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We report here an experimental-computational study of hydrated TiO2anatase nanoparticles interacting with glycine, where we obtain quantitative agreement of the measured adsorption free energies. Ab initio simulations are performed within the tight binding and density functional theory in combination with enhanced free-energy sampling techniques, which exploit the thermodynamic integration of the unbiased mean forces collected on-the-fly along the molecular dynamics trajectories. The experiments adopt a new and efficient setup for electrochemical impedance spectroscopy measurements based on portable screen-printed gold electrodes, which allows fast and in situ signal assessment. The measured adsorption free energy is −30 kJ/mol (both from experiment and calculation), with preferential interaction of the charged NH3+group which strongly adsorbs on the TiO2bridging oxygens. This highlights the importance of the terminal amino groups in the adsorption mechanism of amino acids on hydrated metal oxides. The excellent agreement between computation and experiment for this amino acid opens the doors to the exploration of the interaction free energies for other moderately complex bionano systems.

In high-energy molecular dynamics or Monte Carlo simulations, standard force fields optimized for simulations at ambient temperatures are inadequate. This is largely because their repulsive parts have been regarded as not very significant, even well below zero interaction energies. It is, therefore, not obvious which force fields to resort to for simulating hot gases or plasmas. A force field model that uses the electronic densities of noninteracting atoms or molecules within the pair approximation is introduced. We start by deriving a naïve model that neglects any exchange and correlation effects between the electronic clouds and then correct this model by adding a term calibrated from ab initiocalculations using the CCSD(T)/cc-pVTZ level of theory. The resulting expression for this term can be regarded as a simple exchange–correlation function. We compare the results for the repulsive part of the potential energy hypersurfaces with the force fields commonly used on some dimers of small molecules.

Optimizing a target function over the space of organic molecules is an important problem appearing in many fields of applied science but also a very difficult one due to the vast number of possible molecular systems. We propose an evolutionary Monte Carlo algorithm for solving such problems which is capable of straightforwardly tuning both exploration and exploitation characteristics of an optimization procedure while retaining favorable properties of genetic algorithms. The method, dubbed MOSAiCS (Metropolis Optimization by Sampling Adaptively in Chemical Space), is tested on problems related to optimizing components of battery electrolytes, namely, minimizing solvation energy in water or maximizing dipole moment while enforcing a lower bound on the HOMO–LUMO gap; optimization was carried out over sets of molecular graphs inspired by QM9 and Electrolyte Genome Project (EGP) data sets. MOSAiCS reliably generated molecular candidates with good target quantity values, which were in most cases better than the ones found in QM9 or EGP. While the optimization results presented in this work sometimes required up to 106QM calculations and were thus feasible only thanks to computationally efficient ab initioapproximations of properties of interest, we discuss possible strategies for accelerating MOSAiCS using machine learning approaches.

Robust correlation curves are essential to decipher structural information from IR-vibrational spectra. However, for surface-adsorbed water and hydroxides, few such correlations have been presented in the literature. In this paper, OH vibrational frequencies are correlated against 12 structural descriptors representing the quantum mechanical or geometrical environment, focusing on those external to the vibrating molecule. A nonbiased fitting procedure based on Gaussian process regression (GPR) was used alongside simple analytical functional forms. The training data consist of 217 structurally unique OH groups from 38 water/metal oxide interface systems for MgO, CaO and CeO2, all optimized at the DFT level, and the fully anharmonic and uncoupled OH vibrational signatures were calculated. Among our results, we find the following: (i) The intermolecular R(H···O) hydrogen bond distance is particularly strong, indicating the primary cause of the frequency shift. (ii) Similarly, the electric field along the H-bond vector is also a good descriptor. (iii) Highly detailed machine learning descriptors (ACSF, SOAP) are less intuitive but were found to be more capable descriptors. (iv) Combinations of geometric and QM descriptors give the best predictions, supplying complementary information.

The CO2molecule is weakly bound in water. Here we analyze the influence of a dissolved CO2molecule on the structure and OH vibrational spectra of the surrounding water. From the analysis of ab initiomolecular dynamics simulations (BLYP-D3) we present static (structure, coordination, H-bonding, tetrahedrality) and dynamical (OH vibrational spectra) properties of the water molecules as a function of distance from the solute. We find a weakly oscillatory variation (“ABBA”) in the ‘solution minusbulk water’ spectrum. The origin of these features can largely be traced back to solvent–solute hard-core interactions which lead to variations in density and tetrahedrality when moving from the solute’s vicinity out to the bulk region. The high-frequency peak in the solute-affected spectra is specifically analyzed and found to originate from both water OH groups that fulfill the geometric H-bond criteria, and from those that do not (dangling ones). Effectively, neither is hydrogen-bonded.

