60,917 research outputs found
Equation of state and critical behavior of polymer models: A quantitative comparison between Wertheim's thermodynamic perturbation theory and computer simulations
We present an application of Wertheim's Thermodynamic Perturbation Theory
(TPT1) to a simple coarse grained model made of flexibly bonded Lennard-Jones
monomers. We use both the Reference Hyper-Netted-Chain (RHNC) and Mean
Spherical approximation (MSA) integral equation theories to describe the
properties of the reference fluid. The equation of state, the density
dependence of the excess chemical potential, and the critical points of the
liquid--vapor transition are compared with simulation results and good
agreement is found. The RHNC version is somewhat more accurate, while the MSA
version has the advantage of being almost analytic. We analyze the scaling
behavior of the critical point of chain fluids according to TPT1 and find it to
reproduce the mean field exponents: The critical monomer density is predicted
to vanish as upon increasing the chain length while the critical
temperature is predicted to reach an asymptotic finite temperature that is
attained as . The predicted asymptotic finite critical temperature
obtained from the RHNC and MSA versions of TPT1 is found to be in good
agreement with the point of our polymer model as obtained from the
temperature dependence of the single chain conformations.Comment: to appear in J.Chem.Phy
Trip-Based Public Transit Routing
We study the problem of computing all Pareto-optimal journeys in a public
transit network regarding the two criteria of arrival time and number of
transfers taken. We take a novel approach, focusing on trips and transfers
between them, allowing fine-grained modeling. Our experiments on the
metropolitan network of London show that the algorithm computes full 24-hour
profiles in 70 ms after a preprocessing phase of 30 s, allowing fast queries in
dynamic scenarios.Comment: Minor corrections, no substantial changes. To be presented at ESA
201
Effects of loperamide on the human hypothalamo-pituitary-adrenal axis in vivo and in vitro.
Loperamide, an opiate agonist of high specificity for p-receptors,
was recently reported to suppress ACTH and cortisol levels in normal
subjects, but not in patients with proven ACTH-dependent Cushing’s
disease. However, there is little information on the site of action of
loperamide in the hypothalamo-pituitary-adrenal axis of man. We
investigated the effect of loperamide on pituitary hormone secretion in
uiuo and in vitro. In seven normal subjects, basal ACTH plasma levels
were significantly suppressed 3 h after loperamide administration (16
mg, orally) from 5 + 1 to 2 f 0 pmol/L (P < 0.0001). After the combined
pituitary stimulation test (100 pg human CRH, 100 rg GnRH, 100 pg
GH-releasing hormone, and 200 pg TRH), the ACTH peak (maximum
increase at 30 min) was significantly blunted by loperamide from 9 +
1 to 4 of: 1 pmol/L (P < 0.001) and the area under the curve of ACTH
from O-120 min was reduced from 35 + 5 to 23 + 4 pmol/L.2 h (P <
0.05). In the insulin-hypoglycemia test (0.15 IU/kg BW), neither the
ACTH peak nor the area under the curve of ACTH was affected by
loperamide. In six patients with Cushing’s disease and one patient with
secondary adrenal insufficency due to hypothalamic failure, neither
basal ACTH and cortisol levels nor CRH-stimulated levels were influenced
by loperamide. In four cultured human corticotropic adenomas,
loperamide was not able to reduce basal and CRH-induced ACTH
secretion. In summary, loperamide is able to reduce basal and CRHinduced
ACTH and cortisol levels in normal subjects, but not in
patients with Cushing’s disease or secondary adrenal failure of hypothalamic
origin. Loperamide has no significant effect on insulin-hypoglycemia-
induced ACTH and cortisol levels and, therefore, no effect
on stress-induced elevation of cortisol levels. Loperamide might act at
a suprapituitary site in man in viuo, but, nevertheless, a pituitary site
cannot be excluded
Molecular transport and flow past hard and soft surfaces: Computer simulation of model systems
The properties of polymer liquids on hard and soft substrates are
investigated by molecular dynamics simulation of a coarse-grained bead-spring
model and dynamic single-chain-in-mean-field (SCMF) simulations of a soft,
coarse-grained polymer model. Hard, corrugated substrates are modelled by an
FCC Lennard-Jones solid while polymer brushes are investigated as a
prototypical example of a soft, deformable surface. From the molecular
simulation we extract the coarse-grained parameters that characterise the
equilibrium and flow properties of the liquid in contact with the substrate:
the surface and interface tensions, and the parameters of the hydrodynamic
boundary condition. The so-determined parameters enter a continuum description
like the Stokes equation or the lubrication approximation.Comment: 41 pages, 13 figure
Semiclassical Theory for Parametric Correlation of Energy Levels
Parametric energy-level correlation describes the response of the
energy-level statistics to an external parameter such as the magnetic field.
Using semiclassical periodic-orbit theory for a chaotic system, we evaluate the
parametric energy-level correlation depending on the magnetic field difference.
The small-time expansion of the spectral form factor is shown to be
in agreement with the prediction of parameter dependent random-matrix theory to
all orders in .Comment: 25 pages, no figur
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