complete disagreement with the conventional Nernst equations. The most important facts we describe correctly are: In a region of several hundreds mV above the CV-peak one has a capacitive current plateau with a volume-proportional capacitance of the order of 100 Fcm -3 and a small and almost constant charge transfer resistance; the spin concentration shows an asymmetric peak with a considerable spin concentration in the plateau region. These features arise from increase of the standard potentials with increasing degree of oxidation in our theory, owing to the increase of the P and BP formation energies at high densities.

Paasch, G.; Nguyen, P. H.; Fisher, A. J.

1998-02-01

309

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

310

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

PubMed

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 relatively large (0.05 km2) seagrass beds determined that secondary dispersal is primarily undertaken by the earliest juvenile blue crab instar stages (J1 crabs). These crabs emigrated pelagically from seagrass settlement habitats using nighttime flood tides during average wind conditions (speed approximately 5 m s-1). Moreover, the secondary dispersal of J1 crabs was density-dependent and regulated by intra-cohort (J1) crab density in seagrass. Our results suggest that dispersal occurs rapidly following settlement, and promotes blue crab metapopulation persistence by redistributing juveniles from high-density settlement habitats to areas characterized by low postlarval supply. Collectively, these data indicate that blue crab secondary dispersal is an active process under behavioral control and can alter initial distribution patterns established during settlement. This study highlights the necessity of considering secondary dispersal in ecological studies to improve our understanding of population dynamics of benthic organisms. PMID:15146323

Reyns, Nathalie B; Eggleston, David B

2004-07-01

311

Choice of the density-dependent effective interaction, and. cap alpha. decay of heavy spherical nuclei  

Microsoft Academic Search

The parameters of the density-dependent effective interaction are studied for some nuclei in the vicinity of the doubly magic ²°⁸Pb nucleus. Nuclei differing from magic by +- 2 nucleons and some superfluid nuclei are considered. It is found that the magnitudes of the matrix elements for the zero-range forces (delta forces) are more than three times larger than in the

S. G. Kadmenskii; Y. L. Ratis; K. S. Rybak; V. I. Furman

1978-01-01

312

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

Microsoft Academic Search

Density-dependent relations among the saturation properties of symmetric nuclear matter and hyperonic matter, and properties of hadron-(strange) quark stars are shown by applying the conserving nonlinear sigma-omega-rho hadronic mean-field theory. Nonlinear interactions are renormalized self-consistently as effective coupling constants, effective masses, and sources of equations of motion by maintaining thermodynamic consistency to the mean-field approximation. Effective masses and coupling constants

Hiroshi Uechi; Schun T. Uechi

2011-01-01

313

Temperature dependent spin momentum densities in Ni-Mn-In alloys  

Microsoft Academic Search

The spin-dependent electron momentum densities in Ni2MnIn and Ni2Mn1.4In0.6 shape memory alloy using magnetic Compton scattering with 182.2 keV circularly polarized synchrotron radiation are reported. The magnetic Compton profiles were measured at different temperatures ranging between 10 and 300 K. The profiles have been analyzed mainly in terms of Mn 3d electrons to determine their role in the formation of

B. L. Ahuja; Alpa Dashora; N. L. Heda; K. R. Priolkar; L. Vadkhiya; M. Itou; Nelson Lobo; Y. Sakurai; Aparna Chakrabarti; Sanjay Singh; S. R. Barman

2010-01-01

314

Time-dependent density-functional theory in the projector augmented-wave method  

Microsoft Academic Search

We present the implementation of the time-dependent density-functional theory both in linear-response and in time-propagation formalisms using the projector augmented-wave method in real-space grids. The two technically very different methods are compared in the linear-response regime where we found perfect agreement in the calculated photoabsorption spectra. We discuss the strengths and weaknesses of the two methods as well as their

Michael Walter; Hannu Häkkinen; Lauri Lehtovaara; Martti Puska; Jussi Enkovaara; Carsten Rostgaard

2008-01-01

315

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

316

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

317

High density lipoprotein subfractions in non-insulin- dependent diabetes mellitus and coronary artery disease  

Microsoft Academic Search

High density lipoprotein (HDL) subfractions (2b, 2a, 3a, 3b, and 3c) separated by gradient gel electrophoresis (GGE) and defined by Gaussian summation analysis, and the compositions of HDL2 and HDL,, separated by preparative ultracentrifugation, were studied in four groups of men with or without non-insulin-dependent diabetes mellitus (NIDDM) and coronary artery disease (CAD): group 1 (DM+CAD+, n = 50); group

Mikko Syvanne; Maria Ahola; Sanni Lahdenpera; Juhani Kahri; Kari S. Virtanen; Marja-Riitta Taskinent

318

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

Microsoft Academic Search

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

Pansy D. Patel; Artëm E. Masunov

2009-01-01

319

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

320

Brueckner-Hartree-Fock Calculations Using Density-Dependent Effective Interactions. Application to O16  

Microsoft Academic Search

The effects of allowing variations in the radial orbital wave functions, or major shell mixing, have been investigated within the framework of Brueckner-Hartree-Fock (BHF) theory for O16. The calculation is carried out in the matrix representation using harmonic-oscillator wave functions as a basis. Effective G matrix elements are calculated through the use of a density-dependent, two-body operator following a prescription

Neal E. Reid; Manoj K. Banerjee; G. J. Stephenson

1972-01-01

321

Polarization-dependent gain and saturation energy density in a TE N 2-laser amplifier  

Microsoft Academic Search

The polarization-dependent gain, g0, and saturation energy density, Es, in a TE N2-laser amplifier were measured, using an oscillator–amplifier laser system for different amplifier electrode gap separations, dAMP, of 7, 9 and 4mm and gas pressure of p=77, 60, and 165Torr, respectively. It was realized that for the amplifier with the gap separation of 7 and 9mm, where the pdAMP-value

S. Ghoreyshi; K. Rahimian; A. Hariri

2007-01-01

322

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

323

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

PubMed Central

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

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

2013-01-01

324

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

PubMed

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

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

2006-03-21

325

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

NASA Astrophysics Data System (ADS)

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

Gupta, Vinita; Myers, Anne B.