Corrosion occurring in reinforced concrete has turned into a primary concern of the current century, concrete being the most ubiquitous and predominant material used in the construction industry. Among the many interrelated processes that trigger corrosion of metallic reinforcements, the penetration of chloride ions into the concrete matrix is the most insidious threat. Herein, we developed the first electrochemical device entirely made of paper that allows for the direct, prompt, and noninvasive evaluation of free chloride ion contamination in concrete-based constructions. Our device is based on a three-layer wax-modified filter paper, consisting of two Ag/AgCl screen-printed electrodes that are interfaced by a junction pad in a sandwich-like configuration. Filter paper allows for generating a vertical-flow potentiometric device capable of measuring the electrochemical potential between two solutions containing different concentrations of chloride ions, which are separately drop-cast on the top and bottom layers. After demonstrating the analytical performance of the device, the same principle was applied to the evaluation of the chloride contents in different concrete samples, exploiting paper as a suitable interfacing material for potentiometric measurements on the cement solid surface. Laboratory-prepared concrete samples with known chloride contents were first assessed, and then, the paper-based vertical-flow device was applied to real concrete structures at the Giacomo Manzù Museum (Ardea, Italy) for the evaluation of chloride contamination caused by the proximity to the seaside. The capability of our device to provide timely warning of the risk conditions of concrete-based artifacts was demonstrated.

This paper concerns thin water films and their hydrogen-bond patterns on ionic surfaces. As far as we are aware, this is the first time H-bond correlations for surface water and hydroxide species are presented in the literature while hydrogen-bond relations in the solid state have been scrutinized for at least five decades. Our data set, which was derived using density functional theory, consists of 116 unique surface OH groups–intact water molecules as well as hydroxides–on MgO(001), CaO(001) and NaCl(001), covering the whole range fromstrong toweak tono H-bonds. The intact surface water molecules are found to always be redshifted with respect to the gas-phase water OH vibrational frequency, whereas the surface hydroxide groups are either redshifted (OsH) or blueshifted (OHf) compared to the gas-phase OH–frequency. The surface H-bond relations are compared with the traditional relations for bulk crystals. We find that the “ν(OH) vs R(H···O)” correlation curve for surface water does not coincide with the solid state curve: it is redshifted by about 200 cm–1or more. The intact water molecules and hydroxide groups on the ionic surfaces essentially follow the same H-bond correlation curve.

With the use of the DFT+U method, the properties of Cu adsorbed on the stoichiometric CeO2(111) surface, Cu-doped CeO2(111) (denoted as Cu0.08Ce0.92O2) surface, and CO oxidation on the stoichiometric Cu0.08Ce0.92O2surface are studied systematically. It is found that (i) Cu is stable both as an adsorbed atom on the surface and as dopant in the surface region. Cu adsorbed at the surface is Cu(+I) while Cu as a dopant atom is Cu(+II). (ii) The Cu dopant facilitates O-vacancy formation considerably, while Cu adsorption on the stoichiometric CeO2(111) surface may suppress oxygen vacancy formation. (iii) Physisorbed CO, physisorbed CO2, as well as chemisorbed CO (carbonate) species are observed on the Cu-doped CeO2(111) surface, in contrast, on the clean ceria(111) surface, only physisorbed CO was previously observed. C−O distances, adsorption energies, and surface-induced C−O vibrational frequency shifts were used to characterize these species.

We propose that crystalline ionic hydrates constitute a valuable resource for benchmarking theoretical methods for aqueous ionic systems. Many such structures are known from the experimental literature, and they contain a large variety of water–water and ion–water structural motifs. Here we have collected a data set (CRYSTALWATER50) of 50 structurally unique “in-crystal” water molecules, involved in close to 100 nonequivalent O–H···O hydrogen bonds. A dozen well-known DFT functionals were benchmarked with respect to their ability to describe these experimental structures and their OH vibrational frequencies. We find that the PBE, RPBE-D3, and optPBE-vdW methods give the best H-bond distances and that anharmonic OH frequencies generated from B3LYP//optPBE-vdW energy scans outperform the other methods, i.e., here we performed B3LYP energy scans along the OH stretching coordinate while the rest of the structure was kept fixed at the optPBE-vdW-optimized positions.

The widespread use of ceria-based materials and the need to design suitable strategies to prepare eco-friendly CeO2supports for effective catalytic screening induced us to extend our computational multiscale protocol to the modeling of the hybrid organic/oxide interface between prototypical fluorinated linear alkane chains (polyethylene-like oligomers) and low-index ceria surfaces. The combination of quantum chemistry calculations and classical reactive molecular dynamics simulations provides a comprehensive picture of the interface and discloses, at the atomic level, the main causes of typical adsorption modes. The data show that at room temperature a moderate percentage of fluorine atoms (around 25%) can enhance the interaction of the organic chains by anchoring strongly pivotal fluorines to the channels of the underneath ceria (100) surface, whereas an excessive content can remarkably reduce this interaction because of the repulsion between fluorine and the negatively charged oxygen of the surface.

