5,107 research outputs found
Modeling of Covalent Bonding in Solids by Inversion of Cohesive Energy Curves
We provide a systematic test of empirical theories of covalent bonding in
solids using an exact procedure to invert ab initio cohesive energy curves. By
considering multiple structures of the same material, it is possible for the
first time to test competing angular functions, expose inconsistencies in the
basic assumption of a cluster expansion, and extract general features of
covalent bonding. We test our methods on silicon, and provide the direct
evidence that the Tersoff-type bond order formalism correctly describes
coordination dependence. For bond-bending forces, we obtain skewed angular
functions that favor small angles, unlike existing models. As a
proof-of-principle demonstration, we derive a Si interatomic potential which
exhibits comparable accuracy to existing models.Comment: 4 pages revtex (twocolumn, psfig), 3 figures. Title and some wording
(but no content) changed since original submission on 24 April 199
Order-N Density-Matrix Electronic-Structure Method for General Potentials
A new order-N method for calculating the electronic structure of general
(non-tight-binding) potentials is presented. The method uses a combination of
the ``purification''-based approaches used by Li, Nunes and Vanderbilt, and
Daw, and a representation of the density matrix based on ``travelling basis
orbitals''. The method is applied to several one-dimensional examples,
including the free electron gas, the ``Morse'' bound-state potential, a
discontinuous potential that mimics an interface, and an oscillatory potential
that mimics a semiconductor. The method is found to contain Friedel
oscillations, quantization of charge in bound states, and band gap formation.
Quantitatively accurate agreement with exact results is found in most cases.
Possible advantages with regard to treating electron-electron interactions and
arbitrary boundary conditions are discussed.Comment: 13 pages, REVTEX, 7 postscript figures (not quite perfect
Three Li-rich K giants: IRAS 12327-6523, IRAS 13539-4153, and IRAS 17596-3952
We report on spectroscopic analyses of three K giants previously suggested to
be Li-rich: IRAS 12327-6523, IRAS 13539-4153, and IRAS 17596-3952.
High-resolution optical spectra and the LTE model atmospheres are used to
derive the stellar parameters: (, log , [Fe/H]), elemental
abundances, and the isotopic ratio C/C. IRAS 13539-4153 shows an
extremely high Li abundance of (Li) 4.2, a value ten
times more than the present Li abundance in the local interstellar medium. This
is the third highest Li abundance yet reported for a K giant. IRAS 12327-6523
shows a Li abundances of (Li) 1.4. IRAS 17596-3952 is a
rapidly rotating ( 35 km s) K giant with
(Li) 2.2. Infrared photometry which shows the presence
of an IR excess suggesting mass-loss. A comparison is made between these three
stars and previously recognized Li-rich giants.Comment: 17 pages, 6 figures, accepted for A
Collective vibrational states with fast iterative QRPA method
An iterative method we previously proposed to compute nuclear strength
functions is developed to allow it to accurately calculate properties of
individual nuclear states. The approach is based on the
quasi-particle-random-phase approximation (QRPA) and uses an iterative
non-hermitian Arnoldi diagonalization method where the QRPA matrix does not
have to be explicitly calculated and stored. The method gives substantial
advantages over conventional QRPA calculations with regards to the
computational cost. The method is used to calculate excitation energies and
decay rates of the lowest lying 2+ and 3- states in Pb, Sn, Ni and Ca isotopes
using three different Skyrme interactions and a separable gaussian pairing
force.Comment: 10 pages, 11 figure
Congenital thumb anomalies and the consequences for daily life : patients’ long-term experience after corrective surgery. A qualitative study
Purpose: The aim of the study was to explore patients’ long-term experience of a congenital hand problem, and the consequences for daily life. Method: Fifteen participants with a median age 24 years (17–55 years), born with thumb hypoplasia/aplasia or thumb duplication were interviewed using a semi-structured interview guide. The interviews were subjected to qualitative content analysis. Results: Although the mobility and strength in the thumb/hand(s) varied within the group, hand function was generally described as good. Compensatory strategies were used to overcome practical obstacles. The emotional reactions to being visibly different from peers in early life varied from total acceptance and a sense of pride in being special, to deep distress and social withdrawal. Support from parents, teachers and others was important in facing emotional challenges and practical consequences. Conclusion: The present study highlights the importance of healthcare professionals addressing appearance-related concerns which may have long-term emotional and social consequences for patients born with a thumb anomaly. Implications for RehabilitationAppearance-related concerns and need for emotional support should be fully considered throughout the rehabilitation process to prevent distress and social withdrawal.Effective problem-solving strategies, such as compensation, change in occupational performance and support from others may reduce activity limitations and participation restriction
Design and validation of Segment - freely available software for cardiovascular image analysis
<p>Abstract</p> <p>Background</p> <p>Commercially available software for cardiovascular image analysis often has limited functionality and frequently lacks the careful validation that is required for clinical studies. We have already implemented a cardiovascular image analysis software package and released it as freeware for the research community. However, it was distributed as a stand-alone application and other researchers could not extend it by writing their own custom image analysis algorithms. We believe that the work required to make a clinically applicable prototype can be reduced by making the software extensible, so that researchers can develop their own modules or improvements. Such an initiative might then serve as a bridge between image analysis research and cardiovascular research. The aim of this article is therefore to present the design and validation of a cardiovascular image analysis software package (Segment) and to announce its release in a source code format.</p> <p>Results</p> <p>Segment can be used for image analysis in magnetic resonance imaging (MRI), computed tomography (CT), single photon emission computed tomography (SPECT) and positron emission tomography (PET). Some of its main features include loading of DICOM images from all major scanner vendors, simultaneous display of multiple image stacks and plane intersections, automated segmentation of the left ventricle, quantification of MRI flow, tools for manual and general object segmentation, quantitative regional wall motion analysis, myocardial viability analysis and image fusion tools. Here we present an overview of the validation results and validation procedures for the functionality of the software. We describe a technique to ensure continued accuracy and validity of the software by implementing and using a test script that tests the functionality of the software and validates the output. The software has been made freely available for research purposes in a source code format on the project home page <url>http://segment.heiberg.se</url>.</p> <p>Conclusions</p> <p>Segment is a well-validated comprehensive software package for cardiovascular image analysis. It is freely available for research purposes provided that relevant original research publications related to the software are cited.</p
- …