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The H2O and H2O2 molecules resemble each other in a multitude of ways as has been noted in the literature. Here, we present density functional theory (DFT) calculations for the H2O2(s) and H2O2·2H2O(s) crystals and make selected comparisons with ice polymorphs. The performance of a number of dispersion-corrected density functionals—both self-consistent and a posteriori ones—are assessed, and we give special attention to the D3 correction and its effects. The D3 correction to the lattice energies is large: for H2O2(s) the D3 correction constitutes about 25% of the lattice energy using PBE, much more for RPBE, much less for SCAN, and it primarily arises from non-H-bonded interactions out to about 5 Å.The large D3 corrections to the lattice energies are likely a consequence of several effects: correction for missing dispersion interaction, the ability of D3 to capture and correct various other kinds of limitations built into the underlying DFT functionals, and finally some degree of cell-contraction-induced polarization enhancement. We find that the overall best-performing functionals of the twelve examined are optPBEvdW and RPBE-D3. Comparisons with DFT assessments for ices in the literature show that where the same methods have been used, the assessments largely agree. [ABSTRACT FROM AUTHOR]

Theoretical and experimental vibrational signatures of H2O and OH− (dissociated water) adsorbed on stoichiometric ceria{111} surfaces are compared. The experimental ones were collected from low-coverage experiments in the literature, and the theoretical anharmonic frequencies were generated using density functional theory calculations employing the optPBE-vdW functional for coverages from 0.5 to a few monolayers. It is found that (i) the experiments and our calculations overall agree well, lending credibility to both; (ii) the calculations manage to resolve the large class of H-bonded motifs into frequency classes that can guide experimental assignments; (iii) it is possible to find a geometrical H-bond definition that also captures the OH vibrational frequency downshifts well: R(H⋯O) ≤ 2.5 Å and the O–H⋯O angle θ ≥ 100°; and (iv) the frequency vs electric field relations for water and hydroxides (i.e., dissociated water) follow different and well-separated curves. [ABSTRACT FROM AUTHOR]

Thin film structures of water on the CeO2(111) surface for coverages between 0.5 and 2.0 water monolayers have been optimized and analyzed using density functional theory (optPBE-vdW functional). We present a new 1.0 ML structure that is both the lowest in energy published and features a hydrogen-bond network extending the surface in one-dimension, contrary to what has been found in the literature, and contrary to what has been expected due to the large bulk ceria cell dimension. The adsorption energies for the monolayer and multilayered water structures agree well with experimental temperature programmed desorption results from the literature, and we discuss the stability window of CeO2(111) surfaces covered with 0.5–2.0 ML of water. [ABSTRACT FROM AUTHOR]

Combining experimental spectroscopy and hybrid density functional theory calculations, we show that the incorporation of fluoride ions into a prototypical reducible oxide surface, namely, ceria(111), can induce a variety of nontrivial changes to the local electronic structure, beyond the expected increase in the number of Ce3+ ions. Our resonant photoemission spectroscopy results reveal new states above, within, and below the valence band, which are unique to the presence of fluoride ions at the surface. With the help of hybrid density functional calculations, we show that the different states arise from fluoride ions in different atomic layers in the near surface region. In particular, we identify a structure in which a fluoride ion substitutes for an oxygen ion at the surface, with a second fluoride ion on top of a surface Ce4+ ion giving rise to F 2p states which overlap the top of the O 2p band. The nature of this adsorbate F−–Ce4+ resonant enhancement feature suggests that this bond is at least partially covalent. Our results demonstrate the versatility of anion doping as a potential means of tuning the valence band electronic structure of ceria. [ABSTRACT FROM AUTHOR]

