4,122 research outputs found
The incomplete beta function law for parallel tempering sampling of classical canonical systems
We show that the acceptance probability for swaps in the parallel tempering
Monte Carlo method for classical canonical systems is given by a universal
function that depends on the average statistical fluctuations of the potential
and on the ratio of the temperatures. The law, called the incomplete beta
function law, is valid in the limit that the two temperatures involved in swaps
are close to one another. An empirical version of the law, which involves the
heat capacity of the system, is developed and tested on a Lennard-Jones
cluster. We argue that the best initial guess for the distribution of
intermediate temperatures for parallel tempering is a geometric progression and
we also propose a technique for the computation of optimal temperature
schedules. Finally, we demonstrate that the swap efficiency of the parallel
tempering method for condensed-phase systems decreases naturally to zero at
least as fast as the inverse square root of the dimensionality of the physical
system.Comment: 11 pages, 4 figures; minor changes; to appear in J. Chem. Phy
Possible solution to the riddle of HD 82943 multiplanet system: the three-planet resonance 1:2:5?
We carry out a new analysis of the published radial velocity data for the
planet-hosting star HD82943. We include the recent Keck/HIRES measurements as
well as the aged but much more numerous CORALIE data. We find that the CORALIE
radial velocity measurements are polluted by a systematic annual variation
which affected the robustness of many previous results. We show that after
purging this variation, the residuals still contain a clear signature of an
additional days periodicity. The latter variation leaves
significant hints in all three independent radial velocity subsets that we
analysed: the CORALIE data, the Keck data acquired prior to a hardware upgrade,
and the Keck data taken after the upgrade.
We mainly treat this variation as a signature of a third planet in the
system, although we cannot rule out other interpretations, such as long-term
stellar activity. We find it easy to naturally obtain a stable three-planet
radial-velocity fit close to the three-planet mean-motion resonance 1:2:5, with
the two main planets (those in the 1:2 resonance) in an aligned apsidal
corotation. The dynamical status of the third planet is still uncertain: it may
reside in as well as slightly out of the 5:2 resonance. We obtain the value of
days for its orbital period and of for its
minimum mass, while the eccentric parameters are uncertain.Comment: 18 pages, 5 tables, 18 figures; accepted for publication in MNRA
Evolutionary approach for finding the atomic structure of steps on stable crystal surfaces
The problem addressed here can be concisely formulated as follows: Given a stable surface orientation with a known reconstruction and given a direction in the plane of this surface, find the atomic structure of the steps oriented along that direction. We report a robust and generally applicable variable-number genetic algorithm for determining the atomic configuration of crystallographic steps, and exemplify it by finding structures for several types of monatomic steps on Si(114)-2×1. We show that the location of the step edge with respect to the terrace reconstructions, the step width (number of atoms), and the positions of the atoms in the step region can all be simultaneously determined
The stability of strained H:Si(105) and H:Ge(105) surfaces
We report atomic scale studies of the effect of applied strain and hydrogen
environment on the reconstructions of the (105) Si and Ge surfaces. Surface
energy calculations for monohydride-terminated (001) and (105) reconstructions
reveal that the recently established single-height rebonded model is unstable
not only with respect to (001), but also in comparison to other monohydride
(105) structures. This finding persists for both Si and Ge, for applied biaxial
strains from -4% to 4%, and for nearly the entire relevant domain of the
chemical potential of hydrogen, thus providing an explanation for the recently
observed H-induced destabilization of the Ge(105) surface
Psychological profile of laryngectomized patients
Larynx cancer is one of the most susceptible form of cancer susceptible to induce alteration of the patient’s psychological profile due to the social role that the larynx has in communication. Oral communication is severely impaired even after voice rehabilitation of the laryngectomized patients, so that the social rehabilitation is somewhat not only a medical but also a social problem. The psychological profile of these patients is altered in a way that dealing with the disease is sometimes neglected and the interaction with the outside world in terms of oral communication is totally abandoned. The starting point for depression in these cases is the acknowledgement of the disease and is, in some cases, the entire medical environment. Facial scarring, the inability to verbally interact with other human, as well as the presence of the tracheostoma, are all deciding factors in the presence of a low self-esteem for these particular patients. Psychological counseling is a mandatory approach for laryngectomized patients, in order to improve their ability to cope with cancer and providing better recovery chances
Modeling Tiered Pricing in the Internet Transit Market
ISPs are increasingly selling "tiered" contracts, which offer Internet
connectivity to wholesale customers in bundles, at rates based on the cost of
the links that the traffic in the bundle is traversing. Although providers have
already begun to implement and deploy tiered pricing contracts, little is known
about how such pricing affects ISPs and their customers. While contracts that
sell connectivity on finer granularities improve market efficiency, they are
also more costly for ISPs to implement and more difficult for customers to
understand. In this work we present two contributions: (1) we develop a novel
way of mapping traffic and topology data to a demand and cost model; and (2) we
fit this model on three large real-world networks: an European transit ISP, a
content distribution network, and an academic research network, and run
counterfactuals to evaluate the effects of different pricing strategies on both
the ISP profit and the consumer surplus. We highlight three core findings.
First, ISPs gain most of the profits with only three or four pricing tiers and
likely have little incentive to increase granularity of pricing even further.
Second, we show that consumer surplus follows closely, if not precisely, the
increases in ISP profit with more pricing tiers. Finally, the common ISP
practice of structuring tiered contracts according to the cost of carrying the
traffic flows (e.g., offering a discount for traffic that is local) can be
suboptimal and that dividing contracts based on both traffic demand and the
cost of carrying it into only three or four tiers yields near-optimal profit
for the ISP
Multiscale entanglement in ring polymers under spherical confinement
The interplay of geometrical and topological entanglement in semiflexible
knotted polymer rings confined inside a spherical cavity is investigated using
advanced numerical methods. By using stringent and robust algorithms for
locating knots, we characterize how the knot length lk depends on the ring
contour length, Lc and the radius of the confining sphere, Rc . In the no- and
strong- confinement cases we observe weak knot localization and complete knot
delocalization, respectively. We show that the complex interplay of lk, Lc and
Rc that seamlessly bridges these two limits can be encompassed by a simple
scaling argument based on deflection theory. The same argument is used to
rationalize the multiscale character of the entanglement that emerges with
increasing confinement.Comment: 9 pages 9 figure
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