The effect of biaxial strain on the band structure of two-dimensional silicon nanosheets (Si NSs) with (111), (110), and (001) exposed surfaces was investigated by means of density functional theory calculations. For all the considered Si NSs, an indirect-to-direct band gap transition occurs as the lateral dimensions of Si NSs increase; that is, increasing lateral biaxial strain from compressive to tensile always enhances the direct band gap characteristics. Further analysis revealed the mechanism of the transition which is caused by preferential shifts of the conduction band edge at a specific k-point because of their bond characteristics. Our results explain a photoluminescence result of the (111) Si NSs [U. Kim et al., ACS Nano2011, 5, 2176–2181] in terms of the plausible tensile strain imposed in the unoxidized inner layer by surface oxidation.

The interplay between energetics and entropy in determining defect distributions at ceria(111) is studied using a combination of DFT+Uand lattice Monte Carlo simulations. Our main example is fluorine impurities, although we also present preliminary results for surface hydroxyl groups. A simple classical force-field model was constructed from a training set of DFT+Udata for all symmetrically inequivalent (F−)n(Ce3+)nnearest-neighbor clusters with n= 2 or 3. Our fitted model reproduces the DFT energies well. We find that for an impurity concentration of 15% at 600 K, straight and hooked linear fluorine clusters are surprisingly abundant, with similarities to experimental STM images from the literature. We also find that with increasing temperature the fluorine cluster sizes show a transition from being governed by an attractive potential to being governed by a repulsive potential as a consequence of the increasing importance of the entropy of the Ce3+ions. The distributions of surface hydroxyl groups are noticeably different.

Reducible oxides such as CeO2are challenging to describe with standard density-functional theory (DFT) due to the mixed valence states of the cations; they often require the use of non-standard correction schemes, and/or more computationally expensive methods. This adds a new layer of complexity when it comes to the generation of Slater–Koster tables and the corresponding repulsive potentials for self-consistent density-functional based tight-binding (SCC-DFTB) calculations of such materials. In this work, we provide guidelines for how to set up a parametrization scheme for mixed valence oxides within the SCC-DFTB framework, with a focus on reproducing structural and electronic properties as well as redox reaction energies calculated using a reference DFT method. This parametrization procedure was here used to generate parameters for Ce–O systems, with Ce in its +III or +IV formal oxidation states. The generated parameter set is validated by comparison with DFT calculations for various ceria (CeO2) and reduced ceria (CeO2–x) systems of different dimensionalities ranging from 0D (nanoparticles) to 3D (bulk). As oxygen vacancy defects in ceria are of crucial importance to many technological applications, special focus is directed toward the capability of describing such defects accurately.

Finding transition metal catalysts for effective catalytic conversion of CO to CO2has attracted much attention. MXene as a new 2D layered material of early transition metal carbides, nitrides, and carbo-nitrides is a robust support for achoring metal atoms. In this study, the electronic structure, geometries, thermodynamic stability, and catalytic activity of MXene (Mo2CS2) supported single noble metal atoms (NM = Ru, Rh, Pd, Ir, Pt and Au) have been systematically examined using first-principles calculations and ab initiomolecular dynamic (AIMD) simulations. First, AIMD simulations and phonon spectra demonstrate the dynamic and thermal stabilities of Mo2CS2monolayer. Three likely reaction pathways, Langmuir-Hinshelwood (LH), Eley-Rideal (ER), and Termolecular Eley–Rideal (TER) for CO oxidation on the Ru1- and Ir1@Mo2CS2SACs, have been studied in detail. It is found that CO oxidation mainly proceeds viathe TER mechanism under mild reaction conditions. The corresponding rate-determining steps are the dissociation of the intermediate (OCO-Ru1-OCO) and formation of OCO-Ir1-OCO intermediate. The downshift d-band center of Ru1- and Ir1@Mo2CS2help to enhance activity and improve catalytst stability. Moreover, a microkinetic study predicts a maximum CO oxidation rate of 4.01 × 102 s-1and 4.15 × 103 s-1(298.15 K) following the TER pathway for the Ru1- and Ir1@Mo2CS2catalysts, respectively. This work provides guideline for fabricating and designing highly efficient SACs with superb catalyts using MXene materials.

Hydrate crystals are excellent reference systems to learn about aqueous systems. We have created a database of density functional theory (DFT)-optimized (optPBE-vdW) structures and vibrational frequencies for 101 crystalline hydrate and hydroxide bulk systems and over 300 unique oscillators and use it to explore and discuss the tradeoff between prediction accuracy and insight. Starting from a machine-learning geometrical descriptor, we gradually include more physics/chemistry flavor in the descriptor and examine how the frequency prediction power varies. The most accurate models are the machine-learned model (of modest insight) and a physically motivated model containing the electric field and field gradient. Furthermore, detailed comparisons with experimental correlations show that, where available data exists, our DFT results largely overlap with the experiment. A small blind-test showed that our machine-learned (ML) descriptor model can be used to predict experimental vibrational frequencies based only on the experimental structures and our best-regressed model, with encouraging results.

