Numerical approximation to the Dirichlet problem for the Poisson equation in half lens domain

The solution of the Poisson equation is a crucial step in electronic structure calculations, yielding the electrostatic potential-a key component of the quantum mechanical Hamiltonian. In recent decades, theoretical advances and increases in computer performance have made it possible to simulate the electronic structure of extended systems in complex environments. This requires the solution of more complicated variants of the Poisson equation, featuring nonhomogeneous dielectric permittivities, ionic concentrations with nonlinear dependencies, and diverse boundary conditions. The analytic solutions generally used to solve the Poisson equation in vacuum (or with homogeneous permittivity) are not applicable in these circumstances, and numerical methods must be used. In this work, we present DL_MG, a flexible, scalable, and accurate solver library, developed specifically to tackle the challenges of solving the Poisson equation in modern large-scale electronic structure calculations on parallel computers. Our solver is based on the multigrid approach and uses an iterative high-order defect correction method to improve the accuracy of solutions. Using two chemically relevant model systems, we tested the accuracy and computational performance of DL_MG when solving the generalized Poisson and Poisson-Boltzmann equations, demonstrating excellent agreement with analytic solutions and efficient scaling to ∼109 unknowns and 100s of CPU cores. We also applied DL_MG in actual large-scale electronic structure calculations, using the ONETEP linear-scaling electronic structure package to study a 2615 atom protein-ligand complex with routinely available computational resources. In these calculations, the overall execution time with DL_MG was not significantly greater than the time required for calculations using a conventional FFT-based solver.

In this paper, we extend the work of Daripa et al. [14–16,7] to a larger class of elliptic problems in a variety of domains. In particular, analysis-based fast algorithms to solve inhomogeneous elliptic equations of three different types in three different two-dimensional domains are derived. Dirichlet, Neumann and mixed boundary value problems are treated in all these cases. Three different domains considered are: (i) interior of a circle, (ii) exterior of a circle, and (iii) circular annulus. Three different types of elliptic problems considered are: (i) Poisson equation, (ii) Helmholtz equation (oscillatory case), and (iii) Helmholtz equation (monotone case). These algorithms are derived from an exact formula for the solution of a large class of elliptic equations (where the coefficients of the equation do not depend on the polar angle when written in polar coordinates) based on Fourier series expansion and a one-dimensional ordinary differential equation. The performance of these algorithms is illustrated for several of these problems. Numerical results are presented.

This study introduces a novel fractional-order derivative, termed the Mittag-Leffler–Caputo–Fabrizio (MLCF) fractional derivative, which is characterized by a singular kernel. Symmetry plays a key role in the structure and behavior of fractional operators, and our formulation reflects this by incorporating symmetric properties of the Mittag-Leffler function and its integral representation. To numerically approximate the MLCF derivative, we apply a two-point finite forward difference scheme to estimate the first-order derivative of the function u(λ) within the integral component of the definition. This leads to the construction of a new numerical differentiation scheme. Our analysis demonstrates that the proposed approximation exhibits first-order convergence, with absolute errors decreasing as the time step size h diminishes. These errors are quantified by comparing our numerical results with exact analytical solutions, reinforcing the accuracy of the method.

It is known that the extended Born approximation (ExBorn) is much faster than the method of moments (MoM) in the study of electromagnetic scattering by three-dimensional (3-D) dielectric objects, while it is much more accurate than the Born approximation at low frequencies. Hence, it is more applicable in the low-frequency numerical simulation tools. However, the conventional ExBorn is still too slow to solve large-scale problems because it requires O(N/sup 2/) computational load, where N is the number of unknowns. In this paper, a fast ExBorn algorithm is proposed for the numerical simulation of 3-D dielectric objects buried in a lossy Earth. When the buried objects are discretized with uniform rectangular mesh and the Green's functions are extended appropriately, the computational load can be reduced to O(N log N) using the cyclic convolution, cyclic correlation, and fast Fourier transform (FFT). Numerical analysis shows that the fast ExBorn provides good approximations if the buried target has a small or moderate contrast. If the contrast is large, however, ExBorn will be less accurate. In this case, a preconditioned conjugate-gradient FFT (CG-FFT) algorithm is developed, where the solution of the fast ExBorn is chosen as the initial guess and the preconditioner. Numerical results are given to test the validity and efficiency of the fast algorithms.

A fast algorithm is proposed for the simulation of the transient evolution of a resonant tunneling diode by a multiscale approach. The problem is modeled by the time-dependent Schrodinger--Poisson system. By decomposing the wave function into a nonresonant part and a resonant part, the fast algorithm is designed by combining a standard finite difference method for the Schrodinger--Poisson equation with proper time-dependent and/or nonlinear transmission boundary conditions. In addition, with a suitable interpolation of the nonresonant part, an accurate and fast algorithm is presented for the computation of the resonant part via the projection method. With this two scale decomposition, the new numerical method can save computational time significantly.

In this article, fast time second-order two-grid algorithms are proposed for solving the nonlinear mobile/immobile transport model with the Riemann-Liouville time fractional derivative by using the finite volume element algorithm and the Crank-Nicolson scheme. The fast algorithms involve solving the nonlinear system on the spatial coarse grid (with grid size H) and then solving the linearized system on spatial fine grid (with grid size h). The stability results for the two-grid Crank-Nicolson finite volume element (CNFVE) algorithms are given, and the optimal error estimates in the discrete L ∞ ( L 2 ( Ω ) ) and L 2 ( H 1 ( Ω ) ) norms are obtained. It is shown that the fast two-grid CNFVE algorithms can achieve asymptotically optimal approximation, as long as the spatial coarse and fine grid sizes satisfy H = O ( h 1 / 2 ) (in the discrete L ∞ ( L 2 ( Ω ) ) norm) and H = O ( h 1 / 4 ) (in the discrete L 2 ( H 1 ( Ω ) ) norm). The numerical results are given to validate that the proposed fast algorithms are very effective.

