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Hierarchically combining semi-empirical methods and first-principles calculations we gain a novel and noteworthy picture of the molecular-level mechanisms that govern the water-gas-shift (WGS) and reverse water-gas-shift (r-WGS) reactions on Rh catalysts. Central to this picture is that the WGS and r-WGS follow two different dominant reaction mechanisms: WGS proceeds according to a carboxyl (COOH) mechanism, whereas r-WGS proceeds according to a redox (CO2 {\to} CO + O) mechanism. The obtained results furthermore underscore the danger of common first-principles analyses that focus on a priori selected dominant paths. Not restricted to such bias, our herein proposed hierarchical approach thus constitutes a promising avenue to properly transport and incorporate the ab initio predictive-quality to a new level of system complexity., 11 pages

Electronic structure calculations have emerged as a key contributor in modern heterogeneous catalysis research, though their application in chemical reaction engineering remains largely limited to academia. This perspective aims at encouraging the judicious use of first-principles kinetic models in industrial settings based on a critical discussion of present-day best practices, identifying existing gaps, and defining where further progress is needed.

Postsynthetic metal ion exchange in a benzotriazolate-based MFU-4l(arge) framework leads to a Co(II)-containing framework with open metal sites showing reversible gas-phase oxidation properties.

We present a detailed comparison of the S0, S1 (n -> ��*) and S2 (��-> ��*) potential energy surfaces (PESs) of the prototypical molecular switch azobenzene as obtained by Delta-self-consistent-field (Delta-SCF) Density-Functional Theory (DFT), time-dependent DFT (TD-DFT) and approximate Coupled Cluster Singles and Doubles (RI-CC2). All three methods unanimously agree in terms of the PES topologies, which are furthermore fully consistent with existing experimental data concerning the photo-isomerization mechanism. In particular, sum-method corrected Delta-SCF and TD-DFT yield very similar results for S1 and S2, when based on the same ground-state exchange-correlation (xc) functional. While these techniques yield the correct PES topology already on the level of semi-local xc functionals, reliable absolute excitation energies as compared to RI-CC2 or experiment require an xc treatment on the level of long-range corrected hybrids. Nevertheless, particularly the robustness of Delta-SCF with respect to state crossings as well as its numerical efficiency suggest this approach as a promising route to dynamical studies of larger azobenzene-containing systems., 25 pages, 6 figures

Density-functional theory is used to study the geometric and electronic structure of cationic Si16^+ clusters with a Ti, V or Cr dopant atom. Through unbiased global geometry optimization based on the basin-hopping approach we confirm that a Frank- Kasper polyhedron with the metal atom at the center represents the ground-state isomer for all three systems. The endohedral cage geometry is thus stabilized even though only VSi16^+ achieves electronic shell closure within the prevalent spherical potential model. Our analysis of the electronic structure traces this diminished role of shell closure for the stabilization back to the adaptive capability of the metal- Si bonding, which is more the result of a complex hybridization than the orginally proposed mere formal charge transfer. The resulting flexibility of the metal-Si bond can help to stabilize also "non-magic" cage-dopant combinations, which suggests that a wider range of materials may eventually be cast into this useful geometry for cluster-assembled materials., 13 pages including 5 figures; related publications can be found at http://www.fhi-berlin.mpg.de/th/th.html

Microkinetic modeling of surface chemical reactions still relies heavily on the mean-field based rate equation approach. This approach is expected to be most accurate for systems without appreciable lateral interactions among the adsorbed chemicals, and there in particular for the uniform adlayers resulting in poisoned regimes with predominant coverage of one species. Using first-principles kinetic Monte Carlo simulations and the CO oxidation at RuO(2)(110) as a showcase, we demonstrate that even in this limit mean-field rate equations fail to predict the catalytic activity by orders of magnitude. This deficiency is traced back to the inability to account for the vacancy pair formation that is kinetically driven by the ongoing reactions.

Diffusion and decay of alloyed $\mathrm{Cu}/\mathrm{Ag}$ islands are investigated in the size range from 1 to $40\text{ }\text{ }{\mathrm{nm}}^{2}$ on Ag(100) at room temperature with fast-scanning tunneling microscopy and density functional theory. While islands at sizes above $7\text{ }\text{ }{\mathrm{nm}}^{2}$ show the diffusion and decay behavior expected for dynamics based on single atom hopping, islands smaller than $4\text{ }\text{ }{\mathrm{nm}}^{2}$ diffuse faster and decay slower than predicted by standard theory. This anomalous behavior at unexpected large island sizes is related to a size dependent dealloying of the $\mathrm{Cu}/\mathrm{Ag}$ islands.

