5,511 research outputs found
Pomelo, a tool for computing Generic Set Voronoi Diagrams of Aspherical Particles of Arbitrary Shape
We describe the development of a new software tool, called "Pomelo", for the
calculation of Set Voronoi diagrams. Voronoi diagrams are a spatial partition
of the space around the particles into separate Voronoi cells, e.g. applicable
to granular materials. A generalization of the conventional Voronoi diagram for
points or monodisperse spheres is the Set Voronoi diagram, also known as
navigational map or tessellation by zone of influence. In this construction, a
Set Voronoi cell contains the volume that is closer to the surface of one
particle than to the surface of any other particle. This is required for
aspherical or polydisperse systems.
Pomelo is designed to be easy to use and as generic as possible. It directly
supports common particle shapes and offers a generic mode, which allows to deal
with any type of particles that can be described mathematically. Pomelo can
create output in different standard formats, which allows direct visualization
and further processing. Finally, we describe three applications of the Set
Voronoi code in granular and soft matter physics, namely the problem of
packings of ellipsoidal particles with varying degrees of particle-particle
friction, mechanical stable packings of tetrahedra and a model for liquid
crystal systems of particles with shapes reminiscent of pearsComment: 4 pages, 9 figures, Submitted to Powders and Grains 201
Optimizing the ensemble for equilibration in broad-histogram Monte Carlo simulations
We present an adaptive algorithm which optimizes the statistical-mechanical
ensemble in a generalized broad-histogram Monte Carlo simulation to maximize
the system's rate of round trips in total energy. The scaling of the mean
round-trip time from the ground state to the maximum entropy state for this
local-update method is found to be O([N log N]^2) for both the ferromagnetic
and the fully frustrated 2D Ising model with N spins. Our new algorithm thereby
substantially outperforms flat-histogram methods such as the Wang-Landau
algorithm.Comment: 6 pages, 5 figure
GENGA. II. GPU Planetary N-body Simulations with Non-Newtonian Forces and High Number of Particles
We present recent updates and improvements of the graphical processing unit (GPU) N-body code GENGA. Modern state-of-the-art simulations of planet formation require the use of a very high number of particles to accurately resolve planetary growth and to quantify the effect of dynamical friction. At present the practical upper limit is in the range of 30,000–60,000 fully interactive particles; possibly a little more on the latest GPU devices. While the original hybrid symplectic integration method has difficulties to scale up to these numbers, we have improved the integration method by (i) introducing higher level changeover functions and (ii) code improvements to better use the most recent GPU hardware efficiently for such large simulations. We added treatments of non-Newtonian forces such as general relativity, tidal interaction, rotational deformation, the Yarkovsky effect, and Poynting–Robertson drag, as well as a new model to treat virtual collisions of small bodies in the solar system. We added new tools to GENGA, such as semi-active test particles that feel more massive bodies but not each other, a more accurate collision handling and a real-time openGL visualization. We present example simulations, including a 1.5 billion year terrestrial planet formation simulation that initially started with 65,536 particles, a 3.5 billion year simulation without gas giants starting with 32,768 particles, the evolution of asteroid fragments in the solar system, and the planetesimal accretion of a growing Jupiter simulation. GENGA runs on modern NVIDIA and AMD GPUs
Optimized broad-histogram simulations for strong first-order phase transitions: Droplet transitions in the large-Q Potts model
The numerical simulation of strongly first-order phase transitions has
remained a notoriously difficult problem even for classical systems due to the
exponentially suppressed (thermal) equilibration in the vicinity of such a
transition. In the absence of efficient update techniques, a common approach to
improve equilibration in Monte Carlo simulations is to broaden the sampled
statistical ensemble beyond the bimodal distribution of the canonical ensemble.
Here we show how a recently developed feedback algorithm can systematically
optimize such broad-histogram ensembles and significantly speed up
equilibration in comparison with other extended ensemble techniques such as
flat-histogram, multicanonical or Wang-Landau sampling. As a prototypical
example of a strong first-order transition we simulate the two-dimensional
Potts model with up to Q=250 different states on large systems. The optimized
histogram develops a distinct multipeak structure, thereby resolving entropic
barriers and their associated phase transitions in the phase coexistence region
such as droplet nucleation and annihilation or droplet-strip transitions for
systems with periodic boundary conditions. We characterize the efficiency of
the optimized histogram sampling by measuring round-trip times tau(N,Q) across
the phase transition for samples of size N spins. While we find power-law
scaling of tau vs. N for small Q \lesssim 50 and N \lesssim 40^2, we observe a
crossover to exponential scaling for larger Q. These results demonstrate that
despite the ensemble optimization broad-histogram simulations cannot fully
eliminate the supercritical slowing down at strongly first-order transitions.Comment: 11 pages, 12 figure
Simultaneous measurement of flight time and energy of large matrix-assisted laser desorption ionization ions with a superconducting tunnel junction detector
We evaluated a cryogenically cooled superconducting Nb-Al2O3-Nb tunnel junction (STJ) for use as a molecular ion detector in a matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectrometer. The STJ responds to ion energy and theoretically should detect large molecular ions with a velocity-independent efficiency approaching 100%. The STJ detector produces pulses whose heights are approximately proportional to ion energy, thus the height of a pulse generated by the impact of a doubly charged ion is about twice the height of a singly charged ion pulse. Measurements were performed by bombarding the STJ with human serum albumin (HSA) (66,000 Da) and immunoglobulin (150,000 Da) ions. We demonstrate that pulse height analysis of STJ signals provides a way to distinguish with good discrimination HSA+ from 2HSA2+, whose flight times are coincident. The rise time of STJ detector pulses allows ion flight times to be determined with a precision better than 200 ns, which is a value smaller than the flight time variation typically observed for large isobaric MALDI ions detected with conventional microchannel plate (MCP) detectors. Deflection plates in the flight tube of the mass spectrometer provided a way to aim ions alternatively at a MCP ion detector
