10,197 research outputs found
Nuclear quantum effects in solids using a colored-noise thermostat
We present a method, based on a non-Markovian Langevin equation, to include
quantum corrections to the classical dynamics of ions in a quasi-harmonic
system. By properly fitting the correlation function of the noise, one can vary
the fluctuations in positions and momenta as a function of the vibrational
frequency, and fit them so as to reproduce the quantum-mechanical behavior,
with minimal a priori knowledge of the details of the system. We discuss the
application of the thermostat to diamond and to ice Ih. We find that results in
agreement with path-integral molecular dynamics can be obtained using only a
fraction of the computational effort.Comment: submitted for publicatio
Responsiveness of bovine cumulus-oocyte-complexes (COC) to porcine and recombinant human FSH, and the effect of COC quality on gonadotropin receptor and Cx43 marker gene mRNAs during maturation in vitro
Substantially less development to the blastocyst stage occurs in vitro than in vivo and this may be due to deficiencies in oocyte competence. Although a large proportion of bovine oocytes undergo spontaneous nuclear maturation, less is known about requirements for proper cytoplasmic maturation. Commonly, supraphysiological concentrations of FSH and LH are added to maturation media to improve cumulus expansion, fertilization and embryonic development. Therefore, various concentrations of porcine FSH (pFSH) and recombinant human FSH (rhFSH) were investigated for their effect on bovine cumulus expansion in vitro. Expression of FSHr, LHr and Cx43 mRNAs was determined in cumulus-oocyte complexes to determine whether they would be useful markers of oocyte competence. In serum-free media, only 1000 ng/ml pFSH induced marked cumulus expansion, but the effect of 100 ng/ml pFSH was amplified in the presence of 10% serum. In contrast, cumulus expansion occurred with 1 ng/ml rhFSH in the absence of serum. FSHr mRNA was highest at 0–6 h of maturation, then abundance decreased. Similarly, Cx43 mRNA expression was highest from 0–6 h but decreased by 24 h of maturation. However, the relative abundance of LHr mRNA did not change from 6–24 h of maturation. Decreased levels of FSHr, LHr and Cx43 mRNAs were detected in COCs of poorer quality. In conclusion, expansion of bovine cumulus occurred at low doses of rhFSH in serum-free media. In summary, FSHr, LHr and Cx43 mRNA abundance reflects COC quality and FSHr and Cx43 mRNA expression changes during in vitro maturation; these genes may be useful markers of oocyte developmental competence
Viscoelasticity and Stokes-Einstein relation in repulsive and attractive colloidal glasses
We report a numerical investigation of the visco-elastic behavior in models
for steric repulsive and short-range attractive colloidal suspensions, along
different paths in the attraction-strength vs packing fraction plane. More
specifically, we study the behavior of the viscosity (and its frequency
dependence) on approaching the repulsive glass, the attractive glass and in the
re-entrant region where viscosity shows a non monotonic behavior on increasing
attraction strength. On approaching the glass lines, the increase of the
viscosity is consistent with a power-law divergence with the same exponent and
critical packing fraction previously obtained for the divergence of the density
fluctuations. Based on mode-coupling calculations, we associate the increase of
the viscosity with specific contributions from different length scales. We also
show that the results are independent on the microscopic dynamics by comparing
newtonian and brownian simulations for the same model. Finally we evaluate the
Stokes-Einstein relation approaching both glass transitions, finding a clear
breakdown which is particularly strong for the case of the attractive glass.Comment: 12 pages; sent to J. Chem. Phy
Surrogate-based Real-time Curbside Management for Ride-hailing and Delivery Operations
The present work investigates surrogate model-based optimization for
real-time curbside traffic management operations. An optimization problem is
formulated to minimize the congestion on roadway segments caused by vehicles
stopping on the segment (e.g., ride-hailing or delivery operations) and
implemented in a model predictive control framework. A hybrid simulation
approach where main traffic flows interact with individually modeled stopping
vehicles is adopted. Due to its non-linearity, the optimization problem is
coupled with a meta-heuristic. However, because simulations are time expensive
and hence unsuitable for real-time control, a trained surrogate model that
takes the decision variables as inputs and approximates the objective function
is employed to replace the simulation within the meta-heuristic algorithm.