We compare the stability of various structures of high coverage self-assembled monolayers (SAMs) of short alkylthiolates, S(CH(2))(n-1)CH(3) (= C(n)), on Ag(111) and Au(111). We employ: (i) the ab initio thermodynamics approach based on density functional theory (DFT) calculations, to compare the stability of SAMs of C(1) (with coverages Θ = 3/7 and 1/3) on both substrates, and (ii) a set of pairwise interatomic potentials derived from second-order Møller-Plesset (MP2) perturbation theory calculations, to estimate the role of chain-chain (Ch-Ch) interactions in the structure and stability of SAMs of longer chain alkylthiolates. For C(1)/Ag(111) (C(1)/Au(111)) the SAM with Θ = 3/7 is more (less) stable than for Θ = 1/3 in a wide range of temperatures and pressures in line with experiments. In addition, for the molecular densities of SAMs corresponding to Θ = 3/7 and 1/3, the MP2-based Ch-Ch interaction potential also predicts the different chain orientations observed experimentally in SAMs of alkylthiolates on Ag(111) and Au(111). Thus, for short length alkylthiolates, a simple model based on first principles calculations that separately accounts for molecule-surface (M-S) and Ch-Ch interactions succeeds in predicting the main structural differences between the full coverage SAMs usually observed experimentally on Ag(111) and Au(111).

We present a comprehensive density functional theory study addressing the adsorption, dissociation, and successive diffusion of water molecules on the two regular terminations of ${\text{SrTiO}}_{3}(001)$. Combining the obtained supercell-geometry converged energetics within a first-principles thermodynamics framework we are able to reproduce the experimentally observed hydroxylation of the SrO-termination already at lowest background humidity, whereas the ${\text{TiO}}_{2}$-termination stays free of water molecules in the regime of low water partial pressures. This different behavior is traced back to the effortless formation of energetically very favorable hydroxyl-pairs on the prior termination. Contrary to the prevalent understanding our calculations indicate that at low coverages also the less water-affine ${\text{TiO}}_{2}$-termination can readily decompose water, with the often described molecular state only stabilized toward higher coverages.

We present large-scale density-functional theory (DFT) calculations and temperature programmed desorption measurements to characterize the structural, energetic and vibrational properties of the functionalized molecular switch 3,3',5,5'-tetra-tert-butyl-azobenzene (TBA) adsorbed at Au(111). Particular emphasis is placed on exploring the accuracy of the semi-empirical dispersion correction approach to semi-local DFT (DFT-D) in accounting for the substantial van der Waals component in the surface chemical bond. In line with previous findings for benzene and pure azobenzene at coinage metal surfaces, DFT-D significantly overbinds the molecule, but seems to yield an accurate adsorption geometry as far as can be judged from the experimental data. Comparing the trans adsorption geometry of TBA and azobenzene at Au(111) reveals a remarkable insensitivity of the structural and vibrational properties of the -N=N- moiety. This questions the established view of the role of the bulky tert-butyl-spacer groups for the switching of TBA in terms of a mere geometric decoupling of the photochemically active diazo-bridge from the gold substrate., 9 pages including 6 figures; related publications can be found at http://www.fhi-berlin.mpg.de/th/th.html

Endohedrally doped Si20 fullerenes appear as appealing building blocks for nanoscale materials. We investigate their structural stability with an unbiased and systematic global geometry optimization method within density-functional theory. For a wide range of metal doping atoms, it was sufficient to explore the Born Oppenheimer surface for only a moderate number of local minima to find structures that clearly differ from the initial endohedral cages, but are considerably more favorable in terms of energy. Previously proposed structures are thus all metastable., 4 pages, 1 figure

Optical properties and the geometric structure of self-assembled monolayers of azobenzene-functionalized alkanethiols have been investigated by UV/visible and near edge X-ray absorption fine structure spectroscopy in combination with density-functional theory. By attaching a trifluoro-methyl end group to the chromophore both the molecular tilt and twist angle of the azobenzene moiety are accessible. Based on this detailed structural analysis the energetic shifts observed in optical reflection spectroscopy can be qualitatively described within an extended dipole model. This substantiates sizable excitonic coupling among the azobenzene chromophores as an important mechanism that hinders trans to cis isomerization in densely packed self-assembled monolayers.

We employ normal-incidence x-ray standing wave and temperature programed desorption spectroscopy to derive the adsorption geometry and energetics of the prototypical molecular switch azobenzene at Ag(111). This allows us to assess the accuracy of semiempirical correction schemes as a computationally efficient means to overcome the deficiency of semilocal density-functional theory with respect to long-range van der Waals (vdW) interactions. The obtained agreement underscores the significant improvement provided by the account of vdW interactions, with remaining differences mainly attributed to the neglect of electronic screening at the metallic surface.

We use different semi-empirical dispersion correction schemes to assess the role of long-range van der Waals interactions in the adsorption of the prototypical molecular switch azobenzene (C6H5-N2-C6H5) at the coinage metal surfaces Cu(111), Ag(111) and Au(111). Compared to preceding density-functional theory results employing a semi-local exchange and correlation functional we obtain partly sizable changes of the computed adsorption geometry and energetics. The discomforting scatter in the results provided by the different schemes is largely attributed to the unknown form of the damping function in the semi-empirical correction expression. Using the congeneric problem of the adsorption of benzene as a vehicle to connection with experiment, we cautiously conclude that the account of dispersive interactions at the metal surfaces provided by the various schemes is in the right ballpark, with the more recent, general schemes likely to overbind., 11 pages including 4 figures; related publications can be found at http://www.fhi-berlin.mpg.de/th/th.html

