Periodic Ab Initio Calculations Research Articles

Host–guest interactions in an organic crystal: β-hydroquinone clathrate with Ne and HF guests

Host–guest, guest–guest, and host–host interactions in the β-hydroquinone clathrate crystal have been calculated from periodic and cluster-type ab initio calculations. The potential energy surfaces for the positions of the enclosed guest molecules in the cage have also been studied. The host framework remains virtually unchanged upon guest enclosure. The inclusion energy for the polar HF molecule is about −7 to −9 kcal/mol, in good agreement with our extrapolation from existing calorimetric data for HBr and HCl, and about −1 to −2 kcal/mol for Ne. The guest–guest interaction is small.

Ab initio study on the topological Li insertion in titanium oxide

Abstract Lithium insertion into anatase was studied by XRD, XPS and ab initio calculations. Lithium insertion in anatase was carried out by n-butyllithium and the product was analyzed by XRD and XPS. A distortion of anatase cell by the lithium insertion and removal was estimated by the ab initio periodic pseudo-potential calculations. As a result, a crystallographic transition from tetragonal to orthorhombic along with a considerable hysteretic change of the cell volume were estimated, which is well consistent with the previously reported XRD cell dimensions. In the lithium-removal process, the wide plateau in calculated cell volume was observed for Li x TiO 2 where lithium uptake x ranges from 0.25 to 1.

A theoretical study of CHx chemisorption on the Ru(0001) surface

Abstract The thermodynamics of methane decomposition on the ruthenium (0001) surface has been investigated with ab initio periodic calculations for coverages of 25% and 11%. All surface intermediates are more stable than the gas-phase methane even if the last step of the decomposition path (CH→C+H) is highly endothermic. Amongst all of the surface species, CH appears to be the most stable, in agreement with experiments. All of the surface species (CH x =3,0 and H) adsorb on three-fold sites. Short-range lateral interactions between CH x =3,0 and H are also considered and are found to be mostly repulsive.

Hartree–Fock and DFT calculations of quadrupole coupling constants in water clusters and ice

Periodic ab initio calculations of the 17 O and 2 H quadrupole coupling constants (QCC) and their shifts have been performed for ice VIII and ice IX. Cluster calculations were done for smaller water clusters and chains. The ice VIII crystal structure was optimized at the Hartree–Fock and BLYP (DFT) levels with a 6-311g ** basis. Out of the methods tested here the Hartree–Fock-computed QCC shifts based on the fully optimized Hartree–Fock structure are those which overall agree best (within 5–10%) with experimental shifts. The crystalline surroundings outside the tetrahedral ice cluster accounts for between a third and a half of the observed experimental QCC downshifts for both oxygen and hydrogen in ice VIII.

A UHV STM/STS and ab initio investigation of covellite {001} surfaces

Covellite (CuS) {001} surfaces are examined in ultra-high vacuum using atomically resolved scanning tunneling microscopy and spectroscopy (STM/STS), ultraviolet photoelectron spectroscopy (UPS), and low energy electron diffraction (LEED). The layered structure of covellite is predicted to cleave along two possible basal planes based on a bond critical point analysis of the electron density from ab initio periodic calculations. The electronic structures of the two surfaces are expected to differ primarily based on the presence of copper dangling bond states, which have an impact on the interpretation of STM/STS data. The calculated total density of states for the three-dimensional crystal are in good agreement with He I valence band spectra and the calculations indicate that the highest occupied states are of S 3 p character. Atomically resolved STM images show a periodic hexagonal array of surface sites consistent with the measured surface lattice constant using LEED. Atomically resolved STS spectra show n-type rectifying behavior due to the presence of a Schottky barrier and tip modification of the bending of the bulk bands in the sample. This behavior is indicative of a band gap surface which is due to the lack of copper dangling bonds. In this case, high tunneling current sites are assigned to S sites.

A combined molecular dynamics–ab initio study of H2 adsorption on ideal, relaxed, and temperature-reconstructed MgO(111) surfaces

2D periodic ab initio calculations have been performed for H2 interacting with a temperature-reconstructed MgO(111) surface, created from a molecular dynamics (MD) simulation at 300 K. The temperature effects involve both a long-time reconstruction of the surface and instantaneous vibrational displacements. Chemisorption energies of ∼60 μJ/mol were found. Comparisons with adsorption on ideal and relaxed MgO(111) slabs show that the physi- and chemisorption properties for the H2/MgO(111) system are very sensitive to small topological details in the surface structure. The large surface reconstruction at 300 K involves drastic changes of the surface’s adsorption properties. We also find that a low coordination number of the surface ions is not sufficient to make the site attractive for chemisorption.

Lithium oxide and superionic behaviour—A study using potentials from periodic ab initio calculations

A simple general methodology for obtaining interionic potentials from periodic ab initio calculations is presented, using periodic Hartree-Fock theory as implemented in the program crystal. To test the approach, two-body potentials are generated for Li 2O. Results obtained from our new potential are compared with those from previously suggested empirical potentials, paying most attention to the possibility of superionic behaviour in this material at high temperatures. The application of ab initio Hartree-Fock theory, lattice statics, lattice dynamics and molecular dynamics is able to provide a consistent picture of a superionic transition in lithium oxide at 1100 K. Details of the mechanism of the transition are discussed with the aid of the calculated dispersion curves at high temperature, and individual molecular dynamics trajectories.

