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

▪ Abstract This review surveys the observed properties of interstellar dust grains: the wavelength-dependent extinction of starlight, including absorption features, from UV to infrared; optical lum...

https://www.ifa.hawaii.edu/users/reipurth/reviews/draine.pdf

Interstellar dust grains

IR emission spectra are calculated for dust heated by starlight, for mixtures of amorphous silicate and graphitic grains, including varying amounts of PAH particles. The models are constrained to reproduce the average Milky Way extinction curve. The calculations include the effects of single-photon heating. Updated IR absorption properties for the PAHs are presented that are consistent with observed emission spectra, including those newly obtained by Spitzer. We find a size distribution for the PAHs giving emission band ratios consistent with the observed spectra of the Milky Way and other galaxies. Emission spectra are presented for a wide range of starlight intensities. We calculate how the efficiency of emission into different IR bands depends on PAH size; the strong 7.7 μm emission feature is produced mainly by PAH particles containing Umin. We present graphical procedures using Spitzer IRAC and MIPS photometry to estimate the parameters qPAH, Umin, and γ, the fraction fPDR of the dust luminosity coming from photodissociation regions with U > 100, and the total dust mass Mdust.

https://iopscience.iop.org/article/10.1086/511055/pdf

Infrared Emission from Interstellar Dust. IV. The Silicate-Graphite-PAH Model in the Post-Spitzer Era

Large polycyclic aromatic hydrocarbon (PAH) molecules carry the infrared (IR) emission features that dominate the spectra of most galactic and extragalactic sources. This review surveys the observed mid-IR characteristics of these emission features and summarizes laboratory and theoretical studies of the spectral characteristics of PAHs and the derived intrinsic properties of emitting interstellar PAHs. Dedicated experimental studies have provided critical input for detailed astronomical models that probe the origin and evolution of interstellar PAHs and their role in the universe. The physics and chemistry of PAHs are discussed, emphasizing the contribution of these species to the photoelectric heating and the ionization balance of the interstellar gas and to the formation of small hydrocarbon radicals and carbon chains. Together, these studies demonstrate that PAHs are abundant, ubiquitous, and a dominant force in the interstellar medium of galaxies.

Interstellar Polycyclic Aromatic Hydrocarbon Molecules

We present a quantitative model for the infrared emission from dust in the diffuse interstellar medium. The model consists of a mixture of amorphous silicate grains and carbonaceous grains, each with a wide size distribution ranging from molecules containing tens of atoms to large grains 1 μm in diameter. We assume that the carbonaceous grains have properties like polycyclic aromatic hydrocarbons (PAHs) at very small sizes and graphitic properties for radii a 50 A. On the basis of recent laboratory studies and guided by astronomical observations, we propose astronomical absorption cross sections for use in modeling neutral and ionized PAHs from the far-ultraviolet to the far-infrared. We also propose modifications to the far-infrared emissivity of astronomical silicate. We calculate energy distribution functions for small grains undergoing temperature spikes caused by stochastic absorption of starlight photons using realistic heat capacities and optical properties. Using a grain-size distribution consistent with the observed interstellar extinction, we are able to reproduce the near-IR to submillimeter emission spectrum of the diffuse interstellar medium, including the PAH emission features at 3.3, 6.2, 7.7, 8.6, and 11.3 μm. The model is compared with the observed emission at high Galactic latitudes as well as in the Galactic plane, as measured by the COBE/DIRBE, COBE/FIRAS, IRTS/MIRS, and IRTS/NIRS instruments. The model has 60 × 10-6 of C (relative to H) locked up in PAHs, with 45 × 10-6 of C in a component peaking at ~6 A (NC ≈ 100 carbon atoms) to account for the PAH emission features and with 15 × 10-6 of C in a component peaking at ~50 A to account for the 60 μm flux. The total infrared emission is in excellent agreement with COBE/DIRBE observations at high Galactic latitudes, just as the albedo for our grain model is in accord with observations of the diffuse Galactic light. The aromatic absorption features at 3.3 and 6.2 μm predicted by our dust model are consistent with observations. We calculate infrared emission spectra for our dust model heated by a range of starlight intensities, from 0.3 to 104 times the local interstellar radiation field, and we tabulate the intensities integrated over the SIRTF/IRAC and MIPS bands. We also provide dust opacities tabulated from the extreme-ultraviolet to submillimeter wavelengths.

https://iopscience.iop.org/article/10.1086/323147/pdf

Infrared Emission from Interstellar Dust. II. The Diffuse Interstellar Medium

Theorie de Duley sur les bandes interstellaires diffuses observees entre 677 nm et 542 nm et reponse aux arguments contraires donnes par Nuth et Donn

Vibronic origin for the diffuse band spectrum

Grain temperatures in diffuse clouds and the depletion of interstellar argon

The importance of chemically-active geometrical surface sites on the surfaces of interstellar oxide grains is discussed in terms of their catalytic behaviour in molecular formation. Surface features — such as steps and corners — are considered in detail, and the binding energies of the hydrogen atom and the proton at these sites are estimated. It appears likely that chemical effects of interstellar oxide grains are located not only at point defects discussed elsewhere, but also at these geometrical surface features. Adsorbed surface species, surface reaction schemes, and the possible effects of grain impurities are briefly considered.

Chemical effects of irregular interstellar oxide grains

The invention relates to a moisture generator and a preparation method thereof. The moisture generator comprises a substrate, a first electrode, a nanowire layer and a second electrode which are arranged in sequence, wherein the nanowire layer is arranged between the first electrode and the second electrode, and formed by nanowires in random distribution. The invention also provides a preparationmethod of the moisture generator. The moisture generator provided by the invention adopts a network structured formed by the nanowires, wherein the structure has high hydrophilicity, and a large number of nanometer gaps therein can facilitate moisture diffusion, so that the moisture generator can be relatively high in power output density; and when a substrate of the moisture generator is preparedfrom a flexible material, and when the nanowire layer network structure with high flexibility is matched, the moisture generator can be high in mechanical flexibility and can be pasted on human bodyskin to monitor the human body respiration state in real time, or the moisture generator can be used as a wearable touch electronic screen.

Moisture generator and preparation method thereof

La luminescence des bandes larges emise par des carbones amorphes hydrogenes solides est etudiee en laboratoire. Une comparaison des donnees experimentales avec les observations de poussieres de nuages diffus est realisee

/pdf/very-broad-band-structure-and-emission-by-hydrogenated-10dxcq3s1g.pdf

Walter W. Duley

Papers

Vibronic origin for the diffuse band spectrum

Grain temperatures in diffuse clouds and the depletion of interstellar argon

Chemical effects of irregular interstellar oxide grains

Moisture generator and preparation method thereof

Very broad-band structure and emission by hydrogenated amorphous carbon