It has been suggested in the literature that keto‐to‐enol tautomerization plays a vital role for lignin fragmentation under mild conditions. On the other hand, previous modelling has shown that the adsorbed keto form is more stable than enol on the Pd(111) catalyst. The current density functional theory study of lignin model molecules shows that, in the gas‐phase, keto is more stable than enol, but on the Pd surface, we find enol conformers that are at least as stable as keto. This supports the experimental result that the favourable reaction pathway for lignin depolymerization involves keto‐enol tautomerization. An energy decomposition analysis gives insights concerning the origin of the fine energy balance between the keto and enol forms, where the molecule–surface interaction (−7 eV) and the molecular strain energy (+3 eV) are the main contributors to the adsorption energy. [ABSTRACT FROM AUTHOR]

We present periodic plane-wave density functional theory (DFT) Perdew-Burke-Ernzerhof (PBE-D2) calculations for four highly hydrated crystals, Na2CO3·10H2O, MgSO4·7H2O, MgSO4·11H2O, and Al(NO3)3·9H2O, containing 37 structurally unique water molecules and 74 unique hydrogen bonds. The calculated R(H-O) distances lie in the range 1.60-2.05 Å, the anharmonic OH frequencies in the range 2570-3425 cm-1, and the water dipole moments lie in the range 2.9-4.3 Debye, as calculated from the Wannier function centers and the nuclei. We present the following findings. (i) Our optimized intramolecular r(OH) distances are always larger than the gasphase value and thus more accurate than those derived from neutron diffraction experiments; (ii) The local in situ electric field over the molecule, calculated from the positions of the nuclei and the Wannier centers in the surrounding crystal, appears to be a good descriptor of the pertturbation from the water molecule's surroundings as the internal molecular properties (re, ν, µ) are found to correlate well with the crystalgenerated electric field; (iii) We have added DFT-calculated data points to the well-known experimental 'OH frequency versus R(H - O)' correlation curve in a region where the experimental data points are scarce; (iv) For all 37 water molecules, the Wannier centers located in the lone-pair region, and those located in the OH bonds, displace about equally much due to the polarizing environment. Finally, we propose that our resulting 'OH frequency versus Wannier-type electric field' correlation curve may constitute a useful tool for predicting OH vibrational frequencies from snapshots from PBE-D2-based ab initio molecular dynamics simulations of water-containing systems. [ABSTRACT FROM AUTHOR]

We assess the consequences of the interface model-embedded-cluster or periodic-slab model-on the ability of DFT calculations to describe charge transfer (CT) in a particularly challenging case where periodic-slab calculations indicate a delocalized charge-transfer state. Our example is Cu atom adsorption on ZnO(10-10), and in fact the periodic slab calculations indicate three types of CT depending on the adsorption site: full CT, partial CT, and no CT. Interestingly, when full CT occurs in the periodic calculations, the calculated Cu atom adsorption energy depends on the underlying ZnO substrate supercell size, since when the electron enters the ZnO it delocalizes over as many atoms as possible. In the embedded-cluster calculations, the electron transferred to the ZnO delocalizes over the entire cluster region, and as a result the calculated Cu atom adsorption energy does not agree with the value obtained using a large periodic supercell, but instead to the adsorption energy obtained for a periodic supercell of roughly the same size as the embedded cluster. Different density functionals (of GGA and hybrid types) and basis sets (local atom-centered and planewaves) were assessed, and we show that embedded clusters can be used to model Cu adsorption on ZnO(1010), as long as care is taken to account for the effects of CT. [ABSTRACT FROM AUTHOR]

We have calculated the stabilities of some reactive oxygen species (ROS) in stoichiometric bulk ceria and at the low‐index (111) and (110)‐surfaces, both in vacuum and in the presence of additional O2 molecules. We find that the formation of intrinsic ROS, here oxygen superoxides (O2–) and peroxides (O22–), is always endothermic at vacuum conditions and that the superoxide formation always leads to a higher formation energy than the peroxide formation. In the presence of additional O2 molecules, intrinsic peroxide formation becomes exothermic at the (110)‐surface in conjunction with the formation of extrinsic superoxide ions from adsorbed O2 molecules. This coexistence of intrinsic and extrinsic ROS species is anticipated to be stable at low temperatures, and can be important for understanding the ROS chemistry for nanoceria used in low‐temperature applications. Oxygen‐assisted reduction of the ceria(110) surface. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) Part of Focus Issue on “Functional Oxides” (Eds.: T. Frauenheim, J. M. Knaup, P. Broqvist, S. Ramanathan) While molecular oxygen is only physisorbed on ceria(111) surfaces (right figure), it may trigger intrinsic peroxide formation on ceria(110) surfaces (left figure). Such peroxide species are stabilized by molecular oxygen adsorbed in the form of superoxide species. This coexistence of intrinsic and extrinsic reactive oxygen species (ROS) is anticipated to be stable at low temperatures, and can be important for understanding the ROS chemistry for nanoceria used in low‐temperature applications. [ABSTRACT FROM AUTHOR]

