Distribution by Scientific Domains
Distribution within Engineering

Kinds of Computation

  • ab initio computation
  • accurate computation
  • approximate bayesian computation
  • bayesian computation
  • chemical computation
  • density functional computation
  • density functional theory computation
  • dft computation
  • direct computation
  • efficient computation
  • element computation
  • evolutionary computation
  • fast computation
  • finite element computation
  • flow computation
  • functional computation
  • functional theory computation
  • initio computation
  • large-scale computation
  • matrix computation
  • neural computation
  • numerical computation
  • parallel computation
  • quantum chemical computation
  • quantum computation
  • sensitivity computation
  • theoretical computation
  • theory computation

  • Terms modified by Computation

  • computation complexity
  • computation cost
  • computation effort
  • computation method
  • computation scheme
  • computation time

  • Selected Abstracts


    First page of article [source]


    Dan O. Popa
    ABSTRACT This paper considers a gradient type of iterative algorithm applied to the open loop control for nonlinear affine systems. The convergence of the algorithm relies on the control signal in each iteration be nonsingular. We present an algorithm for computing the singular control for a general class of nonlinear affine systems. Various nonlinear mechanical systems, including nonholonomic systems, are included as examples. [source]

    An elaborate education of basic genetic programming using C++

    Nirod C. Sahoo
    Abstract Evolutionary search is a global search method based on natural selection. In engineering curriculum, these techniques are taught in courses like Evolutionary Computation, Engineering Optimization, etc. Genetic algorithm (GA) is popular among these algorithms. Genetic programming (GP), developed by John Koza, is a powerful extension of GA where a chromosome/computer program (CP) is coded as a rooted point-labeled tree with ordered branches. The search space is the space of all possible CPs (trees) consisting of functions and terminals appropriate to the problem domain. GP uses, like GA, crossover and mutation for evolution. Due to tree-structured coding of individuals, the initial population generation, genetic operators' use, and tree decoding for fitness evaluations demand careful computer programming. This article describes the programming steps of GP implementation (using C++ language) for students' easy understanding with pseudocodes for each step. Two application examples are also illustrated. © 2009 Wiley Periodicals, Inc. Comput Appl Eng Educ 18: 434,448, 2010; View this article online at; DOI 10.1002/cae.20165 [source]

    Isotropic Remeshing with Fast and Exact Computation of Restricted Voronoi Diagram

    Dong-Ming Yan
    Abstract We propose a new isotropic remeshing method, based on Centroidal Voronoi Tessellation (CVT). Constructing CVT requires to repeatedly compute Restricted Voronoi Diagram (RVD), defined as the intersection between a 3D Voronoi diagram and an input mesh surface. Existing methods use some approximations of RVD. In this paper, we introduce an efficient algorithm that computes RVD exactly and robustly. As a consequence, we achieve better remeshing quality than approximation-based approaches, without sacrificing efficiency. Our method for RVD computation uses a simple procedure and a kd -tree to quickly identify and compute the intersection of each triangle face with its incident Voronoi cells. Its time complexity is O(mlog n), where n is the number of seed points and m is the number of triangles of the input mesh. Fast convergence of CVT is achieved using a quasi-Newton method, which proved much faster than Lloyd's iteration. Examples are presented to demonstrate the better quality of remeshing results with our method than with the state-of-art approaches. [source]

    DiFi: Fast 3D Distance Field Computation Using Graphics Hardware

    Avneesh Sud
    We present an algorithm for fast computation of discretized 3D distance fields using graphics hardware. Given a set of primitives and a distance metric, our algorithm computes the distance field for each slice of a uniform spatial grid baly rasterizing the distance functions of the primitives. We compute bounds on the spatial extent of the Voronoi region of each primitive. These bounds are used to cull and clamp the distance functions rendered for each slice. Our algorithm is applicable to all geometric models and does not make any assumptions about connectivity or a manifold representation. We have used our algorithm to compute distance fields of large models composed of tens of thousands of primitives on high resolution grids. Moreover, we demonstrate its application to medial axis evaluation and proximity computations. As compared to earlier approaches, we are able to achieve an order of magnitude improvement in the running time. Categories and Subject Descriptors (according to ACM CCS): I.3.3 [Computer Graphics]: Distance fields, Voronoi regions, graphics hardware, proximity computations [source]

