374 research outputs found
Melting of the Au20 gold cluster : does charge matter?
We investigate the dependence upon charge of the heat capacities of the magic
gold cluster Au20 obtained from density functional based tight binding theory
within parallel tempering molecular dynamics and the multiple histogram method.
The melting temperatures, determined from heat capacity curves, are found to be
1102 K for neutral Au20 and only 866 and 826 K for Au{20} cations and anions
respectively. The present work proves that a single charge quantitatively
affects the thermal properties of the twentymer even for a global property such
as melting
A Sparse SCF algorithm and its parallel implementation: Application to DFTB
We present an algorithm and its parallel implementation for solving a self
consistent problem as encountered in Hartree Fock or Density Functional Theory.
The algorithm takes advantage of the sparsity of matrices through the use of
local molecular orbitals. The implementation allows to exploit efficiently
modern symmetric multiprocessing (SMP) computer architectures. As a first
application, the algorithm is used within the density functional based tight
binding method, for which most of the computational time is spent in the linear
algebra routines (diagonalization of the Fock/Kohn-Sham matrix). We show that
with this algorithm (i) single point calculations on very large systems
(millions of atoms) can be performed on large SMP machines (ii) calculations
involving intermediate size systems (1~000--100~000 atoms) are also strongly
accelerated and can run efficiently on standard servers (iii) the error on the
total energy due to the use of a cut-off in the molecular orbital coefficients
can be controlled such that it remains smaller than the SCF convergence
criterion.Comment: 13 pages, 11 figure
Be Careful What You Ask for: Attacking the Constitutionality of the Clean Air Act Operating Permit Program
This comment explores whether Congress went too far in its effort to guide the states in the control of air pollution through the operating permit program or whether the states merely got what they asked for and cannot use the Constitution as a weapon to roll back the course of pollution control. The comment will focus largely on Title V and the Virginia cases, but the Missouri decision will be used to fill in the many gaps left by the Fourth Circuit in its constitutional analysis. The constitutional issues addressed by the Missouri and Virginia courts are particularly relevant in light of a 1996 regulation under the Clean Water Act (CWA). This regulation amends the requirements for an approvable state pollutant discharge permit system (SPDES) program to include the same opportunity for judicial review by interested persons as is available under Article III of the United States Constitution. Prior to this new regulation, which became effective in 1996, there was no requirement that a state administering an SPDES program include an opportunity for judicial review as broad as Article III standing. Although development of state permit programs is not encouraged as vigorously in the CWA as in the CAA, this new regulation has already caused some controversy in Virginia and elsewhere. For example, in its final rule publication, the EPA noted that several individuals suggested that the regulation may unconstitutionally infringe upon states\u27 rights not to be sued without their consent. In answer to this concern, the EPA stated simply that if states are dissatisfied with the new regulation they could simply leave the administration of a permit program to the federal government. Another concern expressed by those commenting on the rule was that Virginia was being singled out because, at the time the rule was proposed, its judicial review statute enacted to implement the CWA permit program restricted review to an owner aggrieved and effectively barred all others from challenging the state\u27s issuance of a permit. This restriction prompted several environmental groups to petition the EPA to withdraw approval of Virginia\u27s program. The EPA did note in its final rule that Virginia had amended its statute in 1996, but stated that it had not made a final decision as to whether the amendment satisfied the new regulation. It appears from the above discussion that the stage is set for a renewed discussion of citizen\u27s standing to review state environmental agencies\u27 decisions regarding pollution permits. Given the lack of incentive for states to adopt their own SPDES programs, it is unclear whether the same vigorous challenges to the CAA will also plague the CWA. However, there is a possibility that, armed with two favorable decisions, the EPA or Congress may now reexamine other environmental statutes to find new ways to encourage states to regulate the way the EPA itself would. While it is doubtful that this possibility was envisioned by those who, prior