11,659 research outputs found
Lattice thermal expansion and anisotropic displacements in urea, bromomalonic aldehyde, pentachloropyridine and naphthalene
Anisotropic displacement parameters (ADPs) are commonly used in
crystallography, chemistry and related fields to describe and quantify thermal
motion of atoms. Within the very recent years, these ADPs have become
predictable by lattice dynamics in combination with first-principles theory.
Here, we study four very different molecular crystals, namely urea,
bromomalonic aldehyde, pentachloropyridine, and naphthalene, by
first-principles theory to assess the quality of ADPs calculated in the
quasi-harmonic approximation. In addition, we predict both thermal expansion
and thermal motion within the quasi-harmonic approximation and compare the
predictions with experimental data. Very reliable ADPs are calculated within
the quasi-harmonic approximation for all four cases up to at least 200 K, and
they turn out to be in better agreement with experiment than the harmonic ones.
In one particular case, ADPs can even reliably be predicted up to room
temperature. Our results also hint at the importance of normal-mode
anharmonicity in the calculation of ADPs
Benchmarking calculations of excitonic couplings between bacteriochlorophylls
Excitonic couplings between (bacterio)chlorophyll molecules are necessary for
simulating energy transport in photosynthetic complexes. Many techniques for
calculating the couplings are in use, from the simple (but inaccurate)
point-dipole approximation to fully quantum-chemical methods. We compared
several approximations to determine their range of applicability, noting that
the propagation of experimental uncertainties poses a fundamental limit on the
achievable accuracy. In particular, the uncertainty in crystallographic
coordinates yields an uncertainty of about 20% in the calculated couplings.
Because quantum-chemical corrections are smaller than 20% in most biologically
relevant cases, their considerable computational cost is rarely justified. We
therefore recommend the electrostatic TrEsp method across the entire range of
molecular separations and orientations because its cost is minimal and it
generally agrees with quantum-chemical calculations to better than the
geometric uncertainty. We also caution against computationally optimizing a
crystal structure before calculating couplings, as it can lead to large,
uncontrollable errors. Understanding the unavoidable uncertainties can guard
against striving for unrealistic precision; at the same time, detailed
benchmarks can allow important qualitative questions--which do not depend on
the precise values of the simulation parameters--to be addressed with greater
confidence about the conclusions
Mirrors for slow neutrons from holographic nanoparticle-polymer free-standing film-gratings
We report on successful tests of holographically arranged grating-structures
in nanoparticle-polymer composites in the form of 100 microns thin
free-standing films, i.e. without sample containers or covers that could cause
unwanted absorption/incoherent scattering of very-cold neutrons. Despite their
large diameter of 2 cm, the flexible materials are of high optical quality and
yield mirror-like reflectivity of about 90% for neutrons of 4.1 nm wavelength
The XMM-LSS survey: the Class 1 cluster sample over the extended 11 deg and its spatial distribution
This paper presents 52 X-ray bright galaxy clusters selected within the 11
deg XMM-LSS survey. 51 of them have spectroscopic redshifts
(), one is identified at , and all together make
the high-purity "Class 1" (C1) cluster sample of the XMM-LSS, the highest
density sample of X-ray selected clusters with a monitored selection function.