Density functional theory (DFT) calculations, combined with a constrained minima hopping algorithm (global minimum search while preserving the molecular identity), have been performed to investigate important reaction intermediates for the heterogeneously catalyzed β-O-4′ bond cleavage in lignin derivatives. More specifically, we have studied the adsorption properties of a keto tautomer (1-methoxypropan-2-one) and its enol form on a catalytically active Pd(111) surface. In agreement with experiments, we find that for the gas-phase molecules the keto tautomer is the most stable. Interestingly, the enol tautomer has a higher affinity to the Pd catalyst than the keto form, and becomes the most stable molecular form when adsorbed on the catalyst surface. The global minimum complex found on the metal surface corresponds to an enolate structure formed when the enol tautomer chemisorbs onto the surface and donates its π-electrons from the CC region to two adjacent palladium atoms. The actual formation of a chemical bond to the surface in the case of the enol molecule could be the key to understanding why the enol derivative is needed for an efficient β-O-4′ bond cleavage.

We have developed a reactive force-field of the ReaxFF type for stoichiometric ceria (CeO2) and partially reduced ceria (CeO2–x). We describe the parametrization procedure and provide results validating the parameters in terms of their ability to accurately describe the oxygen chemistry of the bulk, extended surfaces, surface steps, and nanoparticles of the material. By comparison with our reference electronic structure method (PBE+U), we find that the stoichiometric bulk and surface systems are well reproduced in terms of bulk modulus, lattice parameters, and surface energies. For the surfaces, step energies on the (111) surface are also well described. Upon reduction, the force-field is able to capture the bulk and surface vacancy formation energies (Evac), and in particular, it reproduces the Evacvariation with depth from the (110) and (111) surfaces. The force-field is also able to capture the energy hierarchy of differently shaped stoichiometric nanoparticles (tetrahedra, octahedra, and cubes), and of partially reduced octahedra. For these reasons, we believe that this force-field provides a significant addition to the method repertoire available for simulating redox properties at ceria surfaces.

We explore a method that can simulate infrared reflection–absorption spectroscopy (IRRAS) spectra for molecules adsorbed on semiconductor surfaces. The method makes it possible to directly correlate experimental spectra with possible adsorbate structures. Our example in this paper is CO adsorbed on rutile TiO2(110). We present simulated IRRAS spectra for coverages in the range from 0.125 to 1.5 monolayer (ML). An explanation is provided for the apparent inconsistency in the literature concerning the tilting geometry of 1 ML CO on this surface. We find that a tilted structure (which is also the lowest-energy configuration) generates IRRAS spectra in excellent agreement with the experimental spectra. Furthermore, we predict the adsorption structure for 1.5 ML CO coverage over TiO2(110), which consists of very weakly bound CO molecules on top of the monolayer. In all cases, our simulation method, which is based on density functional theory (DFT) vibrational calculations, produces s- and p-polarized IRRAS spectra in excellent agreement with the experimental spectra.

The interaction between water molecules and small Cu clusters (up to a size of four atoms) adsorbed on the nonpolar ZnO(101̅0) surface has been studied using hybrid density functional theory. We find that the water molecules can give rise to different scenarios: (i) In contrast to water adsorption on the clean ZnO(101̅0) surface, which occurs molecularly, the first water molecule often preferentially dissociates upon adsorption on the Cu cluster, which may be a key step in the water–gas shift reaction. (ii) While the adsorption of the first water molecule on the adsorbed Cu clusters is always more favorable than the adsorption on the bare ZnO surface, the opposite is true for the second molecule. (iii) As a water molecule adsorbs on the adsorbed Cu atom, it induces charge transfer between the Cu and the ZnO, so that an electron from the Cu atom populates the ZnO conduction band (giving an oxidized Cu species). (iv) Water molecule adsorption on the adsorbed Cu trimer results in a spontaneous dissociation of the Cu trimer into an adsorbed dimer and an adsorbed atom, after which the water molecule adsorbs on the atom, again resulting in the Cu–ZnO charge transfer. We also show that the use of a hybrid density functional gives qualitatively different results as compared to a semilocal density functional for this system, and we explain this in terms of the underestimation of the ZnO band gap obtained with the semilocal functional.

Sulfidation and sulfation of ceria(111) and ceria(110) surfaces are studied with the help of density functional theory (DFT) calculations (PBE+U). Under reducing atmosphere, SO2adsorption leads to the formation of stable surface sulfate species on the (110) surface and sulfides on the (111) surface. A mechanism for sulfur recovery from SO2is presented where SO2reacts with a surface sulfide to form a thiosulfite species. This thiosulfite species is subsequently reduced by an oxygen vacancy and desorbed as S2(g).