Hydrodynamic and magnetohydrodynamic numerical studies of a mechanically forced two-vortex flow inside a sphere are reported. The simulations are performed in the intermediate regime between the laminar flow and developed turbulence, where a hydrodynamic instability is found to generate internal waves with a characteristic m=2 zonal wave number. It is shown that this time-periodic flow acts as a dynamo, although snapshots of the flow as well as the mean flow are not dynamos. The magnetic fields' growth rate exhibits resonance effects depending on the wave frequency. Furthermore, a cyclic self-killing and self-recovering dynamo based on the relative alignment of the velocity and magnetic fields is presented. The phenomena are explained in terms of a mixing of nonorthogonal eigenstates of the time-dependent linear operator of the magnetic induction equation. The potential relevance of this mechanism to dynamo experiments is discussed.

We present a systematic performance analysis of first-principles basin-hopping (BH) runs, with the target to identify all low-energy isomers of small Si and Cu clusters described within density-functional theory. As representative and widely employed move classes we focus on single-particle and collective moves, in which one or all atoms in the cluster at once are displaced in a random direction by some prescribed move distance, respectively. The analysis provides detailed insights into the bottlenecks and governing factors for the sampling efficiency, as well as simple rules-of-thumb for near-optimum move settings, that are intriguingly independent of the distinctly different chemistry of Si and Cu. At corresponding settings, the observed performance of the BH algorithm employing two simple, general-purpose move classes is already very good, and for the small systems studied essentially limited by frequent revisits to a few dominant isomers., 11 pages including 8 figures; related publications can be found at http://www.fhi-berlin.mpg.de/th/th.html

We present a density-functional theory study addressing the energetics and electronic structure properties of isolated oxygen adatoms at the SrTiO3(001) surface. Together with a surface lattice oxygen atom, the adsorbate is found to form a peroxide-type molecular species. This gives rise to a non-trivial topology of the potential energy surface for lateral adatom motion, with the most stable adsorption site not corresponding to the one expected from a continuation of the perovskite lattice. With computed modest diffusion barriers below 1 eV, it is rather the overall too weak binding at both regular SrTiO3(001) terminations that could be a critical factor for oxide film growth applications., Comment: 7 pages including 5 figures; related publications can be found at http://www.fhi-berlin.mpg.de/th/th.html

The oxidized Ag(111) surface has been studied by a combination of experimental and theoretical methods, scanning tunneling microscopy (STM), x-ray photoelectron spectroscopy (XPS), and density functional theory (DFT). A large variety of different surface structures is found, depending on the detailed preparation conditions. The observed structures fall into four classes: (a) individually chemisorbed atomic oxygen atoms, (b) three different oxygen overlayer structures, including the well-known p(4 x 4) phase, formed from the same Ag$_6$ and Ag$_{10}$ building blocks, (c) a c(4 x 8) structure not previously observed, and (d) at higher oxygen coverages structures characterized by stripes along the high-symmetry directions of the Ag(111) substrate. Our analysis provides a detailed explanation of the atomic-scale geometry of the Ag$_6$/Ag$_{10}$ building block structures, and the c(4 x 8) and stripe structures are discussed in detail. The observation of many different and co-existing structures implies that the O/Ag(111) system is characterized by a significantly larger degree of complexity than previously anticipated, and this will impact our understanding of oxidation catalysis processes on Ag catalysts., Comment: 12 pages, 9 figures; peer-reviewed and published version

We present a density-functional theory trend study addressing the binding of the trans-cis conformational switch azobenzene (C6H5-N=N-C6H5) at three coinage metal surfaces. From the reported detailed energetic, geometric, and electronic structure data we conclude that the governing factor for the molecule-surface interaction is a competition between covalent bonding of the central azo (-N=N-) bridge on the one hand and the surface interaction of the two closed-shell phenyl (-C6H5) rings on the other. With respect to this factor the cis conformer exhibits a more favorable gas-phase geometric structure and is thus more stabilized at the studied surfaces. With the overall binding still rather weak the relative stability of the two isomers is thereby reduced at Ag(111) and Au(111). This is significantly different at Cu(111), where the cis bonding is strong enough to even reverse the gas-phase energetic order at the level of the employed semi-local electronic exchange and correlation (xc) functional. While this actual reversal may well be affected by the deficiencies due to the approximate xc treatment, we critically discuss that the rationalization of the general effect of the surface on the meta-stable molecular states is quite robust. This should equally hold for the presented analysis of recent tip-manipulation and photo-excitation isomerization experiments from the view point of the derived bonding mechanism., Comment: 10 pages including 4 figures; related publications can be found at http://www.fhi-berlin.mpg.de/th/th.html

Recently, spin selection rules have been invoked to explain the discrepancy between measured and calculated adsorption probabilities of molecular oxygen reacting with Al(111). In this work, we inspect the impact of nonadiabatic spin transitions on the dynamics of this system from first principles. For this purpose the motion on two distinct potential-energy surfaces associated to different spin configurations and possible transitions between them are inspected by means of the Fewest Switches algorithm. Within this framework we especially focus on the influence of such spin transitions on observables accessible to molecular beam experiments. On this basis we suggest experimental setups that can validate the occurrence of such transitions and discuss their feasibility., Comment: 13 pages, 7 figures