Their X-ray fluxes, averaged gas temperatures (median keV),
luminosities (median ergs/s) and total mass
estimates (median ) are measured, adapting to
the specific signal-to-noise regime of XMM-LSS observations. The redshift
distribution of clusters shows a deficit of sources when compared to the
cosmological expectations, regardless of whether WMAP-9 or Planck-2013 CMB
parameters are assumed. This lack of sources is particularly noticeable at . However, after quantifying uncertainties due to small
number statistics and sample variance we are not able to put firm (i.e. ) constraints on the presence of a large void in the cluster
distribution. We work out alternative hypotheses and demonstrate that a
negative redshift evolution in the normalization of the relation
(with respect to a self-similar evolution) is a plausible explanation for the
observed deficit. We confirm this evolutionary trend by directly studying how
C1 clusters populate the space, properly accounting for selection
biases. We point out that a systematically evolving, unresolved, central
component in clusters and groups (AGN contamination or cool core) can impact
the classification as extended sources and be partly responsible for the
observed redshift distribution.[abridged]Comment: 33 pages, 21 figures, 3 tables ; accepted for publication in MNRA
Point defects and clustering in uranium dioxide by LSDA+U calculations
A comprehensive investigation on point defects and their clustering behavior
in nonstoichiometric uranium dioxide UO2+x is carried out using LSDA+U method
based on density functional theory. Accurate energetic information and charge
transfers available so far are obtained. With these energies that have improved
more than 50% over that of pure GGA and LDA, we show the density functional
theory predicts the predominance of oxygen defects over uranium ones at any
compositions, which is possible only after treated the localized 5f electrons
properly. Calculations also suggest an upper bound of x~0.03 for oxygen
clusters to start off. The volume change induced by point uranium defects is
monotonic but nonlinear, whereas for oxygen defects, increase x always reduces
the system volume linearly, except dimers that require extra space for
accommodation, which has been identified as meta-stable ionic molecule. Though
oxygen dimers usually occupy Willis O'' sites and mimic a single oxygen in
energetics and charge state, they are rare at ambient conditions. Its
decomposition process and vibrational properties have been studied carefully.
To obtain a general clustering mechanism in anion-excess fluorites
systematically, we also analyze the local stabilities of possible basic
clustering modes of oxygen defects. The result shows an unified way to
understand the structure of Willis type and cuboctahedral clusters in UO2+x and
beta-U4O9. Finally we generalize the point defect model to the independent
clusters approximation to include clustering effects, the impact on defect
populations is discussed.Comment: 20 pages, 12 figure
Ab initio investigation on oxygen defect clusters in UO2+x
By first-principles LSDA+U calculations, we revealed that the current
physical picture of defective uranium dioxide suggested solely by neutron
diffraction analysis is unsatisfactory. An understanding based on quantum
theory has been established as a thermodynamical competition among point
defects and cuboctahedral cluster, which naturally interprets the puzzled
origin of the asymmetric O' and O'' interstitials. It also gives a clear and
consistent agreement with most available experimental data. Unfortunately, the
observed high occupation of O'' site cannot be accounted for in this picture
and is still a challenge for theoretical simulations.Comment: 4 pages, 3 figures, title change
Quantum geometry and the Schwarzschild singularity
In homogeneous cosmologies, quantum geometry effects lead to a resolution of
the classical singularity without having to invoke special boundary conditions
at the singularity or introduce ad-hoc elements such as unphysical matter. The
same effects are shown to lead to a resolution of the Schwarzschild
singularity. The resulting quantum extension of space-time is likely to have
significant implications to the black hole evaporation process. Similarities
and differences with the situation in quantum geometrodynamics are pointed out.Comment: 31 pages, 1 figur
Evidence-informed teaching: self-assessment tool for teachers
This tool for teachers is a product of the data collection and analysis, including interviews with 82 teachers, carried out as part of a DfE-funded study Evidence-informed teaching: an evaluation of progress in England (Coldwell et al, 2017). It is designed to help teachers evaluate and consider their levels of interaction with evidence in terms of awareness, engagement and use; and illustrate, based on real examples and quotes, what different levels of interaction look like in more or less evidence-informed schools. It is jointly published with The Chartered College of Teachers and is freely available for download on its website
Degenerate Configurations, Singularities and the Non-Abelian Nature of Loop Quantum Gravity
Degenerate geometrical configurations in quantum gravity are important to
understand if the fate of classical singularities is to be revealed. However,
not all degenerate configurations arise on an equal footing, and one must take
into account dynamical aspects when interpreting results: While there are many
degenerate spatial metrics, not all of them are approached along the dynamical
evolution of general relativity or a candidate theory for quantum gravity. For
loop quantum gravity, relevant properties and steps in an analysis are
summarized and evaluated critically with the currently available information,
also elucidating the role of degrees of freedom captured in the sector provided
by loop quantum cosmology. This allows an outlook on how singularity removal
might be analyzed in a general setting and also in the full theory. The general
mechanism of loop quantum cosmology will be shown to be insensitive to recently
observed unbounded behavior of inverse volume in the full theory. Moreover,
significant features of this unboundedness are not a consequence of
inhomogeneities but of non-Abelian effects which can also be included in
homogeneous models.Comment: 28 pages, 1 figure; v2: extended discussion of singularity removal
and summar
- …