The adsorption and dissociation of a formic acid molecule (HCOOH) on a partially reduced rutile TiO2–x(110) surface and the subsequent transformations of the adsorbed fragments are studied via quantum-mechanical molecular dynamics simulations and climbing-image nudged elastic band (CI-NEB) calculations. The electronic structure methods used are self-consistent-charge density functional tight binding (SCC-DFTB) and DFT+U calculations. We address the apparent lack of consensus in the literature regarding the formic acid adsorbate species that heal the O vacancies, where different experiments have suggested the occurrence of one, two, or no such species types. From our calculations, we propose that the formic acid molecule quickly dissociates on the surface into a formate ion and a proton. If no mechanism exists by which the dissociation products can migrate away from each other, three formate species will coexist on the partially reduced TiO2surface: one majority species bound to the Ti rows and two minority species healing the O vacancies. However, if such a diffusion mechanism does exist, our barrier calculations show that one of the minority species will transform into the other, and only two adsorbate types can be expected on the surface. We also identify a new adsorbate configuration (which we denote C′), where the formate is located on the row of two-coordinated oxygen atoms, healing an O vacancy and accepting an H-bond from the detached H atom.

Using hybrid density functional theory, we investigate structural and electronic properties of small Cunclusters (with n≤ 9) adsorbed on the nonpolar ZnO(101̅0) surface. The Cu clusters grow in a planar fashion up to a size of six atoms, after which the clusters take on a polyhedral shape. We find even–odd alternations with respect to both cluster stability (for n= 1–6) and cluster charge, as a function of the number of atoms. Even-numbered clusters are always charge-neutral, while odd-numbered clusters can become positively charged by donation of an electron to the ZnO conduction band, which can be traced back to the fact that the ionization energies of odd-numbered gas-phase Cu clusters are lower than for even-numbered ones. The most stable adsorbed odd-numbered clusters are neutral and planar for n≤ 3 and positively charged and polyhedral for n≥ 7. For n= 5, both neutral planar and positively charged polyhedral configurations are similarly stable.

We present a simple method, the “band-filling correction”, to calculate accurate adsorption energies (Eads) in the low coverage limit from finite-size supercell slab calculations using DFT. We show that it is necessary to use such a correction if charge transfer takes place between the adsorbate and the substrate, resulting in the substrate bands either filling up or becoming depleted. With this correction scheme, we calculate Eadsof an isolated Cu atom adsorbed on the ZnO(101̅0) surface. Without the correction, the calculated Eadsis highly coverage-dependent, even for surface supercells that would typically be considered very large (in the range from 1 nm × 1 nm to 2.5 nm × 2.5 nm). The correction scheme works very well for semilocal functionals, where the corrected Eadsis converged within 0.01 eV for all coverages. The correction scheme also works well for hybrid functionals if a large supercell is used and the exact exchange interaction is screened.

Even very low levels of sulfur contaminants can degrade the catalytic performance of cerium oxide. Here, the interaction of atomic sulfur with the ceria (111) and (110) surfaces has been studied using first-principles methods. Two sulfoxy species are identified: oxido-sulfate(2-) species (SO2–) on both the CeO2(111) and (110) surfaces and hyposulfite (SO22–) on the (110) surface. Sulfide (S2–) is formed when a surface or a subsurface oxygen atoms is replaced by sulfur. These sulfide species are more stable at the surface. Furthermore, sulfite (SO32–) structures are found when sulfur is made to replace one Ce in the ceria (111) and (110) surfaces. The calculated sulfur diffusion barriers are larger than 1.4 eV for both surfaces, and thus sulfur is essentially immobile, providing a possible explanation for the sulfidation phenomena of the ceria-based catalyst. Thus, we find three different species from interaction of S with ceria which are all, due to their strong binding, capable of poisoning the surface, reduced or unreduced. Our results suggest that under reducing conditions sulfur is likely to be found inthe (111) surface (replacing oxygen) but onthe (110) surface (as SO22–).

The ability to understand the phonon behavior in small metal oxide nanostructures and their surfaces is of great importance for thermal and microelectronic applications in successively smaller devices. Here the development of phonons in successively larger ZnO wurtzite quantum dots (QDs) is investigated. Raman spectroscopic measurements for particles from 3 to 11 nm reveal that the E2Raman active optical phonon at 436 cm–1is the first mode to be developed with a systematic increase with particle size. We also find a broad phonon band at 260–340 cm–1, attributed to surface vibrations. The E1-LO mode at 585 cm–1is the next to be developed while still being strongly suppressed in the confined particles. Other modes found in bulk ZnO are not developed for particles below 11 nm. Results from density functional theory showed an excellent agreement with the experimental molecular vibrations in the zinc acetate precursor and phonon modes in bulk ZnO. To elucidate the vibration behavior and phonon development in the ZnO QDs under nonzero temperature conditions and incorporating surface reconstruction, we performed reactive force field calculations. We show that the experimentally developed phonon modes in the QDs are the ones expected from dynamic theory. In particular, we show that the surface phonon modes in the very outermost surface (5 Å) can explain the observed broad phonon band and give the precise relation between the intensity of the surface and bulk phonons as the particle size increases. Calculations with temperatures between 50K and 1000K also show distinction of temperature effects in the material and that the phonon peaks are not generally shifted when the system is heated and quantum confined but instead reveal a dependence on the symmetry of the phonon mode.

