101,677 research outputs found
Exploration of Reaction Pathways and Chemical Transformation Networks
For the investigation of chemical reaction networks, the identification of
all relevant intermediates and elementary reactions is mandatory. Many
algorithmic approaches exist that perform explorations efficiently and
automatedly. These approaches differ in their application range, the level of
completeness of the exploration, as well as the amount of heuristics and human
intervention required. Here, we describe and compare the different approaches
based on these criteria. Future directions leveraging the strengths of chemical
heuristics, human interaction, and physical rigor are discussed.Comment: 48 pages, 4 figure
Interplay of gas and ice during cloud evolution
During the evolution of diffuse clouds to molecular clouds, gas-phase
molecules freeze out on surfaces of small dust particles to form ices. On dust
surfaces, water is the main constituent of the icy mantle in which a complex
chemistry is taking place. We aim to study the formation pathways and the
composition of the ices throughout the evolution of diffuse clouds. For this
purpose, we use time-dependent rate equations to calculate the molecular
abundances in both gas phase and on solid surfaces (onto dust grains). We fully
consider the gas-dust interplay by including the details of freeze-out,
chemical and thermal desorption, as well as the most important photo-processes
on grain surfaces. The difference in binding energies of chemical species on
bare and icy surfaces is also incorporated into our equations. Using the
numerical code FLASH, we perform a hydrodynamical simulation of a
gravitationally bound diffuse cloud and follow its contraction. We find that
while the dust grains are still bare, water formation is enhanced by grain
surface chemistry which is subsequently released into the gas phase, enriching
the molecular medium. The CO molecules, on the other hand, tend to freeze out
gradually on bare grains. This causes CO to be well mixed and strongly present
within the first ice layer. Once one monolayer of water ice has formed, the
binding energy of the grain surface changes significantly and an immediate and
strong depletion of gas-phase water and CO molecules occur. While hydrogenation
converts solid CO into formaldehyde (HCO) and methanol (CHOH), water
ice becomes the main constituent of the icy grains. Inside molecular clumps
formaldehyde is more abundant than water and methanol in the gas phase owing
its presence in part to chemical desorption.Comment: 19 pages, 10 figures, 9 tables, 23 equations. Accepted for
publication Astronomy & Astrophysics. In version 3: Language edit, added
gas-phase reaction tables, title has change
COEL: A Web-based Chemistry Simulation Framework
The chemical reaction network (CRN) is a widely used formalism to describe
macroscopic behavior of chemical systems. Available tools for CRN modelling and
simulation require local access, installation, and often involve local file
storage, which is susceptible to loss, lacks searchable structure, and does not
support concurrency. Furthermore, simulations are often single-threaded, and
user interfaces are non-trivial to use. Therefore there are significant hurdles
to conducting efficient and collaborative chemical research. In this paper, we
introduce a new enterprise chemistry simulation framework, COEL, which
addresses these issues. COEL is the first web-based framework of its kind. A
visually pleasing and intuitive user interface, simulations that run on a large
computational grid, reliable database storage, and transactional services make
COEL ideal for collaborative research and education. COEL's most prominent
features include ODE-based simulations of chemical reaction networks and
multicompartment reaction networks, with rich options for user interactions
with those networks. COEL provides DNA-strand displacement transformations and
visualization (and is to our knowledge the first CRN framework to do so), GA
optimization of rate constants, expression validation, an application-wide
plotting engine, and SBML/Octave/Matlab export. We also present an overview of
the underlying software and technologies employed and describe the main
architectural decisions driving our development. COEL is available at
http://coel-sim.org for selected research teams only. We plan to provide a part
of COEL's functionality to the general public in the near future.Comment: 23 pages, 12 figures, 1 tabl
Modelling CO formation in the turbulent interstellar medium
We present results from high-resolution three-dimensional simulations of
turbulent interstellar gas that self-consistently follow its coupled thermal,
chemical and dynamical evolution, with a particular focus on the formation and
destruction of H2 and CO. We quantify the formation timescales for H2 and CO in
physical conditions corresponding to those found in nearby giant molecular
clouds, and show that both species form rapidly, with chemical timescales that
are comparable to the dynamical timescale of the gas.
We also investigate the spatial distributions of H2 and CO, and how they
relate to the underlying gas distribution. We show that H2 is a good tracer of
the gas distribution, but that the relationship between CO abundance and gas
density is more complex. The CO abundance is not well-correlated with either
the gas number density n or the visual extinction A_V: both have a large
influence on the CO abundance, but the inhomogeneous nature of the density
field produced by the turbulence means that n and A_V are only poorly
correlated. There is a large scatter in A_V, and hence CO abundance, for gas
with any particular density, and similarly a large scatter in density and CO
abundance for gas with any particular visual extinction. This will have
important consequences for the interpretation of the CO emission observed from
real molecular clouds.