We use density functional theory quantum chemical calculations to model optical properties of small cadmium sulphide clusters in order to explore the formation of trap states and the possibility to achieve surface passivation. The addition of capping oxygen ions on the surface of the CdS quantum dots is found to passivate the single-bonded Cd atoms. Added Cu ions resulted in the formation of deep trap states for certain locations of the copper impurity, while removal of single-bonded Cd atoms by adding S or SH groups leads to a band gap increase. [ABSTRACT FROM AUTHOR]

Interactions between the group XII metals Zn2+ and Cd2+ and amino acid residues play an important role in biology due to the prevalence of the first and the toxicity of the second. Estimates of the interaction energies between the ions and relevant residues in proteins are however difficult to obtain. This study reports on calculated interaction energy curves for small complexes of Zn2+ or Cd2+ and amino acid mimics (acetate, methanethiolate, and imidazole) or water. Given that many applications and models (e.g., force fields, solvation models, etc.) begin with and rely on an accurate description of gas-phase interaction energies, this is where our focus lies in this study. Four density functional theory (DFT)-functionals and MP2 were used to calculate the interaction energies not only at the respective equilibrium distances but also at a relevant range of ion-ligand separation distances. The calculated values were compared with those obtained by CCSD(T). All DFT-methods are found to overestimate the magnitude of the interaction energy compared to the CCSD(T) reference values. The deviation was analyzed in terms of energy components from localized molecular orbital energy decomposition analysis scheme and is mostly attributed to overestimation of the polarization energy. MP2 shows good agreement with CCSD(T) [root mean square error (RMSE) = 1.2 kcal/mol] for the eight studied complexes at equilibrium distance. Dispersion energy differences at longer separation give rise to increased deviations between MP2 and CCSD(T) (RMSE = 6.4 kcal/mol at 3.0 Å). Overall, the results call for caution in applying DFT methods to metalloprotein model complexes even with closed-shell metal ions such as Zn2+ and Cd2+, in particular at ion-ligand separations that are longer than the equilibrium distances. © 2013 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

On the basis of first‐principles calculations, Fe, Co, Ni, Cu, Zn, Ru, Rh, Ag, Ir, Pt, and Au decorated Mo2CO2−δ monolayers are investigated as potential single‐atom catalyst (SAC) candidates for low‐temperature CO oxidation reaction. From a first screening based on intuitive criteria concerning metal sintering, CO poisoning, and O2 adsorption strength, the Zn/Mo2CO2−δ system is selected for further scrutiny by means of reactivity calculations for different CO concentrations. A lower barrier is found for Eley–Rideal reaction mechanism than for the Langmuir–Hinshelwood mechanism. The low Eley–Rideal barrier (0.15 eV) is attributed to the fact that the Zn atom weakens the O‐O bond considerably and the electrophilic attack of CO weakens it further. The main conclusion is that this system is a promising low‐temperature SAC candidate with a lower energy barrier for CO oxidation than noble metal and other 2D SAC systems investigated. [ABSTRACT FROM AUTHOR]

