7,063 research outputs found
Uncertainty in climate science and climate policy
This essay, written by a statistician and a climate scientist, describes our
view of the gap that exists between current practice in mainstream climate
science, and the practical needs of policymakers charged with exploring
possible interventions in the context of climate change. By `mainstream' we
mean the type of climate science that dominates in universities and research
centres, which we will term `academic' climate science, in contrast to `policy'
climate science; aspects of this distinction will become clearer in what
follows.
In a nutshell, we do not think that academic climate science equips climate
scientists to be as helpful as they might be, when involved in climate policy
assessment. Partly, we attribute this to an over-investment in high resolution
climate simulators, and partly to a culture that is uncomfortable with the
inherently subjective nature of climate uncertainty.Comment: submitted as contribution to Conceptual Foundations of
ClimateModeling, Winsberg, E. and Lloyd, E., eds., The University of Chicago
Pres
On the use of simple dynamical systems for climate predictions: A Bayesian prediction of the next glacial inception
Over the last few decades, climate scientists have devoted much effort to the
development of large numerical models of the atmosphere and the ocean. While
there is no question that such models provide important and useful information
on complicated aspects of atmosphere and ocean dynamics, skillful prediction
also requires a phenomenological approach, particularly for very slow
processes, such as glacial-interglacial cycles. Phenomenological models are
often represented as low-order dynamical systems. These are tractable, and a
rich source of insights about climate dynamics, but they also ignore large
bodies of information on the climate system, and their parameters are generally
not operationally defined. Consequently, if they are to be used to predict
actual climate system behaviour, then we must take very careful account of the
uncertainty introduced by their limitations. In this paper we consider the
problem of the timing of the next glacial inception, about which there is
on-going debate. Our model is the three-dimensional stochastic system of
Saltzman and Maasch (1991), and our inference takes place within a Bayesian
framework that allows both for the limitations of the model as a description of
the propagation of the climate state vector, and for parametric uncertainty.
Our inference takes the form of a data assimilation with unknown static
parameters, which we perform with a variant on a Sequential Monte Carlo
technique (`particle filter'). Provisional results indicate peak glacial
conditions in 60,000 years.Comment: superseeds the arXiv:0809.0632 (which was published in European
Reviews). The Bayesian section has been significantly expanded. The present
version has gone scientific peer review and has been published in European
Physics Special Topics. (typo in DOI and in Table 1 (psi -> theta) corrected
on 25th August 2009
The Fast Multipole Method and Point Dipole Moment Polarizable Force Fields
We present an implementation of the fast multipole method for computing
coulombic electrostatic and polarization forces from polarizable force-fields
based on induced point dipole moments. We demonstrate the expected
scaling of that approach by performing single energy point calculations on
hexamer protein subunits of the mature HIV-1 capsid. We also show the long time
energy conservation in molecular dynamics at the nanosecond scale by performing
simulations of a protein complex embedded in a coarse-grained solvent using a
standard integrator and a multiple time step integrator. Our tests show the
applicability of FMM combined with state-of-the-art chemical models in
molecular dynamical systems.Comment: 11 pages, 8 figures, accepted by J. Chem. Phy
Efficient dielectric matrix calculations using the Lanczos algorithm for fast many-body implementations
We present a implementation that assesses the two major bottlenecks
of traditional plane-waves implementations, the summations over conduction
states and the inversion of the dielectric matrix, without introducing new
approximations in the formalism. The first bottleneck is circumvented by
converting the summations into Sternheimer equations. Then, the novel avenue of
expressing the dielectric matrix in a Lanczos basis is developed, which reduces
the matrix size by orders of magnitude while being computationally efficient.
We also develop a model dielectric operator that allows us to further reduce
the size of the dielectric matrix without accuracy loss. Furthermore, we
develop a scheme that reduces the numerical cost of the contour deformation
technique to the level of the lightest plasmon pole model. Finally, the use of
the simplified quasi-minimal residual scheme in replacement of the conjugate
gradients algorithm allows a direct evaluation of the corrections at
the desired real frequencies, without need for analytical continuation. The
performance of the resulting implementation is demonstrated by
comparison with a traditional plane-waves implementation, which reveals a
500-fold speedup for the silane molecule. Finally, the accuracy of our
implementation is demonstrated by comparison with other calculations
and experimental results.Comment: 19 pages, 2 figure
Towards a Semantic Search Engine for Scientific Articles
Because of the data deluge in scientific publication, finding relevant
information is getting harder and harder for researchers and readers. Building
an enhanced scientific search engine by taking semantic relations into account
poses a great challenge. As a starting point, semantic relations between
keywords from scientific articles could be extracted in order to classify
articles. This might help later in the process of browsing and searching for
content in a meaningful scientific way. Indeed, by connecting keywords, the
context of the article can be extracted. This paper aims to provide ideas to
build such a smart search engine and describes the initial contributions
towards achieving such an ambitious goal
Electron-phonon coupling in the C60 fullerene within the many-body GW approach
We study the electron-phonon coupling in the C60 fullerene within the
first-principles GW approach, focusing on the lowest unoccupied t1u three-fold
electronic state which is relevant for the superconducting transition in
electron doped fullerides. It is shown that the strength of the coupling is
significantly enhanced as compared to standard density functional theory
calculations with (semi)local functionals, with a 48% increase of the
electron-phonon potential Vep. The calculated GW value for the contribution
from the Hg modes of 93 meV comes within 4% of the most recent experimental
values. The present results call for a reinvestigation of previous density
functional based calculations of electron-phonon coupling in covalent systems
in general.Comment: 4 pages, 0 figur
Bromophenyl functionalization of carbon nanotubes : an ab initio study
We study the thermodynamics of bromophenyl functionalization of carbon
nanotubes with respect to diameter and metallic/insulating character using
density-functional theory (DFT). On one hand, we show that the activation
energy for the grafting of a bromophenyl molecule onto a semiconducting zigzag
nanotube ranges from 0.73 eV to 0.76 eV without any clear trend with respect to
diameter within numerical accuracy. On the other hand, the binding energy of a
single bromophenyl molecule shows a clear diameter dependence and ranges from
1.51 eV for a (8,0) zigzag nanotube to 0.83 eV for a (20,0) zigzag nanotube.
This is in part explained by the transition from sp2 to sp3 bonding occurring
to a carbon atom of a nanotube when a phenyl is grafted to it and the fact that
smaller nanotubes are closer to a sp3 hybridization than larger ones due to
increased curvature. Since a second bromophenyl unit can attach without energy
barrier next to an isolated grafted unit, they are assumed to exist in pairs.
The para configuration is found to be favored for the pairs and their binding
energy decreases with increasing diameter, ranging from 4.34 eV for a (7,0)
nanotube to 2.27 eV for a (29,0) nanotube. An analytic form for this radius
dependence is derived using a tight binding hamiltonian and first order
perturbation theory. The 1/R^2 dependance obtained (where R is the nanotube
radius) is verified by our DFT results within numerical accuracy. Finally,
metallic nanotubes are found to be more reactive than semiconducting nanotubes,
a feature that can be explained by a non-zero density of states at the Fermi
level for metallic nanotubes.Comment: 7 pages, 5 figures and 3 table
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