Effects of long-range interactions in cyclic cluster calculations of metal oxides

Development of a Cyclic Periodic Wave Function Approach for the Study of Infinitely Periodic Solid-State Systems

The ab initio cyclic periodic wave function (CPWF) approach is developed for the treatment of infinitely periodic systems. Using the full infinite Hamiltonian operator, as well as symmetrically identical basis set wave functions that preserve the translational symmetry of the electron density of the system, this approach can be applied at the Hartree-Fock level, or correlation can be directly included by the usual modes. In this approach, all many-body interactions are included, and no edge effects occur. Initial test calculations of the CPWF method at the ab initio Hartree-Fock level are performed on the chains of hydrogen fluoride molecules.

A Cyclic Periodic Wave Function Approach for the Study of Infinitely Periodic Solid-State Systems: II. Application to Helical Polysaccharides

The cyclic periodic wave function (CPWF) approach is applied at the AM1 and PM3 semiempirical levels of approximation to two infinitely periodic polymer systems in the solid state. The two polysaccharides of interest here are (1→3)-β-d-glucan and (1→3)-β-d-xylan. Our calculated results show excellent agreement with the available data for the two polysaccharides and demonstrate that the use of the CPWF approach at the AM1 and PM3 levels of approximation provides a convenient and reliable method for the study of infinitely periodic bonds of two different types: moderately strong O-H···O hydrogen bonding and strong C-O-C covalent bonding.

A Cyclic Periodic Wave Function Approach for the Study of Infinitely Periodic Solid-State Systems. I. Application to the C-H···π(C≡C) Hydrogen Bonding Systems

The cyclic periodic wave function (CPWF) approach is applied at the AM1 and PM3 semiempirical levels of approximation to infinitely periodic solid-state systems stabilized by weak CH-pi (C-H···π) interactions between repeat units. The reliability of the AM1 and PM3 methods for modeling C-H···π bonding is first demonstrated using two representative dimer systems: the T-shaped ethyne dimer and the T-shaped propyne dimer. The CPWF method is then applied to two different crystal systems that are stabilized by C-H···π interactions: (1) pent-4-ynoic acid solid and (2) a series of three infinite crystal systems-tetrakis(4-ethynylphenyl)methane solid, tetraethynylmethane solid, and tetrabutadiynylmethane solid. A comparison of our results with available data demonstrates that the use of the CPWF approach at the AM1 and PM3 levels of approximation provides a convenient and reliable method for the study of infinitely periodic systems containing very weak C-H···π bonding.

Boron-doped diamond: Investigation of the stability of surface-doping versus bulk-doping using cyclic cluster model calculations

Boron-doped bulk diamond and the boron-doped hydrogen terminated (001) surface of diamond were investigated using the cyclic cluster model. Structure and stability of the hydrogen-terminated (001) surface were calculated and compared with experimental and other theoretical results from the literature. Boron-doping was modeled by substitution of a carbon atom by a boron atom in different positions with increasing distance from the surface up to boron-doped bulk diamond. In agreement with experiments on nanoclusters, boron is most stable in the first surface layers. (c) 2008 Wiley Periodicals, Inc. J Comput Chem, 2008.

ATR-FTIR measurements and quantum chemical calculations concerning the adsorption and photoreaction of oxalic acid on TiO2

The adsorption and photoreaction of oxalic acid on the surface of anatase and rutile TiO2 nanoparticles have been studied using a combined experimental and theoretical approach. In the dark, the experimental adsorption reaches an equilibrium state that can be described as a mixture of adsorbed water and oxalic acid molecules, with the latter forming two different surface complexes on anatase and one on rutile particles. When the system is subsequently illuminated with UV(A) light, the surface becomes enriched with absorbed oxalic acid, which replaces photo-desorbed water molecules, and one of the adsorbed oxalic acid structures on anatase is favoured over the other.