Using a first-principles parameterized lattice-gas Hamiltonian we study the adsorbate ordering behavior at atomic steps of a Pd(100) surface exposed to an oxygen environment. We identify a wide range of gas-phase conditions comprising near atmospheric pressures and elevated temperatures around 900 K, in which the step is decorated by a characteristic O zigzag arrangement. For catalytic processes like the high-temperature combustion of methane that operate under these conditions our approach thus provides first insight into the structure and composition at a prominent defect on the working surface., 5 pages including 3 figures; related publications can be found at http://www.fhi-berlin.mpg.de/th/th.html

In previous work on adsorbate-induced surface core level shifts (SCLSs), the effects caused by O atom adsorption on Rh(111) and Ru(0001) were found to be additive: the measured shifts for first layer Ru atoms depended linearly on the number of directly coordinated O atoms. Density-functional theory calculations quantitatively reproduced this effect, allowed separation of initial and final state contributions, and provided an explanation in terms of a roughly constant charge transfer per O atom. We have now conducted similar measurements and calculations for three well-defined adsorbate and coadsorbate layers containing O and H atoms: (1 x 1)-H, (2 x 2)-(O+H), and (2 x 2)-(O+3H) on Ru(0001). As H is stabilized in fcc sites in the prior two structures and in hcp sites in the latter, this enables us to not only study coverage and coadsorption effects on the adsorbate-induced SCLSs, but also the sensitivity to similar adsorption sites. Remarkably good agreement is obtained between experiment and calculations for the energies and geometries of the layers, as well as for all aspects of the SCLS values. The additivity of the next-neighbor adsorbate-induced SCLSs is found to prevail even for the coadsorbate structures. While this confirms the suggested use of SCLSs as fingerprints of the adsorbate configuration, their sensitivity is further demonstrated by the slightly different shifts unambiguously determined for H adsorption in either fcc or hcp hollow sites., 9 pages including 4 figures; related publications can be found at http://www.fhi-berlin.mpg.de/th/th.html

We performed mixed quantum-classical molecular dynamics simulations based on first-principles potential-energy surfaces to demonstrate that the scattering of a beam of singlet O2 molecules at Al(111) will enable an unambiguous assessment of the role of spin-selection rules for the adsorption dynamics. At thermal energies we predict a sticking probability that is substantially less than unity, with the repelled molecules exhibiting characteristic kinetic, vibrational and rotational signatures arising from the non-adiabatic spin transition., 4 pages including 3 figures; related publications can be found at http://www.fhi-berlin.mpg.de/th/th.html

The possible importance of oxide formation for the catalytic activity of transition metals in heterogenous oxidation catalysis has evoked a lively discussion over the recent years. On the more noble transition metals (like Pd, Pt or Ag) the low stability of the common bulk oxides suggests primarily sub-nanometer thin oxide films, so-called surface oxides, as potential candidates that may be stabilized under gas phase conditions representative of technological oxidation catalysis. We address this issue for the Pd(100) model catalyst surface with first-principles kinetic Monte Carlo (kMC) simulations that assess the stability of the well-characterized (sqrt{5} x sqrt{5})R27 surface oxide during steady-state CO oxidation. Our results show that at ambient pressure conditions the surface oxide is stabilized at the surface up to CO:O2 partial pressure ratios just around the catalytically most relevant stoichiometric feeds (p(CO):p(O2) = 2:1). The precise value depends sensitively on temperature, so that both local pressure and temperature fluctuations may induce a continuous formation and decomposition of oxidic phases during steady-state operation under ambient stoichiometric conditions., Comment: 13 pages including 5 figures; related publications can be found at http://www.fhi-berlin.mpg.de/th/th.html

The measured low initial sticking probability of oxygen molecules at the Al(111) surface that had puzzled the field for many years was recently explained in a non-adiabatic picture invoking spin-selection rules [J. Behler et al., Phys. Rev. Lett. 94, 036104 (2005)]. These selection rules tend to conserve the initial spin-triplet character of the free O2 molecule during the molecule's approach to the surface. A new locally-constrained density-functional theory approach gave access to the corresponding potential-energy surface (PES) seen by such an impinging spin-triplet molecule and indicated barriers to dissociation which reduce the sticking probability. Here, we further substantiate this non-adiabatic picture by providing a detailed account of the employed approach. Building on the previous work, we focus in particular on inaccuracies in present-day exchange-correlation functionals. Our analysis shows that small quantitative differences in the spin-triplet constrained PES obtained with different gradient-corrected functionals have a noticeable effect on the lowest kinetic energy part of the resulting sticking curve., 17 pages including 11 figures; related publications can be found at http://www.fhi-berlin.mpg.de/th/th.html

We present a first-principles atomistic thermodynamics framework to describe the structure, composition, and segregation profile of an alloy surface in contact with a (reactive) environment. The method is illustrated with the application to a ${\mathrm{Ag}}_{3}\mathrm{Pd}(111)$ surface in an oxygen atmosphere, and we analyze trends in segregation, adsorption, and surface free energies. We observe a wide range of oxygen adsorption energies on the various alloy surface configurations, including binding that is stronger than on a $\mathrm{Pd}(111)$ surface and weaker than that on a $\mathrm{Ag}(111)$ surface. This and the consideration of even small amounts of nonstoichiometries in the ordered bulk alloy are found to be crucial to accurately model the Pd surface segregation occurring in increasingly O-rich gas phases.

