Physics - Computational Physics Publications (50)

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Physics - Computational Physics Publications

We present a generalization of the maximum entropy method to the analytic continuation of matrix-valued Green's functions. To treat off-diagonal elements correctly based on Bayesian probability theory, the entropy term has to be extended for non-negative spectral functions. In that way, all matrix elements of the Green's function matrix can be analytically continued; we introduce a computationally cheap element-wise method for this purpose. Read More


Wall modeling is the key technology for making industrial high-Reynolds-number flows accessible to computational analysis. In this work, we present an innovative multiscale approach to hybrid RANS/LES wall modeling, which overcomes the problem of the RANS/LES transition and enables coarse meshes near the boundary. In a layer near the wall, the Navier-Stokes equations are solved for an LES and a RANS component in one single equation. Read More


We have researched the motion of gas in the subnanochannel with functional surface which wettability has a gradient for the fluid by using molecular dynamics simulation. The results show that the gas is driven to flow under a single heat source and without any other work or energy applied to the system. The driving source is owed to the potential gradient of the functional face which keeps the fluid running in the subnanochannel. Read More


In the tensor-network framework, the expectation values of two-dimensional quantum states are evaluated by contracting a double-layer tensor network constructed from initial and final tensor-network states. The computational cost for carrying out this contraction is generally very high, which limits the largest bond dimension of tensor-network states that can be accurately studied to a relatively small value. We propose a dimension reduction scheme to solve this problem by mapping the double-layer tensor network onto an intersected single-layer tensor network. Read More


We describe a simple and effective technique, the Eigenvector Method for Umbrella Sampling (EMUS), for accurately estimating small probabilities and expectations with respect to a given target probability density. In EMUS, we apply the principle of stratified survey sampling to Markov chain Monte Carlo (MCMC) simulation: We divide the support of the target distribution into regions called strata, we use MCMC to sample (in parallel) from probability distributions supported in each of the strata, and we weight the data from each stratum to assemble estimates of general averages with respect to the target distribution. We demonstrate by theoretical results and computational examples that EMUS can be dramatically more efficient than direct Markov chain Monte Carlo when the target distribution is multimodal or when the goal is to compute tail probabilities. Read More


Recently, ParaExp was proposed for the time integration of linear hyperbolic problems. It splits the time interval of interest into sub-intervals and computes the solution on each sub-interval in parallel. The overall solution is decomposed into a particular solution defined on each sub-interval with zero initial conditions and a homogeneous solution propagated by the matrix exponential applied to the initial conditions. Read More


Context. Solar observatories are providing the world-wide community with a wealth of data, covering large time ranges, multiple viewpoints, and returning large amounts of data. In particular, the large volume of SDO data presents challenges: it is available only from a few repositories, and full-disk, full-cadence data for reasonable durations of scientific interest are difficult to download practically due to their size and download data rates available to most users. Read More


Homogeneous droplet nucleation has been studied for almost a century but has not yet been fully understood. In this work, we used the density gradient theory (DGT) and considered the influence of capillary waves (CW) on the predicted size-dependent surface tensions and nucleation rates for selected $n$-alkanes. The DGT model was completed by an equation of state (EoS) based on the perturbed-chain statistical associating fluid theory (PC-SAFT) and compared to the classical nucleation theory and the Peng--Robinson EoS. Read More


The mean axial velocity of lithium irons across the entrance of carbon nanotube VLi is an important factor for the charge-discharge performances of rechargeable Lithium battery. The molecular dynamics simulation method is adopted to evaluate the factors and their effects on VLi which include the diameter of carbon nanotube, functional group type on the port and the number of a given type of functional group. The statistical analysis of the calculation results shows that: In the selected carbon nanotubes of four different diameters, VLi will gradually rise with the increase of CNT diameter due to lithium irons migration resistance decreasing; as the port of CNT is successively modified to hydrogen (-H), hydroxyl (-OH), amino (-NH2) and carboxyl (-COOH), the corresponding migration resistance of lithium ions is enhanced resulting in the dropping of VLi; in comparison to the effect strength of four types of functional groups on VLi, -COOH shows strongest, -NH2 and -OH perform relatively weaker, and the effect difference between -NH2 and -OH is very small, -H displays weakest; When the number of a given non-hydrogen functional group on the port sequentially increases, it also shows a trend that lithium ion migration resistance gradually increases which makes VLi decreases in turn. Read More


