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

To use solar irradiation or interior lighting efficiently, we sought a photocatalyst with high reactivity under visible light. Films and powders of TiO 2- x N x have revealed an improvement over titanium dioxide (TiO 2 ) under visible light (wavelength 2 has proven to be indispensable for band-gap narrowing and photocatalytic activity, as assessed by first-principles calculations and x-ray photoemission spectroscopy.

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Visible-Light Photocatalysis in Nitrogen-Doped Titanium Oxides

Titanium Dioxide Nanomaterials: Synthesis, Properties, Modifications, and Applications

This critical review shows the basis of photocatalytic water splitting and experimental points, and surveys heterogeneous photocatalyst materials for water splitting into H2 and O2, and H2 or O2 evolution from an aqueous solution containing a sacrificial reagent Many oxides consisting of metal cations with d0 and d10 configurations, metal (oxy)sulfide and metal (oxy)nitride photocatalysts have been reported, especially during the latest decade The fruitful photocatalyst library gives important information on factors affecting photocatalytic performances and design of new materials Photocatalytic water splitting and H2 evolution using abundant compounds as electron donors are expected to contribute to construction of a clean and simple system for solar hydrogen production, and a solution of global energy and environmental issues in the future (361 references)

Heterogeneous photocatalyst materials for water splitting

2.3. Evaluation of Photocatalytic Water Splitting 6507 2.3.1. Photocatalytic Activity 6507 2.3.2. Photocatalytic Stability 6507 3. UV-Active Photocatalysts for Water Splitting 6507 3.1. d0 Metal Oxide Photocatalyts 6507 3.1.1. Ti-, Zr-Based Oxides 6507 3.1.2. Nb-, Ta-Based Oxides 6514 3.1.3. W-, Mo-Based Oxides 6517 3.1.4. Other d0 Metal Oxides 6518 3.2. d10 Metal Oxide Photocatalyts 6518 3.3. f0 Metal Oxide Photocatalysts 6518 3.4. Nonoxide Photocatalysts 6518 4. Approaches to Modifying the Electronic Band Structure for Visible-Light Harvesting 6519

Semiconductor-based Photocatalytic Hydrogen Generation

Recent theoretical and experimental evaluations demonstrated that KLi 6 TaO 6 is a good Li-ion conductor. In this study, the energetics and detailed mechanism of Li-ion migration, relevant to the point defects of KLi 6 TaO 6 , were analyzed by performing first-principles calculations. The defect formation energy for a Li vacancy was found to be lower than that for a substitutional defect, which could hinder the achievement of a Li-excess condition, and the interactions between substitutional defects and interstitial Li could raise the migration energy barrier of interstitial Li. These results indicate the limiting factors for achieving high Li-ion conductivity in this material. Based on the insights gained from the investigations on KLi 6 TaO 6 , isostructural materials of the form A Li 6 X O 6 , with various combinations of constituent elements A and X , were each screened on the basis of high stability and low Li-ion migration energy. Twelve structures of the form ( A = K or Rb)Li 6 X O 6 were suggested, of which X was pentavalent. Among them, (K or Rb)Li 6 VO 6 and (K or Rb)Li 6 BiO 6 exhibited low energies for the formation of substitutional defects, which was sufficient to achieve Li-excess. Sn-doped KLi 6 VO 6 and KLi 6 BiO 6 were evaluated using first-principles molecular dynamics, and their Li-ion conductivities at room temperature were comparable with those of garnet-type Li-ion conductors.

Doping Effect and Li-Ion Conduction Mechanism of ALi6XO6 (A = K or Rb and X = Pentavalent): A First-Principles Study

Abstract We examine the effect of isovalent substitutions and co-doping on the ionic dielectric constant of paraelectric titanates (perovskite, Ruddlesden-Popper phases, and rutile) using density functional perturbation theory. Substitutions increase the ionic dielectric constant of the prototype structures, and new dynamically stable structures with ε ion ~ 10 2 –10 4 are reported and analyzed. The boosting of ionic permittivity is attributed to local defect-induced strain, and maximum Ti–O bond length is proposed as a descriptor. The Ti–O phonon mode that is responsible for the large dielectric constant can be tuned by a local strain and symmetry lowering from substitutions. Our findings help explain the recently observed colossal permittivity in co-doped rutile, attributing its intrinsic permittivity boosting solely to the lattice polarization mechanism, without the need to invoke other mechanisms. Finally, we identify new perovskite- and rutile-based systems that can potentially display colossal permittivity. 

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Permittivity boosting by induced strain from local doping in titanates from first principles

Electronics, which harnesses the properties of electrons, has made remarkable progress since its inception and is a cornerstone of modern society. Ionics, which exploits the properties of ions, has also had a profound impact, as demonstrated by the award of the Nobel Prize in Chemistry in 2019 for achievements related to lithium-ion batteries (LIBs). Ionic conduction in solids is the flow of carrier ions through a solid owing to an electrical or chemical bias. Some ionic materials have been studied intensively because their ionic conductivities are higher than those of liquids, even though they are solids. Among various conductive species, fluoride ions are the most promising charge carriers for fluoride-ion batteries (FIBs) as post LIBs. Increasing fluoride-ion conductivity toward the superionic conductive region at room temperature would be a breakthrough for the room-temperature operation of all-solid-state FIBs. This review focuses on fluoride-ion conductors, from the general concept of ions to the characteristics of fluoride ions. Fluoride-ion conductors are classified according to material type and form, and our current understanding, identification of problems, and future directions are discussed from experimental and theoretical physics perspectives.

Ryoji Asahi

Papers

Doping Effect and Li-Ion Conduction Mechanism of ALi6XO6 (A = K or Rb and X = Pentavalent): A First-Principles Study

Permittivity boosting by induced strain from local doping in titanates from first principles

Challenges for fluoride superionic conductors: fundamentals, design, and applications