7,226 research outputs found
Development of an Advanced Force Field for Water using Variational Energy Decomposition Analysis
Given the piecewise approach to modeling intermolecular interactions for
force fields, they can be difficult to parameterize since they are fit to data
like total energies that only indirectly connect to their separable functional
forms. Furthermore, by neglecting certain types of molecular interactions such
as charge penetration and charge transfer, most classical force fields must
rely on, but do not always demonstrate, how cancellation of errors occurs among
the remaining molecular interactions accounted for such as exchange repulsion,
electrostatics, and polarization. In this work we present the first generation
of the (many-body) MB-UCB force field that explicitly accounts for the
decomposed molecular interactions commensurate with a variational energy
decomposition analysis, including charge transfer, with force field design
choices that reduce the computational expense of the MB-UCB potential while
remaining accurate. We optimize parameters using only single water molecule and
water cluster data up through pentamers, with no fitting to condensed phase
data, and we demonstrate that high accuracy is maintained when the force field
is subsequently validated against conformational energies of larger water
cluster data sets, radial distribution functions of the liquid phase, and the
temperature dependence of thermodynamic and transport water properties. We
conclude that MB-UCB is comparable in performance to MB-Pol, but is less
expensive and more transferable by eliminating the need to represent
short-ranged interactions through large parameter fits to high order
polynomials
Left lateralization in autobiographical memory: An fMRI study using the expert archival paradigm
In brain-imaging and behavioural research, studies of autobiographical memory have higher ecological validity than controlled laboratory memory studies. However, they also have less controllability over the variables investigated. Here we present a novel technique—the expert archival paradigm—that increases controllability while maintaining ecological validity. Stimuli were created from games played by two international-level chess masters. We then asked these two players to perform a memory task with stimuli generated from their own games and stimuli generated from other players’ games while they were scanned using fMRI. We found a left lateralised pattern of brain activity which was very similar in both masters. The brain areas activated were the left temporo-parietal junction and left frontal areas. The expert archival paradigm has the advantage of not requiring an interview to assess the participants’ autobiographical memories, and affords the possibility of measuring their accuracy of remembering as well as their brain activity related to remote and recent memories. It can also be used in any field of expertise, including arts, sciences and sports, in which archival data are available
Geology of Hadley Rille
The regional setting, external and internal shape, and materials of Hadley Rille near the Apollo 15 landing site are described. The petrography presented includes lithologies, regolith, talus, and outcrops. The stratigraphy exposed on the rille wall is also considered
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Brain Localisation of Memory Chunks in Chessplayers
Chess experts store domain-specific representations in their long-term memory; due to the activation of such representations, they perform with high accuracy in tasks that require the maintenance of previously seen information. Chunk-based theories of expertise (chunking theory: Chase & Simon, 1973; template theory: Gobet & Simon, 1996) state that expertise is acquired mainly by the acquisition and storage in long-term memory of familiar chunks that allow quick recognition. We tested some predictions of these theories by using fMRI while chessplayers performed a recognition memory task. These theories predict that chessplayers access long-term memory chunks of domain-specific information, which are presumably stored in the temporal lobes. We also predicted that the recognition memory tasks would activate working memory areas in the frontal and parietal lobes. These predictions were supported by the data
Universal persistence exponents in an extremally driven system
The local persistence R(t), defined as the proportion of the system still in
its initial state at time t, is measured for the Bak--Sneppen model. For 1 and
2 dimensions, it is found that the decay of R(t) depends on one of two classes
of initial configuration. For a subcritical initial state, R(t)\sim
t^{-\theta}, where the persistence exponent \theta can be expressed in terms of
a known universal exponent. Hence \theta is universal. Conversely, starting
from a supercritical state, R(t) decays by the anomalous form 1-R(t)\sim
t^{\tau_{\rm ALL}} until a finite time t_{0}, where \tau_{\rm ALL} is also a
known exponent. Finally, for the high dimensional model R(t) decays
exponentially with a non--universal decay constant.Comment: 4 pages, 6 figures. To appear in Phys. Rev.