It is a common problem in lattice QCD calculations of hadron masses with annihilation channels that the signal falls off in time while the noise remains constant. In addition, the disconnected insertion calculation in the three-point function and the calculation of the neutron electric dipole moment with the $\theta$ terms suffer from a noise due to the $\sqrt{V}$ fluctuation. We identify these problems to have the same origin and the $\sqrt{V}$ problem can be resolved by utilizing the cluster decomposition principle. Read More


Magneto-electronic properties of buckled monolayer GaAs is studied by the developed generalized tight-binding model, considering the buckled structure, multi-orbital chemical bondings, spin-orbit coupling, electric field, and magnetic field simultaneously. Three group of spin-polarized Landau levels (LLs) near the Fermi level are induced by the magnetic quantization, whose initial energies, LL degeneracy, energy spacings, magnetic-field-dependence, and spin polarization are investigated. The Landau state probabilities describing the oscillation patterns, localization centers, and node regularities of the dominated/minor orbitals are analyzed, and their energy-dependent variations are discussed. Read More


The current work reports the development of a new general grid generator called Gingred for arbitrary (e.g., number of X-points) 2D magnetic equilibria and plate geometries. Read More


We discuss a backward Monte-Carlo technique for muon transport problem, with emphasis on its application in muography. Backward Monte-Carlo allows exclusive sampling of a final state by reversing the simulation flow. In practice it can be made analogous to an adjoint Monte-Carlo, though it is more versatile for muon transport. Read More


This paper explores in details the capabilities of two model reduction techniques - the Spectral Reduced Order Model (Spectral-ROM) and the Proper Generalised Decomposition (PGD) - to numerically solve moisture diffusion problems. Both techniques assume separated tensorial representation of the solution by a finite sum of function products. The Spectral-ROM fixes a set of spatial basis functions to be the Chebyshev polynomials and then, a system of ordinary differential equations is built to compute the temporal coefficients of the solution using the Galerkin projection method, while the PGD aims at computing directly the basis of functions by minimising the residual. Read More


We present a new algorithm for the discretization of the Vlasov-Maxwell system of equations for the study of plasmas in the kinetic regime. Using the discontinuous Galerkin finite element method for the spatial discretization, we obtain a high order accurate solution for the plasma's distribution function. Time stepping for the distribution function is done explicitly with a third order strong-stability preserving Runge-Kutta method. Read More


Correlated many-body problems ubiquitously appear in various fields of physics such as condensed matter physics, nuclear physics, and statistical physics. However, due to the interplay of the large number of degrees of freedom, it is generically impossible to treat these problems from first principles. Thus the construction of a proper model, namely effective Hamiltonian, is essential. Read More


Rare transitions between long-lived stable states are often analyzed in terms of free energy landscapes computed as functions of a few collective variables. Here, using transitions between geometric phases as example, we demonstrate that the effective dynamics of a system along these variables are an essential ingredient in the description of rare events and that the static perspective provided by the free energy alone may be misleading. In particular, we investigate the disk-to-slab transition in the two-dimensional Ising model starting with a calculation of a two-dimensional free energy landscape and the distribution of committor probabilities. Read More


This paper presents the development of an Adaptive Algebraic Multiscale Solver for Compressible flow (C-AMS) in heterogeneous porous media. Similar to the recently developed AMS for incompressible (linear) flows [Wang et al., JCP, 2014], C-AMS operates by defining primal and dual-coarse blocks on top of the fine-scale grid. Read More


This paper introduces an Algebraic MultiScale method for simulation of flow in heterogeneous porous media with embedded discrete Fractures (F-AMS). First, multiscale coarse grids are independently constructed for both porous matrix and fracture networks. Then, a map between coarse- and fine-scale is obtained by algebraically computing basis functions with local support. Read More


The Rashba effect, a spin splitting in electronic band structure, attracts much attention for the potential applications in spintronics with no requirement of external magnetic field. Realizing one-dimensional (1D) Rashba system is a big challenge due to the difficulties of growing high-quality heavy-metal nanowires or introducing strong spin-orbit coupling (SOC) and broken inversion symmetry in flexible materials. Here, based on first-principles calculations, we propose a pathway to realize the Rashba spin-split by adsorbing Gd atom on zigzag graphene nanoribbons (Gd-ZGNR) and further investigate the magnetic anisotropy energy (MAE). Read More


