scholarly journals Bond Order Densities in Real Space

2019 ◽  
Vol 124 (2) ◽  
pp. 339-352
Author(s):  
José Luis Casals-Sainz ◽  
A. Fernández-Alarcón ◽  
Evelio Francisco ◽  
Aurora Costales ◽  
Ángel Martín Pendás
Keyword(s):  
Bond Order ◽  
Real Space

Bond-Order Potentials for Molybdenum and Niobium: An Assessment of Their Quality

1998 ◽  
Vol 538 ◽  
Author(s):  
M. Mrovec ◽  
V. Vitek ◽  
D. Nguyen-Manh ◽  
D. G. Pettifor ◽  
L. G. Wang ◽  
...  
Keyword(s):  
Bond Order ◽  
Tight Binding ◽  
Direct Calculation ◽  
Real Space ◽  
Central Force ◽  
Full Potential ◽  
Extended Defects ◽  
Many Body ◽  
Deformation Path ◽  
Bond Order Potentials

AbstractThe bond-order potentials (BOP) have been constructed for Mo and Nb. These potentials are based on the real-space parametrized tight-binding method in which diagonalization of the Hamiltonian is avoided by direct calculation of the bond-order. In this scheme the energy consists of three parts: The bond part that comprises contributions of d electrons and introduces into the scheme the covalent character of bonding, the central-force many-body part that reflects the environmental dependence of sp overlap repulsion and a pair-wise contribution. The potentials were tested by calculation of energy differences between the bcc and several alternate structures and by investigating the trigonal deformation path. These calculations have been made in parallel using BOP and the full-potential linearized augmented plane-wave method. The central-force many-body Finnis-Sinclair type potentials have also been included into the study of the deformation path. This evaluation of BOP reveals that the potentials reproduce very closely the ab initio results and are, therefore, very suitable for atomistic studies of extended defects in the transition metals.

scholarly journals Revitalizing the Concept of Bond Order Through Delocalization Measures in Real Space

2018 ◽  
Author(s):  
Carlos Outeiral Rubiera ◽  
Mark Vincent ◽  
Ángel Martín Pendás ◽  
Paul L. A. Popelier
Keyword(s):  
Ab Initio ◽  
Quantum Chemical ◽  
Bond Order ◽  
Large Body ◽  
Real Space ◽  
Chemical Topology ◽  
Delocalization Index ◽  
Consistent Manner ◽  
Quantum Chemical Topology ◽  
Source Of Information

Ab initio quantum chemistry is an independent source of information supplying an ever widening group of experimental chemists. However, bridging the gap between these ab initio data and chemical insight remains a challenge. In particular, there is a need for a bond order index that characterizes novel bonding patterns in a reliable manner, while recovering the familiar effects occurring in well-known bonds. In this article, through a large body of calculations, we show how the delocalization index derived from Quantum Chemical Topology (QCT) serves as such a bond order. This index is defined in a parameter-free, intuitive and consistent manner, and with little qualitative dependency on the level of theory used. The delocalization index is also able to detect the subtler bonding effects that underpin most practical organic and inorganic chemistry. We explore and connect the properties of this index and open the door for its extensive usage in the understanding and discovery of novel chemistry.

scholarly journals Revitalizing the concept of bond order through delocalization measures in real space

2018 ◽  
Vol 9 (25) ◽  
pp. 5517-5529 ◽  
Author(s):  
Carlos Outeiral ◽  
Mark A. Vincent ◽  
Ángel Martín Pendás ◽  
Paul L. A. Popelier
Keyword(s):  
Bond Order ◽  
Real Space ◽  
Quantum Mechanical ◽  
Bond Orders ◽  
Exchange Correlation ◽  
Mechanical Bond ◽  
Topological Atoms

Quantum mechanical bond orders are obtained from integration of the exchange–correlation density between topological atoms.

scholarly journals Revitalizing the Concept of Bond Order Through Delocalization Measures in Real Space