    Description and characterization of a chamber for viewing and quantifying cancer cell chemotaxis

    CYTOSKELETON, Issue 1 2005
    Lilian Soon
    Abstract Direct observations of cancer cell invasion underscore the importance of chemotaxis in invasion and metastasis. Yet, there is to date, no established method for real-time imaging of cancer chemotaxis towards factors clinically correlated with metastasis. A chamber has been designed and tested, called the Soon chamber, which allows the direct observation and quantification of cancer cell chemotaxis. The premise for the design of the Soon chamber is the incorporation of a dam, which creates a steep gradient while retaining stability associated with a pressure-driven system. The design is based on the characteristics of cancer cell motility such as relatively low speeds, and slower motility responses to stimuli compared to classical amoeboid cells like neutrophils and Dictyostelium. We tested MTLn3 breast carcinoma cells in the Soon chamber in the presence of an EGF gradient, obtaining hour-long time-lapses of chemotaxis. MTLn3 cells migrated further, more linearly, and at greater speeds within an EGF gradient compared to buffer controls. Computation of the degree of orientation towards the EGF/buffer source showed that MTLn3 cells were significantly more directional toward the EGF gradient compared to buffer controls. Analysis of the time-lapse data obtained during chemotaxis demonstrated that two populations of cancer cells were present. One population exhibited oscillations in directionality occurring at average intervals of 12 min while the second population exhibited sustained high levels of directionality toward the source of EGF. This result suggests that polarized cancer cells can avoid the need for oscillatory path corrections during chemotaxis. Cell Motil. Cytoskeleton 62:27,34, 2005. © 2005 Wiley-Liss, Inc. [source]

    The Economist as Engineer: Game Theory, Experimentation, and Computation as Tools for Design Economics

    ECONOMETRICA, Issue 4 2002
    Alvin E. Roth
    Economists have lately been called upon not only to analyze markets, but to design them. Market design involves a responsibility for detail, a need to deal with all of a market's complications, not just its principle features. Designers therefore cannot work only with the simple conceptual models used for theoretical insights into the general working of markets. Instead, market design calls for an engineering approach. Drawing primarily on the design of the entry level labor market for American doctors (the National Resident Matching Program), and of the auctions of radio spectrum conducted by the Federal Communications Commission, this paper makes the case that experimental and computational economics are natural complements to game theory in the work of design. The paper also argues that some of the challenges facing both markets involve dealing with related kinds of complementarities, and that this suggests an agenda for future theoretical research. [source]

    Computation of locational and hourly maximum output of a distributed generator connected to a distribution feeder

    Yasuhiro Hayashi
    Abstract Recently, the total number of distributed generation such as photovoltaic generation systems and wind turbine generation systems connected to a distribution network has drastically increased. Distributed generation using renewable energy can reduce the distribution loss and emission of CO2. However, the distribution network with the distributed generators must be operated while maintaining the reliability of the power supply and power quality. In this paper, the authors propose a computational method to determine the maximum output of a distributed generator under operational constraints [(1) voltage limit, (2) line current capacity, and (3) no reverse flow to bank] at arbitrary connection points and hourly periods. In the proposed method, a three-phase iterative load flow calculation is applied to evaluate the above operational constraints. The three-phase iterative load flow calculation has two simple procedures: (Procedure 1) addition of load currents from the terminal node of the feeder to root one, and (Procedure 2) subtraction of voltage drop from the root node of the feeder to terminal one. In order to check the validity of the proposed method, numerical simulations are performed for a distribution system model. Furthermore, the characteristics of locational and hourly maximum output of a distributed generator connected to a distribution feeder are analyzed using several numerical examples. © 2009 Wiley Periodicals, Inc. Electr Eng Jpn, 167(2): 38,47, 2009; Published online in Wiley InterScience ( DOI 10.1002/eej.20610 [source]