We investigate crack propagation in a simple two-dimensional visco-elastic model and find a scaling regime in the relation between the propagation velocity and energy release rate or fracture energy, together with lower and upper bounds of the scaling regime. On the basis of our result, the existence of the lower and upper bounds is expected to be universal or model-independent: the present simple simulation model provides generic insight into the physics of crack propagation, and the model will be a first step towards the development of a more refined coarse-grained model. Relatively abrupt changes of velocity are predicted near the lower and upper bounds for the scaling regime and the positions of the bounds could be good markers for the development of tough polymers, for which we provide simple views that could be useful as guiding principles for toughening polymer-based materials. Read More


A novel approach is presented for fast generation of synthetic seismograms due to microseismic events, using heterogeneous marine velocity models. The partial differential equations (PDEs) for the 3D elastic wave equation have been numerically solved using the Fourier domain pseudo-spectral method which is parallelizable on the graphics processing unit (GPU) cards, thus making it faster compared to traditional CPU based computing platforms. Due to computationally expensive forward simulation of large geological models, several combinations of individual synthetic seismic traces are used for specified microseismic event locations, in order to simulate the effect of realistic microseismic activity patterns in the subsurface. Read More


We introduce a novel method that combines the accuracy of Quantum Monte Carlo simulations with ab-initio Molecular Dynamics, in the spirit of Car-Parrinello. This method is then used for investigating the structure of a two-dimensional layer of hydrogen at $T=0~\text{K}$ and high densities. We find that metallization is to be expected at $r_s \approx 1. Read More


Large eddy simulation (LES) has become the de-facto computational tool for modeling complex reacting flows, especially in gas turbine applications. However, readily usable general-purpose LES codes for complex geometries are typically academic or proprietary/commercial in nature. The objective of this work is to develop and disseminate an open source LES tool for low-Mach number turbulent combustion using the OpenFOAM framework. Read More


Phonon-mediated thermal conductivity, which is of great technological relevance, fundamentally arises due to anharmonic scattering from interatomic potentials. Despite its prevalence, accurate first-principles calculations of thermal conductivity remain challenging, primarily due to the high computational cost of anharmonic interatomic force constant (IFCs) calculations. Meanwhile, the related anharmonic phenomenon of thermal expansion is much more tractable, being computable from the Gr\"{u}neisen parameters associated with phonon frequency shifts due to crystal deformations. Read More


We introduce a lattice gas implementation that is based on coarse-graining a Molecular Dynamics (MD) simulation. Such a lattice gas is similar to standard lattice gases, but its collision operator is informed by an underlying MD simulation. This can be considered an optimal lattice gas implementation because it allows for the representation of any system that can be simulated with MD. Read More


The application of high pressure can fundamentally modify the crystalline and electronic structures of elements as well as their chemical reactivity, which could lead to the formation of novel materials. Here, we explore the reactivity of lithium with sodium under high pressure, using a swarm structure searching techniques combined with first-principles calculations, which identify a thermodynamically stable LiNa compound adopting an orthorhombic oP8 phase at pressure above 355 GPa. The formation of LiNa may be a consequence of strong concentration of electrons transfer from the lithium and the sodium atoms into the interstitial sites, which also leads to opening a relatively wide band gap for LiNa-op8. Read More


Hydrogen-rich compounds are important for understanding the dissociation of dense molecular hydrogen, as well as searching for room temperature Bardeen-Cooper-Schrieffer (BCS) superconductors. A recent high pressure experiment reported the successful synthesis of novel insulating lithium polyhydrides when above 130 GPa. However, the results are in sharp contrast to previous theoretical prediction by PBE functional that around this pressure range all lithium polyhydrides (LiHn (n = 2-8)) should be metallic. Read More


We study segregation of a binary mixture of similarly charged particles under shear using particle-based simulations. We simulate particle dynamics using a discrete-element model including electrostatic interactions and find that particles segregate according to their net charge. Particles that are charged twice as strong as other particles of the same electrical sign are seen more at insulating boundaries with which we shear the system. Read More


We apply the spectral element method to the determination of scattering and bound states of the multichannel Schr\"odinger equation. In our approach the reaction coordinate is discretized on a grid of points whereas the internal coordinates are described by either purely diabatic or locally diabatic (diabatic-by-sector) bases. Bound levels and scattering matrix elements are determined with spectral accuracy using relatively small numbers of points. Read More


