Direct correlation of oxygen adsorption on platinum-electrolyte interfaces with the activity in the oxygen reduction reaction

The oxygen reduction reaction (ORR) on platinum catalysts is essential in fuel cells. Quantitative predictions of the relative ORR activity in experiments, in the range of 1 to 50 times, have remained challenging because of incomplete mechanistic understanding and lack of computational tools to account for the associated small differences in activation energies (<2.3 kilocalories per mole). Using highly accurate molecular dynamics (MD) simulation with the Interface force field (0.1 kilocalories per mole), we elucidated the mechanism of adsorption of molecular oxygen on regular and irregular platinum surfaces and nanostructures, followed by local density functional theory (DFT) calculations. The relative ORR activity is determined by oxygen access to platinum surfaces, which greatly depends on specific water adlayers, while electron transfer occurs at a similar slow rate. The MD methods facilitate quantitative predictions of relative ORR activities of any platinum nanostructures, are applicable to other catalysts, and enable effective MD/DFT approaches.

A Review of Multiscale Computational Methods in Polymeric Materials

Polymeric materials display distinguished characteristics which stem from the interplay of phenomena at various length and time scales. Further development of polymer systems critically relies on a comprehensive understanding of the fundamentals of their hierarchical structure and behaviors. As such, the inherent multiscale nature of polymer systems is only reflected by a multiscale analysis which accounts for all important mechanisms. Since multiscale modelling is a rapidly growing multidisciplinary field, the emerging possibilities and challenges can be of a truly diverse nature. The present review attempts to provide a rather comprehensive overview of the recent developments in the field of multiscale modelling and simulation of polymeric materials. In order to understand the characteristics of the building blocks of multiscale methods, first a brief review of some significant computational methods at individual length and time scales is provided. These methods cover quantum mechanical scale, atomistic domain (Monte Carlo and molecular dynamics), mesoscopic scale (Brownian dynamics, dissipative particle dynamics, and lattice Boltzmann method), and finally macroscopic realm (finite element and volume methods). Afterwards, different prescriptions to envelope these methods in a multiscale strategy are discussed in details. Sequential, concurrent, and adaptive resolution schemes are presented along with the latest updates and ongoing challenges in research. In sequential methods, various systematic coarse-graining and backmapping approaches are addressed. For the concurrent strategy, we aimed to introduce the fundamentals and significant methods including the handshaking concept, energy-based, and force-based coupling approaches. Although such methods are very popular in metals and carbon nanomaterials, their use in polymeric materials is still limited. We have illustrated their applications in polymer science by several examples hoping for raising attention towards the existing possibilities. The relatively new adaptive resolution schemes are then covered including their advantages and shortcomings. Finally, some novel ideas in order to extend the reaches of atomistic techniques are reviewed. We conclude the review by outlining the existing challenges and possibilities for future research.

Mechanisms of subsurface damage and material removal during high speed grinding processes in Ni/Cu multilayers using a molecular dynamics study

Molecular dynamics simulation of Ni/Cu multilayers under grinding process with a diamond tip is performed, with the aim of investigating the subsurface damage and material removal in Ni/Cu multilayers.

A unified set of experimental organometallic data used to evaluate modern theoretical methods

We applied a test set of ligand dissociation enthalpies derived entirely from a unified experimental approach to evaluate the efficacy of various methods for modeling organometallic chemistry.

A methodology for coupling an atomic model with a continuum model using an extended Lagrange function

We propose a hybrid method combining an atomic model and a continuum model, in which the displacement field of the continuum is introduced as a new degree of freedom by extending Andersen's Lagrange function for constant-pressure molecular dynamics. We applied our method to a one-dimensional hybrid model which is composed of an atomic chain and springs. Large-scale fluctuation of the atomic system is found in the hybrid model. The density of states of the phonon is derived, and the large-scale fluctuation induces the generation of a variety of states of phonons. It is shown that the hybrid model proposed by our methodology enables us to perform large-scale simulations without intensive computations.

Multimillion Atom Simulations of Dynamics of Oxidation of an Aluminum Nanoparticle and Nanoindentation on Ceramics

We have developed a first-principles-based hierarchical simulation framework, which seamlessly integrates (1) a quantum mechanical description based on the density functional theory (DFT), (2) multilevel molecular dynamics (MD) simulations based on a reactive force field (ReaxFF) that describes chemical reactions and polarization, a nonreactive force field that employs dynamic atomic charges, and an effective force field (EFF), and (3) an atomistically informed continuum model to reach macroscopic length scales. For scalable hierarchical simulations, we have developed parallel linear-scaling algorithms for (1) DFT calculation based on a divide-and-conquer algorithm on adaptive multigrids, (2) chemically reactive MD based on a fast ReaxFF (F-ReaxFF) algorithm, and (3) EFF-MD based on a space-time multiresolution MD (MRMD) algorithm. On 1920 Intel Itanium2 processors, we have demonstrated 1.4 million atom (0.12 trillion grid points) DFT, 0.56 billion atom F-ReaxFF, and 18.9 billion atom MRMD calculations, with parallel efficiency as high as 0.953. Through the use of these algorithms, multimillion atom MD simulations have been performed to study the oxidation of an aluminum nanoparticle. Structural and dynamic correlations in the oxide region are calculated as well as the evolution of charges, surface oxide thickness, diffusivities of atoms, and local stresses. In the microcanonical ensemble, the oxidizing reaction becomes explosive in both molecular and atomic oxygen environments, due to the enormous energy release associated with Al-O bonding. In the canonical ensemble, an amorphous oxide layer of a thickness of approximately 40 angstroms is formed after 466 ps, in good agreement with experiments. Simulations have been performed to study nanoindentation on crystalline, amorphous, and nanocrystalline silicon nitride and silicon carbide. Simulation on nanocrystalline silicon carbide reveals unusual deformation mechanisms in brittle nanophase materials, due to coexistence of brittle grains and soft amorphous-like grain boundary phases. Simulations predict a crossover from intergranular continuous deformation to intragrain discrete deformation at a critical indentation depth.

Concurrent Coupling of Electronic-Density-Functional, Molecular Dynamics, and Coarse-Grained Particles Schemes for Multiscale Simulation of Nanostructured Materials

Feature sizes of useful electronic devices are becoming smaller and reaching nanometer ranges. There is increasing demand to perform dynamic simulations of such nano-devices with realistic sizes. To date, various kinds of simulation methods have been used for materials and devices including the density-functional theory (DFT) and the molecular dynamics (MD) for atomistic mechanics and the finite element method for continuum mechanics. We review recent progresses in our multiscale, hybrid simulation schemes that combine those methods. The coarse-grained particles (CG) method originally proposed by Rudd and Broughton [Phys. Rev. B58 (1998), p. R5893] has features suitable to such hybridization. We improve the CG method so that it is applicable to realistic nanostructured materials with large deformations. A novel hybridization scheme that couples the DFT method with the MD method is presented, which is applicable to virtually any selection of the DFT region in a wide range of materials. Hybrid DFT-MD simulations of the H2O reaction with nanostructured Si and alumina systems under stresses are performed, to demonstrate significant effects of stress on the chemical reaction.