1998-03-01

326

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

PubMed Central

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

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

2012-01-01

327

Temperature dependence of structure and density for D?O confined in MCM-41-S.  

PubMed

Using neutron diffraction, we have tracked the temperature dependence of structural properties for heavy water confined in the nanoporous silica matrix MCM-41-S. By observing the correlation peak corresponding to the pore-pore distance, which is determined by the scattering contrast between the silica and the water, we monitored the density of the confined water. Concurrently, we studied the prominent first diffraction peak of D(2)O at ? 1.8 ?(-1), which furnishes information on the microscopic arrangement of the water molecules. The data show the presence of a density maximum at ? 275 K (± 10 K), a property similar to bulk water, and the occurrence of a density minimum at ? 180 K (± 10 K). The prominent diffraction peak of D(2)O is found to shift and sharpen over a wide T range from 200 to 270 K, reflecting structural changes that are strongly correlated with the changes in density. We also observe the continuous formation of external ice, arising from water expelled from the pores while expansion takes place within the pores. An efficient method for monitoring the density of the confined D(2)O using a triple-axis spectrometer is demonstrated. PMID:22277370

Kamitakahara, William A; Faraone, Antonio; Liu, Kao-Hsiang; Mou, Chung-Yuan

2012-02-15

328

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

PubMed

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

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

2011-11-01

329

Temperature dependence of structure and density for D2O confined in MCM-41-S  

NASA Astrophysics Data System (ADS)

Using neutron diffraction, we have tracked the temperature dependence of structural properties for heavy water confined in the nanoporous silica matrix MCM-41-S. By observing the correlation peak corresponding to the pore-pore distance, which is determined by the scattering contrast between the silica and the water, we monitored the density of the confined water. Concurrently, we studied the prominent first diffraction peak of D2O at ?1.8 Å-1, which furnishes information on the microscopic arrangement of the water molecules. The data show the presence of a density maximum at ?275 K (±10 K), a property similar to bulk water, and the occurrence of a density minimum at ?180 K (±10 K). The prominent diffraction peak of D2O is found to shift and sharpen over a wide T range from 200 to 270 K, reflecting structural changes that are strongly correlated with the changes in density. We also observe the continuous formation of external ice, arising from water expelled from the pores while expansion takes place within the pores. An efficient method for monitoring the density of the confined D2O using a triple-axis spectrometer is demonstrated.

Kamitakahara, William A.; Faraone, Antonio; Liu, Kao-Hsiang; Mou, Chung-Yuan

2012-02-01

330

A comprehensive approach to age-dependent dosimetric modeling  

SciTech Connect

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

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

1986-01-01

331

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

2011-06-01

332

Density  

NSDL National Science Digital Library

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

Day, Martha M.

2008-05-26

333

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

334

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

335

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

NASA Astrophysics Data System (ADS)

Within the newly updated version of the ultrarelativistic quantum molecular dynamics (UrQMD) model, the transverse-velocity dependence of the elliptic flow of free nucleons from Au197+Au197 collisions at the incident energy 400 MeV/nucleon is studied within different windows of the normalized c.m. rapidity y0. It is found that the elliptic flow difference v2n-v2p and ratio v2n/v2p of neutrons versus protons are sensitive to the density dependence of the symmetry energy, especially the ratio v2n/v2p at small transverse velocity in the intermediate rapidity intervals 0.4<|y0|<0.6. By comparing either transverse-momentum-dependent or integrated FOPI/LAND elliptic flow data of nucleons and hydrogen isotopes with calculations using various Skyrme interactions, all exhibiting similar values of isoscalar incompressibility but very different density dependences of the symmetry energy, a moderately soft to linear symmetry energy is extracted, in good agreement with previous UrQMD or Tübingen QMD model calculations but contrast with results obtained with ?-/?+ yield ratios in the literature.

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

2014-04-01

336

Excitation energies from time-dependent density-functional formalism for small systems  

NASA Astrophysics Data System (ADS)

As a test of the time-dependent local-density approximation (TDLDA), we study the 1?g+ ? 1?u+ excitation of H2 as a function of the nuclear distance d. We find rather accurate results for intermediate d but not for small and large d. At large d, TDLDA fails due to the strong non-locality and energy dependence of the exact functional. The spin-dependent formalism gives a qualitative improvement for large d. To analyze the results, we compare with the 2s ? 2p excitation for He, Li and Be for small and intermediate d and with the Hubbard model for large d. The fairly accurate results for Li and Be are related to the accuracy of the ground-state formalism for a few electrons.

Aryasetiawan, F.; Gunnarsson, O.; Rubio, A.

2002-03-01

337

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

338

Sub-barrier fusion cross-sections for 32S+90,96Zr, using the semiclassical extended Thomas-Fermi approach of energy density formalism  

NASA Astrophysics Data System (ADS)

The fusion excitation functions are studied near and below the Coulomb barrier by modifying nuclear interaction potential. The nuclear potential is obtained in the semiclassical Extended Thomas-Fermi approach of Skyrme energy density formalism for both the parts (the spin-orbit density independent and spin-orbit density dependent) of the Hamiltonian density, since the two behave differently, the spin-orbit density independent part behave mainly attractive and the spin-orbit density dependent mainly as repulsive. The semiclassical expansions of kinetic energy density and spin-orbit density used are allowed up to second order and the two parameter Fermi density is used for the nuclear density. The modification of nuclear potential is done by changing the temperature dependent surface thickness of either target or projectile or both to the best fit. The study is done for the fusion cross-sections for reactions 32S+90Zr and 32S+96Zr as a function of center of mass energy for the arbitrarily chosen Skyrme force SIV.

Verma, Dalip S.

2013-04-01

339

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

340

Density-dependent male mating harassment, female resistance, and male mimicry.  

