Bulletin of the American Physical Society
APS April Meeting 2014
Volume 59, Number 5
Saturday–Tuesday, April 5–8, 2014; Savannah, Georgia
Session U14: Data and Simulations: Methods and Implementation |
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Room: 102 |
Monday, April 7, 2014 3:30PM - 3:42PM |
U14.00001: Sampling saddle points on the free energy surface Amit Samanta We develop an algorithm for finding the saddle points on the free energy surface ``on-the-fly'' without having to find the free energy function itself. This is done by using the general strategy of the heterogeneous multi-scale method, applying a macro-scale solver, here the gentlest ascent dynamics algorithm, with the needed force and Hessian values computed on-the-fly using a micro-scale model such as molecular dynamics. The algorithm is capable of dealing with problems involving many coarse-grained variables. The utility of the algorithm is illustrated by studying the saddle points associated with (a) the isomerization transition of the alanine dipeptide using two coarse-grained variables, specifically the Ramachandran dihedral angles, and (b) the beta-hairpin structure of the alanine decamer using twenty coarse-grained variables, specifically the full set of Ramachandran angle pairs associated with each residue. [Preview Abstract] |
Monday, April 7, 2014 3:42PM - 3:54PM |
U14.00002: An iterative minimization scheme for saddle search Xiang Zhou The gentlest ascent dynamics (E and ZHOU, 2011 *Nonlinearity*) transforms saddles of energy potential into a stable fixed point. Inspired by GAD, in this talk, I introduce a new formulation of iteratively minimizing a sequence of modified potential to find the saddles of the original function. We show that the iteration converges quadratically. An 175-atom example is illustrated as an application. This is the joint work with Weiguo Gao and Jing Leng. [Preview Abstract] |
Monday, April 7, 2014 3:54PM - 4:06PM |
U14.00003: Equation Solution Figures of Merit, Metaheuristic Search, and the Schrodinger Equation Paul MacNeil This presentation deals with: a definition of ``equation error''; a consideration of equation solution figures of merit based on equation error, and on other measures; and the use of metaheuristic techniques in the search for approximate solutions. These considerations are illustrated by application to the Schrodinger equation for a simple system. Models suitable for computation are produced. Computation results are used to compare the consequences of selection of different figures of merit. ``Equation error'' is defined to be the quantity by which an approximate solution fails to satisfy an equation. ``Equation error variance'' is defined to be the squared modulus of the equation error summed/integrated over the domain of interest. (Generalization to sets of equations is straightforward.) In the example, equation error variance is a functional of the Schrodinger wave function. Possible figures of merit include: ground state energy, system geometry, and equation solution variance. The (derivative-free) metaheuristic used to solve the Schrodinger equation has been changed from a genetic algorithm, used in earlier versions of this research, to evolution strategy with covariance matrix adaptation. [Preview Abstract] |
Monday, April 7, 2014 4:06PM - 4:18PM |
U14.00004: Space-Time Finite Element Approach for the Semilinear Wave Equation Hyun Lim, Matthew Anderson, Jung-Han Kimn For certain formulations of partial differential equations, proper time-parallel pre conditioners can be successfully applied in space-time finite element simulations. Such an approach may enable the extraction of more parallelism to better utilize high performance computing resources. In this work, we examine the behavior of the semi linear wave equation in 1 $+$ 1 dimensions using space-time finite elements. We discretize space and time together for the entire domain using a finite element space which does not separate time and space basis functions. We also explore the effectiveness of the time additive Schwarz preconditioner for this problem. We explore the semi linear wave equation at the threshold of singularity formation using p$=$7 for the nonlinear term and search for self-similarity using a non-uniform mesh in both space and time. [Preview Abstract] |
Monday, April 7, 2014 4:18PM - 4:30PM |
U14.00005: Intraband Optical Transitions in InGaAs/GaAs quantum dot and InAs/InGaAs/GaAs Dots-in-Well Venkata Chaganti, Vadym Apalkov We present the results of our numerical analysis of intraband optical transitions within the valance band of pyramidal Quantum Dot (QD) of type In$_{x}$Ga$_{(1-x)}$As/GaAs and conduction band of Pyramidal Quantum Dot-in-Well (DWELL) structure of type InAs/In$_{x}$Ga$_{(1-x)}$As/GaAs. The electronic states and optical transitions within the valence bands of p-doped semiconductor QD were found numerically within 8 band kp model and the intraband optical absorptions within the conduction bands of n-doped semiconductor DWELL structure were obtained within effective mass model with the use of NEXTNANO software and our Fortran program. In application to quantum dot photodetectors, we study how the size of the dot and its composition affect the optical transition within the dot. With