Search results for " Methods"
showing 10 items of 4102 documents
Chemisorption of a molecular oxygen on the UN(001) surface: Ab initio calculations
2010
The results of DFT GGA calculations on oxygen molecules adsorbed upon the (0 0 1) surface of uranium mononitride (UN) are presented and discussed. We demonstrate that O2 molecules oriented parallel to the substrate can dissociate either (i) spontaneously when the molecular center lies above the surface hollow site or atop N ion, (ii) with the activation barrier when a molecule sits atop the surface U ion. This explains fast UN oxidation in air.
Metal Cluster — Surface Interaction: Simple Models and Ab Initio Calculations
1999
We review recent ab initio atomistic calculations on interactions between metal clusters and electronically inert (insulating) substrates. The model system is sodium clusters on the sodium-chloride (001) surface. This system provides an example of weak cluster-support interaction (physisorption) which can however be easily modified by introducing color centers at the surface, resulting in chemisorption of sodium adatom or cluster. The results obtained from atomistic calculations can be used for constructing simple jellium-type models for the adsorbed cluster. These models allow for systematic investigations in a large size-range of clusters on the shell structure, dimensionality, and stabil…
Quantum Mechanical Modelling of Pure and Defective KNbO3 Perovskites
2000
Ab initio electronic structure calculations using the density-functional theory (DFT) are performed for KNbO3 with and without defects. Ferroelectric distortive transitions involve very small changes in energies and are therefore sensitive to DFT-approximations. This is discussed by comparing results obtained with the local density approximation (LDA) to those where generalized gradient approximations (GGA) are used. The results of ab initio calculations for F-type centers and bound hole polarons are compared to those obtained by a semiempirical method of the Intermediate Neglect of the Differential Overlap (INDO), based on the HartreeFock formalism. Supercells with 40 and 320 atoms were us…
Lattice Dynamics in Wurtzite Semiconductors: The Bond Charge Model of CdS
1999
An extension of the adiabatic bond charge model of Rustagi and Weber is used to study the lattice dynamic properties of wurtzite-type compounds. The model has been applied to the description of the phonon dispersion of CdS, which has been recently measured by neutron scattering. The agreement with the neutron data is excellent with a small set of physically meaningful parameters. The eigenvector admixture of the E2 modes, calculated at the G-point, agrees with the experimental values obtained through the isotopic mass dependence of the optical modes and ab initio calculations.
An optical pulse modulator based on an all-fibre mirror
1996
In this article we present an all-fiber Sagnac interferometer modulated in a pulsed regime.
Parallelization of a Lattice Boltzmann Suspension Flow Solver
2002
We have applied a parallel Lattice Boltzmann method to solve the behaviour of the suspension flow. The complex behaviour of the suspension flow cannot be solved by analytical methods, so simulations are the only way to study it. Usually the size of an interesting problem is so big that calculation time on one processor is too long, and this can be solved by parallel program. We have written a parallel suspension flow solver and tested it on massive parallel computers. The measured performance of our program show that the parallelization of suspension particles was successful. We also show that over one million particles can be simulated.
Solubility of Polymers
2011
Detailed knowledge concerning the phase state (homogeneous or coexistence of two or more condensed phases) of polymer containing mixtures is indispensible in virtually any area related to the production or application of macromolecules. In addition to this qualitative information it is for many purposes highly desirable to dispose of quantitative data regarding solvent quality or, more generally, with respect to the thermodynamic interaction between the components of the mixtures. This contribution starts with a brief presentation of the thermodynamic criteria deciding on the phase state and presents the experimental methods used in this area. The next section gives an overview on typical b…
Ab initio calculations of PbTiO 3 /SrTiO 3 (001) heterostructures
2016
We performed ab initio calculations for the PbTiO3/SrTiO3 (001) heterostructures. For both PbO and TiO2-terminations of the PbTiO3 (001) thin film, augmented on the SrTiO3 (001) substrate, the magnitudes of atomic relaxations Δz increases as a function of the number of augmented monolayers. For both terminations of the augmented PbTiO3 (001) nanothin film, all upper, third and fifth monolayers are displaced inwards (Δz is negative), whereas all second, fourth and sixth monolayers are displaced outwards (Δz is positive). The B3PW calculated PbTiO3/SrTiO3 (001) heterostructure band gaps, independently from the number of augmented layers, are always smaller than the PbTiO3 and SrTiO3 bulk band…
Ab initio electronic band structure calculation of InP in the wurtzite phase
2011
Abstract We present ab initio calculations of the InP band structure in the wurtzite phase and compare it with that of the zincblende phase. In both calculations, we use the full potential linearized augmented plane wave method as implemented in the WIEN2k code and the modified Becke-Johnson exchange potential, which provides an improved value of the bandgap. The structural optimization of the wurtizte InP gives a = 0.4150 nm , c = 0.6912 nm , and an internal parameter u = 0.371 , showing the existence of a spontaneous polarization along the growth axis. As compared to the ideal wurtzite structure (that with the lattice parameter derived from the zincblende structure calculations), the actu…
Ab initio Hartree-Fock calculations of LaMnO3 (110) surfaces
2003
We present the results of ab initio Hartree-Fock calculations of the LaMnO3 (110) surface. Using seven-plane slabs, periodic in 2D and containing three formula units, we compare the properties of a stoichiometric surface with oxygen vacancies, and non-stoichiometric, defect-free surfaces, analyze the dispersion of the effective charges near the surface, and calculate the surface energy before and after relaxation, for both ferromagnetric and antiferromagnetic spin orderings in a slab. q 2003 Elsevier Ltd. All rights reserved.