259 research outputs found
Molecular Dynamics Simulations of Solutions at Constant Chemical Potential
Molecular Dynamics studies of chemical processes in solution are of great
value in a wide spectrum of applications, which range from nano-technology to
pharmaceutical chemistry. However, these calculations are affected by severe
finite-size effects, such as the solution being depleted as the chemical
process proceeds, which influence the outcome of the simulations. To overcome
these limitations, one must allow the system to exchange molecules with a
macroscopic reservoir, thus sampling a Grand-Canonical ensemble. Despite the
fact that different remedies have been proposed, this still represents a key
challenge in molecular simulations.
In the present work we propose the Constant Chemical Potential Molecular
Dynamics (CMD) method, which introduces an external force that controls
the environment of the chemical process of interest. This external force,
drawing molecules from a finite reservoir, maintains the chemical potential
constant in the region where the process takes place. We have applied the
CMD method to the paradigmatic case of urea crystallization in aqueous
solution. As a result, we have been able to study crystal growth dynamics under
constant supersaturation conditions, and to extract growth rates and
free-energy barriers.Comment: 8 pages, 8 figures (Supplementary Information: 6 pages, 7 figures).
Typos and labelling corrected Ver. 3: Minor comments added in Sec. 3.
References 13,36,38 added. Minor text changes and typos correcte
Chemical Potential Calculations In Dense Liquids Using Metadynamics
The calculation of chemical potential has traditionally been a challenge in
atomistic simulations. One of the most used approaches is Widom's insertion
method in which the chemical potential is calculated by periodically attempting
to insert an extra particle in the system. In dense systems this method fails
since the insertion probability is very low. In this paper we show that in a
homogeneous fluid the insertion probability can be increased using
metadynamics. We test our method on a supercooled high density binary
Lennard-Jones fluid. We find that we can obtain efficiently converged results
even when Widom's method fails.Comment: 5 pages, 2 figures, Ver 2: typos corrected, Ref. 10 year correcte
Learning Relatedness Measures for Entity Linking
Entity Linking is the task of detecting, in text documents, relevant mentions to entities of a given knowledge base. To this end, entity-linking algorithms use several signals and features extracted from the input text or from the knowl- edge base. The most important of such features is entity relatedness. Indeed, we argue that these algorithms benefit from maximizing the relatedness among the relevant enti- ties selected for annotation, since this minimizes errors in disambiguating entity-linking.
The definition of an e↵ective relatedness function is thus a crucial point in any entity-linking algorithm. In this paper we address the problem of learning high-quality entity relatedness functions. First, we formalize the problem of learning entity relatedness as a learning-to-rank problem. We propose a methodology to create reference datasets on the basis of manually annotated data. Finally, we show that our machine-learned entity relatedness function performs better than other relatedness functions previously proposed, and, more importantly, improves the overall performance of dif- ferent state-of-the-art entity-linking algorithms
On Suggesting Entities as Web Search Queries
The Web of Data is growing in popularity and dimension,
and named entity exploitation is gaining importance in many research
fields. In this paper, we explore the use of entities that can be extracted
from a query log to enhance query recommendation. In particular, we
extend a state-of-the-art recommendation algorithm to take into account
the semantic information associated with submitted queries. Our novel
method generates highly related and diversified suggestions that we as-
sess by means of a new evaluation technique. The manually annotated
dataset used for performance comparisons has been made available to
the research community to favor the repeatability of experiments
Chemical Potential Calculations in Non-Homogeneous Liquids
The numerical computation of chemical potential in dense, non-homogeneous
fluids is a key problem in the study of confined fluids thermodynamics. To this
day several methods have been proposed, however there is still need for a
robust technique, capable of obtaining accurate estimates at large average
densities. A widely established technique is the Widom insertion method, that
computes the chemical potential by sampling the energy of insertion of a test
particle. Non-homogeneity is accounted for by assigning a density dependent
weight to the insertion points. However, in dense systems, the poor sampling of
the insertion energy is a source of inefficiency, hampering a reliable
convergence.
We have recently presented a new technique for the chemical potential
calculation in homogeneous fluids. This novel method enhances the sampling of
the insertion energy via Well-Tempered Metadynamics, reaching accurate
estimates at very large densities. In this paper we extend the technique to the
case of non-homogeneous fluids. The method is successfully tested on a confined
Lennard-Jones fluid. In particular we show that, thanks to the improved
sampling, our technique does not suffer from a systematic error that affects
the classic Widom method for non-homogeneous fluids, providing a precise and
accurate result.Comment: 16 pages, 4 figures Contains a Supplementary Information fil
A scalable hardware and software control apparatus for experiments with hybrid quantum systems
Modern experiments with fundamental quantum systems - like ultracold atoms,
trapped ions, single photons - are managed by a control system formed by a
number of input/output electronic channels governed by a computer. In hybrid
quantum systems, where two or more quantum systems are combined and made to
interact, establishing an efficient control system is particularly challenging
due to the higher complexity, especially when each single quantum system is
characterized by a different timescale. Here we present a new control apparatus
specifically designed to efficiently manage hybrid quantum systems. The
apparatus is formed by a network of fast communicating Field Programmable Gate
Arrays (FPGAs), the action of which is administrated by a software. Both
hardware and software share the same tree-like structure, which ensures a full
scalability of the control apparatus. In the hardware, a master board acts on a
number of slave boards, each of which is equipped with an FPGA that locally
drives analog and digital input/output channels and radiofrequency (RF) outputs
up to 400 MHz. The software is designed to be a general platform for managing
both commercial and home-made instruments in a user-friendly and intuitive
Graphical User Interface (GUI). The architecture ensures that complex control
protocols can be carried out, such as performing of concurrent commands loops
by acting on different channels, the generation of multi-variable error
functions and the implementation of self-optimization procedures. Although
designed for managing experiments with hybrid quantum systems, in particular
with atom-ion mixtures, this control apparatus can in principle be used in any
experiment in atomic, molecular, and optical physics.Comment: 10 pages, 12 figure
Discovering Europeana users’ search behavior
Europeana is a strategic project funded by the European Commission with the goal of making Europe's cultural and scientific heritage accessible to the public. ASSETS is a two-year Best Practice Network co-funded by the CIP PSP Programme to improve performance, accessibility and usability of the Europeana search engine. Here we present a characterization of the Europeana logs by showing statistics on common behavioural patterns of the Europeana users
A PFEM approach to the simulation of landslide generated water-waves
A Particle Finite Element Method is here applied to the simulation of
landslide-water interaction. An elastic-visco-plastic non-Newtonian, Bingham-like constitutive
model has been used to describe the landslide material. Two examples are
shown to show the potential of the approach
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