    Computation of time delay margin for power system small-signal stability

    Saffet AyasunArticle first published online: 19 JUN 200
    Abstract With the extensive use of phasor measurement units (PMU) in the wide-area measurement/monitoring systems (WAMS), time delays have become unavoidable in power systems. This paper presents a direct and exact method to compute the delay margin of power systems with single and commensurate time delays. The delay margin is the maximum amount of time delay that the system can tolerate before it becomes unstable for a given operating point. First, without using any approximation or substitution, the transcendental characteristic equation is converted into a polynomial without the transcendentality such that its real roots coincide with the imaginary roots of the characteristic equation exactly. The resulting polynomial also enables us to easily determine the delay dependency of the system stability and the sensitivities of crossing roots with respect to time delay. Then, an expression in terms of system parameters and imaginary root of the characteristic equation is derived for computing the delay margin. The proposed method is applied to a single-machine-infinite bus (SMIB) power system with an exciter. Delay margins are computed for a wide range of system parameters including generator mechanical power, damping and transient reactance, exciter gain, and transmission line reactance. The results indicate that the delay margin decreases as the mechanical power, exciter gain and line reactance increase while it increases with increasing generator transient reactance Additionally, the relationship between the delay margin and generator damping is found be relatively complex. Finally, the theoretical delay margin results are validated using the time-domain simulations of Matlab. Copyright © 2008 John Wiley & Sons, Ltd. [source]

    Computation of power systems eigenvalues using the modified nodal approach

    B. R. Oswald
    This paper presents a new method of forming state-space equations of large power systems which allows to calculate their eigenvalues. The method does not require any topological tools. It is based on the extended modified nodal approach, which was presented at the IPST '97 in Seattle. The extended modified nodal approach generates a set of algebraic and state-space equations using Kirchhoff's nodal law exclusively. Eliminating the algebraic part of these equations results in the desired state-space equations in an explicit form. [source]

    A learning rule for place fields in a cortical model: Theta phase precession as a network effect

    HIPPOCAMPUS, Issue 7 2005
    Silvia Scarpetta
    Abstract We show that a model of the hippocampus introduced recently by Scarpetta et al. (2002, Neural Computation 14(10):2371,2396) explains the theta phase precession phenomena. In our model, the theta phase precession comes out as a consequence of the associative-memory-like network dynamics, i.e., the network's ability to imprint and recall oscillatory patterns, coded both by phases and amplitudes of oscillation. The learning rule used to imprint the oscillatory states is a natural generalization of that used for static patterns in the Hopfield model, and is based on the spike-time-dependent synaptic plasticity, experimentally observed. In agreement with experimental findings, the place cells' activity appears at consistently earlier phases of subsequent cycles of the ongoing theta rhythm during a pass through the place field, while the oscillation amplitude of the place cells' firing rate increases as the animal approaches the center of the place field and decreases as the animal leaves the center. The total phase precession of the place cell is lower than 360°, in agreement with experiments. As the animal enters a receptive field, the place cells' activity comes slightly less than 180° after the phase of maximal pyramidal cell population activity, in agreement with the findings of Skaggs et al. (1996, Hippocampus 6:149,172). Our model predicts that the theta phase is much better correlated with location than with time spent in the receptive field. Finally, in agreement with the recent experimental findings of Zugaro et al. (2005, Nature Neuroscience 9(1):67,71), our model predicts that theta phase precession persists after transient intrahippocampal perturbation. © 2005 Wiley-Liss, Inc. [source]

    Study on the action of the active earth pressure by variational limit equilibrium method

