2,875 research outputs found
Ab initio molecular dynamics study of collective excitations in liquid HO and DO: Effect of dispersion corrections
The collective dynamics in liquid water is an active research topic
experimentally, theoretically and via simulations. Here, ab initio molecular
dynamics simulations are reported in heavy and ordinary water at temperature
323.15 K, or 50C. The simulations in heavy water were performed both
with and without dispersion corrections. We found that the dispersion
correction (DFT-D3) changes the relaxation of density-density time correlation
functions from a slow, typical of a supercooled state, to exponential decay
behaviour of regular liquids. This implies an essential reduction of the
melting point of ice in simulations with DFT-D3. Analysis of longitudinal (L)
and transverse (T) current spectral functions allowed us to estimate the
dispersions of acoustic and optic collective excitations and to observe the L-T
mixing effect. The dispersion correction shifts the L and T optic (O) modes to
lower frequencies and provides by almost thirty per cent smaller gap between
the longest-wavelength LO and TO excitations, which can be a consequence of a
larger effective high-frequency dielectric permittivity in simulations with
dispersion corrections. Simulation in ordinary water with the dispersion
correction results in frequencies of optic excitations higher than in DO,
and in a long-wavelength LO-TO gap of 24 ps (127 cm).Comment: 14 pages, 9 figure
Profiles of electrostatic potential across the water-vapor, ice-vapor and ice-water interfaces
Ice-water, water-vapor interfaces and ice surface are studied by molecular
dynamics simulations with the SPC/E model of water molecules having the purpose
to estimate the profiles of electrostatic potential across the interfaces. We
have proposed a methodology for calculating the profiles of electrostatic
potential based on a trial particle, which showed good agreement for the case
of electrostatic potential profile of the water-vapor interface of TIP4P model
calculated in another way. The measured profile of electrostatic potential for
the pure ice-water interface decreases towards the liquid bulk region, which is
in agreement with simulations of preferential direction of motion of Li
and F solute ions at the liquid side of the ice-water interface. These
results are discussed in connection with the Workman-Reynolds effect.Comment: 7 pages, 5 figure
Pressure-driven flow of oligomeric fluid in nano-channel with complex structure. A dissipative particle dynamics study
We develop a simulational methodology allowing for simulation of the
pressure-driven flow in the pore with flat and polymer-modified walls. Our
approach is based on dissipative particle dynamics and we combine earlier ideas
of fluid-like walls and reverse flow. As a test case we consider the oligomer
flow through the pore with flat walls and demonstrate good thermostatting
qualities of the proposed method. We found the inhomogeneities in both oligomer
shape and alignment across the pore leading to a non-parabolic velocity
profiles. The method is subsequently applied to a nano-channel decorated with a
polymer brush stripes arranged perpendicularly to the flow direction. At
certain threshold value of a flow force we find a pillar-to-lamellar
morphological transition, which leads to the brush enveloping the pore wall by
a relatively smooth layer. At higher flow rates, the flow of oligomer has
similar properties as in the case of flat walls, but for the narrower effective
pore size. We observe stretching and aligning of the polymer molecules along
the flow near the pore walls.Comment: 14 pages, 12 figure
Solvation force for long ranged wall-fluid potentials
The solvation force of a simple fluid confined between identical planar walls
is studied in two model systems with short ranged fluid-fluid interactions and
long ranged wall-fluid potentials decaying as , for
various values of . Results for the Ising spins system are obtained in two
dimensions at vanishing bulk magnetic field by means of the
density-matrix renormalization-group method; results for the truncated
Lennard-Jones (LJ) fluid are obtained within the nonlocal density functional
theory. At low temperatures the solvation force for the Ising film
is repulsive and decays for large wall separations in the same fashion as
the boundary field , whereas for temperatures larger than
the bulk critical temperature is attractive and the asymptotic decay
is . For the LJ fluid system is always
repulsive away from the critical region and decays for large with the the
same power law as the wall-fluid potential. We discuss the influence of the
critical Casimir effect and of capillary condensation on the behaviour of the
solvation force.Comment: 48 pages, 12 figure
Density functional approach for inhomogeneous star polymers
We propose microscopic density functional theory for inhomogeneous star
polymers. Our approach is based on fundamental measure theory for hard spheres,
and on Wertheim's first- and second-order perturbation theory for the
interparticle connectivity. For simplicity we consider a model in which all the
arms are of the same length, but our approach can be easily extended to the
case of stars with arms of arbitrary lengths.Comment: 4 pages, 3 figures, submitte
Scientific Standards and the Regulation of Genetically Modified Insects
