We report on implementation and performance of the program IMD, designed for short range molecular dynamics simulations on massively parallel computers. After a short explanation of the cell-based algorithm, its extension to parallel computers as well as two variants of the communication scheme are discussed. We provide performance numbers for simulations of different sizes and compare them with values found in the literature. Finally we describe two applications, namely a very large scale simulation with more than 1.23×109 atoms, to our knowledge the largest published MD simulation up to this day and a simulation of a crack propagating in a two-dimensional quasicrystal.

Crack propagation is studied in a two dimensional decagonal model quasicrystal. The simulations reveal the dominating role of highly coordinated atomic environments as structure intrinsic obstacles for both dislocation motion and crack propagation. For certain overloads, these obstacles and the quasiperiodic nature of the crystal result in a specific crack propagation mechanism: The crack tip emits a dislocation followed by a phason wall, along which the material opens up.

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A dislocation moving through a quasicrystal leaves in its wake a fault denoted phason wall. For a two-dimensional model quasicrystal the disregistry energy of this phason wall is studied to determine possible Burgers vectors of the quasicrystalline structure. Unlike periodic crystals, the disregistry energy is an average quantity with large fluctuations on the atomic scale. Therefore the dislocation core structure and mobility cannot be linked to this quantity e.g. by a Peierls-Nabarro model. Atomistic simulations show that dislocation motion is controlled by local obstacles inherent to the atomic structure of the quasicrystal. 61.44.+p,62.20Fe In the last five years it has become possible to grow thermodynamically stable quasicrystals of high structural quality and sizes [1] which have allowed one to measure properties like plasticity and fracture. The data have initiated a wealth of studies, in which the influence of quasiperiodicity on physical properties is being explored. Simultaneously, potential applications of quasicrystals have been discussed, for example as friction reducing, oxidation resistant, non-adhesive coatings, as solar converters or for hydrogen storage [2].The structure of quasicrystals is also responsible for special mechanical properties. On this subject an impressive set of experimental results is available: i) At room temperature quasicrystals are hard as silicon and extremely brittle [3]. ii) At about 80% of the melting temperature they become plastically deformable up to 30% and soften upon straining [4]. iii) Stress relaxation experiments reveal unusually high values for the activation enthalpy and for the activation volume [5]. iv) For AlMnPd it has been proven that the deformation is governed by dislocation motion [6].In this letter we provide atomistic insight into the mechanical behavior of quasicrystals. It will turn out that two structural features govern the motion of dislocations: the faults in the wake of dislocations and cluster-like obstacles in the glide planes.Quasiperiodic structures can be obtained as cuts of an irrationally oriented n-dimensional hyperplane (physical space) through a d-dimensional periodic crystal [7], where d > n is the number of incommensurate length scales [8]. Due to the periodicity of the hyper-crystal, dislocations also exist in quasicrystals [9]. The d-dimensional Burgers vectors carry an n-dimensional component b || in physical space and a (d − n)-dimensional component b ⊥ in the complementing orthogonal space. A moving dislocation in a two-dimensional quasicrystal leaves in its wake a phason-wall, separating two areas with a phase difference b ⊥ , which gives rise to faults in the pattern and therefore differences in the atomic environments compared to the perfect quasicrystal. Atomic jumps move these faults and the wall may consequently vanish by diffusive processes.Recently, we performed numerical simulations of plastic deformation [10,11] on a two-dimensional binary model quasicrystal displayed in Fig. 1. We observed that the phason-wall r...

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