    Li Xinggao
    Abstract Within the framework of limiting equilibrium approach, the problem of active earth pressure on rigid retaining wall is formulated in terms of the calculus of variations by means of Lagrange multipliers. It is transcribed as the functional of extreme-value problem by two undetermined function arguments, and is further transformed into determining the minimax solution of restrained functions incorporating the geometrical relations of the problem. The function of (fmincon) in the optimization toolbox of MATLAB 6.1 can be used to find the minimax solution. Computation results show there exist two kinds of modes of failure sliding along plane surface and rotating around log-spiral cylinder surface when the soil behind the walls reaches the critical active state. The magnitude of active earth pressure in the case of translational mode is less than that in the case of rotational mode. The location of action point of earth pressure in the case of translational mode is at or below height of the wall, and in the case of rotational mode, is above height of the wall. Preliminary study indicates a pair of numbers by two theoretical modes can be regarded as an interval estimation of active pressure. Copyright © 2009 John Wiley & Sons, Ltd. [source]

    Computation of the J -integral for large strains

    Ágnes Horváth
    Abstract The phenomenon of failure by catastrophic crack propagation in structural materials poses problems of design and analysis in many fields of engineering. Cracks are present to some degree in all structures. They may exist as basic defects in the constituent materials or they may be induced in construction or during service life. Using the finite element method, a lot of papers deal with the calculation of stress intensity factors for two- and three-dimensional geometries containing cracks of different shapes under various loadings to elastic bodies. In order to increase the accuracy of the results, special elements have been used. They are described together with methods for calculating the stress intensity factors from the computed results. At the vicinity of a crack tip, the strains are not always small, but they may also be large. In this case, the J -integral can also be applied to characterize the cracks in elastic or elastic,plastic bodies. This paper describes the computation of the two-dimensional J -integral for large strains to elastic and elastic,plastic bodies and represents some numerical examples. Copyright © 2007 John Wiley & Sons, Ltd. [source]

    Computation of mould filling processes with a moving Lagrangian interface technique

    Marcela Cruchaga
    Abstract Computation of non-isothermal flow problems involving moving interfaces is presented. A Lagrangian interface technique, defined in the context of a fixed-mesh finite element formulation for incompressible flows, is employed to update the interface position. A global mass-corrector algorithm is used to accurately enforce the global mass conservation. The Navier,Stokes equations are solved with an improved sub-element integration technique to more accurately account for sudden changes in the fluid properties across the interface. The method described is applied to two mould filling problems. Copyright © 2002 John Wiley & Sons, Ltd. [source]

    Computation of a few smallest eigenvalues of elliptic operators using fast elliptic solvers

    Janne Martikainen
    Abstract The computation of a few smallest eigenvalues of generalized algebraic eigenvalue problems is studied. The considered problems are obtained by discretizing self-adjoint second-order elliptic partial differential eigenvalue problems in two- or three-dimensional domains. The standard Lanczos algorithm with the complete orthogonalization is used to compute some eigenvalues of the inverted eigenvalue problem. Under suitable assumptions, the number of Lanczos iterations is shown to be independent of the problem size. The arising linear problems are solved using some standard fast elliptic solver. Numerical experiments demonstrate that the inverted problem is much easier to solve with the Lanczos algorithm that the original problem. In these experiments, the underlying Poisson and elasticity problems are solved using a standard multigrid method. Copyright © 2001 John Wiley & Sons, Ltd. [source]

    On the computation of steady-state compressible flows using a discontinuous Galerkin method

