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

다중혈관 관상동맥 환자에서 y-문합을 이용하여 양쪽 내흉동맥만을 사용한 우회술의 조기 성적

인공고관절의 세라믹 볼 헤드의 기계적 안정성 평가를 위한 3 차원 유한요소 해석

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Introduction To Electrodynamics

心臓病診療プラクティス 16. 心不全を癒す

When effective medical treatment and vaccination are not available, non-pharmaceutical interventions such as social distancing, home quarantine and far-reaching shutdown of public life are the only available strategies to prevent the spread of epidemics. Based on an extended SEIR (susceptible-exposed-infectious-recovered) model and continuous-time optimal control theory, we compute the optimal non-pharmaceutical intervention strategy for the case that a vaccine is never found and complete containment (eradication of the epidemic) is impossible. In this case, the optimal control must meet competing requirements: First, the minimization of disease-related deaths, and, second, the establishment of a sufficient degree of natural immunity at the end of the measures, in order to exclude a second wave. Moreover, the socio-economic costs of the intervention shall be kept at a minimum. The numerically computed optimal control strategy is a single-intervention scenario that goes beyond heuristically motivated interventions and simple “flattening of the curve”. Careful analysis of the computed control strategy reveals, however, that the obtained solution is in fact a tightrope walk close to the stability boundary of the system, where socio-economic costs and the risk of a new outbreak must be constantly balanced against one another. The model system is calibrated to reproduce the initial exponential growth phase of the COVID-19 pandemic in Germany.

/pdf/beyond-just-flattening-the-curve-optimal-control-of-39ll28k8z5.pdf

Beyond just “flattening the curve”: Optimal control of epidemics with purely non-pharmaceutical interventions

For lighting applications, organic light-emitting diodes (OLED) need much higher brightness than for displays, leading to self-heating. Due to the temperature-activated transport in organic semiconductors, this can result in brightness inhomogeneities and catastrophic failure. Here, it is shown that due to the strong electrothermal feedback of OLEDs, the common spatial current and voltage distribution is completely changed, requiring advanced device modeling and operation concepts. This study clearly demonstrates the effect of S-shaped negative differential resistance in OLEDs induced by self-heating. As a consequence, for increasing voltage, regions with declining voltages are propagating through the device, and even more interestingly, a part of these regions show even decreasing currents, leading to strong local variation in luminance. The expected breakthrough of OLED lighting technology will require an improved price performance ratio, and the realization of modules with very high brightness but untainted appearance is considered to be an essential step into this direction. Thus, a deeper understanding of the control of electrothermal feedback will help to make OLEDs in lighting more competitive.

Feel the heat: Nonlinear electrothermal feedback in organic LEDs

In 1950, van Roosbroeck introduced the fundamental semiconductor device equations as a system of three nonlinearly coupled partial differential equations (PDEs). They describe the semiclassical transport of free electrons and holes in a self-consistent electric field using a drift-diffusion approximation. This chapter introduces a method for discretizing the van Roosbroeck system which is close to the physicist's approach to derive PDEs based on a subdivision of the computational domain into representative elementary volumes or control volumes. The method has two main ingredients: a geometry-based approach to obtain a system describing communicating control volumes; and a consistent description of the fluxes between two adjacent control volumes. When applied to the drift-diffusion formulation, compared to various variants of the stabilized finite element method, the two-point flux finite volume scheme is outstanding in the sense that it guarantees positivity of densities and absense of unphysical oscillations. Spin-polarized drift-diffusion models have been proposed for the description of spintronic devices.

Drift-Diffusion Models

We demonstrate electric bistability induced by the positive feedback of self-heating onto the thermally activated conductivity in a two-terminal device based on the organic semiconductor ${\mathrm{C}}_{60}$. The central undoped layer with a thickness of 300 nm is embedded between thinner $n$-doped layers adjacent to the contacts, minimizing injection barriers. The observed current-voltage characteristics follow the general theory for thermistors described by an Arrhenius-like conductivity law. Our findings include hysteresis phenomena and are of general relevance for the entire material class since most organic semiconductors can be described by a thermally activated conductivity.

/pdf/self-heating-bistability-and-thermal-switching-in-organic-4q9jb53ut3.pdf

Thomas Koprucki

Papers

Beyond just “flattening the curve”: Optimal control of epidemics with purely non-pharmaceutical interventions

Feel the heat: Nonlinear electrothermal feedback in organic LEDs

Drift-Diffusion Models

Self-heating, bistability, and thermal switching in organic semiconductors.

Self-heating, bistability, and thermal switching in organic semiconductors