On the structure and stability ofTi-zeolites. A comparison of cluster and periodicab initio calculations

A periodic Hartree-Fock study of Ti-containing zeolites was performed. The zeolitic framework was modeled by means of the chabazite structure. Two framework compositions were considered (TiO( Si1Ti )40 and 0.5) and the corresponding periodic structures were optimized in order to estimate the geometric and energetic features of the Ti sites. Cluster models were also considered for comparison. It arises that the main structural distortion of the zeolite lattice upon SiOTi substitution comes from the increment of the T-site volume. From the obtained results, the limitations of the different models are discussed.

A hybrid Gaussian and plane wave density functional scheme

A density functional theory-based algorithm for periodic and non-periodic ab initio calculations is presented. This scheme uses pseudopotentials in order to integrate out the core electrons from the problem. The valence pseudo-wavefunctions are expanded in Gaussian-type orbitals and the density is represented in a plane wave auxiliary basis. The Gaussian basis functions make it possible to use the efficient analytical integration schemes and screening algorithms of quantum chemistry. Novel recursion relations are developed for the calculation of the matrix elements of the density-dependent Kohn-Sham self-consistent potential. At the same time the use of a plane wave basis for the electron density permits efficient calculation of the Hartree energy using fast Fourier transforms, thus circumventing one of the major bottlenecks of standard Gaussian based calculations. Furthermore, this algorithm avoids the fitting procedures that go along with intermediate basis sets for the charge density. The performance and accuracy of this new scheme are discussed and selected examples are given.

Ab initio calculated electron densities for tetrahedral sheet [H2Si2O5]∞ in phyllosilicates

Periodic ab initio Hartree-Fock LCAO calculations have been carried out on the two dimensional sheet of SiO4 tetrahedra, representing one of the basic constituting units of layer silicates, using Huzinaga's DZP basis sets. The influence of the basis set on the chemical bonding picture is characterized by Mulliken atomic charges and by electron density maps. Silicon atomic charges ∼ +1.6 ¦e¦ are more realistic than those ∼ +2.4 ¦e¦ reported for smaller basis sets. The silicon d orbital population is found to be 0.6 in close agreement with molecular data. Electron density maps indicate the absence of charge density in the center of the ditrigonal cavity. The charge buildup of nonbonding basal oxygen orbitals is directed mainly downwards perpendicular to the sheet plane.

Electronic structure and properties of AlN

The electronic structure of the wurtzite-type phase of aluminum nitride has been investigated by means of periodic ab initio Hartree-Fock calculations. The binding energy, lattice parameters (a,c), and the internal coordinate (u) have been calculated. All structural parameters are in excellent agreement with the experimental data. The electronic structure and bonding in AlN are analyzed by means of density-of-states projections and electron-density maps. The calculated values of the bulk modulus, its pressure derivative, the optical-phonon frequencies at the center of the Brillouin zone, and the full set of elastic constants are in good agreement with the experimental data.

Energetics, structure, and compressibility of NaF determined by the periodic Hartree–Fock method

Basis set development methods for molecular cluster calculations were used to develop basis sets for periodic ab initio calculations of the energy-dependent properties of ionic crystals. Two extended, Gaussian basis sets for closed-shell sodium and fluorine ions were developed to compare with two minimal basis sets in calculating the crystal structures, enthalpies of formation, and high-pressure compression behavior of the B1 and B2 phases of NaF. Accurate calculation of the energy-dependent properties to within experimental error required the use of the larger, extended basis sets. Enthalpies of formation for NaF–B1 at 0 K and 1 bar were calculated to be between −921 and −929 kJ/mol, which agrees well with an experimental value (−927±10 kJ/mol) estimated from calorimetric data. For the B1 phase, the zero-pressure bulk modulus was calculated to be between 0.485 and 0.499 Mbar, as compared to the measured value of 0.464±0.062 Mbar. The calculated molar volume at zero pressure agrees to within 0.1% of the experimental value. For the B2 phase, the zero-pressure bulk modulus was nearly the same as that for the B1 phase, ranging from 0.494 to 0.517 Mbar. However, this calculated bulk modulus for the B2 phase was considerably smaller than the experimental value of 1.03±0.19 Mbar estimated from high-pressure compression measurements. Our calculations also suggest the V02/V01 of the B2 phase is nearer to 0.871–0.895 than to the experimental estimate of 0.811. The compression curves for both phases agreed, for the most part, with the experimental data to within the experimental error. The calculated pressure for the NaF B1/B2 transition is between 0.26 and 0.29 Mbar, slightly higher than the averaged experimental value (0.23±0.03 Mbar) but within a pressure range where both phases metastably coexist along the compression curve. The calculated volume change accompanying the phase transition (−10.1%) is in good agreement with experimental data (−10.4%). The apparent discrepancies between the zero-pressure bulk modulus and the compression curves of the B2 phase are shown to be related to the range of molar volumes used in deriving the zero-pressure properties from an equation of state.

An inverse photoemission study of empty electronic states in TiO 2

Empty electronic states in rutile (TiO 2) have been studied by inverse photoemission spectroscopy at (110) and (001) surfaces. The spectra reveal a 2 eV splitting of the empty Ti 3d bands into π ∗ and σ ∗ components, with an overall Ti 3d bandwidth of order 6 eV. These values are in good agreement with periodic ab initio Hartree-Fock calculations.

Periodic ab initio determination of interatomic potentials for alumina

Periodic ab initio Hartree-Fock calculations are used to obtain two-body potential parameters for alpha -Al2O3. The approach, which is generally applicable to crystalline solids, yields potential models that are superior to empirical parameterizations.