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

J. Appl. Cryst.の発刊に際して

We show nanoscale phase stabilization of CsPbI 3 quantum dots (QDs) to low temperatures that can be used as the active component of efficient optoelectronic devices. CsPbI 3 is an all-inorganic analog to the hybrid organic cation halide perovskites, but the cubic phase of bulk CsPbI 3 (α-CsPbI 3 )—the variant with desirable band gap—is only stable at high temperatures. We describe the formation of α-CsPbI 3 QD films that are phase-stable for months in ambient air. The films exhibit long-range electronic transport and were used to fabricate colloidal perovskite QD photovoltaic cells with an open-circuit voltage of 1.23 volts and efficiency of 10.77%. These devices also function as light-emitting diodes with low turn-on voltage and tunable emission.

http://science.sciencemag.org/content/sci/354/6308/92.full.pdf

Quantum dot-induced phase stabilization of α-CsPbI3 perovskite for high-efficiency photovoltaics.

The aim of this critical review is to provide a broad but digestible overview of mechanochemical synthesis, i.e. reactions conducted by grinding solid reactants together with no or minimal solvent. Although mechanochemistry has historically been a sideline approach to synthesis it may soon move into the mainstream because it is increasingly apparent that it can be practical, and even advantageous, and because of the opportunities it provides for developing more sustainable methods. Concentrating on recent advances, this article covers industrial aspects, inorganic materials, organic synthesis, cocrystallisation, pharmaceutical aspects, metal complexes (including metal–organic frameworks), supramolecular aspects and characterization methods. The historical development, mechanistic aspects, limitations and opportunities are also discussed (314 references).

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Mechanochemistry: opportunities for new and cleaner synthesis

A set of general guidelines for structure refinement using the Rietveld (whole-profile) method has been formulated by the International Union of Crystallography Commission on Powder Diffraction. The practical rather than the theoretical aspects of each step in a typical Rietveld refinement are discussed with a view to guiding newcomers in the field. The focus is on X-ray powder diffraction data collected on a laboratory instrument, but features specific to data from neutron (both constant-wavelength and time-of-flight) and synchrotron radiation sources are also addressed. The topics covered include (i) data collection, (ii) background contribution, (iii) peak-shape function, (iv) refinement of profile parameters, (v) Fourier analysis with powder diffraction data, (vi) refinement of structural parameters, (vii) use of geometric restraints, (viii) calculation of e.s.d.'s, (ix) interpretation of R values and (x) some common problems and possible solutions.

/pdf/rietveld-refinement-guidelines-e684kr9ri6.pdf

Rietveld refinement guidelines

A new procedure for simultaneous refinement of structural and microstructural disorder parameters for polycrystalline materials is proposed. It is based on the Rietveld method combined with Fourier analysis for broadened peaks. Crystallite size and shape and r.m.s. microstrain are regarded as fitting parameters, replacing the well known formula of Caglioti, Paoletti & Ricci [Nucl. Instrum. Methods (1958), 3, 223–228] for the angular dependence of the peak width. In particular, from these microstructural disorder parameters, by inverting the Warren–Averbach procedure [Warren & Averbach (1950). J. Appl. Phys. 21, 595–599; (1952), 23, 1059] for a single peak, it is possible to obtain the parameters of the pseudo-Voigt (pV) functions employed to fit the experimental data. The anisotropy of the crystallite size and microstrain is also taken into account. The method has been tested on three materials with different degrees of crystallization: tetragonal ZrO2 (P42/nmc, a = 3.5961, c = 5.1770 A, Vc = 66.95 A3, Z = 2, Rwp = 0.077, M ≃100 A, 〈∊2〉1/2 ≃ 3 × 10−3); tetragonal Zr0.82Ce0.18O2 (P42/nmc, a = 3.6419, c = 5.2440 A, Vc = 69.556 A3, Z = 2, Rwp = 0.0654, M ≃1000 A, 〈∊2〉1/2 ≃8 × 10−4); α-Al2O3 (R{\bar 3}c, a = 4.7605, c = 12.9956 A, Vc = 255.05 A3, Z = 6, Rwp = 0.0684, M ≃ 1400 A, 〈∊2〉1/2 ≃7 × 10−4).