    Hong Luo
    Abstract Computation of compressible steady-state flows using a high-order discontinuous Galerkin finite element method is presented in this paper. An accurate representation of the boundary normals based on the definition of the geometries is used for imposing solid wall boundary conditions for curved geometries. Particular attention is given to the impact and importance of slope limiters on the solution accuracy for flows with strong discontinuities. A physics-based shock detector is introduced to effectively make a distinction between a smooth extremum and a shock wave. A recently developed, fast, low-storage p -multigrid method is used for solving the governing compressible Euler equations to obtain steady-state solutions. The method is applied to compute a variety of compressible flow problems on unstructured grids. Numerical experiments for a wide range of flow conditions in both 2D and 3D configurations are presented to demonstrate the accuracy of the developed discontinuous Galerkin method for computing compressible steady-state flows. Copyright © 2007 John Wiley & Sons, Ltd. [source]

    Computation of the matrix exponential and its derivatives by scaling and squaring

    T. C. FungArticle first published online: 4 FEB 200
    Abstract In this paper, a simple method based on the scaling and squaring technique for the evaluation of the matrix exponential and its derivatives is presented. A more general formulation with non-constant first derivatives is considered here. Both higher order and mixed derivatives are investigated. The method is based directly on the property of the exponential function and does not require the use of perturbation formulae for eigenvalues and eigenvectors. The approach provides a simple and direct algorithm for the computation of the matrix exponential and its derivatives regardless of degeneracy in the spectral decomposition of the matrix argument. If the derivatives are taken with respect to the entries of the matrix argument, the first and second linearizations can be obtained directly. Copyright © 2004 John Wiley & Sons, Ltd. [source]

    Computation of heat transfer enhancement in a plate-fin heat exchanger with triangular inserts and delta wing vortex generator

    Gulshan Sachdeva
    Abstract Longitudinal vortices disrupt the growth of the thermal boundary layer, thereby the vortex generators producing the longitudinal vortices are well known for the enhancement of heat transfer in compact heat exchangers. The present investigation determines the heat transfer characteristics with secondary flow analysis in plate fin triangular ducts with delta wing vortex generators. This geometrical configuration is investigated for various angles of attack of the wing i.e. 15°, 20°, 26° and 37° and Reynolds numbers 100 and 200. The constant wall temperature boundary condition is used. The solution of the complete Navier Stokes equation and the energy equation is carried out using the staggered grid arrangement. The performance of the combination of triangular secondary fins and delta wing with stamping on slant surfaces has also been studied. Copyright © 2009 John Wiley & Sons, Ltd. [source]

    On pressure separation algorithms (PSepA) for improving the accuracy of incompressible flow simulations

    S. Turek
    Abstract We investigate a special technique called ,pressure separation algorithm' (PSepA) (see Applied Mathematics and Computation 2005; 165:275,290 for an introduction) that is able to significantly improve the accuracy of incompressible flow simulations for problems with large pressure gradients. In our numerical studies with the computational fluid dynamics package FEATFLOW (, we mainly focus on low-order Stokes elements with nonconforming finite element approximations for the velocity and piecewise constant pressure functions. However, preliminary numerical tests show that this advantageous behavior can also be obtained for higher-order discretizations, for instance, with Q2/P1 finite elements. We analyze the application of this simple, but very efficient, algorithm to several stationary and nonstationary benchmark configurations in 2D and 3D (driven cavity and flow around obstacles), and we also demonstrate its effect to spurious velocities in multiphase flow simulations (,static bubble' configuration) if combined with edge-oriented, resp., interior penalty finite element method stabilization techniques. Copyright © 2008 John Wiley & Sons, Ltd. [source]

    Computation of strongly swirling confined flows with cubic eddy-viscosity turbulence models

    Xiaodong Yang Graduate Student
    Abstract An investigation on the predictive performance of four cubic eddy-viscosity turbulence models for two strongly swirling confined flows is presented. Comparisons of the prediction with the experiments show clearly the superiority of cubic models over the linear k,,model. The linear k,,model does not contain any mechanism to describe the stabilizing effects of swirling motion and as a consequence it performs poorly. Cubic models return a lower level of Reynolds stresses and the combined forced-free vortex profiles of tangential velocity close to the measurements in response to the interaction between swirl-induced curvature and stresses. However, a fully developed rotating pipe flow is too simple to contain enough flow physics, so the calibration of cubic terms is still a topic of investigation. It is shown that explicit algebraic stress models require fewer calibrations and contain more flow physics. Copyright © 2003 John Wiley & Sons, Ltd. [source]

