A periodic density functional study of the location of titanium within TS-1

The thermodynamics of the substitution of titanium within the silicalite framework to form TS-1 has been investigated using periodic density functional theory. In contrast to previous force field and ab initio cluster studies, the favoured tetrahedral sites are found to be T8 and T10 at the level of one titanium per unit cell, which accords with the best information currently available from diffraction studies. At lower concentrations, T4 and T11 are also important sites for substitution. Bond lengths for titanium to neighbouring oxygens are also found to be in good agreement with information from EXAFS. The present work suggests that the titanium distribution in TS-1 is not so greatly at variance with the thermodynamic site preferences as has been proposed on the basis of some previous theoretical studies

A first principles investigation of lithium intercalation in TiO2-B

The intercalation of lithium into the polymorph of titania, TiO2-B, has been examined using firstprinciples methods, based on the Generalized Gradient Approximation within density functionaltheory. Three symmetry unique sites have been identified for the preferential location of lithium withinthe structure at low concentration, as well as the diffusion pathways between these sites. Lithium isfound to bind most favourably at a site close to the titania octahedral layer, while the lowest activationenergy for diffusion of 27 kJ mol-1 is found for diffusion along the open channel parallel to the b axis ofthe material. The need to activate lithium towards diffusion through the population of higher energybinding sites within the channel, along with the larger barrier for lithium to migrate through thesidewalls of nanotubular TiO2-B, provides an explanation for many of the observed experimentalelectrochemical properties of this potential battery material

Three-dimensional kinetic Monte Carlo simulation of crystal growth from solution

The growth of urea crystals from water and methanol solutions has been studied with kinetic Monte Carlo simulations. Parameters for the simulations were derived from atomistic molecular dynamics simulations of the growth and dissolution of urea from water and methanol solutions. This approach allows the effect of solvation on the growth and dissolution kinetics to be fully included while extending the size of the simulation to the micrometre length scale and millisecond timescale

Atomistic theory and simulation of the morphology and structure of ionic nanoparticles

Computational techniques are widely used to explore the structure and properties of nanomaterials. This review surveys the application of both quantum mechanical and force field based atomistic simulation methods to nanoparticles, with a particular focus on the methodologies available and the ways in which they can be utilised to study structure, phase stability and morphology. The main focus of this article is on partially ionic materials, from binary semiconductors through to mineral nanoparticles, with more detailed considered of three examples, namely titania, zinc sulphide and calcium carbonate

Atomistic simulation of Mg2SiO4 and Mg2GeO4 spinels: a new model

We have developed a new interatomic potential model for the simulation of ringwoodite, the high-pressure phase of Mg2SiO4, and its low-pressure analogue, Mg2GeO4 spinel. The main novelty is the addition of a breathing shell model that enables us to accurately describe the structural and elastic parameters of both spinels up to 15PGs. Our model has also been applied to the two other Mg2SiO4 polymorphs in order to test its transferability. We find that although it is able to reproduce the structure and physical properties of wadsleyite, the breathing shell description is less successful with forsterite. the Mott-Littleton method has been used to calculate the energy of the intrinsic point defects in both spinels. The results indicate that these phases are likely to have the same defect population with teh MgO partial Schottky defect predominating

Implementation of a Z-matrix approach within the SIESTA periodic boudnary conditions code and its application to surface adsorption

We implement a flexible Z-matrix approach in the density functional theory (DFT) periodic boundary conditions code, SIESTA. This allows a mixture of Z-matrix and Cartesian coordinates to be used for geometry specification and optimisation. In addition, geometry constraints in the form of fixed coordinates and fixed linear relationships between coordinates can be specified. A Z-matrix approach in condensed phase calculations can be advantageous, for example in studying molecular adsorption onto a surface, both in terms of flexibility and efficiency.We demonstrate our implementation for the case of thiol adsorption on the Au(111) surface

From fused aromatics to graphene-like nanoribbons: The effects of multiple terminal groups, length and symmetric pathways on charge transport

class of molecular ribbons, with almost-ideal charge transmission, that is weakly dependent on the anchoring structure or electrode crystalline orientation and easy to synthesize has been identified. Charge transport through two sets of aromatic nanoribbons, based on the pyrene and perylene motifs, has been investigated using density functional theory combined with the nonequilibrium Green's function method. The effects of wire length and multiple terminal thiolate groups at the junction with gold leads have been examined. For the oligopyrene series, an exponential drop in the conductance with the increase of the wire length is found. In contrast, the oligoperylene series of nanoribbons, with dual thiolate groups, exhibits no visible length dependence, indicating that the contacts are the principal source of the resistance. Between the Au(001) leads, the transmission spectra of the oligoperylenes display a continuum of highly conducting channels and the resulting conductance is nearly independent of the bias. The predictions are robust against artefacts from the exchange-correlation potential, as evidenced from the self-interaction corrected calculations. Therefore, oligoperylene nanoribbons show the potential to be the almost-ideal wires for molecular circuitry