Several modeling techniques (i.e., linear regression, polynomial regression,
neural network, radial basis network, regression tree ensemble, and Gaussian
process regression) are compared based on their accuracy in reproducing
solutions to the problem and computational tractability for real-time control
under different configurations of simulation parameters. It is found that
Gaussian process regression is the most suited for use as a surrogate model for
the given problem. Finally, a realistic application with multiple ride-hailing
vehicle operations is presented. The proposed approach for controlling the stop
positions of vehicles is able to achieve an improvement of 20.65% over the
uncontrolled case. The example shows the potential of the proposed approach in
reducing the negative impacts of stopping vehicles and favorable computational
properties
The influence of bond-rigidity and cluster diffusion on the self-diffusion of hard spheres with square-well interaction
Hard spheres interacting through a square-well potential were simulated using
two different methods: Brownian Cluster Dynamics (BCD) and Event Driven
Brownian Dynamics (EDBD). The structure of the equilibrium states obtained by
both methods were compared and found to be almost the identical. Self diffusion
coefficients () were determined as a function of the interaction strength.
The same values were found using BCD or EDBD. Contrary the EDBD, BCD allows one
to study the effect of bond rigidity and hydrodynamic interaction within the
clusters. When the bonds are flexible the effect of attraction on is
relatively weak compared to systems with rigid bonds. increases first with
increasing attraction strength, and then decreases for stronger interaction.
Introducing intra-cluster hydrodynamic interaction weakly increases for a
given interaction strength. Introducing bond rigidity causes a strong decrease
of which no longer shows a maximum as function of the attraction strength
Scaling of dynamics with the range of interaction in short-range attractive colloids
We numerically study the dependence of the dynamics on the range of
interaction for the short-range square well potential. We find that,
for small , dynamics scale exactly in the same way as thermodynamics,
both for Newtonian and Brownian microscopic dynamics. For interaction ranges
from a few percent down to the Baxter limit, the relative location of the
attractive glass line and the liquid-gas line does not depend on . This
proves that in this class of potentials, disordered arrested states (gels) can
be generated only as a result of a kinetically arrested phase separation.Comment: 4 pages, 4 figure
Review of recent advances in index flood estimation
International audienceIndex flood estimation for regional flood frequency analysis needs to be based on the information available. The most appropriate method depends on the specific application and its choice requires a problem-oriented analysis. This paper presents a simple theoretical framework to deal with index flood estimation for a specific river site. The methodological approaches available for the purpose are reviewed. For each, the information required is specified and the reliability of the estimate, particularly desirable in risk analysis and management, is discussed. Where flood observations are lacking, indirect estimation must be undertaken using scenarios including those commonly met in hydrological practice; generally, these depend on the amount and type of information available. For each scenario, the methodologies are outlined, in order of the expected degree of complexity. After a guided analysis, an investigator can adopt the method providing the best tradeoff between effort in collecting and handling data and the resultant reliability which can be expected. Keywords: direct and indirect methods, index flood estimation, reliability, scenarios
Non-gaussian effects in the cage dynamics of polymers
The correlation between the fast cage dynamics and structural relaxation is
investigated in a model polymer system. It is shown that the cage vibration
amplitude, as expressed by the Debye-Waller factor, and the relaxation time
collapse on a single universal curve with a simple analytic form
when the temperature, the density, the chain length and the monomer-monomer
interaction potential are changed. For the physical states with the same
coincidence of the mean-square displacement, the intermediate
scattering function and the non-Gaussian parameter is observed in a wide time
window spanning from the ballistic regime to the onset of the Rouse dynamics
driven by the chain connectivity. The role of the non-Gaussian effects is
discussed.Comment: 8 pages, 5 figure
Linear Ramps of Interaction in the Fermionic Hubbard Model
We study the out of equilibrium dynamics of the Fermionic Hubbard Model
induced by a linear ramp of the repulsive interaction from the metallic
state through the Mott transition. To this extent we use a time dependent
Gutzwiller variational method and complement this analysis with the inclusion
of quantum fluctuations at the leading order, in the framework of a slave
spin theory. We discuss the dynamics during the ramp and the issue of
adiabaticity through the scaling of the excitation energy with the ramp
duration . In addition, we study the dynamics for times scales longer
than the ramp time, when the system is again isolated and the total energy
conserved. We establish the existence of a dynamical phase transition analogous
to the one present in the sudden quench case and discuss its properties as a
function of final interaction and ramp duration. Finally we discuss the role of
quantum fluctuations on the mean field dynamics for both long ramps, where spin
wave theory is sufficient, and for very short ramps, where a self consistent
treatment of quantum fluctuations is required in order to obtain relaxation.Comment: v2: 19 pages, 14 figures, published versio
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