to the 1990 amendments, requested some direction in implementing the CAA, it may be an answer that many citizens, companies, and state officials will get in the end. Since the imposition of sanctions is complete or imminent in at least two states and the new CWA regulations threaten to further drag out the debate, the constitutionality of the relevant CAA provisions is a major concern. To effectively examine the issues, Part II of this Comment provides background information, including the relevant provisions of the CAA and their legislative histories, regulatory action taken by the EPA in implementing the state permit program, the EPA\u27s decisions with regard to Virginia\u27s proposed permit program, and case law relevant to the constitutional issues. The constitutional arguments made by Missouri, Virginia, the EPA, and amici, and the courts\u27 analyses of those arguments, are set out in Part III. Part IV consists of a legal analysis of the courts\u27 opinions. Part discusses the policy reasons supporting a finding of constitutionality and possible impacts of a ruling in favor of Virginia and Missouri. Finally, Part V also concludes that the courts were correct in ruling against Virginia and Missouri for the legal and practical reasons previously discussed
Stacked clusters of polycyclic aromatic hydrocarbon molecules
Clusters of polycyclic aromatic hydrocarbon (PAH) molecules are modelled
using explicit all-atom potentials using a rigid body approximation. The PAH's
considered range from pyrene (C10H8) to circumcoronene (C54H18), and clusters
containing between 2 and 32 molecules are investigated. In addition to the
usual repulsion-dispersion interactions, electrostatic point-charge
interactions are incorporated, as obtained from density functional theory
calculations. The general electrostatic distribution in neutral or singly
charged PAH's is reproduced well using a fluctuating charges analysis, which
provides an adequate description of the multipolar distribution. Global
optimization is performed using a variety of methods, including basin-hopping
and parallel tempering Monte Carlo. We find evidence that stacking the PAH
molecules generally yields the most stable motif. A structural transition
between one-dimensional stacks and three-dimensional shapes built from mutiple
stacks is observed at larger sizes, and the threshold for this transition
increases with the size of the monomer. Larger aggregates seem to evolve toward
the packing observed for benzene in bulk.Difficulties met in optimizing these
clusters are analysed in terms of the strong anisotropy of the molecules. We
also discuss segregation in heterogeneous clusters and vibrational properties
in the context of astrophysical observations.Comment: 12 pages, 7 figure
Modelling the spatial organization of cell proliferation in the developing central nervous system
How far is neuroepithelial cell proliferation in the developing central
nervous system a deterministic process? Or, to put it in a more precise way,
how accurately can it be described by a deterministic mathematical model? To
provide tracks to answer this question, a deterministic system of transport and
diffusion partial differential equations, both physiologically and spatially
structured, is introduced as a model to describe the spatially organized
process of cell proliferation during the development of the central nervous
system. As an initial step towards dealing with the three-dimensional case, a
unidimensional version of the model is presented. Numerical analysis and
numerical tests are performed. In this work we also achieve a first
experimental validation of the proposed model, by using cell proliferation data
recorded from histological sections obtained during the development of the
optic tectum in the chick embryo
Extended Red Emission and the evolution of carbonaceaous nanograins in NGC 7023
Extended Red Emission (ERE) was recently attributed to the photo-luminescence
of either doubly ionized Polycyclic Aromatic Hydrocarbons (PAH), or
charged PAH dimers. We analysed the visible and mid-infrared (mid-IR) dust
emission in the North-West and South photo-dissociation regions of the
reflection nebula NGC 7023.Using a blind signal separation method, we extracted
the map of ERE from images obtained with the Hubble Space Telescope, and at the
Canada France Hawaii Telescope. We compared the extracted ERE image to the
distribution maps of the mid-IR emission of Very Small Grains (VSGs), neutral
and ionized PAHs (PAH and PAH) obtained with the Spitzer Space
Telescope and the Infrared Space Observatory. ERE is dominant in transition
regions where VSGs are being photo-evaporated to form free PAH molecules, and
is not observed in regions dominated by PAH. Its carrier makes a minor
contribution to the mid-IR emission spectrum. These results suggest that the
ERE carrier is a transition species formed during the destruction of VSGs.
