6,873 research outputs found
Properties of low-dimensional collective variables in the molecular dynamics of biopolymers
The description of the dynamics of a complex, high-dimensional system in
terms of a low-dimensional set of collective variables Y can be fruitful if the
low dimensional representation satisfies a Langevin equation with drift and
diffusion coefficients which depend only on Y. We present a computational
scheme to evaluate whether a given collective variable provides a faithful
low-dimensional representation of the dynamics of a high-dimensional system.
The scheme is based on the framework of finite-difference Langevin-equation,
similar to that used for molecular-dynamics simulations. This allows one to
calculate the drift and diffusion coefficients in any point of the
full-dimensional system. The width of the distribution of drift and diffusion
coefficients in an ensemble of microscopic points at the same value of Y
indicates to which extent the dynamics of Y is described by a simple Langevin
equation. Using a simple protein model we show that collective variables often
used to describe biopolymers display a non-negligible width both in the drift
and in the diffusion coefficients. We also show that the associated effective
force is compatible with the equilibrium free--energy calculated from a
microscopic sampling, but results in markedly different dynamical properties
Robust Optimization in Simulation: Taguchi and Response Surface Methodology
Optimization of simulated systems is tackled by many methods, but most methods assume known environments. This article, however, develops a 'robust' methodology for uncertain environments. This methodology uses Taguchi's view of the uncertain world, but replaces his statistical techniques by Response Surface Methodology (RSM). George Box originated RSM, and Douglas Montgomery recently extended RSM to robust optimization of real (non-simulated) systems. We combine Taguchi's view with RSM for simulated systems, and apply the resulting methodology to classic Economic Order Quantity (EOQ) inventory models. Our results demonstrate that in general robust optimization requires order quantities that differ from the classic EOQ.Pareto frontier;bootstrap;Latin hypercube sampling
An observable for vacancy characterization and diffusion in crystals
To locate the position and characterize the dynamics of a vacancy in a
crystal, we propose to represent it by the ground state density of a quantum
probe quasi-particle for the Hamiltonian associated to the potential energy
field generated by the atoms in the sample. In this description, the h^2/2mu
coefficient of the kinetic energy term is a tunable parameter controlling the
density localization in the regions of relevant minima of the potential energy
field. Based on this description, we derive a set of collective variables that
we use in rare event simulations to identify some of the vacancy diffusion
paths in a 2D crystal. Our simulations reveal, in addition to the simple and
expected nearest neighbor hopping path, a collective migration mechanism of the
vacancy. This mechanism involves several lattice sites and produces a long
range migration of the vacancy. Finally, we also observed a vacancy induced
crystal reorientation process
Finite Symmetry of Leptonic Mass Matrices
We search for possible symmetries present in the leptonic mixing data from
SU(3) subgroups of order up to 511. Theoretical results based on symmetry are
compared with global fits of experimental data in a chi-squared analysis,
yielding the following results. There is no longer a group that can produce all
the mixing data without a free parameter, but a number of them can accommodate
the first or the second column of the mixing matrix. The only group that fits
the third column is . It predicts and
, in good agreement with experimental results.Comment: Version to appear in Physical Review
Revelations of Folies through Geometric Transformations
This article presents an activity carried out in a course on representation in a masterâs degree in architecture, which aims to train students in the practice and theory of geometric transformation for the production of shapes, using a case study
from contemporary architectural design: the Folies of the Parc de la Villette
Natural and man-made terrestrial electromagnetic noise: an outlook
The terrestrial environment is continuously exposed to electromagnetic radiations which set up a «background»
electromagnetic noise. Within the Non Ionizing Radiation band (NIR), i.e. for frequencies lower than 300 GHz,
this background can have a natural or an artificial origin. Natural origins of electromagnetic radiations are generally
atmospheric or cosmic while artificial origins are technological applications, power transmission, communications,
etc. This paper briefly describes the natural and man-made electromagnetic noise in the NIR band.
Natural noise comes from a large variety of sources involving different physical phenomena and covering a wide
range of frequencies and showing various propagation characteristics with an extremely broad range of power
levels. Due to technological growth man-made electromagnetic noise is nowadays superimposed on natural
noise almost everywhere on Earth. In the last decades man-made noise has increased dramatically over and
above the natural noise in residential and business areas. This increase has led some scientists to consider possible
negative effects of electromagnetic waves on human life and living systems in general. Accurate measurements
of natural and man-made electromagnetic noise are necessary to understand the relative power levels in
the different bands and their influence on life
Building lighting energy consumption modelling with hybrid neural-statistic approaches
"In the proposed work we aim at modelling building lighting energy consumption. We compared several classical methods to the latest Artificial Intelligence. modelling technique: Artificial Neural Networks Ensembling (ANNE). Therefore, in this study we show how we built the ANNE and a new hybrid model based on the. statistical-ANNE combination. Experimentation has been carried out over a three. months data set coming from a real office building located in the ENEA âCasacciaâ. Research Centre. Experimental results show that the proposed hybrid statistical-ANNE approach can get a remarkable improvement with respect to the best classical method(the statistical one).
A decomposition approach for multidimensional knapsacks with family-split penalties
The optimization of Multidimensional Knapsacks with Family-Split Penalties has been introduced in the literature as a variant of the more classical Multidimensional Knapsack and Multi-Knapsack problems. This problem deals with a set of items partitioned in families, and when a single item is picked to maximize the utility, then all items in its family must be picked. Items from the same family can be assigned to different knapsacks, and in this situation split penalties are paid. This problem arises in real applications in various fields. This paper proposes a new exact and fast algorithm based on a specific Combinatorial Benders Cuts scheme. An extensive experimental campaign computationally shows the validity of the proposed method and its superior performance compared to both commercial solvers and state-of-the-art approaches. The paper also addresses algorithmic flexibility and scalability issues, investigates challenging cases, and analyzes the impact of problem parameters on the algorithm behavior. Moreover, it shows the applicability of the proposed approach to a wider class of realistic problems, including fixed costs related to each knapsack utilization. Finally, further possible research directions are considered
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