Volume-controlled buckling of thin elastic shells: Application to crusts formed on evaporating partially-wetted droplets
Motivated by the buckling of glassy crusts formed on evaporating droplets of
polymer and colloid solutions, we numerically model the deformation and
buckling of spherical elastic caps controlled by varying the volume between the
shell and the substrate. This volume constraint mimics the incompressibility of
the unevaporated solvent. Discontinuous buckling is found to occur for
sufficiently thin and/or large contact angle shells, and robustly takes the
form of a single circular region near the boundary that `snaps' to an inverted
shape, in contrast to externally pressurised shells. Scaling theory for shallow
shells is shown to well approximate the critical buckling volume, the
subsequent enlargement of the inverted region and the contact line force.Comment: 7 pages in J. Phys. Cond. Mat. spec; 4 figs (2 low-quality to reach
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required
Electron-vibration interaction in transport through atomic gold wires
We calculate the effect of electron-vibration coupling on conduction through
atomic gold wires, which was measured in the experiments of Agra\"it et al.
[Phys. Rev. Lett. 88, 216803 (2002)]. The vibrational modes, the coupling
constants, and the inelastic transport are all calculated using a tight-binding
parametrization and the non-equilibrium Green function formalism. The
electron-vibration coupling gives rise to small drops in the conductance at
voltages corresponding to energies of some of the vibrational modes. We study
systematically how the position and height of these steps vary as a linear wire
is stretched and more atoms are added to it, and find a good agreement with the
experiments. We also consider two different types of geometries, which are
found to yield qualitatively similar results. In contrast to previous
calculations, we find that typically there are several close-lying drops due to
different longitudinal modes. In the experiments, only a single drop is usually
visible, but its width is too large to be accounted for by temperature.
Therefore, to explain the experimental results, we find it necessary to
introduce a finite broadening to the vibrational modes, which makes the
separate drops merge into a single, wide one. In addition, we predict how the
signatures of vibrational modes in the conductance curves differ between linear
and zigzag-type wires.Comment: 19 pages, 12 figure
Metabolic and Vascular Imaging Biomarkers in Down Syndrome Provide Unique Insights Into Brain Aging and Alzheimer Disease Pathogenesis
People with Down syndrome (DS) are at high risk for developing Alzheimer disease (AD). Neuropathology consistent with AD is present by 40 years of age and dementia may develop up to a decade later. In this review, we describe metabolic and vascular neuroimaging studies in DS that suggest these functional changes are a key feature of aging, linked to cognitive decline and AD in this vulnerable cohort. FDG-PET imaging in DS suggests systematic reductions in glucose metabolism in posterior cingulate and parietotemporal cortex. Magentic resonance spectroscopy studies show consistent decreases in neuronal health and increased myoinositol, suggesting inflammation. There are few vascular imaging studies in DS suggesting a gap in our knowledge. Future studies would benefit from longitudinal measures and combining various imaging approaches to identify early signs of dementia in DS that may be amenable to intervention
Ab Initio Study of Screw Dislocations in Mo and Ta: A new picture of plasticity in bcc transition metals
We report the first ab initio density-functional study of screw
dislocations cores in the bcc transition metals Mo and Ta. Our results suggest
a new picture of bcc plasticity with symmetric and compact dislocation cores,
contrary to the presently accepted picture based on continuum and interatomic
potentials. Core energy scales in this new picture are in much better agreement
with the Peierls energy barriers to dislocation motion suggested by
experiments.Comment: 3 figures, 3 table
Granular Pressure and the Thickness of a Layer Jamming on a Rough Incline
Dense granular media have a compaction between the random loose and random
close packings. For these dense media the concept of a granular pressure
depending on compaction is not unanimously accepted because they are often in a
"frozen" state which prevents them to explore all their possible microstates, a
necessary condition for defining a pressure and a compressibility
unambiguously. While periodic tapping or cyclic fluidization have already being
used for that exploration, we here suggest that a succession of flowing states
with velocities slowly decreasing down to zero can also be used for that
purpose. And we propose to deduce the pressure in \emph{dense and flowing}
granular media from experiments measuring the thickness of the granular layer
that remains on a rough incline just after the flow has stopped.Comment: 10 pages, 2 figure
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