The hydroxylation structural features of the first adsorption layer and its connection to proton transfer reactivity have been studied for the ZnO−liquid water interface at room temperature. Molecular dynamics simulations employing the ReaxFF forcefield were performed for water on seven ZnO surfaces with varying step concentrations. At higher water coverage a higher level of hydroxylation was found, in agreement with previous experimental results. We have also calculated the free energy barrier for transferring a proton to the surface, showing that stepped surfaces stabilize the hydroxylated state and decrease the water dissociation barrier. On highly stepped surfaces the barrier is only 2 kJ/mol or smaller. Outside the first adsorption layer no dissociation events were found during almost 100 ns of simulation time; this indicates that these reactions are much more likely if catalyzed by the metal oxide surface. Also, when exposed to a vacuum, the less stepped surfaces stabilize adsorption beyond monolayer coverage.

First-principles electronic structures calculations of the adsorption properties of Cu and Ni on the ceria(111) surface are presented. The adatoms (Cu, Ni) are adsorbed strongly at the hollow site on the CeO2(111) support. Metal induced gap states (MIGS) appear in the O2p-Ce4f gaps and the Cu and Ni adatoms are oxidized to Cu+ and Ni+ mainly by their next nearest neighbor Ce ion, which experiences a conversion of Ce4+→Ce3+. The bonding mechanisms for the Cu-ceria(111) and Ni-ceria(111) systems are proposed.

The uncoupled anharmonic OH-stretching vibrational frequency for the layered mineral Mg(OH)2(brucite) has been calculated in the pressure range 0−22 GPa. Quantum-mechanical electronic structure (DFT) calculations were performed, followed by quantum-mechanical vibrational energy calculations. The following findings emerged: (1) The calculated dν(OH)/dP slope is -4 cm-1/GPa, in agreement with the experimental literature value [taken as the average between the Raman and IR-measured slopes for Mg(OH)2]. (2) The calculated ν(OH) vs. R(O···O) correlation is linear and the slope is much smaller than that of traditional H-bond correlation curves in the literature. (3) The main origin of the small dν/dP and dν/dR(O···O) slopes is the small electric field variation as the mineral layers are pressed toward each other. (4) At high pressure, the OH− ions show some tendency to be tilted with respect to the c axis, and a larger tilt angle leads to a larger ν(OH) downshift. (5) The pressure variation of the D quadrupole coupling constant is approximately -1 kHz/GPa

The solvation shell structure and dynamics of a single Cu2ion in a periodic box with 32 water molecules under ambient conditions has been investigated using Car–Parrinello molecular dynamics simulations in a time-window of 18 ps. Five-fold coordination with four equidistant equatorial water molecules at 2.00 and one axial water molecule at 2.45 from the Cu2ion is found. A “hole” without water molecules is found on the opposite side of the axial water. The ion–water bonding character for the equatorial water molecules is different from that of the axial water molecules, as shown by a localized orbital analysis of the electronic structure. Moreover, the calculated OD stretching vibrational band for the equatorial water molecules lies ca. 175 cm−1below the axial-water band, in good agreement with experimental data. The equatorial-water band lies below, and the axial-water band above, the pure liquid D2O band, also in agreement with experimental data.

Liquid DMSO has been investigated by means of X-ray and neutron diffraction, quantum-chemical calculations and molecular dynamics MD simulations. A new analytical all-atom pair potential for DMSO–DMSO interactions was developed from quantum-chemical calculations and employed in the simulations. The MD-derived total structure function agrees favourably with those obtained from the X-ray and neutron diffraction experiments. An analysis of the local structure shows a rather unstructured liquid with a slight preference for antiparallel ordering of the S–O dipoles.

We present periodic ab initio calculations for ferroelectric NaNO₂. The spontaneous polarisation (P_s) has been calculated with three different models based on: (i) point charges; (ii) a multipolar expansion of the charge distribution; and (iii) the Berry phase approach. Both the Hartree–Fock and LDA Hamiltonians were employed. Within the Hartree–Fock scheme, at the optimised geometry, we obtain P_s values of 20.3, 13.0 and 16.4 µC cm−2 with the three models, compared to the experimental value of ~11.9 µCcm−2. The Berry-phase approach at the Hartree–Fock level gives a value very close to experiment (namely 12 µCcm−2) when the experimental structure is used. At the optimised LDA structure, the LDA P_s values are 16.8, 10.0 and 16.9 µC cm−2 with models (i)–(iii). The optimised lattice parameters at the Hartree–Fock level are slightly shorter (between 0 and 4%) than those determined experimentally from X-ray-diffraction, while the lattice parameters of the LDA-optimised structure are up to 10% smaller than the experiment. The calculated lattice energies are 679 and 964 kJ mol−1 at the HF and LDA levels, compared with 729 kJ mol−1 from the experiment. Charge densities, Mulliken charges and dipole moments are discussed. Finally, we have studied the mechanism for the phase transformation in-between the ferroelectric and paraelectric crystal structures; a rotation of the NO₂ group around the c-axis gives the lowest energy barrier.