Experimental releases of genetically modified (GM) insects are reportedly being evaluated in various countries, including Brazil, the Cayman Islands (United Kingdom), France, Guatemala, India, Malaysia, Mexico, Panama, Philippines, Singapore, Thailand, the United States of America, and Vietnam. GM mosquitoes (Aedes aegypti) have already been released for field trials into inhabited areas in the Cayman Islands (2009–?), Malaysia (2010–2011), and Brazil (2011–2012). Here, we assess the regulatory process in the first three countries permitting releases (Malaysia, US, and the Cayman Islands) in terms of pre-release transparency and scientific quality. We find that, despite 14 US government–funded field trials over the last 9 years (on a moth pest of cotton), there has been no scientific publication of experimental data, and in only two instances have permit applications been published. The world's first environmental impact statement (EIS) on GM insects, produced by US authorities in 2008, is found to be scientifically deficient on the basis that (1) most consideration of environmental risk is too generic to be scientifically meaningful; (2) it relies on unpublished data to establish central scientific points; and (3) of the approximately 170 scientific publications cited, the endorsement of the majority of novel transgenic approaches is based on just two laboratory studies in only one of the four species covered by the document. We find that it is not possible to determine from documents publically available prior to the start of releases if obvious hazards of the particular GM mosquitoes released in Malaysia, the Cayman Islands, and Brazil received expert examination. Simple regulatory measures are proposed that would build public confidence and stimulate the independent experimental studies that environmental risk assessments require. Finally, a checklist is provided to assist the general public, journalists, and lawmakers in determining, from documents issued by regulators prior to the start of releases, whether permit approval is likely to have a scientifically high quality basi
Normal Intelligence in Female and Male Patients with Congenital Adrenal Hyperplasia
We provide evidence regarding the nature, causes, and consequences of intelligence in patients with 21-hydroxylase deficient congenital adrenal hyperplasia (CAH). Intelligence and quality of life (psychological adjustment) were measured on multiple occasions from childhood to young adulthood in 104 patients with CAH (62 females, 42 males) and 88 unaffected relatives (31 females, 57 males). Information on disease severity (CAH type, age at diagnosis, genital virilization for girls) and salt-wasting crises was obtained from medical records. There was no evidence of intellectual deficit in either female or male patients with CAH. Intelligence was not significantly associated with psychological adjustment or disease characteristics. CAH itself does not appear to increase risk for poor intellectual function. In a sample of patients with generally good disease control, intelligence is not related to adjustment problems, disease severity, or salt-wasting crises
Velocity autocorrelations across the molecular-atomic fluid transformation in hydrogen under pressure
Non-monotonous changes in velocity autocorrelations across the transformation
from molecular to atomic fluid in hydrogen under pressure are studied by ab
initio molecular dynamics simulations at the temperature 2500 K. We report
diffusion coefficients in a wide range of densities from purely molecular fluid
up to metallic atomic fluid phase. An analysis of contributions to the velocity
autocorrelation functions from the motion of molecular centers-of-mass,
rotational and intramolecular vibrational modes is performed, and a crossover
in the vibrational density of intramolecular modes across the transition is
discussed.Comment: 7 pages, 5 figure
A new procedure for microarray experiments to account for experimental noise and the uncertainty of probe response
Although microarrays are routine analysis tools in biomedical research, theystill yield noisy output that often requires experimental confirmation. Manystudies have aimed at optimizing probe design and statistical analysis totackle this problem. However, less emphasis has been placed on controlling thenoise inherent to the experimental approach. To address this problem, weinvestigate here a procedure that controls for such experimental variance andcombine it with an assessment of probe performance. Two custom arrays were usedto evaluate the procedure: one based on 25mer probes from an Affymetrix designand the other based on 60mer probes from an Agilent design. To assessexperimental variance, all probes were replicated ten times. To assess probeperformance, the probes were calibrated using a dilution series of targetmolecules and the signal response was fitted to an absorption model. We foundthat significant variance of the signal could be controlled by averaging acrossprobes and removing probes that are nonresponsive. Thus, a more reliable signalcould be obtained using our procedure than conventional approaches. We suggestthat once an array is properly calibrated, absolute quantification of signalsbecomes straight forward, alleviating the need for normalization and referencehybridizations.<br
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