Our group has recently developed a finite element model of a nanoparticle-mediated optical breakdown phenomena. Previously, this model was used to analyze the role of the nanoparticle morphology and the wavelength dependence of a nanoparticle-mediated optical breakdown threshold during near-infrared ps and fs pulse exposures. In this study, we provide a theoretical insight into the optoporation efficiency of live cells and bubble formation threshold during nanoparticle-mediated optical breakdown. Read More


2017May
Affiliations: 1CD Laboratory for Multi-Scale Modelling of Multiphase Processes, Johannes Kepler University Linz, Austria, 2Department of Particulate Flow Modelling, Johannes Kepler University Linz, 3CD Laboratory for Multi-Scale Modelling of Multiphase Processes, Johannes Kepler University Linz, Austria, 4Department of Particulate Flow Modelling, Johannes Kepler University Linz

The large time and length scales and, not least, the vast number of particles involved in industrial-scale simulations inflate the computational costs of the Discrete Element Method (DEM) excessively. Coarse grain models can help to lower the computational demands significantly. However, for effects that intrinsically depend on particle size, coarse grain models fail to correctly predict the behaviour of the granular system. Read More


The recently introduced acoustic ray-tracing semiclassical (RTS) method is validated for a set of practically relevant boundary conditions. RTS is a frequency domain geometrical method which directly reproduces the acoustic Green's function. As previously demonstrated for a rectangular room and weakly absorbing boundaries with a real and frequency-independent impedance, RTS is capable of modeling also the lowest modes of such a room, which makes it a useful method for low frequency sound field modeling in enclosures. Read More


Molecular dynamics (MD) simulations allow the exploration of the phase space of biopolymers through the integration of equations of motion of their constituent atoms. The analysis of MD trajectories often relies on the choice of collective variables (CVs) along which the dynamics of the system is projected. We developed a graphical user interface (GUI) for facilitating the interactive choice of the appropriate CVs. Read More


We propose a new semi-Lagrangian Vlasov-Poisson solver. It employs elements of metric to follow locally the flow and its deformation, allowing one to find quickly and accurately the initial phase-space position $Q(P)$ of any test particle $P$, by expanding at second order the geometry of the motion in the vicinity of the closest element. It is thus possible to reconstruct accurately the phase-space distribution function at any time $t$ and position $P$ by proper interpolation of initial conditions, following Liouville theorem. Read More


We report the results of an extended search for planar Newtonian periodic three-body orbits with vanishing angular momentum, that has led to more than 150 new topologically distinct orbits, which is more than three-fold increase over the previously known ones. Each new orbit defines an infinite family of orbits with non-vanishing angular momenta. We have classified these orbits in ten algebraically defined sequences. Read More


In this paper, we have proposed a modified Marker-And-Cell (MAC) method to investigate the problem of an unsteady 2-D incompressible flow with heat and mass transfer at low, moderate, and high Reynolds numbers with no-slip and slip boundary conditions. We have used this method to solve the governing equations along with the boundary conditions and thereby to compute the flow variables, viz. $u$-velocity, $v$-velocity, $P$, $T$, and $C$. Read More


This is an elementary introduction to infinite-dimensional probability. In the lectures, we compute the exact mean values of some functionals on C[0,1] and L[0,1] by considering these functionals as infinite-dimensional random variables. The results show that there exist the complete concentration of measure phenomenon for these mean values since the variances are all zeroes. Read More


Wave functions of a new functional kind have been proposed for Helium-like atoms in this work . These functions explicitly depend on interelectronic and hyperspherical coordinates. The best ground state energy for the Helium atom $ -2. Read More


Accurate path integral Monte Carlo or molecular dynamics calculations of isotope effects have until recently been expensive because of the necessity to reduce three types of errors present in such calculations: statistical errors due to sampling, path integral discretization errors, and thermodynamic integration errors. While the statistical errors can be reduced with virial estimators and path integral discretization errors with high-order factorization of the Boltzmann operator, here we propose a method for accelerating isotope effect calculations by eliminating the integration error. We show that the integration error can be removed entirely by changing particle masses stochastically during the calculation and by using a piecewise linear umbrella biasing potential. Read More