We have performed periodic Hartree-Fock calculations for OH groups adsorbed on the MgO(001) surface considering different surface coverages. Six types of OH groups are discussed: OH m, OH . , H+, H . and hydrogen-bonded OH and H. It is found that when both OH and H are present on the surface, the two groups are best described as OH m . We suggest that the highest-frequency fundamental band (∼3750 cm m 1 in the experimental OH spectrum) is assigned to OH m groups adsorbed on top of Mg 2+ , while H + adsorbed on top of O 2 m give rise to the broader band at ∼3550 cm m 1 .

A molecular dynamics simulation of a 0.28 molar aqueous AlCl3 solution was performed with the flexible Bopp, Jancsó, and Heinzinger (BJH) model for water and a newly developed three-body potential for Al³⁺–(H2O)2 interactions derived from ab initio calculations. The simulations extended over 25 ps at an average temperature of 300 K. The structural properties of the solution are discussed on the basis of radial distribution functions and the orientation of the water molecules. Dynamical properties and OH frequency shifts were calculated from autocorrelation functions for the hydration shells of the ions. © 2000 John Wiley & Sons, Inc. Int J Quant Chem 80: 892–906, 2000

Hydrated Al³⁺ ions [Al(H2O)n]³⁺, n=1–6, were examined with ab initio self-consistent field (SCF) calculations. The relative contributions of two-, three-, and higher-body terms to the total interaction energy for an [Al(H2O)6]³⁺ complex were calculated. The sum of all three-body contributions amounts to ~30% of the sum of all pair-additive contributions and is opposite in sign. The three-body energy contributions were also derived for two types of [Al(H2O)2]³⁺ complexes. In the first type, both water molecules reside in the first hydration shell of Al³⁺ and in the second type there is one in the first shell and one in the second. Altogether 15,500 triplets were investigated and analytical two- and three-body potential energy functions were derived via a fitting procedure.  ©1999 John Wiley & Sons, Inc. Int J Quant Chem 75: 659–669, 1999

The potential surface for the interaction between a rigid formate ion and a rigid water molecule has been investigated byab initio methods. An analytical potential expression was derived to fit the 591 calculated SCF energies. The global minimum on the surface is -16.3 kcal/mol and corresponds to a bifurcated bonding situation.

The potential energy surfaces for the interatomic interaction in the Li+HCOO- system have been investigated byab initio methods within the rigid-molecule approximation. Analytical potential expressions were fitted to 133 calculated SCF energies for the Li+-HCOO- interaction, 42 SCF energies for the Li+-Li+ interaction, and 332 SCF energies for the HCOO--HCOO- interaction. The global minimum on the Li+-HCOO- surface is -170 kcal/mol and corresponds to the lithium ion lying on the C2 axis of the formate ion at 2.2 Å from the carbon atom on the oxygen side. The cation-cation and anion-anion interactions are repulsive everywhere, although the potential surface is markedly anisotropic for the HCOO--HCOO- interaction.

The cation influence on the water molecule in the Li+·H2O, Be2+·H2O, Mg2+·H2O and A13+·H2O complexes has been studied by means of quantum-mechanical ab initio calculations. A number of general trends are noted. (1) The calculated equilibrium water O-H distances increase with increasing binding energies, i.e. in the order Li+, Mg2+, Be2+, Al3+. The H-O-H angles differ by about ±1 ° from the calculated equilibrium angle for the free H2O molecule; the variation has no systematic trend. (2) The electron density redistribution accompanying the change in the internal H2O geometry in these complexes is considerably smaller than the redistribution brought about by the direct influence of the external field. (3) The harmonic O-H stretching force constant decreases with increased cation-water bonding. (4) The qualitative features of the density changes are very similar for the four complexes. The magnitudes of the interactions follow the relation Li+ < Mg2+ < Be2+? Al3+. An increased polarization of the H2O molecule occurs with electron migration from the H atoms towards the O atom and an accumulation of electron charge approximately at the centre of the Men+—O bond, especially in Be2+·H2O and A13+·H2O. An electron deficiency is found in the lone-pair region.

SCF and MCSCF calculations of infrared OH stretching band intensities are reported for the Mg2+OH2 and Zn2+OH2 complexes and for the free water molecule. The results are utilized in a discussion on the origin of the observed differences in infrared band intensities for hydrated ions in aqueous solutions.

