49 research outputs found
A Self-Consistent First-Principles Technique Having Linear Scaling
An algorithm for first-principles electronic structure calculations having a
computational cost which scales linearly with the system size is presented. Our
method exploits the real-space localization of the density matrix, and in this
respect it is related to the technique of Li, Nunes and Vanderbilt. The density
matrix is expressed in terms of localized support functions, and a matrix of
variational parameters, L, having a finite spatial range. The total energy is
minimized with respect to both the support functions and the elements of the L
matrix. The method is variational, and becomes exact as the ranges of the
support functions and the L matrix are increased. We have tested the method on
crystalline silicon systems containing up to 216 atoms, and we discuss some of
these results.Comment: 12 pages, REVTeX, 2 figure
Insights into the function of silver as an oxidation catalyst by ab initio, atomistic thermodynamics
To help understand the high activity of silver as an oxidation catalyst,
e.g., for the oxidation of ethylene to epoxide and the dehydrogenation of
methanol to formaldehyde, the interaction and stability of oxygen species at
the Ag(111) surface has been studied for a wide range of coverages. Through
calculation of the free energy, as obtained from density-functional theory and
taking into account the temperature and pressure via the oxygen chemical
potential, we obtain the phase diagram of O/Ag(111). Our results reveal that a
thin surface-oxide structure is most stable for the temperature and pressure
range of ethylene epoxidation and we propose it (and possibly other similar
structures) contains the species actuating the catalysis. For higher
temperatures, low coverages of chemisorbed oxygen are most stable, which could
also play a role in oxidation reactions. For temperatures greater than about
775 K there are no stable oxygen species, except for the possibility of O atoms
adsorbed at under-coordinated surface sites Our calculations rule out thicker
oxide-like structures, as well as bulk dissolved oxygen and molecular
ozone-like species, as playing a role in the oxidation reactions.Comment: 15 pages including 9 figures, Related publications can be found at
http://www.fhi-berlin.mpg.de/th/paper.htm
Sub-surface Oxygen and Surface Oxide Formation at Ag(111): A Density-functional Theory Investigation
To help provide insight into the remarkable catalytic behavior of the
oxygen/silver system for heterogeneous oxidation reactions, purely sub-surface
oxygen, and structures involving both on-surface and sub-surface oxygen, as
well as oxide-like structures at the Ag(111) surface have been studied for a
wide range of coverages and adsorption sites using density-functional theory.
Adsorption on the surface in fcc sites is energetically favorable for low
coverages, while for higher coverage a thin surface-oxide structure is
energetically favorable. This structure has been proposed to correspond to the
experimentally observed (4x4) phase. With increasing O concentrations, thicker
oxide-like structures resembling compressed Ag2O(111) surfaces are
energetically favored. Due to the relatively low thermal stability of these
structures, and the very low sticking probability of O2 at Ag(111), their
formation and observation may require the use of atomic oxygen (or ozone, O3)
and low temperatures. We also investigate diffusion of O into the sub-surface
region at low coverage (0.11 ML), and the effect of surface Ag vacancies in the
adsorption of atomic oxygen and ozone-like species. The present studies,
together with our earlier investigations of on-surface and
surface-substitutional adsorption, provide a comprehensive picture of the
behavior and chemical nature of the interaction of oxygen and Ag(111), as well
as of the initial stages of oxide formation.Comment: 17 pages including 14 figures, Related publications can be found at
http://www.fhi-berlin.mpg.de/th/paper.htm
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Developmental trajectories of infants with multiplex family risk for Autism: a Baby Siblings Research Consortium Study
Importance: Autism spectrum disorder (ASD) is a neurodevelopmental disorder associated with
different genetic etiologies. Prospective examination of familial-risk infants informs
understanding of developmental trajectories preceding ASD diagnosis, potentially improving
early detection.
Objective: Compare outcomes and trajectories associated with varying familial risk for ASD
across first 3 years of life.
Design and Setting: Longitudinal, prospective observational study. Data from 11 sites in Baby
Siblings Research Consortium (BSRC) database included. Data collected between 2003-2015.
Infants followed for 3 years. Analyses conducted in 2018.
Participants: Of initial 1,008 infants from BSRC database, 573 removed due to missing
necessary data, diagnostic discrepancies, or only one older sibling. 435 younger siblings of
children with ASD included; 355 from single-incidence families (1 sibling with ASD and 1+
sibling without ASD) and 80 from multiplex families (2+ siblings with ASD). No group
differences in major demographics.
Exposure: Number of ASD-siblings.
Main Outcomes and Measures: Outcomes included ASD symptoms, cognitive abilities, and
adaptive skills. Diagnosis (ASD/no-ASD) given at 36-month outcome. No-ASD group classified
as atypical (developmental delays and/or social-communication concerns) or typical for some
analyses. Generalized linear mixed models examined developmental trajectories by ASD
outcome and familial-risk group.
Results: In the 435 analyzed participants (age range at outcome: 32-43 months; 57% male),
children from multiplex families were more likely than those from single-incidence families to
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be classified as ASD (36% vs. 16%, p<.001) and less likely as typical (33% vs. 57%, p<.001),
with similar rates of atypical classifications (31% vs. 27%, p=.49). No differences in ASD
symptoms between multiplex and single-incidence groups, after controlling for ASD outcome
(p=.18). During infancy, differences in cognitive and adaptive abilities observed based upon
ASD outcome in single-incidence group only (ps<.001-.04). At 36 months, multiplex/no-ASD
group had lower cognitive abilities than single-incidence/no-ASD group (p=.02), and multiplex
had lower adaptive abilities than single-incidence, after controlling for ASD outcome (p=.02).
Conclusions and Relevance: Infants with a multiplex family history of ASD should be
monitored early and often and referred for early intervention at the first sign of concern. Direct
examination of genetic contributions to neurodevelopmental phenotypes in infants with familial
risk for ASD is needed
Stochastic method for automatic recognition of topics
The field of topic spotting in conversational speech has received growing attention in recent years. The goal of this field is to develop a system that can identify topics of interest among large volumes of speech data. In order to cope with practical considerations, researchers are concentrating on phoneme-based methods, which eliminate the need for topic specific data to be hand-transcribed. A number of different phoneme-based approaches have recently been proposed, of which the Euclidean Nearest Wrong Neighbour (ENWN) system has yielded the most promising experimental results. A phoneme-based topic spotter makes use of a phoneme recognizer to transcribe the speech data. The main problem of this approach is that the accuracy of such transcriptions is very poor. Typically, only between 40 and 50 percent of the phonemes are transcribed correctly. It is therefore important to compensate for the low quality of the transcriptions. However, existing techniques make no use of statistical modelling to compensate for transcription errors. In this research, a Stochastic Method for Automatic Recognition of Topics (SMART) was developed to address the abovementioned problem. The resulting system is an extension of the existing ENWN algorithm. Comparative results indicate an improvement of SMART over ENWN characterized by a 26% reduction in ROC (receiver operating characteristic) error area. This difference was found to be statistically significant.Articl
Academic writing: Whose expectations?
10.1177/003368820103200104RELC Journal32152-7
On feminist methodology
There is now a considerable literature advocating a feminist methodology. This article summarises the features of such a methodology under four headings: the ubiquitous social significance of gender, the validity of experience as against method, the rejection of hierarchy in the research relationship, and the adoption of the emancipation of women as the goal of research and the criterion of validity. The arguments supporting each of these themes are assessed. The conclusion reached is that while some of these arguments are convincing the overall case for a feminist methodology is not