Three parallel algorithms for classical molecular dynamics are presented. The first assigns each processor a fixed subset of atoms; the second assigns each a fixed subset of inter-atomic forces to compute; the third assigns each a fixed spatial region. The algorithms are suitable for molecular dynamics models which can be difficult to parallelize efficiently—those with short-range forces where the neighbors of each atom change rapidly. They can be implemented on any distributed-memory parallel machine which allows for message-passing of data between independently executing processors. The algorithms are tested on a standard Lennard-Jones benchmark problem for system sizes ranging from 500 to 100,000,000 atoms on several parallel supercomputers--the nCUBE 2, Intel iPSC/860 and Paragon, and Cray T3D. Comparing the results to the fastest reported vectorized Cray Y-MP and C90 algorithm shows that the current generation of parallel machines is competitive with conventional vector supercomputers even for small problems. For large problems, the spatial algorithm achieves parallel efficiencies of 90% and a 1840-node Intel Paragon performs up to 165 faster than a single Cray C9O processor. Trade-offs between the three algorithms and guidelines for adapting them to more complex molecular dynamics simulations are also discussed.

Fast parallel algorithms for short-range molecular dynamics

Fast radio bursts, milliseconds-duration radio bursts predominantly originating from cosmological distances, figure among the unsolved puzzles of contemporary astrophysics. The rapid accumulation of observational data has generated an equally intense theoretical activity toward the understanding of the physical processes at the origin of these events. This review presents a thorough survey of the current knowledge about fast radio bursts, starting with the generic constraints that can be placed on theoretical models based on current observations and plasma physics considerations, then moving to a critical discussion of coherent radiation mechanisms and source models currently debated in the scientific community. 

The physics of fast radio bursts

Computer Simulations in Condensed Matter: Systems: From Materials to Chemical Biology. Volume 1

Using molecular dynamics simulation of a standard coarse-grained polymer glass model, we investigate by means of the stress-fluctuation formalism the shear modulus μ as a function of temperature T and sampling time Δt. While the ensemble-averaged modulus μ(T) is found to decrease continuously for all Δt sampled, its standard deviation δμ(T) is nonmonotonic, with a striking peak at the glass transition. Confirming the effective time-translational invariance of our systems, μ(Δt) can be understood using a weighted integral over the shear-stress relaxation modulus G(t). While the crossover of μ(T) gets sharper with an increasing Δt, the peak of δμ(T) becomes more singular. It is thus elusive to predict the modulus of a single configuration at the glass transition.

Shear Modulus and Shear-Stress Fluctuations in Polymer Glasses.


 Fast radio bursts (FRBs) are observed to be highly polarized. Most have high linear polarization but a small fraction show significant circular polarization. We systematically investigate a variety of polarization mechanisms of FRBs within the magnetar theoretical framework considering two emission sites inside and outside the magnetosphere. For each site, we discuss both intrinsic radiation mechanisms and propagation effects. Inside the magnetosphere, we investigate the polarization properties of both coherent curvature radiation and inverse Compton scattering by charged bunches and conclude that both mechanisms produce 100% linear polarization at an on-axis geometry but can produce circular polarization if the viewing angle is off axis. The lack of circular polarization for the majority of bursts requires that the bunches have a large transverse dimension size. Resonant cyclotron absorption within magnetosphere may produce high circular polarization if electrons and positrons have an asymmetric Lorentz factor distribution. Outside the magnetosphere, the synchrotron maser emission mechanism in general produces highly linearly polarized emission. Circular polarization would appear at off-beam angles but the flux is greatly degraded and such bursts are not detectable at cosmological distances. Synchrotron absorption in a nebula with ordered magnetic field may reduce the circular polarization degree. Cyclotron absorption in a strongly magnetized medium may generate significant circular polarization. Faraday conversion in a medium with field reversal can convert one polarization mode to another. The two absorption processes require stringent physical conditions. Significant Faraday conversion may be realized in a magnetized dense environment involving binary systems or supernova remnants.

/pdf/polarization-of-fast-radio-bursts-radiation-mechanisms-and-3e0hjz3b.pdf

Polarization of Fast Radio Bursts: radiation mechanisms and propagation effects

We report the properties of more than 600 bursts (including cluster-bursts) detected from the repeating fast radio burst (FRB) source FRB 20201124A with the Five-hundred-meter Aperture Spherical radio Telescope during an extremely active episode on UTC 2021 September 25–28, in a series of four papers. The observations were carried out in the band of 1.0–1.5 GHz by using the center beam of the L-band 19-beam receiver. We monitored the source in sixteen 1 hr sessions and one 3 hr session spanning 23 days. All the bursts were detected during the first four days. In this first paper of the series, we perform a detailed morphological study of 624 bursts using the two-dimensional frequency-time “waterfall” plots, with a burst (or cluster-burst) defined as an emission episode during which the adjacent emission peaks have a separation shorter than 400 ms. The duration of a burst is therefore always longer than 1 ms, with the longest up to more than 120 ms. The emission spectra of the sub-bursts are typically narrow within the observing band with a characteristic width of ∼277 MHz. The center frequency distribution has a dominant peak at about 1091.9 MHz and a secondary weak peak around 1327.9 MHz. Most bursts show a frequencydownward-drifting pattern. Based on the drifting patterns, we classify the bursts into five main categories: downward drifting (263) bursts, upward drifting (3) bursts, complex (203), no drifting (35) bursts, and no evidence for drifting (121) bursts. Subtypes are introduced based on the emission frequency range in the band (low, middle, high and wide) as well as the number of components in one burst (1, 2, or multiple). We measured a varying scintillation bandwidth from about 0.5 MHz at 1.0 GHz to 1.4 MHz at 1.5 GHz with a spectral index of 3.0.

https://iopscience.iop.org/article/10.1088/1674-4527/ac98f8/pdf

FAST Observations of an Extremely Active Episode of FRB 20201124A: I. Burst Morphology

We reconsider numerically the two-dimensional version of the Kob-Andersen model (KA2d) with a fraction of 80% of large spheres. A constant moderate pressure is imposed while the temperature T is systematically quenched from the liquid limit through the glass transition at T-g approximate to 0.3 down to very low temperatures. Monodisperse Lennard-Jones (mdLJ) bead systems, forming a crystal phase at low temperatures, are used to highlight several features of the KA2d model. As can be seen, e.g. from the elastic shear modulus G(T), determined using the stress-fluctuation formalism, our KA2d model is a good glass-former. A continuous cusp-singularity, G(T) proportional to (1 - T/T-g)(alpha) with alpha approximate to 0.6, is observed in qualitative agreement with other recent numerical and theoretical work, however in striking conflict with the additive jump discontinuity predicted by mode-coupling theory.

Glass transition of two-dimensional 80–20 Kob–Andersen model at constant pressure

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FAST Observations of an Extremely Active Episode of FRB 20201124A: I. Burst Morphology

Glass transition of two-dimensional 80–20 Kob–Andersen model at constant pressure