PubMed

Genetic variation in female resistance and tolerance to male mating harassment can affect the outcome of sexually antagonistic mating interactions. We investigated female mating rates and male mating harassment in natural populations of a damselfly (Ischnura elegans). This damselfly species has a heritable sex-limited polymorphism in females, where one of the morphs is a male mimic (androchrome females). The three female morphs differ in mating rates, and these differences are stable across populations and years. However, the degree of premating resistance toward male mating attempts varied across generations and populations. Male mating harassment of the female morphs changed in a density-dependent fashion, suggesting that male mate preferences are plastic and vary with the different morph densities. We quantified morph differences in male mating harassment and female fecundity, using path analysis and structural equation modeling. We found variation between the morphs in the fitness consequences of mating, with the fecundity of one of the nonmimetic morphs declining with increasing male mating harassment. However, androchrome females had lower overall fecundity, presumably reflecting a cost of male mimicry. Density-dependent male mating harassment on the morphs and fecundity costs of male mimicry are thus likely to contribute to the maintenance of this female polymorphism. PMID:19382852

Gosden, Thomas P; Svensson, Erik I

2009-06-01

341

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

SciTech Connect

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

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

1991-05-01

342

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

343

Structure of electric double layers: A self-consistent weighted-density-functional approach  

Microsoft Academic Search

A self-consistent weighted-density-functional approach is developed for the structure of electric double layer using the restricted primitive model which corresponds to charged hard sphere ions and a continuum solvent. The one-particle correlation function of this inhomogeneous system is evaluated using suitably averaged weighted densities for the short range hard sphere as well as the long range electrical components. The hard-sphere

Chandra N. Patra; Swapan K. Ghosh

2002-01-01

344

Assessment of dispersion-corrected density functional approaches for extended systems  

Microsoft Academic Search

Standard density functional (DFT) methods are known to fail in describing the long range van der Waals interactions, and currently, there is a great interest in incorporating dispersion corrections in density functionals. Recently, Tkatchenko and Scheffler introduced a new scheme where dispersion corrections are included by a summation of damped interatomic C6\\/R^6 terms. However, contrary to the DFT-D2 approach of

Wissam Al-Saidi; Vamsee Voora; Ken Jordan

2011-01-01

345

Density functional theory screened-exchange approach for investigating electronical properties of graphene-related materials  

Microsoft Academic Search

We present ab initio calculations of the band structure of graphene and of short zigzag graphene nanoribbons (ZGNRs) by the screened-exchange-local-density approximation (LDA) method within the framework of density functional theory. The inclusion of nonlocal electron-electron interactions in this approach results in a renormalization of the electronic band structure and the Fermi velocity compared to calculations within LDA gives good

Roland Gillen; John Robertson

2010-01-01

346

Exploring the Dependence of the Exchange-Correlation Energy on the Laplacian of the Electron Density  

NASA Astrophysics Data System (ADS)

It has been observed that the deviation of exchange-correlation (XC) energy from the local density approximation (LDA) model in quantum Monte Carlo simulations of several systems correlates to the local Laplacian of the density [1] in a roughly linear fashion. We examine the effects on the prediction of typical material properties of the linear dependence of the XC energy density on the Laplacian of the density. To do this we focus on slowly varying systems within LDA and generalized gradient approximation models and compare these models using the Laplacian. We use the pseudopotential code Abinit to examine selected systems such as the AE6 test set and some typical solids such as Si, Cu, Na and NaCl. We then observe the differences between predictions of properties, particularly atomization energies and bond lengths of molecules and cohesive energies and lattice constants of solids, between standard DFT models and models using the Laplacian.[4pt] [1] A. C. Cancio and M. Y. Chou, Phys. Rev. B. 74, 081202 (2006)

Ross, Seth; Cancio, A. C.

2010-03-01

347

Density-dependent phytotoxicity of impatiens pallida plants exposed to extracts of Alliaria petiolata.  

PubMed

Invasive plants are by definition excellent competitors, either indirectly through competition for resources or directly through allelopathic inhibition of neighboring plants. Although both forms of competition are commonly studied, attempts to explore the interactions between direct and indirect competition are rare. We monitored the effects of several doses of extracts of Alliaria petiolata, a Eurasian invader in North America, on the growth of Impatiens pallida, a North American native, at several planting densities. The density-dependent phytotoxicity model predicts that as plant density increases, individual plant size will decrease, unless a toxin is present in the soil. In this case, individual plant size is predicted to increase as plant density increases, as plants share a limited toxin dose. We tested this model using fractions of an A. petiolata extract enriched in flavonoids or glucosinolates, as well as a combined fraction. The flavonoid-enriched fraction and the combined fraction suppressed I. pallida growth but only when applied at a dose eight times higher than that expected in the field. When treated with a dose equivalent to estimated field exposure levels, I. pallida growth was not distinguishable from that of control plants that received no extract, showing that indirect competition for resources was more important for determining the growth of I. pallida than direct allelopathic inhibition by A. petiolata. This is an important reminder that, even though many plants have the demonstrated potential to exert strong allelopathic effects, those effects may not always be apparent when other forms of competition are considered as well. PMID:19381723

Barto, E Kathryn; Cipollini, Don

2009-04-01

348

Density-dependent parasitism of the soil-borne nematodeCriconemella xenoplax by the nematophagous fungusHirsutella rhossiliensis.  

PubMed

Spatial sampling was used to investigate temporal density-dependent parasitism of the plant-parasitic nematodeCriconemella xenoplax byHirsutella rhossiliensis in three peach orchards on eight sample dates. The patches of soil in which the nematode and fungus interacted were assumed to possess similar density-dependent dynamics and to be small, independent, and asynchronous. Furthermore, sampling of separate patches was assumed to provide similar information with respect to density dependence as would temporal (repeated) sampling of the same patch. Percent parasitism was dependent on the number ofC. xenoplax/100 cm(3) soil (P=0.0001). The slope was unaffected by orchard or date but ranged from 0.0001 to 0.0043 depending on distance from the irrigation furrow. The relative shallowness of the slope and the large variation in percent parasitism not explained by nematode density suggest thatH. rhossiliensis is a weak regulator ofC. xenoplax population density. PMID:24197247

Jaffee, B A; Gaspard, J T; Ferris, H

1989-03-01

349

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

350

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

PubMed

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

Autschbach, Jochen

2011-12-01

351

Divergent approaches to the treatment of dyslipidemia with low levels of high-density lipoprotein cholesterol.  