Simultaneous structure and size-strain refinement by the Rietveld method

A new approach for the modelling of diffraction patterns without using analytical profile functions is described and tested on ball milled f.c.c. Ni powder samples. The proposed whole powder pattern modelling (WPPM) procedure allows a one-step refinement of microstructure parameters by a direct modelling of the experimental pattern. Lattice parameter and defect content, expressed as dislocation density, outer cut-off radius, contrast factor, twin and deformation fault probabilities), can be refined together with the parameters (mean and variance) of a grain-size distribution. Different models for lattice distortions and domain size and shape can be tested to simulate or model diffraction data for systems as different as plastically deformed metals or finely dispersed crystalline powders. TEM pictures support the conclusions obtained by WPPM and confirm the validity of the proposed procedure.

Whole powder pattern modelling.

1 Line Profile Analysis: A Historical Overview- 2 Convolution Based Profile Fitting- 3 Whole Powder Pattern Modelling: Theory and Applications- 4 Full Profile Analysis of X-ray Diffraction Patterns for Investigation of Nanocrystalline Systems- 5 Crystallite Size and Residual Strain/Stress Modeling in Rietveld Refinement- 6 The Quantitative Determination of the Crystalline and the Amorphous Content by the Rietveld Method: Application to Glass Ceramics with Different Absorption Coefficients- 7 Quantitative Analysis of Amorphous Fraction in the Study of the Microstructure of Semi-crystalline Materials- 8 A Bayesian/Maximum Entropy Method for the Certification of a Nanocrystallite-Size NIST Standard Reference Material- 9 Study of Submicrocrystalline Materials by Diffuse Scattering in Transmitted Wave- 10 Determining the Dislocation Contrast Factor for X-ray Line Profile Analysis- 11 X-ray Peak Broadening Due to Inhomogeneous Dislocation Distributions- 12 Determination of Non-uniform Dislocation Distributions in Polycrystalline Materials- 13 Line Profile Fitting: The Case of fcc Crystals Containing Stacking Faults- 14 Diffraction Elastic Constants and Stress Factors Grain Interaction and Stress in Macroscopically Elastically Anisotropic Solids The Case of Thin Films- 15 Interaction between Phases in Co-deforming Two-Phase Materials: The Role of Dislocation Arrangements- 16 Grain Surface Relaxation Effects in Powder Diffraction- 17 Interface Stress in Polycrystalline Materials- 18 Problems Related to X-Ray Stress Analysis in Thin Films in the Presence of Gradients and Texture- 19 Two-Dimensional XRD Profile Modelling in Imperfect Epitaxial Layers- 20 Three-Dimensional Reciprocal Space Mapping: Application to Polycrystalline CVD Diamond

Diffraction analysis of the microstructure of materials

Integral breadth methods for line profile analysis are reviewed, including modifications of the Williamson–Hall method recently proposed for the specific case of dislocation strain broadening. Two cases of study, supported by the results of a TEM investigation, are considered in detail: nanocrystalline ceria crystallized from amorphous precursors and highly deformed nickel powder produced by extensive ball milling. A further application concerns a series of Fe–Mo powder specimens that were ball milled for increasing time. Traditional and modified Williamson–Hall methods confirm their merits for a rapid overview of the line broadening effects and possible understanding of the main causes. However, quantitative results are generally not reliable. Limits in the applicability of integral breadth methods and reliability of the results are discussed in detail.

Paolo Scardi

Papers

Rietveld refinement guidelines

Simultaneous structure and size-strain refinement by the Rietveld method

Whole powder pattern modelling.

Diffraction analysis of the microstructure of materials

Line broadening analysis using integral breadth methods: a critical review