Validation of Density Functionals for Adsorption Energies on Transition Metal Surfaces

The quantitative prediction of adsorption energies of radicals and molecules on surfaces is essential for the design and understanding of heterogeneous catalytic processes. A recent paper by Wellendorff et al. collected an experimental database of 39 reaction energies involving adsorption energies on transition metal surfaces that can be used as benchmarks for testing quantum mechanical electronic structure methods, and we compared the experimental data to Kohn–Sham density functional calculations with six exchange–correlation functionals. In this paper, we rearranged the data into two categories: open-shell radical adsorption reactions and closed-shell molecular adsorption reactions. We recalculated the adsorption energies with PBE, and we also calculated them with three functionals, M06-L, GAM, and MN15-L, that were not studied in the Wellendorff et al. paper; then we compared our results to the benchmark data. Of the nine functionals that have been compared to the databases, we find that BEEF-vdW, GAM, and RPBE perform best for the open-shell radical adsorption reactions, and MN15-L performs best for the closed-shell molecular adsorption, followed by BEEF-vdW and M06-L

Partial Ionic Character beyond the Pauling Paradigm: Metal Nanoparticles

A canonical perspective on the chemical bond is the Pauling paradigm: a bond in a molecule containing only identical atoms has no ionic character. However, we show that homonuclear silver clusters have very uneven charge distributions (for example, the C2v structure of Ag4 has a larger dipole moment than formaldehyde or acetone), and we show how to predict the charge distribution from coordination numbers and Hirshfeld charges. The new charge model is validated against Kohn–Sham calculations of dipole moments with four approximations for the exchange–correlation functional. We report Kohn–Sham studies of the binding energies of CO on silver monomer and silver clusters containing 2–18 atoms. We also find that an accurate charge model is essential for understanding the site dependence of binding. In particular we find that atoms with more positive charges tend to have higher binding energies, which can be used for guidance in catalyst modeling and design. Thus, the nonuniform charge distribution of silver clusters predisposes the site preference of binding of carbon monoxide, and we conclude that nonuniform charge distributions are an important property for understanding binding of metal nanoparticles in general

Validation of Methods for Computational Catalyst Design: Geometries, Structures, and Energies of Neutral and Charged Silver Clusters

We report a systematic study of small silver clusters, Ag_<i>n</i>, Ag_<i>n</i>⁺, and Ag_<i>n</i>^–, <i>n</i> = 1–7. We studied all possible isomers of clusters with <i>n</i> = 5–7. We tested 42 exchange–correlation functionals, and we assess these functionals for their accuracy in three respects: geometries (quantitative prediction of internuclear distances), structures (the nature of the lowest-energy structure, for example, whether it is planar or nonplanar), and energies. We find that the ingredients of exchange–correlation functionals are indicators of their success in predicting geometries and structures: local exchange–correlation functionals are generally better than hybrid functionals for geometries; functionals depending on kinetic energy density are the best for predicting the lowest-energy isomer correctly, especially for predicting two-dimensional to three-dimenstional transitions correctly. The accuracy for energies is less sensitive to the ingredient list. Our findings could be useful for guiding the selection of methods for computational catalyst design

Thermodynamics of Metal Nanoparticles: Energies and Enthalpies of Formation of Magnesium Clusters and Nanoparticles as Large as 1.3 nm

The major obstacle that prevents reliable electronic structure studies of nanoparticles is the rapid increasing computational cost for benchmark calculations using coupled-cluster methods. We show that a CCSD­(T) scheme with an MP2/CBS correction can reproduce accurate cohesive energies for magnesium clusters, and this scheme is much less computationally demanding than other reliable methods, so it is applied to Mg_<i>n</i> with <i>n</i> up to 19, which enters the realm of nanoparticles. (The diameters of all Mg clusters <i>n</i> ≥ 11 are >1 nm). With the extended benchmark data, we validate exchange–correlation functionals into the nanoparticle regime and use the two best-validated functionals to calculate the enthalpy of formation of Mg₂₈, with a diameter of 1.30 nm. We also calculated the enthalpy of formation of all Mg clusters and nanoparticles from Mg₂ to Mg₁₉. This kind of reliable thermodynamic data on size-selected metal nanoparticles has been hard to come by, either experimentally or theoretically, but it is badly needed to support applications in catalysis, electrochemistry, and other technologies

Retraction of “Challenges in Predicting Aqueous Solubility of Organic Molecules Using the COSMO-RS Model”

Retraction of “Challenges in Predicting Aqueous Solubility of Organic Molecules Using the COSMO-RS Model

Retraction of “Challenges in Predicting Aqueous Solubility of Organic Molecules Using the COSMO-RS Model”

Retraction of “Challenges in Predicting Aqueous Solubility of Organic Molecules Using the COSMO-RS Model

Retraction of “Challenges in Predicting Aqueous Solubility of Organic Molecules Using the COSMO-RS Model”

Retraction of “Challenges in Predicting Aqueous Solubility of Organic Molecules Using the COSMO-RS Model

Atomic Oxygen Recombination at Surface Defects on Reconstructed (0001) α‑Quartz Exposed to Atomic and Molecular Oxygen

The surface chemistry of silica is strongly affected by the nature of chemically active sites (or defects) occurring on the surface. Here, we employ quantum mechanical electronic structure calculations to study an uncoordinated silicon defect, a non-bridging oxygen defect, and a peroxyl defect on the reconstructed (0001) surface of α-quartz. We characterized the spin states and energies of the defects, and calculated the reaction profiles for atomic oxygen recombination at the defects. We elucidated the diradical character by analyzing the low-lying excited states using multireference wave function methods. We show that the diradical defects consist of weakly coupled doublet radicals, and the atomic oxygen recombination can take place through a barrierless process at defects. We have delineated the recombination mechanism and computed the formation energy of the peroxyl and non-bridging oxygen defects. We found that key recombination reaction paths are barrierless. In addition, we characterize the electronically excited states that may play a role in the chemical and physical processes that occur during recombination on these surface defect sites

Density Functional Theory of the Water Splitting Reaction on Fe(0): Comparison of Local and Nonlocal Correlation Functionals

Metal clusters have broad applicability in catalysis due to their unique reactivity and chemical selectivity, and density functional theory has become an important method for understanding catalysis and attempting to design better catalysts. In the present paper, a main focus is on the correlation part of the exchange-correlation functional, and we tested the reliability of the Kohn–Sham density functional theory with local correlation functionals and with the nonlocal random phase approximation (RPA) correlation functional for the water splitting reaction on monatomic Fe(0) and, by implication, for transition-metal-catalyzed reactions more generally. We computed four barrier heights and six energies of reaction in the catalytic mechanism. If the results are judged by deviation from CCSD­(T) calculations, it is found that many modern exchange-correlation (xc) functionals (about half of the functionals tested) with local correlation perform better than those using RPA nonlocal correlation; for example, the PWB6K, B97-3, ωB97X-D, MPW1K, M06-2X, and M05-2X hybrid xc functionals with local correlation have overall mean unsigned deviations of 1.9 kcal/mol or less from the CCSD­(T) results, in comparison to a mean unsigned deviation of 3.5 kcal/mol for EXX-RPA@PBE. We also find significant differences between the predictions for catalysis at the Fe(100) surface. This work provides guidance and challenges for future theoretical investigations of transition-metal catalysis