PubMed

Further therapy should be considered for patients who have reached target low-density lipoprotein cholesterol (LDL-C) levels but have low plasma levels of high-density lipoprotein cholesterol (HDL-C) since rates of coronary artery disease continue to be high for a sizable proportion of this population. Lifestyle approaches include dietary changes, smoking cessation, moderate alcohol intake, concomitant drug reassessment, and specific coexistent conditions. Aggressive statin therapy still leaves a high residual rate of cardiac events. Other drug approaches include estrogen, fibrate, niacin therapy, and combinations of these agents. Recent results from an ongoing trial of combination extended-release prescription niacin plus lovastatin are very promising. PMID:11374858

Pearson, T A

2000-12-21

352

Dendritic spine dynamics in synaptogenesis after repeated LTP inductions: dependence on pre-existing spine density.  

PubMed

Not only from our daily experience but from learning experiments in animals, we know that the establishment of long-lasting memory requires repeated practice. However, cellular backgrounds underlying this repetition-dependent consolidation of memory remain largely unclear. We reported previously using organotypic slice cultures of rodent hippocampus that the repeated inductions of LTP (long-term potentiation) lead to a slowly developing long-lasting synaptic enhancement accompanied by synaptogenesis distinct from LTP itself, and proposed this phenomenon as a model system suitable for the analysis of the repetition-dependent consolidation of memory. Here we examined the dynamics of individual dendritic spines after repeated LTP-inductions and found the existence of two phases in the spines' stochastic behavior that eventually lead to the increase in spine density. This spine dynamics occurred preferentially in the dendritic segments having low pre-existing spine density. Our results may provide clues for understanding the cellular bases underlying the repetition-dependent consolidation of memory. PMID:23739837

Oe, Yuki; Tominaga-Yoshino, Keiko; Hasegawa, Sho; Ogura, Akihiko

2013-01-01

353

Dendritic spine dynamics in synaptogenesis after repeated LTP inductions: Dependence on pre-existing spine density  

PubMed Central

Not only from our daily experience but from learning experiments in animals, we know that the establishment of long-lasting memory requires repeated practice. However, cellular backgrounds underlying this repetition-dependent consolidation of memory remain largely unclear. We reported previously using organotypic slice cultures of rodent hippocampus that the repeated inductions of LTP (long-term potentiation) lead to a slowly developing long-lasting synaptic enhancement accompanied by synaptogenesis distinct from LTP itself, and proposed this phenomenon as a model system suitable for the analysis of the repetition-dependent consolidation of memory. Here we examined the dynamics of individual dendritic spines after repeated LTP-inductions and found the existence of two phases in the spines' stochastic behavior that eventually lead to the increase in spine density. This spine dynamics occurred preferentially in the dendritic segments having low pre-existing spine density. Our results may provide clues for understanding the cellular bases underlying the repetition-dependent consolidation of memory.

Oe, Yuki; Tominaga-Yoshino, Keiko; Hasegawa, Sho; Ogura, Akihiko

2013-01-01

354

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

NASA Astrophysics Data System (ADS)

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

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

2011-08-01

355

Temperature dependent spin momentum densities in Ni-Mn-In alloys.  

PubMed

The spin-dependent electron momentum densities in Ni(2)MnIn and Ni(2)Mn(1.4)In(0.6) shape memory alloy using magnetic Compton scattering with 182.2 keV circularly polarized synchrotron radiation are reported. The magnetic Compton profiles were measured at different temperatures ranging between 10 and 300 K. The profiles have been analyzed mainly in terms of Mn 3d electrons to determine their role in the formation of the total spin moment. We have also computed the spin polarized energy bands, partial and total density of states, Fermi surfaces and spin moments using full potential linearized augmented plane wave and spin polarized relativistic Korringa-Kohn-Rostoker methods. The total spin moments obtained from our magnetic Compton profile data are explained using both the band structure models. The present Compton scattering investigations are also compared with magnetization measurements. PMID:21403357

Ahuja, B L; Dashora, Alpa; Heda, N L; Priolkar, K R; Vadkhiya, L; Itou, M; Lobo, Nelson; Sakurai, Y; Chakrabarti, Aparna; Singh, Sanjay; Barman, S R

2010-11-10

356

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

357

Angular momentum dependence of the nuclear level-density parameter around Z~50  

NASA Astrophysics Data System (ADS)

?-particle evaporation spectra and ?-ray multiplicities were measured for various target-projectile systems corresponding to residual nuclei in the range of ZR=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 ?-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.

2008-11-01

358

Thickness-dependent carrier density at the surface of SrTiO3 (111) slabs  

NASA Astrophysics Data System (ADS)

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

Sivadas, N.; Dixit, H.; Cooper, Valentino R.; Xiao, Di

2014-02-01

359

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

SciTech Connect

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

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

2014-01-01

360

Conservation laws and constitutive relations for density-gradient-dependent viscous fluids  

NASA Astrophysics Data System (ADS)

Conservation laws and constitutive relations for a density-gradient-dependent viscous fluid as a multipolar continuum obeying an entropy inequality with generalized entropy flux and supply density are considered in this paper. A decomposition of the rate of work of dipolar stress, which reveals the contribution of various parts of this stress to the energy equation, is used to discuss the inconsistencies between the results obtained here and those obtained by Bluestein and Green [1] on the basis of the pioneering work of Green and Rivlin [8]. Furthermore, we discuss the connection between the model presented here and the materials of Korteweg type considered by Dunn and Serrin [6]. In particular, we relate the rate of work of dipolar stress and the interstitial working introduced by Dunn and Serrin [6].

Mehrabadi, M. M.; Cowin, S. C.; Massoudi, M.

2005-05-01

361

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

PubMed

We extend our previous definition of the metric ?r for electronic excitations in the framework of the time-dependent density functional theory [C. A. Guido, P. Cortona, B. Mennucci, and C. Adamo, J. Chem. Theory Comput. 9, 3118 (2013)], by including a measure of the difference of electronic position variances in passing from occupied to virtual orbitals. This new definition, called ?, permits applications in those situations where the ?r-index is not helpful: transitions in centrosymmetric systems and Rydberg excitations. The ?-metric is then extended by using the Natural Transition Orbitals, thus providing an intuitive picture of how locally the electron density changes during the electronic transitions. Furthermore, the ? values give insight about the functional performances in reproducing different type of transitions, and allow one to define a "confidence radius" for GGA and hybrid functionals. PMID:24628146

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

2014-03-14

362

Determining the Density Content of Symmetry Energy and Neutron Skin: An Empirical Approach  

NASA Astrophysics Data System (ADS)

The density dependence of nuclear symmetry energy remains poorly constrained. Starting from precise empirical values of the nuclear volume and surface symmetry energy coefficients and the nuclear saturation density, we show how in the ambit of microscopic calculations with different energy density functionals, the value of the symmetry energy slope parameter L along with that for neutron skin can be put in tighter bounds. The value of L is found to be L=64±5MeV. For Pb208, the neutron skin thickness comes out to be 0.188±0.014fm. Knowing L, the method can be applied to predict neutron skin thicknesses of other nuclei.