Temperature programmed desorption (TPD) and re- action (TPR) spectroscopy is a frequently employed tool that provides unparalleled insight into the binding en- ergetics of adsorbates or reactants at solid surfaces by recording the amount of desorbing species while ramp- ing the substrate temperature (1). The obtained experi- mental data does contain information about the spatial arrangement of the surface species before and during the ramp, but only in an indirect way. Prevalent experimen- tal data analysis procedures neglect such dependencies and rely almost entirely on a mean-field approach, arriv- ing at expressions considering only the averaged surface coveragesi of species i (1). Obvious breakdowns of the mean-field assumption come from strong lateral interac- tions among the adsorbates or extrinsic defect sites. The intricate consequences on TPD and TPR spectra are well documented in the literature (2) and have made surface scientists broadly aware of the limitations of the preva- lent mean-field data analysis procedures. With the ex- plicit lattice structure not taken into account, another fundamental source of deficiency of mean-field theories are the neglected correlations between the locations of the active surface sites themselves, i.e. that the mere way how the active sites are arranged on the surface is not accounted for. Within the general move towards the atomic-scale characterization of ever more complex and nanostructured surfaces exhibiting a wide variety of dif- ferent surface sites this will become an increasingly im- portant factor also for the analysis of TPD and TPR spectra. We illustrate this here for the catalytic CO ox- idation at RuO2(110), which is a showcase for a system with only very small lateral interactions. Without appre- ciable thermodynamic driving force for segregation (and thus for inhomogeneities in the adlayer) one would gen- erally expect a mean-field picture to be valid. Instead we demonstrate that the neglect of the essentially one- dimensional trench like arrangement of the most active surface sites in a mean-field analysis of a set of experi- mental TPR data (3) for this system leads to qualitatively wrong conclusions about the reactivity of the various sur- cus site bridge site bridge sites cus sites

Self-assembled monolayers (SAMs) of 4-trifluoromethyl-azobenzene-4′-methyleneoxy-alkanethiols (CF3– C6H4–N=N–C6H4–O–(CH2) n –SH on (111)-oriented poly-crystalline gold films on mica were examined by X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS). The spectra are analyzed with the help of density-functional-theory calculations of the isolated molecule. Only one doublet is detected in the sulphur 2p spectra of the investigated SAMs, consistent with a thiolate bond of the molecule to the gold surface. The C 1s XP spectra and the corresponding XAS π * resonance exhibit a rich structure which is assigned to the carbon atoms in the different chemical surroundings. Comparing XPS binding energies of the azobenzene moiety and calculated initial-state shifts reveals comparable screening of all C 1s core holes. While the carbon 1s XPS binding energy lies below the π *-resonance excitation-energy, the reversed order is found comparing core ionization and neutral core excitation of the nitrogen 1s core-hole of the azo group. This surprising difference in core-hole binding energies is interpreted as site-dependent polarization screening and charge transfer among the densely packed aromatic moieties. We propose that a quenching of the optical excitation within the molecular layer is thus one major reason for the low trans to cis photo-isomerization rate of azobenzene in aromatic-aliphatic SAMs.

Using the catalytic CO oxidation at RuO2(110) as a showcase, we employ first-principles kinetic Monte Carlo simulations to illustrate the intricate effects on temperature programmed reaction spectroscopy data brought about by the mere correlations between the locations of the active sites at a nanostructured surface. Even in the absence of lateral interactions, this nanostructure alone can cause inhomogeneities that cannot be grasped by prevalent mean-field data analysis procedures, which thus lead to wrong conclusions on the reactivity of the different surface species.

Phenomenological kinetics (PK) is widely used in the study of the reaction rates in heterogeneous catalysis, and it is an important aid in reactor design. PK makes simplifying assumptions: It neglects the role of fluctuations, assumes that there is no correlation between the locations of the reactants on the surface, and considers the reacting mixture to be an ideal solution. In this article we test to what extent these assumptions damage the theory. In practice the PK rate equations are used by adjusting the rate constants to fit the results of the experiments. However, there are numerous examples where a mechanism fitted the data and was shown later to be erroneous or where two mutually exclusive mechanisms fitted well the same set of data. Because of this, we compare the PK equations to “computer experiments” that use kinetic Monte Carlo (kMC) simulations. Unlike in real experiments, in kMC the structure of the surface, the reaction mechanism, and the rate constants are known. Therefore, any discrepancy between PK and kMC must be attributed to an intrinsic failure of PK. We find that the results obtained by solving the PK equations and those obtained from kMC, while using the same rate constants and the same reactions, do not agree. Moreover, when we vary the rate constants in the PK model to fit the turnover frequencies produced by kMC, we find that the fit is not adequate and that the rate constants that give the best fit are very different from the rate constants used in kMC. The discrepancy between PK and kMC for the model of CO oxidation used here is surprising since the kMC model contains no lateral interactions that would make the coverage of the reactants spatially inhomogeneous. Nevertheless, such inhomogeneities are created by the interplay between the rate of adsorption, of desorption, and of vacancy creation by the chemical reactions.

