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

We demonstrate that a monolayer graphene membrane is impermeable to standard gases including helium. By applying a pressure difference across the membrane, we measure both the elastic constants and the mass of a single layer of graphene. This pressurized graphene membrane is the world's thinnest balloon and provides a unique separation barrier between 2 distinct regions that is only one atom thick.

/pdf/impermeable-atomic-membranes-from-graphene-sheets-xtis5huy2d.pdf

Impermeable atomic membranes from graphene sheets.

SINCE the invention of the scanning tunnelling microscope1, the value of establishing a physical connection between the macroscopic world and individual nanometre-scale objects has become increasingly evident, both for probing these objects2–4 and for direct manipulation5–7 and fabrication8–10 at the nanometre scale. While good progress has been made in controlling the position of the macroscopic probe of such devices to sub-angstrom accuracy, and in designing sensitive detection schemes, less has been done to improve the probe tip itself4. Ideally the tip should be as precisely defined as the object under investigation, and should maintain its integrity after repeated use not only in high vacuum but also in air and water. The best tips currently used for scanning probe microscopy do sometimes achieve sub-nanometre resolution, but they seldom survive a 'tip crash' with the surface, and it is rarely clear what the atomic configuration of the tip is during imaging. Here we show that carbon nanotubes11,12 might constitute well defined tips for scanning probe microscopy. We have attached individual nanotubes several micrometres in length to the silicon cantilevers of conventional atomic force microscopes. Because of their flexibility, the tips are resistant to damage from tip crashes, while their slenderness permits imaging of sharp recesses in surface topography. We have also been able to exploit the electrical conductivity of nanotubes by using them for scanning tunnelling microscopy.

Nanotubes as nanoprobes in scanning probe microscopy

The Interaction of Water with Solid Surfaces: Fundamental Aspects Revisited

A concept for molecular electronics exploiting carbon nanotubes as both molecular device elements and molecular wires for reading and writing information was developed. Each device element is based on a suspended, crossed nanotube geometry that leads to bistable, electrostatically switchable ON/OFF states. The device elements are naturally addressable in large arrays by the carbon nanotube molecular wires making up the devices. These reversible, bistable device elements could be used to construct nonvolatile random access memory and logic function tables at an integration level approaching 10 12 elements per square centimeter and an element operation frequency in excess of 100 gigahertz. The viability of this concept is demonstrated by detailed calculations and by the experimental realization of a reversible, bistable nanotube-based bit.

/pdf/carbon-nanotube-based-nonvolatile-random-access-memory-for-2m51guev4k.pdf

Carbon nanotube-based nonvolatile random access memory for molecular computing

Summary Abstract: Classical trajectory calculations of photon‐stimulated desorption of ions from alkali–halides

Laser Spectroscopy of Molecular Ions in Plasmas

We discuss specific examples of desorption due to electron or photon bombardment of solids, in which the repulsive interactions that result in particle expulsion can be clearly identified. A particularly interesting example is the desorption of positive ions from alkali-halides. Classical trajectory calculations are used to provide a picture of the atomic-scale motion during desorption. The results show that dynamic distortions of the lattice play a crucial role in the desorption process.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S1946427400548222

Dynamics of Electron and Photon Stimulated Desorption

Diabatic potential energy curves are reported for ions interacting with metal surfaces. The calculations are based on a local density functional approach, which fully accounts for screening by the electrons within the metal valence band. Results for Ar+ are used to predict yields and energy distributions for Ar atoms which desorb upon ionization of physisorbed Ar. The calculated potentials provide a substantially improved description of the desorption process compared to previous models based on a classical image potential for the ion-surface interaction. Classical and quantal treatments of desorption are compared, and quantum effects in the dynamics of desorption are discussed.

Image-Screening of Positive Ions at Metal Surfaces, and Electronically Stimulated Desorption of Noble Gas Atoms

We present experimental results which suggest a new mechanism for the production of excited atoms and ions by electron bombardment of alkali-halides. Doppler shift measurements show that the electronically excited atoms have a thermal velocity distribution in equilibrium with the surface temperature. Measurements of the absolute yield of excited atoms, the distribution of population among the excited states, and the dependence of yield on incident electron current support a model in which excited atoms are produced by gas-phase collisions between desorbed ground-state atoms and secondary electrons. Similarly, gas-phase ionization of ground-state neutrals by secondary electrons accounts for a substantial portion of the positive ions produced by electron bombardment of alkali-halides.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S1946427400549987

Robert E. Walkup

Papers

Summary Abstract: Classical trajectory calculations of photon‐stimulated desorption of ions from alkali–halides

Laser Spectroscopy of Molecular Ions in Plasmas

Dynamics of Electron and Photon Stimulated Desorption

Image-Screening of Positive Ions at Metal Surfaces, and Electronically Stimulated Desorption of Noble Gas Atoms

Excited-Atom Production by Electron Bombardment of Alkali-Halides