2018 ◽  
Author(s):  
Carlos Outeiral Rubiera ◽  
Mark Vincent ◽  
Ángel Martín Pendás ◽  
Paul L. A. Popelier
Keyword(s):  
Ab Initio ◽  
Quantum Chemical ◽  
Bond Order ◽  
Large Body ◽  
Real Space ◽  
Chemical Topology ◽  
Delocalization Index ◽  
Consistent Manner ◽  
Quantum Chemical Topology ◽  
Source Of Information

Ab initio quantum chemistry is an independent source of information supplying an ever widening group of experimental chemists. However, bridging the gap between these ab initio data and chemical insight remains a challenge. In particular, there is a need for a bond order index that characterizes novel bonding patterns in a reliable manner, while recovering the familiar effects occurring in well-known bonds. In this article, through a large body of calculations, we show how the delocalization index derived from Quantum Chemical Topology (QCT) serves as such a bond order. This index is defined in a parameter-free, intuitive and consistent manner, and with little qualitative dependency on the level of theory used. The delocalization index is also able to detect the subtler bonding effects that underpin most practical organic and inorganic chemistry. We explore and connect the properties of this index and open the door for its extensive usage in the understanding and discovery of novel chemistry.

scholarly journals Real space bond orders are energetic descriptors

2018 ◽  
Vol 20 (23) ◽  
pp. 16231-16237 ◽  
Author(s):  
A. Martín Pendás ◽  
E. Francisco
Keyword(s):  
Bond Order ◽  
Real Space ◽  
Bond Orders ◽  
Chemical Interactions ◽  
Position Space

Orbital invariant position space techniques are used to show a theoretical link between the conventional concept of bond order and the energetics of chemical interactions.

scholarly journals A Bond-Order Potential Incorporating Analytic Screening Functions for the Molybdenum Silicides

2004 ◽  
Vol 842 ◽  
Author(s):  
Marc J. Cawkwell ◽  
Matous Mrovec ◽  
Duc Nguyen-Manh ◽  
David G. Pettifor ◽  
Vaclav Vitek
Keyword(s):  
Crystal Structure ◽  
High Temperatures ◽  
Bond Order ◽  
Tight Binding ◽  
Dislocation Core ◽  
Real Space ◽  
Internal Degree ◽  
Extended Defects ◽  
Ambient Conditions ◽  
Bond Order Potential

ABSTRACTThe intermetallic compound MoSi2, which adopts the C11b crystal structure, and related alloys exhibit an excellent corrosion resistance at high temperatures but tend to be brittle at room and even relatively high temperatures. The limited ductility of MoSi2 in ambient conditions along with the anomalous temperature dependence of the critical resolved shear stress (CRSS) of the {110)<111], {011)<100] and {010)<100] slip systems and departure from Schmid law behavior of the {013)<331] slip system can all be attributed to complex dislocation core structures. We have therefore developed a Bond-Order Potential (BOP) for MoSi2 for use in the atomistic simulation of dislocations and other extended defects. BOPs are a real-space, O(N), two-center orthogonal tight-binding formalism that are naturally able to describe systems with mixed metallic and covalent bonding. In this development novel analytic screening functions have been adopted to properly describe the environmental dependence of bond integrals in the open, bcc-based C11b crystal structure. A many-body repulsive term is included in the model that allows us to fit the elastic constants and negative Cauchy pressures of MoSi2. Due to the internal degree of freedom in the position of the Si atoms in the C11b structure which is a function of volume, it was necessary to adopt a self-consistent procedure in the fitting of the BOP. The constructed BOP is found to be an excellent description of cohesion in C11b MoSi2 and we have carefully assessed its transferability to other crystal structures and stoichiometries, notably C 40, C 49 and C 54 MoSi2, A15 and D03 Mo3Si and D8m Mo5Si3 by comparing with ab initio structural optimizations.