The possible formation of oxides or thin oxide films (surface oxides) on late transition-metal surfaces has recently been recognized as an essential ingredient when aiming to understand catalytic oxidation reactions under technologically relevant gas phase conditions. Using $\mathrm{CO}$ oxidation at $\mathrm{Pd}(100)$ as an example, we investigate the composition and structure of this model catalyst surface over a wide range of $(T,p)$ conditions within a multiscale modeling approach where density-functional theory is linked to thermodynamics. The results show that under the catalytically most relevant gas phase conditions a thin surface oxide is the most stable ``phase'' and that the system is actually very close to a transition between this oxidic state and a reduced state in terms of a $\mathrm{CO}$-covered $\mathrm{Pd}(100)$ surface.

Título del postprint: Coulomb parameters and photoemission for the molecular metal TTF-TCNQ, We employ density-functional theory to calculate realistic parameters for an extended Hubbard model of the molecular metal TTF-TCNQ. Considering both intra- and intermolecular screening in the crystal, we find significant longer-range Coulomb interactions along the molecular stacks, as well as inter-stack coupling. We show that the long-range Coulomb term of the extended Hubbard model leads to a broadening of the spectral density, likely resolving the problems with the interpretation of photoemission experiments using a simple Hubbard model only, J.M. is grateful to the Ramón y Cajal program of Ministerio de Ciencia y Tecnología, Spain for financial support through contract CTQ2005-09385. The Lanczos runs were performed on the Jülich BlueGene/L under grant JIFF22

Our paper determines the lowest-energy structure of a RuO2 110 surface in thermodynamic equilibrium with an oxygenrich environment. The limit of least oxygen-containing gas phase conditions considered O-poor limit is motivated by the Gibbs free energy of formation of the bulk oxide Gf. Employing the value of Gf 0,0 at 0 K, we stated the error as opposed to using the temperature and pressure dependent value Gf T , p as 0.63 eV at 1000 K and 1 atm. This value was erroneously deduced from the CRC Handbook,1 due to inconsistent entries in the Gf T , p° =1 atm table, mixing decadic and natural logarithms in the stated interpolation formulae for rutile structured oxides. Using the proper decadic logarithm in the interpolation formula for RuO2 the correct variation up to 1000 K and 1 atm is 1.78 eV. 2 In addition, Eq. 19 in our paper contains an erroneous factor of 2 in the denominator, which leads to an estimate of the vibrational contribution to the surface free energy in Fig. 1 that is too small by a factor of 2. In the temperature range up to 600 K discussed in our paper, this contribution remains nevertheless below 10 meV/A2. Therefore neither error affects the conclusions drawn. We thank Ian Grey for making us aware of the inconsistent entries in the CRC Handbook.

We employ a multiscale modeling approach to study the surface structure and composition of a Pd(100) model catalyst in reactive environments. Under gas phase conditions representative of technological CO oxidation (~1 atm, 300-600 K) we find the system on the verge of either stabilizing sub-nanometer thin oxide structures or CO adlayers at the surface. Under steady-state operation this suggests the presence or continuous formation and reduction of oxidic patches at the surface, which could be key to understand the observable catalytic function., 4 pages including 2 figures; related publications can be found at http://www.fhi-berlin.mpg.de/th/th.html

The accurate description of molecule-surface interactions requires a detailed knowledge of the underlying potential-energy surface (PES). Recently, neural networks (NNs) have been shown to be an efficient technique to accurately interpolate the PES information provided for a set of molecular configurations, e.g. by first-principles calculations. Here, we further develop this approach by building the NN on a new type of symmetry functions, which allows to take the symmetry of the surface exactly into account. The accuracy and efficiency of such symmetry-adapted NNs is illustrated by the application to a six-dimensional PES describing the interaction of oxygen molecules with the Al(111) surface., Comment: 13 pages including 8 figures; related publications can be found at http://www.fhi-berlin.mpg.de/th/th.html

If metal surfaces are exposed to sufficiently oxygen-rich environments, oxides start to form. Important steps in this process are the dissociative adsorption of oxygen at the surface, the incorporation of O atoms into the surface, the formation of a thin oxidic overlayer and the growth of the once formed oxide film. For the oxidation process of late transition metals (TM), recent experimental and theoretical studies provided a quite intriguing atomic-scale insight: The formed initial oxidic overlayers are not merely few atomic-layer thin versions of the known bulk oxides, but can exhibit structural and electronic properties that are quite distinct to the surfaces of both the corresponding bulk metals and bulk oxides. If such nanometer or even sub-nanometer surface oxide films are stabilized in applications, new functionalities not scalable from the known bulk materials could correspondingly arise. This can be particularly important for oxidation catalysis, where technologically relevant gas phase conditions are typically quite oxygen-rich. In such environments surface oxides may even form naturally in the induction period, and actuate then the reactive steady-state behavior that has traditionally been ascribed to the metal substrates. Corresponding aspects are reviewed by focusing on recent progress in the modeling and understanding of the oxidation behavior of late TMs, using particularly the late 4d series from Ru to Ag to discuss trends.

