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

Electrospinning uses electrical forces to produce polymer fibres with nanometre-scale diameters. Electrospinning occurs when the electrical forces at the surface of a polymer solution or melt overcome the surface tension and cause an electrically charged jet to be ejected. When the jet dries or solidifies, an electrically charged fibre remains. This charged fibre can be directed or accelerated by electrical forces and then collected in sheets or other useful geometrical forms. More than 20 polymers, including polyethylene oxide, nylon, polyimide, DNA, polyaramid, and polyaniline, have been electrospun in our laboratory. Most were spun from solution, although spinning from the melt in vacuum and air was also demonstrated. Electrospinning from polymer melts in a vacuum is advantageous because higher fields and higher temperatures can be used than in air.

https://iopscience.iop.org/article/10.1088/0957-4484/7/3/009/pdf

Nanometre diameter fibres of polymer, produced by electrospinning

A review was presented to demonstrate a historical description of the synthesis of light-emitting conjugated polymers for applications in electroluminescent devices. Electroluminescence (EL) was first reported in poly(para-phenylene vinylene) (PPV) in 1990 and researchers continued to make significant efforts to develop conjugated materials as the active units in light-emitting devices (LED) to be used in display applications. Conjugated oligomers were used as luminescent materials and as models for conjugated polymers in the review. Oligomers were used to demonstrate a structure and property relationship to determine a key polymer property or to demonstrate a technique that was to be applied to polymers. The review focused on demonstrating the way polymer structures were made and the way their properties were controlled by intelligent and rational and synthetic design.

Synthesis of Light-Emitting Conjugated Polymers for Applications in Electroluminescent Devices

Although it has long been recognized that dynamics in supercooled liquids might be spatially heterogeneous, only in the past few years has clear evidence emerged to support this view. As a liquid is cooled far below its melting point, dynamics in some regions of the sample can be orders of magnitude faster than dynamics in other regions only a few nanometers away. In this review, the experimental work that characterizes this heterogeneity is described. In particular, the following questions are addressed: How large are the heterogeneities? How long do they last? How much do dynamics vary between the fastest and slowest regions? Why do these heterogeneities arise? The answers to these questions influence practical applications of glass-forming materials, including polymers, metallic glasses, and pharmaceuticals.

Spatially Heterogeneous Dynamics in Supercooled Liquids

Intermolecular And Surface Forces

Scanning tunneling microscopy (STM) studies at the interface between the basal plane of graphite and organic solutions or melts of long chain alkanes and alkyl derivatives reveal that the molecules order in lamellae with the main molecular axes oriented parallel to the substrate. Here we employ molecular dynamics (MD) simulations to obtain more details on the molecular order and dynamics within the alkane lamellae as a function of density. We find that the orientation of the molecular carbon zigzag planes relative to the graphite is governed by a subtle interplay of packing and entropic effects. In addition, we consider multiple layer adsorption and investigate the rapid loss of order with increasing distance from the interface. Finally, we study the diffusive behavior of an isolated long chain alkane, C350H702, on graphite, which is of interest in the context of STM imaging of isolated macromolecules at interfaces. The sensitive dependence on atomic parameters renders MD simulations a valuable complement...

Molecular dynamics simulations of ordered alkane chains physisorbed on graphite

Molten salts are used as heat transfer fluids and for short-term heat energy storage in solar power plants. Experiments show that the specific heat capacity of the base salt may be significantly enhanced by adding small amounts of certain nanoparticles. This effect, which is technically interesting and economically important, is not yet understood. This paper presents a critical discussion of the existing attendant experimental literature and the phenomenological models put forward thus far. A common assumption, the existence of nanolayers surrounding the nanoparticles, which are thought to be the source of, in some cases, the large increase of a nanofluid’s specific heat capacity is criticized and a different model is proposed. The model assumes that the influence of the nanoparticles in the surrounding liquid is of long range. The attendant long-range interfacial layers may interact with each other upon increase of nanoparticle concentration. This can explain the specific heat maximum observed by different groups, for which no other theoretical explanation appears to exist.

/pdf/on-the-specific-heat-capacity-enhancement-in-nanofluids-4qqgkp7jxk.pdf

On the specific heat capacity enhancement in nanofluids

We carry out molecular-dynamics simulations of the water–NaCl(100) interface. The study includes the bulk interface at T=298 K and thin physisorbed films for coverages ranging from 0 to 1.5 at T=140 K. We use an efficient SPC/E based fluctuating charge water model to account for polarization effects. The water model is tested calculating cluster, gas, and liquid phase properties of neat water as well as structural and dynamic properties of solutions containing Na+-, Cl−-, and Ca2+-ions. For the bulk water–NaCl(100) system we analyze the surface induced hydration shell structure as well as residence times and the dipole orientation autocorrelation near the surface. At the low temperature we calculate the coverage dependence of the isosteric heat of adsorption, which is compared to available experimental data, including the coverage dependence of the adsorbate structure and dynamics. We note that our simulations support the formation of the (1×1) superstructure seen in helium scattering experiments.

A molecular-dynamics simulation study of water on NaCl(100) using a polarizable water model

Isotropic, nematic, and columnar ordering in systems of persistent flexible hard rods.

The gas-isotropic liquid-nematic liquid phase behavior of the Stockmayer fluid is studied using molecular dynamics simulation together with a mean field lattice model. We obtain coexistence curves of the Stockmayer fluid over a wide range of dipole strengths, temperatures, and densities, including the transition from the isotropic liquid to the ferroelectric liquid. In our simulations we do not observe the disappearance of the isotropic gas-isotropic liquid coexistence at high dipole strength contrary to earlier findings based on Monte Carlo techniques. Even though the formation of reversible dipole chains strongly affects the location of the critical point, it does not lead to its disappearance. These results are supported by a mean field lattice model which yields good qualitative, and in parts quantitative, agreement with our simulations. In addition, we also investigate the gas-isotropic liquid phase behavior for different polarizabilities.

Reinhard Hentschke

Papers

Molecular dynamics simulations of ordered alkane chains physisorbed on graphite

On the specific heat capacity enhancement in nanofluids

A molecular-dynamics simulation study of water on NaCl(100) using a polarizable water model

Isotropic, nematic, and columnar ordering in systems of persistent flexible hard rods.

Phase behavior of the Stockmayer fluid via molecular dynamics simulation.