increasing the QD size the absorption spectra are shifted to lower energies. The optical spectra are more sensitive to X-polarized light, with corresponding intensity one order magnitude greater than the absorption intensity of Z-polarized light. In application to DWELL photodetectors we study how the size of the dot and the position of the dot in the well affect the optical transitions within the system. For small QD size (\textless 12nm), the main optical transitions occur either between the QD and quantum well states or between the QD and substrate states. The wavelengths of optical transitions for such small QDs vary between 2 $\mu $m and 6 $\mu $m. DWELL systems are more sensitive to X-polarized light which has intensities 2 orders of magnitude higher than the absorption intensity for Z-polarized light. [Preview Abstract] |
Monday, April 7, 2014 4:30PM - 4:42PM |
U14.00006: Density-Independent Technique for Modeling Self-Propelled Particles Paul Ohmann, Daniel Schubring We present an algorithm for modeling a two-dimensional system of self-propelled particles independent of their density. In this system, we define the nearest neighbors of each particle in terms of a Voronoi tessellation. Specifically, each particle is associated with a region closer to that individual than to any other; we call this region a cell. Neighbors are then defined as those individuals in adjacent cells, no matter their metric distances -- and it is these neighbors that influence the subsequent motion of a particle. This density-independent model holds promise in realistically simulating flocking behavior; however, a challenge in developing these simulations is with the efficiency of the updates. We present an algorithm to efficiently update these systems using Delaunay triangulation. [Preview Abstract] |
Monday, April 7, 2014 4:42PM - 4:54PM |
U14.00007: Medical Impairment and Computational Reduction to a Single Whole Person Impairment (WPI) Rating Value Jerry Artz, John Alchemy, Anne Weilepp, Michael Bongiovanni, Kumar Siddhartha A medical, biophysics, engineering collaboration has produced a standardized cloud-based application for creating automated WPI ratings. The project assigns numerical values to injuries/illness in accordance with the American Medical Association Guides to the Evaluation of Permanent Impairment, Fifth Edition, AMA Press handbook, 5$^{\mathrm{th}}$ edition (with 63 medical contributors and 89 medical reviewers). The AMA Guide serves as the industry standard for assigning impairment values for 32 US states and 190 other countries. Clinical medical data is collected using a menu-driven user interface which is computationally combined into a single numeric value. A medical doctor performs a biometric analysis and enters the quantitative data into a mobile device. The data is analyzed using proprietary validation algorithms, and a WPI Impairment rating is created. The findings are imbedded into a formalized medicolegal report in a matter of minutes. This particular presentation will concentrate upon the WPI rating of the spine---cervical, thoracic, and lumbar. Both common rating techniques will be presented---i.e., Diagnosis Related Estimates (DRE) and Range of Motion (ROM). [Preview Abstract] |
Monday, April 7, 2014 4:54PM - 5:06PM |
U14.00008: The Extent of the Superglass Phase of Binary Mixtures Sea Hoon Lim, Bob Bell In this work, we attempt to map the extent of the superglass phase of Kob-Anderson Lennard-Jones (KALJ) binary mixtures via Path Integral Monte Carlo (PIMC). At low temperatures, KALJ binary mixtures are capable of avoiding crystallization, yet exhibit superfluidity only for certain parameterizations of the KALJ potential. Using PIMC, we observe superfluidity in our mixtures for $\varepsilon \le $ 1.375 $\varepsilon _{\mathrm{He}}$ . For $\varepsilon $ \textgreater 1.375 $\varepsilon _{\mathrm{He}}$, exchange among particles is dramatically reduced. Future work will explore the dynamics of our mixtures for $\varepsilon \le $ 1.375 $\varepsilon_{\mathrm{He}}$ to ascertain whether they are not just superfluid, but glassy as well. [Preview Abstract] |
Monday, April 7, 2014 5:06PM - 5:18PM |
U14.00009: By-passing the sign-problem in Fermion Path Integral Monte Carlo simulations by use of high-order propagators Siu A. Chin The sign-problem in PIMC simulations of non-relativistic fermions increases in serverity with the number of fermions and the number of beads (or time-slices) of the simulation. A large of number of beads is usually needed, because the conventional primitive propagator is only second-order and the usual thermodynamic energy-estimator converges very slowly from below with the total imaginary time. The Hamiltonian energy-estimator, while more complicated to evaluate, is a variational upper-bound and converges much faster with the total imaginary time, thereby requiring fewer beads. This work shows that when the Hamiltonian estimator is used in conjunction with fourth-order propagators with optimizable parameters, the ground state energies of 2D parabolic quantum-dots with approximately 10 completely polarized electrons can be obtain with ONLY 3-5 beads, before the onset of severe sign problems. [Preview Abstract] |
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