We employ a first-principles lattice-gas Hamiltonian (LGH) approach to determine the lateral interactions between O atoms adsorbed on the Pd(100) surface. With these interactions we obtain an ordering behavior at low coverage that is in quantitative agreement with experimental data. Uncertainties in the approach arise from the finite LGH expansion and from the approximate exchange-correlation (xc) functional underlying the employed density-functional theory energetics. We carefully scrutinize these uncertainties and conclude that they primarily affect the on-site energy, which rationalizes the agreement with the experimental critical temperatures for the order-disorder transition. We also investigate the validity of the frequently applied assumption that the ordering energies can be represented by a sum of pair terms. Restricting our LGH expansion to just pairwise lateral interactions, we find that this results in effective interactions which contain spurious contributions that are of equal size, if not larger than any of the uncertainties e.g. due to the approximate xc functional., Comment: 14 pages including 7 figures; related publications can be found at http://www.fhi-berlin.mpg.de/th/th.html

It is shown that the errors of present-day exchange-correlation (xc) functionals are rather short ranged. For extended systems the correction can therefore be evaluated by analyzing properly chosen clusters and employing highest-quality quantum chemistry methods. The xc correction rapidly approaches a universal dependence with cluster size. The method is applicable to bulk systems as well as to defects in the bulk and at surfaces. It is demonstrated here for CO adsorption at transition-metal surfaces, where present-day xc functionals dramatically fail to predict the correct adsorption site, and for the crystal bulk cohesive energy., slightly revised version: 4 pages including 3 figures; related publications can be found at http://www.fhi-berlin.mpg.de/th/th.html

We present a density-functional theory study addressing the on-surface adsorption of oxygen at the $\mathrm{Pd}(11N)$ $(N=3,5,7)$ vicinal surfaces, which exhibit (111) steps and (100) terraces of increasing width. We find the binding to be predominantly governed by the local coordination at the adsorption site. This leads to very similar bonding properties at the threefold step sites of all three vicinal surfaces, while the binding at the central fourfold hollow site in the four atomic row terrace of Pd(117) is already very little disturbed by the presence of the neighboring steps.

Non-adiabatic effects play an important role in many chemical processes. In order to study the underlying non-adiabatic potential-energy surfaces (PESs), we present a locally-constrained density-functional theory approach, which enables us to confine electrons to sub-spaces of the Hilbert space, e.g. to selected atoms or groups of atoms. This allows to calculate non-adiabatic PESs for defined charge and spin states of the chosen subsystems. The capability of the method is demonstrated by calculating non-adiabatic PESs for the scattering of a sodium and a chlorine atom, for the interaction of a chlorine molecule with a small metal cluster, and for the dissociation of an oxygen molecule at the Al(111) surface., 11 pages including 7 figures; related publications can be found at http://www.fhi-berlin.mpg.de/th/th.html

Scanning tunneling microscopy (STM) and density-functional theory are used to reexamine the structure of the renowned p(4×4)-O/Ag(111) surface oxide. The accepted structural model [C. I. Carlisle et al., Phys. Rev. Lett. 84, 3899 (2000)] is incompatible with the enhanced resolution of the current STM measurements. An "Ag6 model" is proposed that is more stable than its predecessor and accounts for the coexistence of the p(4×4) and a novel c(3×5sqrt(3))rect phase. This coexistence is an indication of the dynamic complexity of the system that until now has not been appreciated.

We describe a first-principles statistical mechanics approach enabling us to simulate the steady-state situation of heterogeneous catalysis. In a first step, density-functional theory together with transition-state theory is employed to obtain the energetics of the relevant elementary processes. Subsequently the statistical mechanics problem is solved by the kinetic Monte Carlo method, which accounts for the correlations, fluctuations, and spatial distributions of the chemicals at the surface of the catalyst under steady-state conditions. Applying this approach to the catalytic oxidation of CO at RuO2110, we determine the surface atomic structure and composition in reactive environments ranging from ultra-high vacuum UHV to technologically relevant conditions, i.e., up to pressures of several atmospheres and elevated temperatures. We also compute the CO2 formation rates turnover frequencies. The results are in quantitative agreement with all existing experimental data. We find that the high catalytic activity of this system is intimately connected with a disordered, dynamic surface “phase” with significant compositional fluctuations. In this active state the catalytic function results from a self-regulating interplay of several elementary processes.