Agrawal, B. K.; De, J. N.; Samaddar, S. K.

2012-12-01

363

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

364

Reduced Glial and Neuronal Packing Density in the Orbitofrontal Cortex in Alcohol Dependence and Its Relationship with Suicide and Duration of Alcohol Dependence  

PubMed Central

Background Reduced metabolism, blood flow, and tissue volume have been detected in the dorsolateral prefrontal cortex (dlPFC) of neurologically intact alcoholic subjects and these deficits are accompanied by lower density of neurons and glial cells. Another prefrontal region, the orbitofrontal cortex (ORB), functionally and structurally differentiated from the dlPFC, and heavily involved in decision-making processes, also shows functional alterations in alcoholic subjects. However, it is unknown whether changes in the packing density of neurons or glial cells also occur in the ORB and whether that density may be related to the increased suicide probability of alcoholic subjects or to the duration of alcohol dependence. Methods The present study used a 3-dimensional cell-counting method in postmortem brain tissue to determine the packing density of neurons and glial cells in the ORB (area 47) of 15 subjects with alcohol dependence (8 suicides, 7 nonsuicides) and 8 normal controls and to determine whether cell density is correlated with suicide and duration of alcohol dependence. Results There was a significantly lower density of both neurons (by 27%) and glial cells (by 25%) in the ORB of alcoholic subjects compared with controls. Packing density of either neurons or glial cells was not significantly different in alcoholic suicides compared with alcoholic nonsuicides. Age was not correlated with neuronal or glial density in either group. However, the duration of alcohol dependence and the ratio of that duration to the length of life span were significantly and negatively correlated to the overall density of neurons. Conclusion The present results indicate that alcohol dependence is associated with a decrease in the packing density of neurons and glia in the ORB and that the reduction in neuronal but not glial density progresses with the duration of alcohol dependence.

Miguel-Hidalgo, Jose J.; Overholser, James C.; Meltzer, Herbert Y.; Stockmeier, Craig A.; Rajkowska, Grazyna

2010-01-01

365

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

USGS Publications Warehouse

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

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

2004-01-01

366

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

367

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

SciTech Connect

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

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

2011-12-14

368

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

NASA Astrophysics Data System (ADS)

We employ time-dependent density functional theory to study the optical properties of a squaraine sensitized TiO 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 TiO slab exposing anatase (1 0 1) surfaces, both for the clean slab and for the surface covered by the squaraine dye. Those spectra are compared to recent experimental data. The main absorption peaks are analyzed, concluding that both direct and indirect charge injection mechanisms are active in this system.

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

2009-06-01

369

Adiabatic Approximation in Nonperturbative Time-Dependent Density-Functional Theory  

SciTech Connect

We construct the exact exchange-correlation potential of time-dependent density-functional theory and the approximation to it that is adiabatic but exact otherwise. For the strong-field double ionization of the Helium atom these two potentials are virtually identical. Thus, memory effects play a negligible role in this paradigm process of nonlinear, nonperturbative electron dynamics. We identify the regime of high-frequency excitations where the adiabatic approximation breaks down and explicitly calculate the nonadiabatic contribution to the exchange-correlation potential.

Thiele, M.; Kuemmel, S. [Physikalisches Institut, Universitaet Bayreuth, D-95440 Bayreuth (Germany); Gross, E. K. U. [Theoretische Physik, Freie Universitaet Berlin, D-14195 Berlin (Germany)

2008-04-18

370

Time-Dependent Density Functional Response Theory for Electronic Chiroptical Properties of Chiral Molecules  

NASA Astrophysics Data System (ADS)

Methodology to calculate electronic chiroptical properties from time-dependent density functional theory (TDDFT) is outlined. Applications of TDDFT to computations of electronic circular dichroism, optical rotation, and optical rotatory dispersion are reviewed. Emphasis is put on publications from 2005 to 2010, but much of the older literature is also cited and discussed. The determination of the absolute configuration of chiral molecules by combined measurements and computations is an important application of TDDFT chiroptical methods and discussed in some detail. Raman optical activity (ROA) spectra are obtained from normal-mode derivatives of the optical rotation tensor and other linear response tensors. A few selected (ROA) benchmarks are reviewed.

Autschbach, Jochen; Nitsch-Velasquez, Lucia; Rudolph, Mark

371

Time-dependent density functional theory molecular dynamics simulation of doubly charged uracil in gas phase  

NASA Astrophysics Data System (ADS)

We use time-dependent density functional theory and Born-Oppenheimer molecular dynamics methods to investigate the fragmentation of doubly ionized uracil in gas phase. Different initial electronic excited states of the dication are obtained by removing electrons from different inner-shell orbitals of the neutral species. We show that shape-equivalent orbitals lead to very different fragmentation patterns revealing the importance of the intramolecular chemical environment. The results are in good agreement with ionion coincidence measurements of uracil collision with 100 keV protons.

López-Tarifa, Pablo; Hervé du Penhoat, Marie-Anne; Vuilleumier, Rodophe; Gaigeot, Marie-Pierre; Rothlisberger, Ursula; Tavernelli, Ivano; Le Padellec, Arnaud; Champeaux, Jean-Philippe; Alcamí, Manuel; Moretto-Capelle, Patrick; Martín, Fernando; Politis, Marie-Françoise

2014-02-01

372

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

Microsoft Academic Search

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

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

2001-01-01

373

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

NASA Astrophysics Data System (ADS)

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

Herbert, John Michael

374

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

NASA Astrophysics Data System (ADS)

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

Olsen, Thomas; Thygesen, Kristian S.

