Methane transformation to carbon and hydrogen on Pd(100): Pathways and energetics from density functional theory calculations

Density functional theory with gradient corrections has been employed to study the reaction pathways and the reaction energetics for the transformations of CH4 to C and H on a Pd(100) surface. On examination of transition state structures identified in each elementary reaction, a clear relationship between the valencies of the CHx fragments and the locations of the transition states emerges. The higher the valency of the CHx fragment, the higher the coordination number of the CHx with the surface atoms. The calculated reaction energetics are in good agreement with the experiments. In addition, calculation results are also used to illustrate an interesting issue concerning the CH3 stability on Pd surfaces. (C) 2002 American Institute of Physics.

Reduced-size polarized basis sets for calculations of molecular electric properties. III. Second-row atoms

Reduced-size polarized (ZmPolX) basis sets are developed for the second-row atoms X = Si, P, S, and Cl. The generation of these basis sets follows from a simple physical model of the polarization effect of the external electric field which leads to highly compact polarization functions to be added to the chosen initial basis set. The performance of the ZmPolX sets has been investigated in calculations of molecular dipole moments and polarizabilities. Only a small deterioration of the quality of the calculated molecular electric properties has been found. Simultaneously the size of the present reduced-size ZmPolX basis sets is about one-third smaller than that of the usual polarized (PolX) sets. This reduction considerably widens the range of applications of the ZmPolX sets in calculations of molecular dipole moments, dipole polarizabilities, and related properties.

Effect of quantization of vibrations on the structural properties of crystals

We study the structural effects produced by the quantization of vibrational degrees of freedom in periodic crystals at zero temperature. To this end we introduce a methodology based on mapping a suitable subspace of the vibrational manifold and solving the Schrödinger equation in it. A number of increasingly accurate approximations ranging from the quasiharmonic approximation (QHA) to the vibrational self-consistent field (VSCF) method and the exact solution are described. A thorough analysis of the approximations is presented for model monatomic and hydrogen-bonded chains, and results are presented for a linear H-F chain where the potential-energy surface is obtained via first-principles electronic structure calculations. We focus on quantum nuclear effects on the lattice constant and show that the VSCF is an excellent approximation, meaning that correlation between modes is not extremely important. The QHA is excellent for covalently bonded mildly anharmonic systems, but it fails for hydrogen-bonded ones. In the latter, the zero-point energy exhibits a nonanalytic behavior at the lattice constant where the H atoms center, which leads to a spurious secondary minimum in the quantum-corrected energy curve. An inexpensive anharmonic approximation of noninteracting modes appears to produce rather good results for hydrogen-bonded chains for small system sizes. However, it converges to the incorrect QHA results for increasing size. Isotope effects are studied for the first-principles H-F chain. We show how the lattice constant and the H-F distance increase with decreasing mass and how the QHA proves to be insufficient to reproduce this behavior.

Atomistic studies of interactions between the dominant lattice dislocations and γ/γ-lamellar boundaries in lamellar γ-TiAl

This paper reports on atomistic simulations of the interactions between the dominant lattice dislocations in ?-TiAl (<1 0 1] superdislocations) with all three kinds of ?/?-lamellar boundaries in polysynthetically twinned (PST) TiAl. The purpose of this study is to clarify the early stage of lamellar boundary controlled plastic deformation in PST TiAl. The interatomic interactions in our simulations are described by a bond order potential for L10-TiAl which provides a proper quantum mechanical description of the bonding. We are interested in the dislocation core geometries that the lattice produces in proximity to lamellar boundaries and the way in which these cores are affected by the elastic and atomistic effects of dislocation-lamellar boundary interaction. We study the way in which the interfaces affect the activation of ordinary dislocation and superdislocation slip inside the ?-lamellae and transfer of plastic deformation across lamellar boundaries. We find three new phenomena in the atomic-scale plasticity of PST TiAl, particularly due to elastic and atomic mismatch associated with the 60° and 120° ?/?-interfaces: (i) two new roles of the ?/?-interfaces, i.e. decomposition of superdislocations within 120° and 60° interfaces and subsequent detachment of a single ordinary dislocation and (ii) blocking of ordinary dislocations by 60° and 120° interfaces resulting in the emission of a twinning dislocation.

Evolution of Clay Morphology in Polypropylene/Montmorillonite Nanocomposites upon Equi-biaxial Stretching: A Solid-State NMR and TEM Approach