High-resolution structure determination by ALCHEMI

1983 ◽  
Vol 41 ◽  
pp. 178-181 ◽  
Author(s):  
Peter G. Self ◽  
Peter R. Buseck
Keyword(s):  
Site Occupancy ◽  
Real Space ◽  
Unit Cell ◽  
Bragg Angle ◽  
Electron Current ◽  
High Resolution Structure ◽  
Beam Incident ◽  
Crystallographic Planes ◽  
Electron Beam Incident ◽  
Resolution Structure

ALCHEMI (Atom Location by CHanneling Enhanced Microanalysis) enables the site occupancy of atoms in single crystals to be determined. In this article the fundamentals of the method for both EDS and EELS will be discussed. Unlike HRTEM, ALCHEMI does not place stringent resolution requirements on the microscope and, because EDS clearly distinguishes between elements of similar atomic number, it can offer some advantages over HRTEM. It does however, place certain constraints on the crystal. These constraints are: a) the sites of interest must lie on alternate crystallographic planes, b) the projected charge density on the alternate planes must be significantly different, and c) there must be at least one atomic species that lies solely on one of the planes.An electron beam incident on a crystal undergoes elastic scattering; in reciprocal space this is seen as a diffraction pattern and in real space this is a modulation of the electron current across the unit cell. When diffraction is strong (i.e., when the crystal is oriented near to the Bragg angle of a low-order reflection) the electron current at one point in the unit cell will differ significantly from that at another point.

An Image Reconstruction System Applied to High Voltage Microscopy

1975 ◽  
Vol 33 ◽  
pp. 292-293
Author(s):  
D. E. Johnson
Keyword(s):  
High Voltage ◽  
Prior Information ◽  
Block Diagram ◽  
Three Dimensional ◽  
Real Space ◽  
Two Dimensional ◽  
Efficient Manner ◽  
Fourier Methods ◽  
Tilt Series ◽  
Methods Of Reconstruction

Increased specimen penetration; the principle advantage of high voltage microscopy, is accompanied by an increased need to utilize information on three dimensional specimen structure available in the form of two dimensional projections (i.e. micrographs). We are engaged in a program to develop methods which allow the maximum use of information contained in a through tilt series of micrographs to determine three dimensional speciman structure.In general, we are dealing with structures lacking in symmetry and with projections available from only a limited span of angles (±60°). For these reasons, we must make maximum use of any prior information available about the specimen. To do this in the most efficient manner, we have concentrated on iterative, real space methods rather than Fourier methods of reconstruction. The particular iterative algorithm we have developed is given in detail in ref. 3. A block diagram of the complete reconstruction system is shown in fig. 1.

Correlation averaging of two-dimensional crystals: basic strategy and refinements

1986 ◽  
Vol 44 ◽  
pp. 136-139
Author(s):  
W. Baumeister ◽  
R. Rachel ◽  
R. Guckenberger ◽  
R. Hegerl
Keyword(s):  
Low Dose ◽  
Signal To Noise Ratio ◽  
Real Space ◽  
Fourier Domain ◽  
Two Dimensional ◽  
Signal To Noise ◽  
Unit Cells ◽  
Lateral Displacements ◽  
Established Technique ◽  
Crystalline Array

IntroductionCorrelation averaging (CAV) is meanwhile an established technique in image processing of two-dimensional crystals /1,2/. The basic idea is to detect the real positions of unit cells in a crystalline array by means of correlation functions and to average them by real space superposition of the aligned motifs. The signal-to-noise ratio improves in proportion to the number of motifs included in the average. Unlike filtering in the Fourier domain, CAV corrects for lateral displacements of the unit cells; thus it avoids the loss of resolution entailed by these distortions in the conventional approach. Here we report on some variants of the method, aimed at retrieving a maximum of information from images with very low signal-to-noise ratios (low dose microscopy of unstained or lightly stained specimens) while keeping the procedure economical.

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