2014-05-01

375

Long-range excitations in time-dependent density functional theory.  

PubMed

Adiabatic time-dependent density functional theory fails for excitations of a heteroatomic molecule composed of two open-shell fragments at large separation. Strong frequency dependence of the exchange-correlation kernel is necessary for both local and charge-transfer excitations. The root of this is the static correlation created by the step in the exact Kohn-Sham ground-state potential between the two fragments. An approximate nonempirical kernel is derived for excited molecular dissociation curves at large separation. Our result is also relevant when the usual local and semilocal approximations are used for the ground-state potential, as static correlation there arises from the coalescence of the highest occupied and lowest unoccupied orbital energies as the molecule dissociates. PMID:17115742

Maitra, Neepa T; Tempel, David G

2006-11-14

376

The time-dependent local density approximation for collective excitations of atomic clusters  

NASA Astrophysics Data System (ADS)

We discuss the calculation of collective excitations in atomic clusters using the time-dependent local density approximation. In principle there are many formulations of the TDLDA, but we have found that a particularly efficient method for large clusters is to use a coordinate space mesh and the algorithms for the operators and the evolution equations that had been developed for the nuclear time-dependent Hartree-Fock theory. The TDLDA works remarkably well to describe the strong excitations in alkali metal clusters and in carbon clusters. We show as an example the benzene molecule, which has two strong features in its spectrum. The systematics of the linear carbon chains is well reproduced, and may be understood in rather simple terms.

Bertsch, G. F.; Yabana, K.

1997-06-01

377

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

PubMed

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

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

2012-11-01

378

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

PubMed Central

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

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

2012-01-01

379

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

PubMed

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

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

2014-01-01

380

A graphical unitary group approach-based hybrid density functional theory multireference configuration interaction method  

Microsoft Academic Search

A hybrid density functional theory multireference configuration interaction hybrid model (DFT\\/MRCI) method for computing electronic excitation energies in heavy-element atomic and molecular systems has been developed within a graphical unitary group approach (GUGA) based configuration interaction framework. Implemented in the COLUMBUS suite of programs, the method advances several new capabilities for evaluating systems involving several electronic spin states, relativistic effects

Eric V. Beck; Eric A. Stahlberg; Larry W. Burggraf; Jean-Philippe Blaudeau

2008-01-01

381

Structure of electric double layers: A simple weighted density functional approach  

Microsoft Academic Search

A simple weighted density functional approach is developed for inhomogeneous ionic fluids and applied to the structure of the electric double layer using the restricted primitive model where the ions are considered to be charged hard spheres of equal diameter. The formalism is nonperturbative with both hard-sphere and electrical contributions to the one-particle correlation function evaluated through a suitably averaged

Chandra N. Patra

1999-01-01

382

Choosing the smoothing parameter in a fourier approach to nonparametric deconvolution of a density estimate  

Microsoft Academic Search

In this note we derive a weighted non-linear least squares procedure for choosing the smoothing parameter in a Fourier approach to deconvolution of a density estimate. The method has the advantage over a previous procedure in that it is robust to the range of frequencies over which the model is fitted. A simulation study with different parametric forms for the

J. Barry; P. Diggle

1995-01-01

383

Adatom density kinetic Monte Carlo: A hybrid approach to perform epitaxial growth simulations  

Microsoft Academic Search

We describe an alternative approach to perform growth simulations that combines the kinetic Monte Carlo (KMC) method with elements from continuum and rate equations. Similar to the KMC method it takes the atomistic structure of the growing surface fully into account but is based on the adatom density rather than on explicit trajectories of the adatoms. As will be demonstrated,

L. Mandreoli; J. Neugebauer; R. Kunert; E. Schöll

2003-01-01

384

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

NASA Astrophysics Data System (ADS)

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

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

2014-02-01

385

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

SciTech Connect

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

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

2001-05-24

386

Tobacco retailer proximity and density and nicotine dependence among smokers with serious mental illness.  

PubMed

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

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

2014-08-01

387

Seasonality selects for more acutely virulent parasites when virulence is density dependent  

PubMed Central

Host condition is often likely to influence parasite virulence. Furthermore, condition may often be correlated with host density, and therefore, it is important to understand the role of density-dependent virulence (DDV). We examine the consequences of DDV to the evolution of parasites in both seasonal and non-seasonal environments. In particular, we consider seasonality in host birth rate that results in a fluctuating host density and therefore a variable virulence. We show that parasites are selected for lower exploitation, and therefore lower transmission and virulence as the strength of DDV increases without seasonality. This is an important insight from our models; DDV has the opposite effect on the evolution of parasites to that of higher baseline mortality. Our key result is that although seasonality does not affect the evolution of virulence in classical models, with DDV parasites in seasonal environments are predicted to evolve to be more acute. This suggests that in more seasonal environments wildlife disease is likely to be more rather than less virulent if DDV is widespread.

Donnelly, R.; Best, A.; White, A.; Boots, M.

2013-01-01

388

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

PubMed

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

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

2014-02-27

389

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

NASA Astrophysics Data System (ADS)

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

Oliveira, Micael J. T.; Nogueira, Fernando; Marques, Miguel A. L.; Rubio, Angel

2009-12-01

390

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

NASA Astrophysics Data System (ADS)

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

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

2014-07-01

391

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

PubMed

We introduce a generic form of time-dependent density functional theory (TDDFT) to describe ion diffusion in electrochemical systems to account for steric effects and electrostatic correlations neglected in the Poisson-Nernst-Planck equations. An efficient numerical algorithm is proposed to analyze the charging kinetics of electric double layers in model electrochemical systems that consist of spherical ions in a dielectric continuum confined between two planar electrodes. By comparing the theoretical predictions from TDDFT and conventional electrokinetic methods for constant-voltage charging of the model electrochemical cells, we demonstrate that thermodynamic non-ideality plays a pivotal role in electrodiffusion even at relatively low electrolyte concentrations, and this effect cannot be captured by the lattice-gas model for the excluded volume effects. In particular, TDDFT predicts 'wave-like' variation of the ionic density profiles that has not been identified in previous investigations. At conditions where there are no significant correlations between electric double layers from opposite electrodes, the charging kinetics follows an exponential behavior with a linear dependence of the relaxation time on the cell thickness in excellent agreement with the equivalent circuit model. However, the conventional electrokinetic model breaks down when the electrodes are at small separation, in particular for systems with low ionic strength or high charging voltage. We also find that ionic screening retards the charging kinetics at low salt concentrations, but has the opposite effect at large salt concentrations. PMID:24920008

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

2014-07-16

392

Density-dependent and density-independent mortality of the western corn rootworm: impact on dose calculations of rootworm-resistant Bt corn.  