Solid-state NMR and TEM were used to quantitatively examine the evolution of clay morphology upon equibiaxial stretching of polypropylene/montmorillonite (PP-MMT) nanocomposites up to a stretch ratio (?= final length/initial length) of 3.5. 1 H spin-lattice relaxation times were measured by the saturation-recovery sequence. For the nanocomposites, initial portions of the magnetization recovery
curves (e~20 ms) were found to depend on v t, indicative of diffusion-limited relaxation and in agreement with calculations based on estimates of the spin-diffusion barrier radius surrounding the paramagnetic centers in the clay, the electron-nucleus coupling constant, and the spin-diffusion coefficient. Initial slopes of these magnetization recovery curves directly correlated with the fraction of clay/polymer interface. New clay surface was exposed as a near linear function of strain. Long-time portions of the magnetization recovery curves yielded information on the average interparticle separations, which decreased slowly before reaching a plateau at ?=~2.5 as particles aligned. TEM images supported these findings and were used to define and quantify degrees of exfoliation and homogeneity from the NMR data. Exfoliation, defined as (platelets/ stack)-1, increased from 0.38 (unstretched) to 0.80 at ? = 3.5 for PP-MMT nanocomposites stretched at
150 C and 16 s-1. A lower stretch temperature, 145 C, which is slightly below melting onset, led to an exfoliation degree of 0.87 at ?= 2.8, consistent with the ability of higher melt viscosities to allow for higher shear stress transfer. Exposure of new clay surface is attributed to aggregate breakup and orientation at low strains (? e ~2) and to platelets sliding apart at higher strains.

A density functional theory study of the surface relaxation and reactivity of Cu2O(100)

Density functional theory has been used to investigate the surface relaxation of Cu2O(100) and the adsorption of NO. The calculations indicate the formation of surface copper dimers on relaxation coupled with a large contraction of the spacing between the first and second layers. Local density of states for atoms in the top three layers shows that the third layer copper atoms have the greatest change in bonding character. Adsorption energies have been calculated for the N-down and O-down adsorption of NO on the Cu2O(100) surface. These indicate that N-down adsorption is favoured and that in this case NO-lattice oxygen interactions dominate the adsorbate structure. (C) 2000 Elsevier Science B.V. All rights reserved.

Upper and lower bounds of 180 degrees unit element of a ridge waveguide: Calculations and measurements

The upper and lower bounds on the actual solution of any microwave structure is of general interest. The purpose of this letter is to compare some calculations using the mode-matching and finite-element methods, with some measurements on a 180 degrees ridge waveguide insert between standard WR62 rectangular waveguides. The work suggests that the MMM produces an upper bound, while the FEM places a lower bound on the measurement. (C) 2001 John Wiley & Sons, Inc.

Crystal structure and ab initio calculations of a cyano-carbamimidic acid ethyl ester

The structure of one tautomer (amine form) of cyano-carbamimidic acid ethyl ester or (amino-ethoxy-methylidene)aminoformonitrile (CAS: 13947-84-7) was determined by single crystal X-ray diffraction. Ab initio quantum chemical calculations at the B3LYP, MP2 and G3 levels were performed to investigate the stability and the formation of the different tautomers and conformers. The calculations indicate that the amine form is the more stable tautomer, showing a high degree of election conjugation. The most stable amine conformer located by the calculations corresponds to the crystallized structure. On the contrary, in the less stable imine form, the conjugation is separated by a N2-C2 single bond. (C) 2007 Elsevier B.V. All rights reserved.

An analysis of geometric uncertainty calculations for prostate radiotherapy in clinical practice.

Margins are used in radiotherapy to assist in the calculation of planning target volumes. These margins can be determined by analysing the geometric uncertainties inherent to the radiotherapy planning and delivery process. An important part of this process is the study of electronic portal images collected throughout the course of treatment. Set-up uncertainties were determined for prostate radiotherapy treatments at our previous site and the new purpose-built centre, with margins determined using a number of different methods. In addition, the potential effect of reducing the action level from 5 mm to 3 mm for changing a patient set-up, based on off-line bony anatomy-based portal image analysis, was studied. Margins generated using different methodologies were comparable. It was found that set-up errors were reduced following relocation to the new centre. Although a significant increase in the number of corrections to a patient's set-up was predicted if the action level was reduced from 5 mm to 3 mm, minimal reduction in patient set-up uncertainties would be seen as a consequence. Prescriptive geometric uncertainty analysis not only supports calculation and justification of the margins used clinically to generate planning target volumes, but may also best be used to monitor trends in clinical practice or audit changes introduced by new equipment, technology or practice. Simulations on existing data showed that a 3 mm rather than a 5 mm action level during off-line, bony anatomy-based portal imaging would have had a minimal benefit for the patients studied in this work.

A Cyclotomic Lattice Based Quasi-Orthogonal STBC for Eight Transmit Antennas

In this letter, we propose a lattice-based full diversity design for rate-one quasi-orthogonal space time block codes (QSTBC) to obtain an improved diversity product for eight transmit antennas where the information bits are mapped into 4-D lattice points instead of the common modulation constellations. Particularly, the diversity product of the proposed code is directly determined by the minimum Euclidean distance of the used lattice and can be improved by using the lattice packing. We show analytically and by using simulation results that the proposed code achieves a larger diversity product than the rate-one QSTBCs reported previously.

Achieving Space and Time Diversity by Using Lattice Constellation Based Joint Alamouti Coding

In this letter, we propose a simple space-time code to simultaneously achieve both the space and time diversities over time dispersive channels by using two-dimensional lattice constellations and Alamouti codes. The proposed scheme still reserves full space diversity and double-real-symbols joint maximum likelihood decoding which has the similar computation complexity as the Alamouti code.