    Computation of an unsteady complex geometry flow using novel non-linear turbulence models

    Paul G. Tucker
    Abstract Non-linear zonal turbulence models are applied to an unsteady complex geometry flow. These are generally found to marginally improve predicted turbulence intensities. However, relative to linear models, convergence is mostly difficult to achieve. Clipping of some non-linear Reynolds stress components is required along with velocity field smoothing or alternative measures. Smoothing is naturally achieved through multilevel convergence restriction operators. As a result of convergence difficulties, generally, non-linear model computational costs detract from accuracy gains. For standard Reynolds stress model results, again computational costs are prohibitive. Also, mean velocity profile data accuracies are found worse than for a simple mixing length model. Of the non-linear models considered, the explicit algebraic stress showed greatest promise with respect to accuracy and stability. However, even this shows around a 30% error in total (the sum of turbulence and unsteadiness) intensity. In strong contradiction to measurements the non-linear and Reynolds models predict quasi-steady flows. This is probably a key reason for the total intensity under-predictions. Use of LES in a non-linear model context might help remedy this modelling aspect. Copyright © 2003 John Wiley & Sons, Ltd. [source]

    Computation of turbulent free-surface flows around modern ships

    Tingqiu Li
    Abstract This paper presents the calculated results for three classes of typical modern ships in modelling of ship-generated waves. Simulations of turbulent free-surface flows around ships are performed in a numerical water tank, based on the FINFLO-RANS SHIP solver developed at Helsinki University of Technology. The Reynolds-averaged Navier,Stokes (RANS) equations with the artificial compressibility and the non-linear free-surface boundary conditions are discretized by means of a cell-centred finite-volume scheme. The convergence performance is improved with the multigrid method. A free surface is tracked using a moving mesh technology, in which the non-linear free-surface boundary conditions are given on the actual location of the free surface. Test cases recommended are a container ship, a US Navy combatant and a tanker. The calculated results are compared with the experimental data available in the literature in terms of the wave profiles, wave pattern, and turbulent flow fields for two turbulence models, Chien's low Reynolds number k,,model and Baldwin,Lomax's model. Furthermore, the convergence performance, the grid refinement study and the effect of turbulence models on the waves have been investigated. Additionally, comparison of two types of the dynamic free-surface boundary conditions is made. Copyright © 2003 John Wiley& Sons, Ltd. [source]

    Linearized and non-linear acoustic/viscous splitting techniques for low Mach number flows

    Mohammad Farshchi
    Abstract Computation of the acoustic disturbances generated by unsteady low-speed flow fields including vortices and shear layers is considered. The equations governing the generation and propagation of acoustic fluctuations are derived from a two-step acoustic/viscous splitting technique. An optimized high order dispersion,relation,preserving scheme is used for the solution of the acoustic field. The acoustic field generated by a corotating vortex pair is obtained using the above technique. The computed sound field is compared with the existing analytic solution. Results are in good agreement with the analytic solution except near the centre of the vortices where the acoustic pressure becomes singular. The governing equations for acoustic fluctuations are then linearized and solved for the same model problem. The difference between non-linear and linearized solutions falls below the numerical error of the simulation. However, a considerable saving in CPU time usage is achieved in solving the linearized equations. The results indicate that the linearized acoustic/viscous splitting technique for the simulation of acoustic fluctuations generation and propagation by low Mach number flow fields seems to be very promising for three-dimensional problems involving complex geometries. Copyright © 2003 John Wiley & Sons, Ltd. [source]

    Computation of unsteady viscous incompressible flows in generalized non-inertial co-ordinate system using Godunov-projection method and overlapping meshes