Right Inferior Frontal Activation During Alcohol-Specific Inhibition Increases With Craving and Predicts Drinking Outcome in Alcohol Use Disorder
Alcohol use disorder (AUD) is characterized by enhanced cue-reactivity and the opposing control processes being insufficient. The ability to inhibit reactions to alcohol-related cues, alcohol-specific inhibition, is thus crucial to AUD; and trainings strengthening this ability might increase treatment outcome. The present study investigated whether neurophysiological correlates of alcohol-specific inhibition (I) vary with craving, (II) predict drinking outcome in AUD and (III) are modulated by alcohol-specific inhibition training. A total of 45 recently abstinent patients with AUD and 25 controls participated in this study. All participants underwent functional magnetic resonance imaging (fMRI) during a Go-NoGo task with alcohol-related as well as neutral conditions. Patients with AUD additionally participated in a double-blind RCT, where they were randomized to either an alcohol-specific inhibition training or an active control condition (non-specific inhibition training). After the training, patients participated in a second fMRI measurement where the Go-NoGo task was repeated. Percentage of days abstinent was assessed as drinking outcome 3 months after discharge from residential treatment. Whole brain analyses indicated that in the right inferior frontal gyrus (rIFG), activation related to alcohol-specific inhibition varied with craving and predicted drinking outcome at 3-months follow-up. This neurophysiological correlate of alcohol-specific inhibition was however not modulated by the training version. Our results suggest that enhanced rIFG activation during alcohol-specific (compared to neutral) inhibition (I) is needed to inhibit responses when craving is high and (II) fosters sustained abstinence in patients with AUD. As alcoholspecific rIFG activation was not affected by the training, future research might investigate whether potential training effects on neurophysiology are better detectable with other methodological approaches
Heisenberg-limited metrology with information recycling
Information recycling has been shown to improve the sensitivity of atom interferometers by exploiting atom-light entanglement. In this Rapid Communication, we apply information recycling to an interferometer where the input quantum state has been partially transferred from some donor system. We demonstrate that when the quantum state of this donor system is from a particular class of number-correlated Heisenberg-limited states, information recycling yields a Heisenberg-limited phase measurement. Crucially, this result holds irrespective of the fraction of the quantum state transferred to the interferometer input and also for a general class of number-conserving quantum-state-transfer processes, including ones that destroy the first-order phase coherence between the branches of the interferometer. This result could have significant applications in Heisenberg-limited atom interferometry, where the quantum state is transferred from a Heisenberg-limited photon source, and in optical interferometry where the loss can be monitored
Feedback-optimized parallel tempering Monte Carlo
We introduce an algorithm to systematically improve the efficiency of
parallel tempering Monte Carlo simulations by optimizing the simulated
temperature set. Our approach is closely related to a recently introduced
adaptive algorithm that optimizes the simulated statistical ensemble in
generalized broad-histogram Monte Carlo simulations. Conventionally, a
temperature set is chosen in such a way that the acceptance rates for replica
swaps between adjacent temperatures are independent of the temperature and
large enough to ensure frequent swaps. In this paper, we show that by choosing
the temperatures with a modified version of the optimized ensemble feedback
method we can minimize the round-trip times between the lowest and highest
temperatures which effectively increases the efficiency of the parallel
tempering algorithm. In particular, the density of temperatures in the
optimized temperature set increases at the "bottlenecks'' of the simulation,
such as phase transitions. In turn, the acceptance rates are now temperature
dependent in the optimized temperature ensemble. We illustrate the
feedback-optimized parallel tempering algorithm by studying the two-dimensional
Ising ferromagnet and the two-dimensional fully-frustrated Ising model, and
briefly discuss possible feedback schemes for systems that require
configurational averages, such as spin glasses.Comment: 12 pages, 14 figure
Dynamics of the Wang-Landau algorithm and complexity of rare events for the three-dimensional bimodal Ising spin glass
We investigate the performance of flat-histogram methods based on a
multicanonical ensemble and the Wang-Landau algorithm for the three-dimensional
+/- J spin glass by measuring round-trip times in the energy range between the
zero-temperature ground state and the state of highest energy. Strong
sample-to-sample variations are found for fixed system size and the
distribution of round-trip times follows a fat-tailed Frechet extremal value
distribution. Rare events in the fat tails of these distributions corresponding
to extremely slowly equilibrating spin glass realizations dominate the
calculations of statistical averages. While the typical round-trip time scales
exponential as expected for this NP-hard problem, we find that the average
round-trip time is no longer well-defined for systems with N >= 8^3 spins. We
relate the round-trip times for multicanonical sampling to intrinsic properties
of the energy landscape and compare with the numerical effort needed by the
genetic Cluster-Exact Approximation to calculate the exact ground state
energies. For systems with N >= 8^3 spins the simulation of these rare events
becomes increasingly hard. For N >= 14^3 there are samples where the
Wang-Landau algorithm fails to find the true ground state within reasonable
simulation times. We expect similar behavior for other algorithms based on
multicanonical sampling.Comment: 9 pages, 12 figure
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