PubMed

The percentage of viable eggs of the western corn rootworm, Diabrotica virgifera virgifera LeConte, which survived to the adult stage was evaluated for the effect of egg density in 2005 and 2007 in central Missouri. In 2005, each plot was 2.44 by 3.05 m and contained 64 maize (corn), Zea mays L., plants. In 2007, plots were 3.05 by 3.05 m and again contained 64 corn plants. Seven egg densities (2,400, 1,200, 600, 300, 100, 50, and 25 viable eggs per 30.5 cm) were evaluated with four to six replications in each year in a completely randomized design. In 2007 only, an additional row was infested near each plot to evaluate plant damage. In both years, there was no correlation of infestation level and percentage of emergence between infestation levels of 25-600 viable eggs per 30.5 cm, indicating that density-dependent mortality did not occur at these egg densities. In 2005, 8.04% of the viable eggs established on a corn plant and produced an adult at these lower infestation rates. In 2007, this value was 2.9%. Regardless of egg density, approximately 92-97% failed to establish and produce adults (density-independent mortality). In 2005 and in the combined analysis, as viable egg densities increased from 600 to 2400 per 30.5 cm there was a significant decrease in percentage of emergence. In a broken line analysis of the 2005 data, the point where density-dependent mortality began in the combined analysis was 851 eggs per 30.5 cm with a 95% confidence interval from 678 to 1024. That year density-dependent mortality was important at high infestations and killed 54.4% of those larvae that successfully established on a plant at the highest egg density. However, little or no density-dependent mortality occurred at infestation levels <850 viable eggs per 30.5 cm in either year of the study. Combining data from both years with all previously published data in a broken line analysis indicated that density-dependent mortality began at approximately 800 viable eggs per 30.5 cm. These data are discussed in terms of dose calculations for products targeting the western corn rootworm. PMID:20214371

Hibbard, Bruce E; Meihls, Lisa N; Ellersieck, Mark R; Onstad, David W

2010-02-01

393

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

NASA Technical Reports Server (NTRS)

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

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

2002-01-01

394

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

395

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

PubMed

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

Li, Zhendong; Liu, Wenjian

2012-01-14

396

Plasmonic excitations in quantum-sized sodium nanoparticles studied by time-dependent density functional calculations  

NASA Astrophysics Data System (ADS)

The plasmonic properties of spherelike bcc Na nanoclusters ranging from Na15 to Na331 have been studied by real-time time-dependent local density approximation calculations. The optical absorption spectrum, density response function, and static polarizability are evaluated. It is shown that the effect of the ionic background (ionic species and lattice) of the clusters accounts for the remaining discrepancy in the principal (surface plasmon) absorption peak energy between the experiments and previous calculations based on a jellium background model. The ionic background effect also pushes the critical cluster size where the maximum width of the principal peak occurs from Na40 predicted by the previous jellium model calculations to Na65. In the volume mode clusters (Na27, Na51, Na65, Na89, and Na113) in which the density response function is dominated by an intense volume mode, a multiple absorption peak structure also appears next to the principal peak. In contrast, the surface mode clusters of greater size (Na169, Na229, Na283, and Na331) exhibit a smoother and narrower principal absorption peak because their surface plasmon energy is located well within that of the unperturbed electron-hole transitions, and their density responses already bear resemblance to that of classical Mie theory. Moreover, it is found that the volume plasmon that exists only in finite-size particles gives rise to the long absorption tail in the UV region. This volume plasmon manifests itself in the absorption spectrum even for clusters as large as Na331 with an effective diameter of ˜3.0 nm.

Li, Jian-Hao; Hayashi, Michitoshi; Guo, Guang-Yu

2013-10-01

397

Dependency of irradiation damage density on tritium migration behaviors in Li2TiO3  

NASA Astrophysics Data System (ADS)

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

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

2014-04-01

398

Predominance of large low density lipoprotein particles and lower fractional esterification rate of cholesterol in high density lipoprotein in children with insulin-dependent diabetes mellitus  

Microsoft Academic Search

Coronary artery disease (CAD) is a major cause of death in patients with insulin-dependent diabetes mellitus. Qualitative\\u000a changes in low density lipoprotein (LDL) and high density lipoprotein (HDL) are thought to be important for evaluating the\\u000a risk for CAD. In the present study, we evaluated LDL particle size (LDL-size) by 2%–16% gradient gel electrophoresis, along\\u000a with conventional lipids and apolipoproteins,

T. Ohta; S. Nishiyama; T. Nakamura; K. Saku; K. K. Maung; I. Matsuda

1998-01-01

399

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

SciTech Connect

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

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

2013-09-14

400

The Column Density Variance in Turbulent Interstellar Media: A Fractal Model Approach  

NASA Astrophysics Data System (ADS)

Fractional Brownian motion structures are used to investigate the dependency of column density variance (?2 ln N ) in the turbulent interstellar medium on the variance of three-dimensional density (?2 ln ?) and the power-law slope of the density power spectrum. We provide quantitative expressions to infer the three-dimensional density variance, which is not directly observable, from the observable column density variance and spectral slope. We also investigate the relationship between the column density variance and sonic Mach number (M s) in the hydrodynamic (HD) regime by assuming the spectral slope and density variance to be functions of sonic Mach number, as obtained from the HD turbulence simulations. They are related by the expression ?2 ln N = A?ln ? 2 = Aln (1 + b 2 M 2 s), suggested by Burkhart & Lazarian for the magnetohydrodynamic case. The proportional constant A varies from ?0.2 to ?0.4 in the HD regime as the turbulence forcing parameter b increases from 1/3 (purely solenoidal forcing) to 1 (purely compressive forcing). It is also discussed that the parameter A is lowered in the presence of a magnetic field.