Finally, we also examine the temperature structure of the simulated gas. We
show that the molecular gas is not isothermal. Most of it has a temperature in
the range of 10--20 K, but there is also a significant fraction of warmer gas,
located in low-extinction regions where photoelectric heating remains
effective.Comment: 37 pages, 15 figures; minor revisions, matches version accepted by
MNRA
Design and Development of Software Tools for Bio-PEPA
This paper surveys the design of software tools for the Bio-PEPA process algebra. Bio-PEPA is a high-level language for modelling biological systems such as metabolic pathways and other biochemical reaction networks. Through providing tools for this modelling language we hope to allow easier use of a range of simulators and model-checkers thereby freeing the modeller from the responsibility of developing a custom simulator for the problem of interest. Further, by providing mappings to a range of different analysis tools the Bio-PEPA language allows modellers to compare analysis results which have been computed using independent numerical analysers, which enhances the reliability and robustness of the results computed.
Hardware acceleration of reaction-diffusion systems:a guide to optimisation of pattern formation algorithms using OpenACC
Reaction Diffusion Systems (RDS) have widespread applications in computational ecology, biology, computer graphics and the visual arts. For the former applications a major barrier to the development of effective simulation models is their computational complexity - it takes a great deal of processing power to simulate enough replicates such that reliable conclusions can be drawn. Optimizing the computation is thus highly desirable in order to obtain more results with less resources. Existing optimizations of RDS tend to be low-level and GPGPU based. Here we apply the higher-level OpenACC framework to two case studies: a simple RDS to learn the ‘workings’ of OpenACC and a more realistic and complex example. Our results show that simple parallelization directives and minimal data transfer can produce a useful performance improvement. The relative simplicity of porting OpenACC code between heterogeneous hardware is a key benefit to the scientific computing community in terms of speed-up and portability
Quantifying the implicit process flow abstraction in SBGN-PD diagrams with Bio-PEPA
For a long time biologists have used visual representations of biochemical
networks to gain a quick overview of important structural properties. Recently
SBGN, the Systems Biology Graphical Notation, has been developed to standardise
the way in which such graphical maps are drawn in order to facilitate the
exchange of information. Its qualitative Process Diagrams (SBGN-PD) are based
on an implicit Process Flow Abstraction (PFA) that can also be used to
construct quantitative representations, which can be used for automated
analyses of the system. Here we explicitly describe the PFA that underpins
SBGN-PD and define attributes for SBGN-PD glyphs that make it possible to
capture the quantitative details of a biochemical reaction network. We
implemented SBGNtext2BioPEPA, a tool that demonstrates how such quantitative
details can be used to automatically generate working Bio-PEPA code from a
textual representation of SBGN-PD that we developed. Bio-PEPA is a process
algebra that was designed for implementing quantitative models of concurrent
biochemical reaction systems. We use this approach to compute the expected
delay between input and output using deterministic and stochastic simulations
of the MAPK signal transduction cascade. The scheme developed here is general
and can be easily adapted to other output formalisms
Approximations for modelling CO chemistry in GMCs: a comparison of approaches
We examine several different simplified approaches for modelling the
chemistry of CO in three-dimensional numerical simulations of turbulent
molecular clouds. We compare the different models both by looking at the
behaviour of integrated quantities such as the mean CO fraction or the
cloud-averaged CO-to-H2 conversion factor, and also by studying the detailed
distribution of CO as a function of gas density and visual extinction. In
addition, we examine the extent to which the density and temperature
distributions depend on our choice of chemical model.
We find that all of the models predict the same density PDF and also agree
very well on the form of the temperature PDF for temperatures T > 30 K,
although at lower temperatures, some differences become apparent. All of the
models also predict the same CO-to-H2 conversion factor, to within a factor of
a few. However, when we look more closely at the details of the CO
distribution, we find larger differences. The more complex models tend to
produce less CO and more atomic carbon than the simpler models, suggesting that
the C/CO ratio may be a useful observational tool for determining which model
best fits the observational data. Nevertheless, the fact that these chemical
differences do not appear to have a strong effect on the density or temperature
distributions of the gas suggests that the dynamical behaviour of the molecular
clouds on large scales is not particularly sensitive to how accurately the
small-scale chemistry is modelled.Comment: 18 pages, 10 figures. Minor revisions, including the addition of a
comparison of simulated and observed C/CO ratios. Accepted by MNRA
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