Adsorption of Functionalized Benzoic Acids on MgSO4⋅H2O (100)

Using a combination of ab initio and semiempirical methods, adsorption problems on surfaces with large unit cells and low symmetry can still be studied. Here, a hybrid approach of density functional theory (DFT) and Hartree-Fock (HF) was used. As an example, we determined the geometry and the electronic properties of benzoic acid (BA), salicylic acid (SA) and para-salicylic acid (p-SA) adsorbed on MgSO(4).H(2)O (100), which are used as conditioner molecules for the electrostatic separation of minerals. Contrary to general expectations, these molecules are chemisorbed, with binding energies around 1.9 eV, forming bonds through the carboxylic O atom of the COOH groups in a nonplanar geometry, although the surface is a stoichiometric wide-band-gap insulator and the molecules stay intact. In contrast, a planar adsorption geometry turned out to be nonbonding. Bonding takes place by means of surface-molecule resonances due to the overlap of the valence band with molecular orbitals, assisted by a small charge-transfer molecule to the surface of around 0.15e. These combined interactions cause an intramolecular twist between the COOH group and the benzene ring.

Molecular Dynamics Study of the Thermal Entropy in Mixed Zinc Chalcogenides

Molecular dynamics (MD) calculations were performed to determine the vibrational contribution to the entropy of mixing and its importance for the mixing of ZnO/ZnS and ZnS/Zn3P2. These systems were modeled by cyclic clusters Zn48O48, Zn32S32, and Zn48P32. The mixed cyclic clusters considered were Zn48O47S, Zn32S31O, Zn33S30P2, and Zn47S2P30. For each of the clusters, the entropy was calculated in the range of the experimental temperature of the mixing process. The convergence of the entropy with respect to the number of MD steps was studied. Finally, the thermal part of the entropy of mixing was determined, and its dependence on the number of MD steps was investigated. It was found that the thermal entropy is important for the Gibbs free energy of mixing near the miscibility gaps.

Anion substitution in zinc chalcogenides

Anion substitution effects on the structure and energy of zinc chalcogenides were studied with the semiempirical molecular orbital method MSINDO. Cyclic clusters of different sizes were chosen as model systems. The convergence of the bulk properties of the perfect clusters with increasing cluster size was tested. Single and multiple substitution of oxygen atoms in zinc oxide by sulfur and of sulfur atoms in zinc sulfide by oxygen served to determine the energetics of substitution for these two cases. It was found that the substitution of oxygen by sulfur in ZnO is easier than the substitution of sulfur by oxygen in ZnS in agreement with experimental results. The interaction between two oxygen atoms vs. two selenium atoms in zinc sulfide was investigated. Oscillations of the cluster energy in dependence of the distance between the two doping atoms were observed. These are explained by the relative sites of the doping atoms in the crystal lattice. The magnitude of the oscillations is smaller in ZnS:Se than in ZnS:O, because the difference between the anion radii of S2- and Se2- is smaller than between S2- and O2-. This is also reflected in the band gap.

Miscibility of Zinc Sulfide and Zinc Phosphide

Solid solutions in the system zinc sulfide/zinc phosphide (Zn(2+)(x)S(2-2xP(2x)) were investigated using the cyclic cluster model within the semiempirical MSINDO method. Results of cyclic cluster calculations for binding energies of the perfect ZnS and Zn(3)P(2) are presented and compared with the experimental data. The miscibility of ZnS and Zn(3)P(2) over the whole composition range of 0 < x < 1 was investigated by calculating the Gibbs free energy of mixing Delta(M)G for different values of x. A miscibility gap was found at both ends of the composition range and compared with experimental data.

Structural and Electronic Properties of Li2B4O7

The reliability of various quantum-chemical approaches for the calculation of bulk properties of lithium tetraborate Li(2)B(4)O(7) was examined. Lattice parameters and the electronic structure obtained with density-functional theory (DFT), with DFT-Hartree-Fock (HF) hybrid methods, and with the semiempirical method MSINDO were compared to available experimental data. We also compared the results at DFT level using different wave functions, either based on linear combinations of atom-centered orbitals (LCAO), or on plane waves, as implemented in the crystalline orbital programs CRYSTAL and VASP. The basis set dependence of calculated properties was investigated for the LCAO method. In the plane wave approach ultrasoft pseudopotentials (US PP), and projector-augmented wave (PAW) potentials were used to represent the core electrons. For all methods under consideration, the calculated Li(2)B(4)O(7) structure parameters are close to each other and agree within a few percent with measured values. A more pronounced method dependence was found for the band structure, the band gap and the cohesive energy. Closest agreement between theoretical and experimental results for the band gap was obtained with the DFT-HF hybrid methods while pure DFT methods underestimate and HF based methods overestimate the measured value. It was found that the calculated band gap strongly depends on the atomic basis set in the LCAO approach. The description of the core electrons considerably affects the cohesive energy obtained with the plane wave approach. Atomic charges based on a Mulliken analysis were compared to effective charges obtained from Raman spectroscopy. Electron density maps are used to analyze the character of B-O and Li-O interactions.