We scrutinize the accuracy of the pseudopotential approximation in density-functional theory (DFT) calculations of surfaces by systematically comparing to results obtained within a full-potential setup. As model system we choose the CO oxidation at a RuO2(110) surface and focus in particular on the adsorbate binding energies and reaction barriers as target quantities for the comparison. Rather surprisingly, the major reason for discrepancy does not result from the neglected semi-core state relaxation in the frozen-core approximation, but from an inadequate description of the local part of the Ru pseudopotential, responsible for the scattering of f like waves. Tiny, seemingly irrelevant, imprecisions appearing in these properties can have a noticeable influence on the surface energetics. At least for the present example, we obtain excellent agreement between both approaches, if the pseudopotential describes these scattering properties accurately., 8 pages including 3 figures; related publications can be found at http://www.fhi-berlin.mpg.de/th/th.html

Pd(111) has recently been shown to exhibit a propensity to form a sub-nanometer thin surface oxide film already well before a full monolayer coverage of adsorbed O atoms is reached on the surface. Aiming at an atomic-scale understanding of this finding, we study the initial oxygen incorporation into the Pd(111) surface using density-functional theory. We find that oxygen incorporation into the sub-surface region starts at essentially the same coverage as formation of the surface oxide. This implies that the role of sub-surface oxygen should be considered as that of a metastable precursor in the oxidation process of the surface. The mechanisms found to play a role towards the ensuing stabilization of an ordered oxidic structure with a mixed on-surface/sub-surface site occupation follow a clear trend over the late 4d transition metal series, as seen by comparing our data to previously published studies concerned with oxide formation at the basal surface of Ru, Rh and Ag. The formation of a linearly aligned O-TM-O trilayered structure (TM = Ru, Rh, Pd, Ag), together with an efficient coupling to the underlying substrate seem to be key ingredients in this respect., Comment: 8 pages including 5 figures; related publications can be found at http://www.fhi-berlin.mpg.de/th/th.html

Previous and present "academic" research aiming at atomic scale understanding is mainly concerned with the study of individual molecular processes possibly underlying materials science applications. Appealing properties of an individual process are then frequently discussed in terms of their direct importance for the envisioned material function, or reciprocally, the function of materials is somehow believed to be understandable by essentially one prominent elementary process only. What is often overlooked in this approach is that in macroscopic systems of technological relevance typically a large number of distinct atomic scale processes take place. Which of them are decisive for observable system properties and functions is then not only determined by the detailed individual properties of each process alone, but in many, if not most cases also the interplay of all processes, i.e. how they act together, plays a crucial role. For a "predictive materials science modeling with microscopic understanding", a description that treats the statistical interplay of a large number of microscopically well-described elementary processes must therefore be applied. Modern electronic structure theory methods such as DFT have become a standard tool for the accurate description of individual molecular processes. Here, we discuss the present status of emerging methodologies which attempt to achieve a (hopefully seamless) match of DFT with concepts from statistical mechanics or thermodynamics, in order to also address the interplay of the various molecular processes. The new quality of, and the novel insights that can be gained by, such techniques is illustrated by how they allow the description of crystal surfaces in contact with realistic gas-phase environments.

The structural, vibrational, energetic, and electronic properties of hydrogen at the stoichiometric $\mathrm{Ru}{\mathrm{O}}_{2}(110)$ termination are studied using density-functional theory. The oxide surface is found to bind both molecular and dissociated ${\mathrm{H}}_{2}$. Apart from the most stable configuration in the form of hydroxyl groups (monohydrides) at the undercoordinated ${\mathrm{O}}^{\mathrm{br}}$ surface anions, we also identify a molecular state at the undercoordinated ${\mathrm{Ru}}^{\mathrm{cus}}$ atoms (dihydrogen) and a waterlike species (dihydride) at the ${\mathrm{O}}^{\mathrm{br}}$ sites. Hydrogen adsorption at ${\mathrm{O}}^{\mathrm{br}}$ sites increases the reactivity of the neighboring ${\mathrm{Ru}}^{\mathrm{cus}}$ sites, which are believed to play a key role in catalytic oxidation reactions.

As a first step towards understanding the morphology of PdO crystals we performed a systematic full-potential density-functional theory study of all possible $(1\ifmmode\times\else\texttimes\fi{}1)$ terminations of the low-index surfaces of tetragonal PdO. Applying the concept of first-principles atomistic thermodynamics we analyze the composition, structure and stability of these PdO orientations in equilibrium with an arbitrary oxygen environment. Within the studied subset of $(1\ifmmode\times\else\texttimes\fi{}1)$ geometries the polar PdO-terminated PdO(100) orientation turns out to be surprisingly stable over the whole range of experimentally accessible gas phase conditions. Setting up a constrained Wulff construction within the compiled data set, this PdO(100)-PdO facet correspondingly dominates the obtained polyhedron by far. The real PdO crystallite shape will however likely be affected by surface reconstructions, which are not covered by the present study.

The turn-over frequency of the catalytic oxidation of CO at RuO2(110) was calculated as function of temperature and partial pressures using ab initio statistical mechanics. The underlying energetics of the gas-phase molecules, dissociation, adsorption, surface diffusion, surface chemical reactions, and desorption were obtained by all-electron density-functional theory. The resulting CO2 formation rate [in the full (T, p_CO, p_O2)-space], the movies displaying the atomic motion and reactions over times scales from picoseconds to seconds, and the statistical analyses provide insights into the concerted actions ruling heterogeneous catalysis and open thermodynamic systems in general., Comment: 4 pages including 3 figures, Related publications can be found at http://www.fhi-berlin.mpg.de/th/paper.html