1,954 research outputs found
Topological methods for searching barriers and reaction paths
We present a family of algorithms for the fast determination of reaction
paths and barriers in phase space and the computation of the corresponding
rates. The method requires the reaction times be large compared to the
microscopic time, irrespective of the origin - energetic, entropic, cooperative
- of the timescale separation. It lends itself to temperature cycling as in
simulated annealing and to activation-relaxation routines. The dynamics is
ultimately based on supersymmetry methods used years ago to derive Morse
theory. Thus, the formalism automatically incorporates all relevant topological
information.Comment: 4 pages, 4 figures, RevTex
A consistent hierarchy of generalized kinetic equation approximations to the chemical master equation applied to surface catalysis
We develop a hierarchy of approximations to the master equation for systems
that exhibit translational invariance and finite-range spatial correlation.
Each approximation within the hierarchy is a set of ordinary differential
equations that considers spatial correlations of varying lattice distance; the
assumption is that the full system will have finite spatial correlations and
thus the behavior of the models within the hierarchy will approach that of the
full system. We provide evidence of this convergence in the context of one- and
two-dimensional numerical examples. Lower levels within the hierarchy that
consider shorter spatial correlations, are shown to be up to three orders of
magnitude faster than traditional kinetic Monte Carlo methods (KMC) for
one-dimensional systems, while predicting similar system dynamics and steady
states as KMC methods. We then test the hierarchy on a two-dimensional model
for the oxidation of CO on RuO2(110), showing that low-order truncations of the
hierarchy efficiently capture the essential system dynamics. By considering
sequences of models in the hierarchy that account for longer spatial
correlations, successive model predictions may be used to establish empirical
approximation of error estimates. The hierarchy may be thought of as a class of
generalized phenomenological kinetic models since each element of the hierarchy
approximates the master equation and the lowest level in the hierarchy is
identical to a simple existing phenomenological kinetic models
Yang-Mills fields on CR manifolds
We study pseudo Yang-Mills fields on a compact strictly pseudoconvex CR
manifold.Comment: 52 page
Physical mechanism of the (tri)critical point generation
We discuss some ideas resulting from a phenomenological relation recently
declared between the tension of string connecting the static quark-antiquark
pair and surface tension of corresponding cylindrical bag. This relation
analysis leads to the temperature of vanishing surface tension coefficient of
the QGP bags at zero baryonic charge density as T_\sigma = 152.9 +- 4.5 MeV. We
develop the view point that this temperature value is not a fortuitous
coincidence with the temperature of (partial) chiral symmetry restoration as
seen in the lattice QCD simulations. Besides, we argue that T_\sigma defines
the QCD (tri)critical endpoint temperature and claim that a negative value of
surface tension coefficient recently discovered is not a sole result, but
should also exist in ordinary liquids at the supercritical temperatures.Comment: Talk given at the Conference "Critical Point and Onset of
Deconfinement (CPOD)" that held on August 23 - 29, 2010, JINR, Dubna, Russia.
Contains minimal change
N=2 supergravity in five dimensions revisited
We construct matter-coupled N=2 supergravity in five dimensions, using the
superconformal approach. For the matter sector we take an arbitrary number of
vector-, tensor- and hyper-multiplets. By allowing off-diagonal vector-tensor
couplings we find more general results than currently known in the literature.
Our results provide the appropriate starting point for a systematic search for
BPS solutions, and for applications of M-theory compactifications on Calabi-Yau
manifolds with fluxes.Comment: 35 pages; v.2: A sign changed in a bilinear fermion term in (5.7
Evaluating Molecular Mechanical Potentials for Helical Peptides and Proteins
Multiple variants of the AMBER all-atom force field were quantitatively evaluated with respect to their ability to accurately characterize helix-coil equilibria in explicit solvent simulations. Using a global distributed computing network, absolute conformational convergence was achieved for large ensembles of the capped A21 and Fs helical peptides. Further assessment of these AMBER variants was conducted via simulations of a flexible 164-residue five-helix-bundle protein, apolipophorin-III, on the 100 ns timescale. Of the contemporary potentials that had not been assessed previously, the AMBER-99SB force field showed significant helix-destabilizing tendencies, with beta bridge formation occurring in helical peptides, and unfolding of apolipophorin-III occurring on the tens of nanoseconds timescale. The AMBER-03 force field, while showing adequate helical propensities for both peptides and stabilizing apolipophorin-III, (i) predicts an unexpected decrease in helicity with ALA→ARG+ substitution, (ii) lacks experimentally observed 310 helical content, and (iii) deviates strongly from average apolipophorin-III NMR structural properties. As is observed for AMBER-99SB, AMBER-03 significantly overweighs the contribution of extended and polyproline backbone configurations to the conformational equilibrium. In contrast, the AMBER-99φ force field, which was previously shown to best reproduce experimental measurements of the helix-coil transition in model helical peptides, adequately stabilizes apolipophorin-III and yields both an average gyration radius and polar solvent exposed surface area that are in excellent agreement with the NMR ensemble
Regulatory control and the costs and benefits of biochemical noise
Experiments in recent years have vividly demonstrated that gene expression
can be highly stochastic. How protein concentration fluctuations affect the
growth rate of a population of cells, is, however, a wide open question. We
present a mathematical model that makes it possible to quantify the effect of
protein concentration fluctuations on the growth rate of a population of
genetically identical cells. The model predicts that the population's growth
rate depends on how the growth rate of a single cell varies with protein
concentration, the variance of the protein concentration fluctuations, and the
correlation time of these fluctuations. The model also predicts that when the
average concentration of a protein is close to the value that maximizes the
growth rate, fluctuations in its concentration always reduce the growth rate.
However, when the average protein concentration deviates sufficiently from the
optimal level, fluctuations can enhance the growth rate of the population, even
when the growth rate of a cell depends linearly on the protein concentration.
The model also shows that the ensemble or population average of a quantity,
such as the average protein expression level or its variance, is in general not
equal to its time average as obtained from tracing a single cell and its
descendants. We apply our model to perform a cost-benefit analysis of gene
regulatory control. Our analysis predicts that the optimal expression level of
a gene regulatory protein is determined by the trade-off between the cost of
synthesizing the regulatory protein and the benefit of minimizing the
fluctuations in the expression of its target gene. We discuss possible
experiments that could test our predictions.Comment: Revised manuscript;35 pages, 4 figures, REVTeX4; to appear in PLoS
Computational Biolog
Self-organizing robot formations using velocity potential fields commands for material transfer
Mobile robot formations differ in accordance with the mission, environment, and robot abilities. In the case of decentralized control, the ability to achieve the shapes of these formations needs to be built in the controllers of each autonomous robot. In this paper, self-organizing formations control for material transfer is investigated, as an alternative to automatic guided vehicles. Leader–follower approach is applied for controllers design to drive the robots toward the goal. The results confirm the ability of velocity potential approach for motion control of both self-organizing formations
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