Toward an Understanding of the Formation of Vanadia−Titania Catalysts

Structures of hydrated vanadia species on the TiO2-anatase surfaces were investigated using the semiempirical molecular orbital method MSINDO. The (101), (001), and (100) surfaces of anatase were considered. They were modeled by appropriate two-dimensional cyclic clusters of TiO2. Monomeric and dimeric hydrated vanadia species on the anatase surfaces were simulated by adsorbing VO4H3 and V2O7H4 molecules, respectively. Different adsorption structures were considered, and their stabilities at 300 and 600 K were tested by constant-temperature Born-Oppenheimer molecular dynamics simulations in the framework of MSINDO. Structural features of the vanadia-titania catalysts found in extended X-ray absorption fine structure, secondary ion mass spectrometry, IR, Raman, and NMR spectroscopy and conductivity experiments can be explained by the present calculations.

Properties of Ternary Insulating Systems: The Electronic Structure of MgSO4·H2O

Structural and electronic properties of (100)-oriented MgSO(4) and MgSO(4).H(2)O surfaces and the adsorption of water on the latter were investigated theoretically with a combination of ab initio and semiempirical methods. Ab initio electronic structure calculations were based on a density functional theory (DFT)-Hartree-Fock (HF) hybrid approach. The semiempirical method MSINDO was used for the determination of the local adsorption geometry of the water molecule. With the hybrid method good agreement was obtained with the experimental band gap of 7.4 eV determined with electron energy loss spectroscopy of polycrystalline MgSO(4).H(2)O samples under ultrahigh vacuum conditions. The valence bands of the (100) surfaces of both MgSO(4) and MgSO(4).H(2)O are formed mainly by the O2p levels, whereas the S2p states contribute to the lower part of the conduction band. The preferred adsorption site of water at MgSO(4).H(2)O (100) is above a surface Mg atom. The water molecule is stabilized by two additional hydrogen bonds with surface atoms. Only small differences between the electronic structure of MgSO(4).H(2)O and MgSO(4) were observed. Also, the molecular adsorption of water on the MgSO(4).H(2)O surface leads to only small shifts of the electronic energy levels.

Molecular dynamics investigation of oxygen vacancy diffusion in rutile

Oxygen vacancy diffusion in rutile was studied by Born-Oppenheimer molecular dynamics techniques in the framework of the semiempirical molecular orbital method MSINDO. Migration of an oxygen vacancy from the rutile (110) surface towards the bulk was simulated. The metadynamics technique was employed to accelerate the diffusion processes. In this way, transition state structures and activation energies for the diffusion processes were obtained. Rate constants and the time scale of diffusion processes were estimated for different temperatures using the calculated activation energy. It was found that the vacancies in the bulk are less stable than on the surface. The feasibility of oxygen vacancy diffusion under experimental conditions is discussed.

Models for the treatment of crystalline solids and surfaces

Crystalline solids and surfaces have become a subject of growing interest. The difficulty of a comprehensive description of a variety of phenomena by a single method has led to the development of many models. These models can be classified as nonperiodic and periodic models. The former include free clusters, saturated clusters, and embedded clusters. The latter two models serve to remove the boundary effects of the free clusters. No perfect avoidance of such effects can be achieved in this way. The cyclic cluster model overcomes this difficulty in a natural way. It is periodic with a finite periodicity. An embedding can take into account a long-range effect in ionic crystals. Previous periodic approaches relied on the large unit cell model, which is related to the supercell approach. For perfect crystals the conventional unit cell approach is a well-known standard. However, its disadvantage is the unphysical periodicity of defects, which is avoided in the cyclic cluster model. The present article presents a description of these models together with selective applications to solid-state systems and surfaces.