Seon, Kwang-Il

2012-12-01

401

THE COLUMN DENSITY VARIANCE IN TURBULENT INTERSTELLAR MEDIA: A FRACTAL MODEL APPROACH  

SciTech Connect

Fractional Brownian motion structures are used to investigate the dependency of column density variance ({sigma}{sup 2}{sub lnN}) in the turbulent interstellar medium on the variance of three-dimensional density ({sigma}{sup 2}{sub ln{rho}}) and the power-law slope of the density power spectrum. We provide quantitative expressions to infer the three-dimensional density variance, which is not directly observable, from the observable column density variance and spectral slope. We also investigate the relationship between the column density variance and sonic Mach number (M{sub s}) in the hydrodynamic (HD) regime by assuming the spectral slope and density variance to be functions of sonic Mach number, as obtained from the HD turbulence simulations. They are related by the expression {sigma}{sup 2}{sub lnN} = A{sigma}{sub ln{rho}} {sup 2} = Aln (1 + b {sup 2} M{sup 2}{sub s}), suggested by Burkhart and Lazarian for the magnetohydrodynamic case. The proportional constant A varies from Almost-Equal-To 0.2 to Almost-Equal-To 0.4 in the HD regime as the turbulence forcing parameter b increases from 1/3 (purely solenoidal forcing) to 1 (purely compressive forcing). It is also discussed that the parameter A is lowered in the presence of a magnetic field.

Seon, Kwang-Il, E-mail: kiseon@kasi.re.kr [Korea Astronomy and Space Science Institute, Daejeon 305-348 (Korea, Republic of); Astronomy and Space Science Major, University of Science and Technology, Daejeon 305-350 (Korea, Republic of)

2012-12-20

402

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

PubMed Central

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

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

2014-01-01

403

Hybrid large-eddy simulation/Lagrangian filtered-density-function approach for simulating turbulent combustion  

SciTech Connect

A consistent hybrid large-eddy simulation/filtered-density-function approach (LES-FDF) is formulated for variable-density low-Mach-number flows. The LES-FDF approach has been proposed as a suitable method for finite-rate-chemistry-based predictive modeling of turbulent reactive flows. Due to the large computational grid associated with LES, use of Lagrangian schemes is numerically expensive. In this work, a highly efficient parallel Lagrangian implementation is used for the simulation of a nonpremixed flame. This bluff-body-stabilized flame is characterized by complex flow fields that interact strongly with the combustion mechanism. A LES grid size of 1 million computational cells and roughly 15 million notional particles is used to simulate a time-accurate variable-density flow. The hybrid approach predicts the time-averaged velocity and root mean square (RMS) velocity components quite accurately. Species profiles including hydroxyl radical compare well with experimental data. Consistency and accuracy are established by comparing particle and Eulerian density, mixture fraction, and RMS mixture fraction fields. Scalar FDFs at select locations are shown to be well approximated by the presumed beta function used in typical combustion LES.

Raman, Venkatramanan; Pitsch, Heinz [Center for Turbulence Research, Stanford University, Stanford, CA 94305 (United States); Fox, Rodney O. [Department of Chemical Engineering, Iowa State University, Ames, IA 50010 (United States)

2005-10-01

404

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

NASA Astrophysics Data System (ADS)

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

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

2011-11-01

405

Position and Current Dependence of Charge-Density-Wave Polarization Dynamics  

NASA Astrophysics Data System (ADS)

We have studied the frequency and position dependence of charge-density-wave (CDW) polarization by simulating the response to square-waves of variable amplitude and frequency using parameters appropriate for niobium triselenide at T = 90 K, in its upper CDW state. For these simulations, we have numerically solved the phase-slip augmented diffusion model introduced by Adelman et al (Phys. Rev. B 53, 1833 (1996)) for time domain studies. At each position in the sample, the frequency dependence was fit to a modified harmonic oscillator expression and the position and current dependence of the fitting parameters determined. In particular, both the delay time (1/resonant frequency) and relaxation time decrease with increasing current (and phase-slip rate) and increase with distance from the contact, with the delay time vanishing adjacent to the contact, as experimentally observed with electro-optic measurements in blue bronze. No decay of the polarization at long times is observed however, in contrast to electro-optic results.

Ladino, L.; Brill, J. W.

2008-03-01

406

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

407

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

NASA Astrophysics Data System (ADS)

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

Wang, Yan; Hirschfeld, Peter; Graser, Siegfried

2011-03-01

408

Analytical blowup solutions to the pressureless Navier-Stokes-Poisson equations with density-dependent viscosity in RN  

NASA Astrophysics Data System (ADS)

We study the pressureless Navier-Stokes-Poisson equations with density-dependent viscosity. With the extension of the blowup solutions for the Euler-Poisson equations, the analytical blowup solutions, in radial symmetry, in RN (N >= 2) are constructed.

Yuen, Manwai

2009-09-01

409

A Multiple Motives Approach to Tobacco Dependence: The Wisconsin Inventory of Smoking Dependence Motives (WISDM-68)  

ERIC Educational Resources Information Center

The dependence construct fills an important explanatory role in motivational accounts of smoking and relapse. Frequently used measures of dependence are either atheoretical or grounded in a unidimensional model of physical dependence. This research creates a multidimensional measure of dependence that is based on theoretically grounded motives for…

Piper, Megan E.; Piasecki, Thomas M.; Federman, E. Belle; Bolt, Daniel M.; Smith, Stevens S.; Fiore, Michael C.; Baker, Timothy C.

2004-01-01

410

Patchy recruitment patterns in marine invertebrates: a spatial test of the density-dependent hypothesis in the bivalve Spisula ovalis  

Microsoft Academic Search

Density-dependent and density-independent processes have been shown to influence the population dynamics of marine invertebrates,\\u000a especially recruitment. However, their relative importance has not been evaluated in natural populations. High adult densities\\u000a have been suggested to inhibit recruitment, especially in suspension-feeders which may ingest incoming larvae. Age structure\\u000a and juvenile abundance were investigated in the bivalve Spisula ovalis in order to

Patric David; Patrick Berthou;