    H. Pan
    Abstract Time-dependent incompressible Navier,Stokes equations are formulated in generalized non-inertial co-ordinate system and numerically solved by using a modified second-order Godunov-projection method on a system of overlapped body-fitted structured grids. The projection method uses a second-order fractional step scheme in which the momentum equation is solved to obtain the intermediate velocity field which is then projected on to the space of divergence-free vector fields. The second-order Godunov method is applied for numerically approximating the non-linear convection terms in order to provide a robust discretization for simulating flows at high Reynolds number. In order to obtain the pressure field, the pressure Poisson equation is solved. Overlapping grids are used to discretize the flow domain so that the moving-boundary problem can be solved economically. Numerical results are then presented to demonstrate the performance of this projection method for a variety of unsteady two- and three-dimensional flow problems formulated in the non-inertial co-ordinate systems. Copyright © 2002 John Wiley & Sons, Ltd. [source]

    Interaction of a two-level cyclic XY n -spin model with a two-mode cavity field in off-resonant states

    Horacio Grinberg
    Abstract The interaction of the XY n -spin cyclic model with a two-mode cavity field in the rotating-wave approximation is investigated in the framework of a generalized Jaynes,Cummings two-level system consisting of the vacuum state and a thermally averaged manifold of excited sates. Computation of the energy of this manifold allows this interaction to be examined in off-resonant states. Time evolution of the population inversion, photon distribution, and temperature distribution for an excited initial state are computed via second- and third-order perturbation expansion of the time evolution operator matrix elements for the excited and ground states, respectively and for an ideal squeezed initial coherent state of the cavity field. It was assumed that the two modes have initially the same photon distribution. The pattern of the spin population inversion appears as a manifestation of multiple and complicated inerferences, which is mathematically reflected in a double discrete summation that appears in the calculation of the dynamics. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2008 [source]

    Computation of the eigenvalues of the one-dimensional Schrödinger equation by symplectic methods

    Z. Kalogiratou
    Abstract The computation of high-state eigenvalues of the one-dimensional time-independent Schrödinger equation is considered by symplectic integrators. The Schrödinger equation is first transformed into a Hamiltonian canonical equation. Yoshida-type symplectic integrators are used as well as symplectic integrators based on the Magnus expansion. Numerical results are obtained for a wide range of eigenstates of the one-dimensional harmonic oscillator, the doubly anharmonic oscillator, and the Morse potential. The eigenvalues found by the symplectic methods are compared with the eigenvalues produced by Numerov-type methods. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2006 [source]

    Computation of STM images of carbon nanotubes

    P. Lambin
    Abstract Scanning tunneling microscopy (STM) is the only probing technique that allows for the investigation of both the topography and the electronic structure of carbon nanosystems at a subnanometer resolution. The interpretation of the STM images of carbon nanostructures involves complications that are normally absent in the study of planar crystalline surfaces. The complications typically appear from a number of quantum effects responsible for distortions in the microscope image of a nano-object. Because of these difficulties, computer simulation plays an extremely important role in the analysis of experimental data. In the current article, we report on two theoretical approaches developed for aiding in the interpretation and understanding of the formation of the STM image of a nanotube: first, the quantum mechanical dynamics of a wave packet, which allows for the modeling of the flow of the tunneling current between a tip and a nanotube supported by a substrate; and, second, a tight-binding perturbation theory that allows for the explicit calculation of realistic STM images and scanning tunneling spectra of carbon nanostructures. An atlas of computed STM images is provided for a series of 27 single-wall nanotubes with diameter around 1.3 nm. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem, 2003 [source]