One-dimensional potential energy expansions in stretching, bending and rigid-body translational and rotational coordinates up to fourth order have been calculated for bound H2O molecules by abinitio MO LCAO SCF methods. Water in crystalline ice and LiOH·H2O have been simulated by calculations on (H2O)4·H2O and (Li+)2(OH−)2H2O clusters, each surrounded by point charges. The force constants ƒ 2, ƒ 3and ƒ 4in the potential energy expression V=ƒ 2Δr3+ƒ 3Δr3+ƒ 4Δr4were calculated, where Δris a simultaneous stretching of the OH bonds in the water molecule. The force constants were found to decrease linearly with the equilibrium OH distance. Anharmonicity constants Xwere calculated for rigid-body vibrations of H2O in LiOH·H2O and found to be 14 cm−1for the rotational vibration and less than 1 cm−1for the translational vibration.

Molecular dynamics simulations of clusters of Li⁺ and Be²⁺ cations with up to 12 water molecules were performed calculating the particle trajectories using Hartree–Fock-derived forces. It was found that independent of the starting configuration tetrahedral clusters are obtained in less than 1 ps. Only for Li⁺ clusters, transition states with five and three water molecules are found. We discuss the structures of the clusters and the vibrational dynamics of the water molecules and compare them with other ab initio simulations, with simulations using analytical potential functions, and with static calculations. Water-dissociation (hydrogen transfer) reactions which take place at elevated temperatures were investigated. © 1997 John Wiley & Sons, Inc. Int J Quant Chem 65: 803–816, 1997

The correlation between intramolecular bond length and vibrational frequency shifts was calculated at the MP4(aug-cc-PVTZ) ab initio level for a number of molecules (LiH, BH, HF, OH⁻, HDO, BF, CN⁻, and HCI) exposed to uniform electric fields in the range from −0.10 to +0.10 au. The “ω vs. re” correlation curves always consist of two branches, each approximately linear. The slopes for the molecules investigated here vary between −2500 and −16600 cm⁻¹/Å. The slopes are well described by an expression containing only the free-molecule second- and third-order force constants and the reduced mass for the stretching mode. Experimental data for polar molecules can be expected to show deviations from a linear “ω vs. re” correlation (i) for molecules where the maximum of the frequency vs. field curve occurs at a positive field and (ii) for molecules where the maximum of the frequency vs. field curve falls on the negative-field side but very close to the zero-field case, and (iii) in bonding situations when there is much electron overlap. As opposed to uniform-field situations, anharmonicity and electronic overlap have a substantial influence on the “frequency vs. re” slopes in molecular environments. © 1997 John Wiley & Sons, Inc. Int J Quant Chem 63: 537–546, 1997

Vibrational spectra for the O—H stretching motion of HDO molecules in different surroundings have been calculated by quantum mechanical ab initio methods and compared with experimental spectra. The free water molecule, water chains, and ion–water clusters are discussed. Solvent effects on OH vibrations in liquid water have been calculated as well as “in‐crystal” OH frequencies in some ice and ionic crystalline hydrate structures. The importance of nonadditivity effects, electron correlation (at the mp2 level), and long‐range interactions for the total frequency downshift is demonstrated. It is shown that the inclusion of these effects, in conjunction with a variational quantum mechanical treatment of the anharmonic vibrational stretching motion (force constants up to the fourth order), yields vibrational frequencies in quantitative agreement with experiment for a wide range of aqueous systems.

The influence of an external pressure on the solvent exchange mechanisms around the Na ion in a dilute NaCl(aq) solution has been investigated through MD simulations at 0 and 200 MPa. At zero pressure most of the exchange processes (≈ 75%) can be classified in terms of associative 6-7-6 mechanisms. At 200 MPa the exchange rate is 20% higher and the mechanistic processes become more involved. The activation volume for the 6-7-6 exchange process at zero pressure was calculated from the MD simulation using Voronoi polyhedra, and gives a negative sign for Δ V . This supports the prevailing notion that a negative activation volume is indicative of an associative exchange mechanism.

EXAFS studies of metal ions with hydration numbers higher than six in aqueous solution, often show asymmetric distribution of the metal‐oxygen bond distances. The hydration number can be determined from a correlation with the bond distance. The mean Ca‐O distance 2.46(1) Å shows the calcium(II) ion to be eight‐hydrated in a wide asymmetric distribution. Theoretically calculated EXAFS oscillations for individual snapshots from an MD simulation show large variations. The scandium(III) ion is surrounded by two groups of about eight water molecules, with the mean Sc‐O distance 2.185(6) Å. The yttrium(III) ion coordinates eight waters in an asymmetric distribution at 2.368(5) Å, and the lanthanum(III) ion 6 + 3 water molecules at 2.52(2) and 2.65(3) Å, respectively. For the the uranium(IV) and thorium(IV) ions, the M‐O distances 2.42(1) and 2.45(1) Å, respectively, indicate hydration numbers close to 10.

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