Path integral implementation of the quantum instanton approximation currently belongs among the most accurate methods for computing quantum rate constants and kinetic isotope effects, but its use has been limited due to the rather high computational cost. Here we demonstrate that the efficiency of quantum instanton calculations of the kinetic isotope effects can be increased by orders of magnitude by combining two approaches: The convergence to the quantum limit is accelerated by employing high-order path integral factorizations of the Boltzmann operator, while the statistical convergence is improved by implementing virial estimators for relevant quantities. After deriving several new virial estimators for the high-order factorization and evaluating the resulting increase in efficiency, using $\mathrm{\cdot H_{\alpha}+H_{\beta}H_{\gamma}\rightarrow H_{\alpha}H_{\beta}+\cdot H_{\gamma }}$ reaction as an example, we apply the proposed method to obtain several kinetic isotope effects on $\mathrm{CH_{4}+\cdot H\rightleftharpoons\cdot CH_{3}+H_{2}}$ forward and backward reactions. Read More


In this paper we present a new variable time step criterion for the velocity-Verlet algorithm allowing to correctly simulate the dynamics of charged particles exchanging energy via Coulomb collisions while minimising simulation time. We present physical arguments supporting the use of the criterion along with numerical results proving its validity. We numerically show that $\bar{\textrm{H}}^{+}$ ions with 18 meV initial energy can be captured and sympathetically cooled by a Coulomb crystal of $\textrm{Be}^{+}$ and $\textrm{HD}^{+}$ in less than 10 ms, an important result for the GBAR project. Read More


In spite of their intrinsic one-dimensional nature matrix product states have been systematically used to obtain remarkably accurate results for two-dimensional systems. Motivated by basic entropic arguments favoring projected entangled-pair states as the method of choice, we assess the relative performance of infinite matrix product states and infinite projected entangled-pair states on cylindrical geometries. By considering the Heisenberg and half-filled Hubbard models on the square lattice as our benchmark cases, we evaluate their variational energies as a function of both bond dimension as well as cylinder width. Read More


The expedient design of precision components in aerospace and other high-tech industries requires simulations of physical phenomena often described by partial differential equations (PDEs) without exact solutions. Modern design problems require simulations with a level of resolution difficult to achieve in reasonable amounts of time---even in effectively parallelized solvers. Though the scale of the problem relative to available computing power is the greatest impediment to accelerating these applications, significant performance gains can be achieved through careful attention to the details of memory communication and access. Read More


The most popular and widely used subtract-with-borrow generator, also known as RANLUX, is reimplemented as a linear congruential generator using large integer arithmetic with the modulus size of 576 bits. Modern computers, as well as the specific structure of the modulus inferred from RANLUX, allow for the development of a fast modular multiplication -- the core of the procedure. This was previously believed to be slow and have too high cost in terms of computing resources. Read More


For over a decade now, physical and energy constraints have limited clock speed improvements in commodity microprocessors. Instead, chipmakers have been pushed into producing lower-power, multi-core processors such as GPGPU, ARM and Intel MIC. Broad-based efforts from manufacturers and developers have been devoted to making these processors user-friendly enough to perform general computations. Read More


We investigate roles of electron correlation effects in the determination of the $g_j$ factors of the $4s ~ ^2S_{1/2}$, $4p ~ ^2P_{1/2}$, $4p ~ ^2P_{3/2}$, $3d ~ ^2D_{3/2}$, and $3d ~ ^2D_{5/2}$ states, representing to different parities and angular momenta, of the Ca$^+$ ion. Correlation contributions are highlighted with respect to the mean-field values evaluated using the Dirac-Hartree-Fock method, relativistic second order many-body theory, and relativistic coupled-cluster (RCC) theory with the singles and doubles approximation considering only the linear terms and also accounting for all the non-linear terms. This shows that it is difficult to achieve reasonably accurate results employing an approximated perturbative approach. Read More


Most recent exciting experimental advances introduced buckled and flat borophene nanomembranes as new members to the advancing family of two-dimensional (2D) materials. Borophene, is the boron atom analogue of graphene with interesting properties suitable for a wide variety of applications. In this investigation, we conducted extensive first-principles density functional theory simulations to explore the application of four different flat borophene films as anode materials for Al, Mg, Na or Li-ion batteries. Read More


In this review, we look at the concepts and state-of-the-art concerning the analysis of micro and nanoresonators from the underlying concept of their natural resonances, also called quasi-normal modes (QNMs). It is these modes with complex frequencies that are responsible for the spectral response and temporal dynamics of the resonators. They are initially excited by the driving near or far-field, then loaded before exponentially decaying in time due to power leakage or absorption. Read More


Finding an easy-to-build coils set has been a critical issue for stellarator design for decades. Conventional approaches assume a toroidal "winding" surface. We'll investigate if the existence of winding surface unnecessarily constrains the optimization, and a new method to design coils for stellarators is presented. Read More