    Computation on symmetry-invariant bases

    Jian Wu
    Abstract There is standard methodology available to facilitate electronic structure computations on a space that is invariant under a symmetry group. Here, we focus on additional consequences that arise if the basis itself is invariant under the symmetry group (i.e., in the case that application of symmetry operations to each basis vector yields, up to proportionality, a single basis vector). In illustration of the formal development, examples are considered where the symmetries are point-group symmetries and the basis vectors are Slater determinants over singly occupied atomic orbitals, as for an open-shell valence bond (VB) model. Several other types of examples are mentioned, e.g., a basis of chemically motivated resonance structures, as for a VB model, or an orbital basis of atomic orbitals for a one-electron Hückel-type model. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem 94: 7,22, 2003 [source]

    Design of irrigation water supply systems using the Q,C feasibility domain concept: I. Introduction and theory,

    Gideon Sinai
    irrigation; alimentation en eau; qualité de l'eau; systèmes d'irrigation; analyse de la contamination des réseaux; débit de l'eau Abstract The Q,C Feasibility Domain (QCFD) was defined and proposed as a tool for design of multiquality irrigation water supply systems. It determines all feasible combinations of water discharge and water quality, and can be represented by a point, a line, or an area in a diagram of water discharge versus solute flow rate (a Q,J diagram). The shape of the QCFD is the result of dilution of two or more flows from sources of different water quality. (assuming conservative substances) Several types of QCFDs were analyzed at sources, inner nodes of a network, and of consumer outlets. The effect of water discharge constraints (due to flow limitations in the network) on the QCFDs was formulated and analyzed. Computation of QCFDs of dilution junctions by vector addition of their inflows was described. The method was extended numerically to nonlinear mixing due to dependence of water salinity. Use of this method enables computation of QCFDs for inner nodes in networks, including dilution junctions. The effect of network topology and flow direction was discussed. Application and demonstration will follow in the next paper in this series. Copyright © 2008 John Wiley & Sons, Ltd. Le domaine de faisabilité Q,C (QCFD) a été défini et proposé comme un outil pour la conception des systèmes d'alimentation en eau d'irrigation de qualités multiples. Il détermine toutes les combinaisons faisables de débit et de qualité de l'eau, et peut être représenté par un point, une ligne, ou un secteur dans un diagramme débit-concentration (un diagramme de Q,J). La forme du QCFD est le résultat de la dilution de deux écoulements ou plus provenant de sources de qualité différente (en supposant la conservation des quantités). Plusieurs types de QCFD ont été analysés aux sources, n,uds, et sorties du réseau. L'effet des contraintes de débit (dues aux limitations dans le réseau) sur le QCFD a été formulé et analysé. On décrit le calcul de QCFD aux jonctions par l'addition des vecteurs d'apports. La méthode a été étendues numériquement aux mélanges non linéaires du fait de la liaison avec la salinité de l'eau. L'utilisation de cette méthode permet le calcul de QCFD aux n,uds intérieurs des réseaux, y compris les jonctions de dilution. L'effet de la topologie de réseau et du sens d'écoulement a été discuté. L'application et la démonstration suivront dans le prochain papier de cette série. Copyright © 2008 John Wiley & Sons, Ltd. [source]

    Computation and analysis of multiple structural change models

    Jushan Bai
    In a recent paper, Bai and Perron (1998) considered theoretical issues related to the limiting distribution of estimators and test statistics in the linear model with multiple structural changes. In this companion paper, we consider practical issues for the empirical applications of the procedures. We first address the problem of estimation of the break dates and present an efficient algorithm to obtain global minimizers of the sum of squared residuals. This algorithm is based on the principle of dynamic programming and requires at most least-squares operations of order O(T2) for any number of breaks. Our method can be applied to both pure and partial structural change models. Second, we consider the problem of forming confidence intervals for the break dates under various hypotheses about the structure of the data and the errors across segments. Third, we address the issue of testing for structural changes under very general conditions on the data and the errors. Fourth, we address the issue of estimating the number of breaks. Finally, a few empirical applications are presented to illustrate the usefulness of the procedures. All methods discussed are implemented in a GAUSS program. Copyright © 2002 John Wiley & Sons, Ltd. [source]