A comparison study on thermal performance enhancement of corrugated oil cooler with internal turbulators – Numerical and experimental approach

Integral representations are derived from a differential-type boundary value problem using a fundamental solution. A set of integral representations is equivalent to a set of differential equations. If the boundary conditions are substituted into the integral representations, the integral equations are obtained, and the unknown variables are determined by solving the integral equations. In other words, an integral-type boundary value problem is derived from the integral representations. An effective and flexible finite element algorithm is easily obtained from the integral-type boundary value problem. In the present paper, integral representations are obtained for the diffusion of a material or heat in the sea, where the convective velocity and diffusion constant change in space and time. A new numerical solution of an advection-diffusion equation is proposed based integral representations using the fundamental solution of the primary space-differential operator, and the numerical results are shown. An innovative generalization of the integral representation method: generalized integral representation method is also proposed. The numerical examples are given to verify the theory.

This brief presents a novel fast algorithm derivation and structure design of analysis and synthesis quadrature mirror filterbanks (SQMFs) on the spectral band replication in Digital Radio Mondiale (DRM). After the preprocedure and postprocedure, a Fourier-transform-based computational kernel was required to construct two types of fast algorithms that offered certain advantages. The Proposed-I method employs a modified split-radix fast Fourier transform (FFT) for analysis quadrature mirror filterbank (AQMF) to reduce the number of additions at the last stage of the butterfly and adopts a split-radix FFT to calculate the SQMF coefficients. The Proposed-II method used the compact structure of the variable-length recursive DFT to realize the kernel procedure for the proposed fast AQMF and SQMF algorithms. In addition, a well-known lifting scheme was applied to reduce numerous multiplication and addition calculations. Compared with the original calculations for the long transform length, all multiplication, addition, and coefficient operations for the Proposed-I method (i.e., AQMF + SQMF) had 91.65%, 79.81%, and 97.22% reductions, respectively. However, for the Proposed-II method, the total reductions of multiplication, addition, and coefficient operations were 64.16%, 21.53%, and 97.12%, respectively. Compared with the fast SQMF algorithm by Huang , the Proposed-I method for SQMF reduces 58.33% of the multiplication, 65% of the addition, and 67.19% of the coefficients. Therefore, the proposed fast quadrature mirror filterbank algorithm is a better solution than other approaches for future DRM applications.

A new two-dimensional analytical model for the potential distribution and depletion-layer width for the short-gate GaAs MESFET's has been presented in this paper. The solution of the two-dimensional Poisson's equation has been considered as the superposition of the solution of one-dimensional Poisson's equation in the lateral direction and the two-dimensional Laplace equation with suitable boundary conditions. The remarkable feature of the proposed model is that, in the hand the simplicity of the mathematical expressions and in the other hand the acceptable distribution for the potential and charge in the channel. The numerical results have been presented for the potential distribution and depletion-layer width for different parameters such as the drain-source voltage, gate-length, active-layer thickness and doped profile. It is observed that for the GaAs MESFET's, as the gate-length is decreased, the 2-D potential is increased and this demonstrates the importance of the two-dimensional analysis for the short-gate devices.

Improved algorithm for calculating high accuracy values of the Chandrasekhar function

A new numerical approach for the time-dependent wave and heat equations as well as for the time-independent Poisson equation on irregular domains has been developed. Trivial Cartesian meshes and simple 9-point stencil equations with unknown coefficients are used for 2-D irregular domains. The calculation of the coefficients of the stencil equations is based on the minimization of the local truncation error of the stencil equations and yields the optimal order of accuracy. The treatment of the Dirichlet and Neumann boundary conditions in the new approach is related to the development of high-order boundary conditions with the stencils that include the same or a smaller number of grid points compared to that for the regular 9-point internal stencils. At similar 9-point stencils, the accuracy of the new approach is two orders higher than that for the linear finite elements. The numerical results for irregular domains in Part 2 of the paper also show that at the same number of degrees of freedom, the new approach is even much more accurate than the quadratic and cubic finite elements with much wider stencils. Similar to our recent results on regular domains, the order of the accuracy of the new approach for the Poisson equation on irregular domains with square Cartesian meshes is higher than that with rectangular Cartesian meshes. The new approach can be directly applied to other partial differential equations.

Plastic encapsulated transistors : C. F. Maguire, F. J. Koons and Q. T. Jarrett. Solid State Technol., August (1971), p. 37

A finite element formulation for the hydrodynamic semiconductor device equations

Transformations of the involution type are considered in the space $R^l$, $l\geq 2$. The matrix properties of these transformations are investigated. The structure of the matrix under consideration is determined and it is proved that the matrix of these transformations is determined by the elements of the first row. Also, the symmetry of the matrix under study is proved. In addition, the eigenvectors and eigenvalues of the matrix under consideration are found explicitly. The inverse matrix is also found and it is proved that the inverse matrix has the same structure as the main matrix. The properties of the nonlocal analogue of the Laplace operator are introduced and studied as applications of the transformations under consideration. For the corresponding nonlocal Poisson equation in the unit ball, the solvability of the Dirichlet and Neumann boundary value problems is investigated. A theorem on the unique solvability of the Dirichlet problem is proved, an explicit form of the Green's function and an integral representation of the solution are constructed, and the order of smoothness of the solution of the problem in the Hölder class is found. Necessary and sufficient conditions for the solvability of the Neumann problem, an explicit form of the Green's function, and the integral representation are also found.