22,888 research outputs found
Structure determination from powder data : Mogul and CASTEP
When solving the crystal structure of complex molecules from powder data, accurately locating the global minimum can be challenging, particularly where the number of internal degrees of freedom is large. The program Mogul provides a convenient means to access typical torsion angle ranges for fragments related to the molecule of interest. The impact that the application of modal torsion angle constraints has on the structure determination process of two structure solution attempts using DASH is presented. Once solved, accurate refinement of a molecular structure against powder data can also present challenges. Geometry optimisation using density functional theory in CASTEP is shown to be an effective means to locate hydrogen atom positions reliably and return a more accurate description of molecular conformation and intermolecular interactions than global optimisation and Rietveld refinement alone
The face-space duality hypothesis: a computational model
Valentine's face-space suggests that faces are represented in a psychological multidimensional space according to their perceived properties. However, the proposed framework was initially designed as an account of invariant facial features only, and explanations for dynamic features representation were neglected. In this paper we propose, develop and evaluate a computational model for a twofold structure of the face-space, able to unify both identity and expression representations in a single implemented model. To capture both invariant and dynamic facial features we introduce the face-space duality hypothesis and subsequently validate it through a mathematical presentation using a general approach to dimensionality reduction. Two experiments with real facial images show that the proposed face-space: (1) supports both identity and expression recognition, and (2) has a twofold structure anticipated by our formal argument
Cognitive representations of disability behaviours in people with mobility limitations : consistency with theoretical constructs
Disability is conceptualised as behaviour by psychological theory and as a result of bodily impairment by medical models. However, how people with disabilities conceptualise those disabilities is unclear. The purpose of this study was to examine disability representations in people with mobility disabilities. Thirteen people with mobility disabilities completed personal repertory grids (using the method of triads) applied to activities used to measure disabilities. Ten judges with expertise in health psychology then examined the correspondence between the elicited disability constructs and psychological and medical models of disability. Participants with mobility disabilities generated 73 personal constructs ofdisability. These constructs were judged consistent with the content of two psychological models, namely the theory of planned behaviour and social cognitive theory and with the main medical model of disability, the International Classification of Functioning Disability and Health.Individuals with activity limitations conceptualise activities in a manner that is compatible with both psychological and medical models. This ensures adequate communication in contexts where the medical model is relevant, e.g. clinical contexts, as well as in everyday conversation about activities and behaviours. Finally, integrated models of disability may be of value for theory driven interdisciplinary approaches to disability and rehabilitation
Nuclear Stopping as A Probe to In-medium Nucleon-nucleon Cross Section in Intermediate Energy Heavy Ion Collisions
Using an isospin-dependent quantum molecular dynamics, nuclear stopping in
intermediate heavy ion collisions has been studied. The calculation has been
done for colliding systems with different neutron-proton ratios in beam energy
ranging from 15MeV/u to 150MeV/u. It is found that, in the energy region from
above Fermi energy to 150MeV/u, nuclear stopping is very sensitive to the
isospin dependence of in-medium nucleon-nucleon cross section, but insensitive
to symmetry potential. From this investigation, we propose that nuclear
stopping can be used as a new probe to extract the information on the isospin
dependence of in-medium nucleon-nucleon cross section in intermediate energy
heavy ion collisions
Theory of high-energy emission from the pulsar/Be-star system PSR 125963 I: radiation mechanisms and interaction geometry
We study the physical processes of the PSR B1259-63 system containing a 47 ms
pulsar orbiting around a Be star in a highly eccentric orbit. Motivated by the
results of a multiwavelength campaign during the January 1994 periastron
passage of PSR B1259-63, we discuss several issues regarding the mechanism of
high-energy emission. Unpulsed power law emission from the this system was
detected near periastron in the energy range 1-200 keV. We find that the
observed high energy emission from the PSR B1259-63 system is not compatible
with accretion or propeller-powered emission. Shock-powered high-energy
emission produced by the pulsar/outflow interaction is consistent with all high
energy observations. By studying the evolution of the pulsar cavity we
constrain the magnitude and geometry of the mass outflow outflow of the Be
star. The pulsar/outflow interaction is most likely mediated by a collisionless
shock at the internal boundary of the pulsar cavity. The system shows all the
characteristics of a {\it binary plerion} being {\it diffuse} and {\it compact}
near apastron and periastron, respectively. The PSR B1259-63 cavity is subject
to different radiative regimes depending on whether synchrotron or inverse
Compton (IC) cooling dominates the radiation of electron/positron pairs
advected away from the inner boundary of the pulsar cavity. The highly
non-thermal nature of the observed X-ray/gamma-ray emission near periastron
establishes the existence of an efficient particle acceleration mechanism
within a timescale shown to be less than s. A synchrotron/IC
model of emission of e\pm-pairs accelerated at the inner shock front of the
pulsar cavity and adiabatically expanding in the MHD flow provides an excellent
explanation of the observed time variableX-ray flux and spectrum from the PSRComment: 68 pages, accepted for publication in the Astrophys. J. on Aug. 26,
199
Wealth Condensation in Pareto Macro-Economies
We discuss a Pareto macro-economy (a) in a closed system with fixed total
wealth and (b) in an open system with average mean wealth and compare our
results to a similar analysis in a super-open system (c) with unbounded wealth.
Wealth condensation takes place in the social phase for closed and open
economies, while it occurs in the liberal phase for super-open economies. In
the first two cases, the condensation is related to a mechanism known from the
balls-in-boxes model, while in the last case to the non-integrable tails of the
Pareto distribution. For a closed macro-economy in the social phase, we point
to the emergence of a ``corruption'' phenomenon: a sizeable fraction of the
total wealth is always amassed by a single individual.Comment: 4 pages, 1 figur
Isospin relaxation time in heavy-ion collisions at intermediate energies
Using an isospin-dependent transport model, we have studied the isospin and
momentum relaxation times in the heavy residues formed in heavy-ion collisions
at intermediate energies. It is found that only at incident energies below the
Fermi energy, chemical or thermal equilibrium can be reached before dynamical
instability is developed in the heavy residues. Also, the isospin relaxation
time is shorter (longer) than that for momentum at beam energies lower (higher)
than the Fermi energy.Comment: 8 pages Latex + 2 ps Figs.; Phys. Rev. C in pres
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