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year-2015

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Publications during 2015

Article Reference Reaction network of methanol synthesis over Cu/ZnO Nanocatalysts
The eciency of industrial methanol synthesis from syngas results from a complex scenario of surface chemical reactions in the presence of dynamical morphological changes of the catalyst material in response to the chemical and physical properties of the gas phase, which are believed to explain the superior performance of the Cu/ZnO catalyst. Yet, the applied conditions of elevated temperatures and pressures substantially hamper in situ experimental access and, therefore, detailed understanding of the underlying reaction mechanism(s) and active site(s). Here, part of this huge space of possibilities emerging from the structural and chemical congurations of both, adsorbates and continuously altering Cu/ZnO catalyst material, is successfully explored by pure computational means. Using our molecular dynamics approach to computational heterogeneous catalysis, being based on advanced ab initio simulations in conjunction with thermodynamically optimized catalyst models, the resulting mapping of the underlying free energy landscape discloses an overwhelmingly rich network of parallel, competing and reverse reaction channels. After having analyzed various pathways that directly lead from CO2 to methanol, not only specic Cu/ZnO interface sites but also the near surface region over the catalyst surface were identied as key to some pivotal reaction steps in the global reaction network. Analysis of the mechanistic details and electronic structure along individual steps unveils three distinct mechanisms of surface chemical reactions being all at work, namely Eley-Rideal, Langmuir-Hinshelwood, and Mars-van-Krevelen. Importantly, the former and latter mechanisms can only be realized upon including systematically the near surface region and dynamical transformations of catalyst sites, respectively, in the reaction space throughout all simulations.
Article Reference Utilizing fast multipole expansions for efficient and accurate quantum-classical molecular dynamics simulations
Recently, a novel approach to hybrid quantum mechanics/molecular mechanics (QM/MM) molecular dynamics (MD) simulations has been suggested [Schwörer et al., J. Chem. Phys. 138, 244103 (2013)]. Here, the forces acting on the atoms are calculated by grid-based density functional theory (DFT) for a solute molecule and by a polarizable molecular mechanics (PMM) force field for a large solvent environment composed of several 103-105 molecules as negative gradients of a DFT/PMM hybrid Hamiltonian. The electrostatic interactions are efficiently described by a hierarchical fast multipole method (FMM). Adopting recent progress of this FMM technique [Lorenzen et al., J. Chem. Theory Comput. 10, 3244 (2014)], which particularly entails a strictly linear scaling of the computational effort with the system size, and adapting this revised FMM approach to the computation of the interactions between the DFT and PMM fragments of a simulation system, here, we show how one can further enhance the efficiency and accuracy of such DFT/PMM-MD simulations. The resulting gain of total performance, as measured for alanine dipeptide (DFT) embedded in water (PMM) by the product of the gains in efficiency and accuracy, amounts to about one order of magnitude. We also demonstrate that the jointly parallelized implementation of the DFT and PMM-MD parts of the computation enables the efficient use of high-performance computing systems. The associated software is available online.
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