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A new software package, RASPA, for simulating adsorption and diffusion of molecules in flexible nanoporous materials is presented. The code implements the latest state-of-the-art algorithms for molecular dynamics and Monte Carlo (MC) in various ensembles including symplectic/measure-preserving integrators, Ewald summation, configurational-bias MC, continuous fractional component MC, reactive MC and Baker's minimisation. We show example applications of RASPA in computing coexistence properties, adsorption isotherms for single and multiple components, self-and collective diffusivities, reaction systems and visualisation. The software is released under the GNU General Public License.

The thermal-expansion properties [1] of substances are very important in materials design; for example, cracks form when joined materials expand or contract by different amounts upon heating. The most famous example of a substance that contracts when heated is ice: it transforms into water, which has a higher density than ice. Negative thermal expansion (NTE) in solids is relatively rare, although examples have been found in zeolites.[2] The underlying physics of NTE remains poorly understood. Herein, we show, on the basis of molecular simulations, that the recently synthesized isoreticular metal-organic frameworks (IRMOFs) consistently have negative thermal-expansion coefficients and are by far the most contracting materials known. Our simulations point to two competing effects: a local effect, where all bond lengths increase with temperature, and a second long-range effect, where the thermal movement of the linker molecules leads to a shorter average distance between corners upon heating.MOFs are a new class of nanoporous materials that have good stability, large void volumes, and well-defined tailorable cavities of uniform size. Their potential appears great, because these are precisely the properties needed for catalysis, separations, and storage/release applications. [3] MOFs generally consist of metal or metal-oxygen vertices interconnected by rigid or semirigid organic molecules. A large variety of MOFs, featuring different linker molecules and different types of bonding between the vertices with the linkers, have been produced by various research groups. The specific examples shown in Figure 1 are IRMOFs developed by Yaghi and co-workers. [4][5][6][7][8] In general, the IRMOFs consist of zinc-oxygen complexes connected by carboxylate-terminated linkers, forming a three-dimensional lattice of cubic cavities.Molecular simulations of adsorption in MOFs have shown very good agreement with experiment, [9][10][11][12][13] and it is interesting to note that simulations of diffusion in MOFs preceded experiments by almost two years. [9,14] In addition to predicting macroscopic observables, simulations can also provide useful molecular-level insights. To systematically investigate the thermal properties of MOFs, we herein simulate the (cubic) structures of several IRMOFs of varying linker length. We obtain information about the unit-cell length (L) as a function of temperature and about adsorbate loadings (q) as a function of pressure. We show that the experimental data scattered in the literature (for different adsorbate loadings and temperatures) are, in fact, consistent, and we elucidate the different effects of temperature and loading on L at the microscopic level.Various models for MOF flexibility have recently appeared. [15][16][17] Our flexible framework model for IRMOF-1, IRMOF-10, and IRMOF-16 is described in the Supporting Information. It is similar in spirit to the model of Greathouse and Allendorf, [15] but differs in the treatment of the carboxylate group and has the advantage of being calibrated to ex...

Oxide surfaces are important for applications in catalysis and thin film growth. An important frontier in solid-state inorganic chemistry is the prediction of the surface structure of an oxide. Comparatively little is known about atomic arrangements at oxide surfaces at present, and there has been considerable discussion concerning the forces that control such arrangements. For instance, one model suggests that the dominant factor is a reduction of Coulomb forces; another favours minimization of 'dangling bonds' by charge transfer to states below the Fermi energy. The surface structure and properties of SrTiO(3)--a standard model for oxides with a perovskite structure--have been studied extensively. Here we report a solution of the 2 x 1 SrTiO(3) (001) surface structure obtained through a combination of high-resolution electron microscopy and theoretical direct methods. Our results indicate that surface rearrangement of TiO(6-x) units into edge-sharing blocks determines the SrO-deficient surface structure of SrTiO(3). We suggest that this structural concept can be extended to perovskite surfaces in general.

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