• The article present explicit simulations of temperature-programmed desorption (TPD) experiments through a first-principles derived micro-kinetic model for nanopowders. • The simulations show that oxygen adsorption on ceria is strongly coordination-, and coverage dependent. • Metastable five-coordinated ridge Ce ions offer a plausible explanation for the broad spectra recorded in the TPD experiments and suggests a possible co-existence of Ce3+-ions and superoxide ions even in an oxidative environment. • Our simulations support the notation that small ceria nanoparticles display a supercharged oxygen storage capacity with a broad desorption peak. Density functional theory calculations (DFT), coupled with microkinetic modelling, have been used to simulate Temperature Programmed Desorption (TPD) experiments for calcined ceria nanopowders with the aim to gain insight into the chemistry governing their high redox activity. Our simulations consider two main nanoparticle models. One is a perfect ceria octahedron supercharged with adsorbed oxygen molecules turned into superoxide ions, as has previously been used to explain the enhanced oxygen storage capacity (OSC) in nanoceria. The other model is a variant where we have introduced oxygen vacancies under ridge Ce ions, thereby reducing their coordination numbers to five. The results from our microkinetic modelling suggest that including such five-coordinated Ce adsorption sites results in a TPD spectrum that better matches the experimental counterpart in terms of both peak position and width. In addition, this new structural model allows for the co-existence of Ce 3 + ions, superoxide ions and O 2 molecules, as seen in experiments in the literature. [ABSTRACT FROM AUTHOR]

Binding energies of beryllium and tungsten atoms on surfaces of the alloys Be 2 W and Be 12 W were obtained from density functional theory calculations. Values of 4.08–5.63 eV for beryllium and 6.81–10.04 eV for tungsten were obtained. An analytical force field agrees for beryllium, but its tungsten surface atoms are too strongly bound. The surface binding energies of Be and W on Be 12 W surfaces is slightly smaller than on the pure Be and W surfaces, respectively. For higher tungsten content, i.e. for Be 2 W, the situation is more complicated. For some surfaces of this alloy the surface binding energies are enhanced while for others they are diminished, compared to the pure metal surfaces. The dependency of the cohesive energy on the mole fraction follows a linear relationship. [ABSTRACT FROM AUTHOR]

We present a theoretical study of a range of surface defects for the most abundant polar ZnO(0001)/(0001) surfaces using a tight binding approach with self-consistent charges (SCC-DFTB). We find that a combination of triangular pits at the Zn-terminated surface and a strongly ordered hexagonal defect pattern at the O-terminated surface constitutes a very stable reconstruction, in excellent agreement with experimental findings. On the whole, the SCC-DFTB method describes the polar surfaces of ZnO very well, and at a low computational cost which allows for the investigation of larger - and more realistic - surface structures compared to previous studies. Such large-scale calculations show that, at the Zn-terminated surface, the reconstruction results in a high density of one-layer deep triangular pit-like defects and surface vacancies which allow for a high configurational freedom and a vast variety of defect motifs. We also present extensive tests of the performance of the SCC-DFTB method in comparison with DFT results. [ABSTRACT FROM AUTHOR]

In a recent Brief Report, Grünebohm et al. [Phys. Rev. B 84, 132105 (2011)] report that they fail to reproduce the A2u ferroelectric instability of TiO2 in the rutile structure calculated with density functional theory within the Perdew-Burke-Ernzerhof (PBE) generalized gradient approximation (GGA) by Montanari et al. [Chem. Phys. Lett. 364, 528 (2002)]. We demonstrate that this disagreement arises from an erroneous treatment of Ti 3s and 3p semicore electrons as core in their calculations. Fortuitously the effect of the frozen-semicore pseudopotential cancels the phonon instability of the PBE exchange correlation, and the combination yields phonon frequencies similar to the local density approximation harmonic values. Grünebohm et al. also attempted and failed to reproduce the soft acoustic phonon mode instability under (110) strain reported by Mitev et al. [Phys. Rev. B 81, 134303 (2010)]. For this mode the combination of PBE-GGA and frozen semicore yields a small imaginary frequency of 9.8i. The failure of Grünebohm et al. to find this mode probably arose from numerical limitations of the geometry optimization approach in the presence of a shallow double well potential; the optimization method is not suitable for locating such instabilities. [ABSTRACT FROM AUTHOR]