Singly ionized PAH dimers appear as good candidates but PAH molecules
seem to be excluded.Comment: Accepted for publication in A&
Detection of the buckminsterfullerene cation (C60+) in space
In the early 90s, C60+ was proposed as the carrier of two diffuse
interstellar bands (DIBs) at 957.7 and 963.2 nm, but a firm identification
still awaits gas-phase spectroscopic data. Neutral C60, on the other hand, was
recently detected through its infrared emission bands in the interstellar
medium and evolved stars. In this contribution, we present the detection of
C60+ through its infrared vibrational bands in the NGC 7023 nebula, based on
spectroscopic observations with the Spitzer space telescope, quantum chemistry
calculation, and laboratory data from the literature. This detection supports
the idea that C60+ could be a DIB carrier, and provides robust evidence that
fullerenes exist in the gas-phase in the interstellar medium. Modeling efforts
to design specific observations, combined with new gas-phase data, will be
essential to confirm this proposal. A definitive attribution of the 957.7 and
963.2 nm DIBs to C60+ would represent a significant step forward in the field.Comment: To appear in "Proceedings of IAU 297 symposium on the Diffuse
Interstellar Bands", eds. J. Cami and N. Cox (5 pages
Mapping the structural diversity of C60 carbon clusters and their infrared spectra
The current debate about the nature of the carbonaceous material carrying the
infrared (IR) emission spectra of planetary and proto-planetary nebulae,
including the broad plateaus, calls for further studies on the interplay
between structure and spectroscopy of carbon-based compounds of astrophysical
interest. The recent observation of C60 buckminsterfullerene in space suggests
that carbon clusters of similar size may also be relevant. In the present work,
broad statistical samples of C60 isomers were computationally determined
without any bias using a reactive force field, their IR spectra being
subsequently obtained following local optimization with the
density-functional-based tight-binding theory. Structural analysis reveals four
main structural families identified as cages, planar polycyclic aromatics,
pretzels, and branched. Comparison with available astronomical spectra
indicates that only the cage family could contribute to the plateau observed in
the 6-9 micron region. The present framework shows great promise to explore and
relate structural and spectroscopic features in more diverse and possibly
hydrogenated carbonaceous compounds, in relation with astronomical
observations
The Genetic Code Degeneration I: Rules Governing the Code Degeneration and the Spatial Organization of the Codon Informative Properties
The present work is devoted to describe a set of rules explaining the
discriminating versus non-discriminating behavior of the di-basic stages and to
characterize the role of each base in determining such a behavior. Bases are
analyze as dual entities characterized by its chemical type and the number of H
bonds involved in the codon-anticodon interaction. A codon is characterized as
an asymmetric informative entity whose global informative capacity results from
the spatially organized combinatory of the 6 proper-ties assigned by the 3
bases
NGF, TrkA-P and neuroprotection after a hypoxic event in the developing central nervous system
A decrease in the concentration of oxygen in the blood and tissues (hypoxia) produces important, sometimes irreversible, damages in the central nervous system (CNS) both during development and also postnatally. The present work aims at analyzing the expression of nerve growth factor (NGF) and p75 and the activation of TrkA in response to an acute normobaric hypoxic event and to evaluate the possible protective role of exogenous NGF. The developing chick optic tectum (OT), a recognized model of corticogenesis, was used as experimental system by means of in vivo and in vitro studies. Based on identification of the period of highest sensitivity of developmental programmed cell death (ED15) we show that hypoxia has a mild but reproducible effect that consist of a temporal increase of cell death 6 h after the end of a hypoxic treatment. Cell death was preceded by a significant early increase in the expression of Nerve Growth Factor (NGF) and its membrane receptor p75. In addition, we found a biphasic response of TrkA activation: a decrease during hypoxia followed by an increase â4 h later- that temporally coincide with the interval of NGF overexpression. To test the NGF - NGF receptors role in hypoxic cell death, we quantified, in primary neuronal cultures derived from ED15 OT, the levels of TrkA activation after an acute hypoxic treatment. A significant decline in the level of TrkA activation was observed during hypoxia followed, 24 h later, by significant cell death. Interestingly, this cell death can be reverted if TrkA inactivation during hypoxia is suppressed by the addition of NGF. Our results suggest that TrkA activation may play an important role in the survival of OT neurons subjected to acute hypoxia. The role of TrkA in neuronal survival after injury may be advantageously used for the generation of neuroprotective strategies to improve prenatal insult outcomes.Fil: Bogetti, MarĂa Eugenia. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas. Oficina de CoordinaciĂłn Administrativa Houssay. Instituto de BiologĂa Celular y Neurociencia "Prof. Eduardo de Robertis". Universidad de Buenos Aires. Facultad de Medicina. Instituto de BiologĂa Celular y Neurociencia; ArgentinaFil: Pozo Devoto, Victorio Martin. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas. Oficina de CoordinaciĂłn Administrativa Houssay. Instituto de BiologĂa Celular y Neurociencia "Prof. Eduardo de Robertis". Universidad de Buenos Aires. Facultad de Medicina. Instituto de BiologĂa Celular y Neurociencia; ArgentinaFil: Rapacioli, Melina. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas. Oficina de CoordinaciĂłn Administrativa Houssay. Instituto de Neurociencia Cognitiva. FundaciĂłn Favaloro. Instituto de Neurociencia Cognitiva; ArgentinaFil: Flores, Domingo Vladimir. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas. Oficina de CoordinaciĂłn Administrativa Houssay. Instituto de Neurociencia Cognitiva. FundaciĂłn Favaloro. Instituto de Neurociencia Cognitiva; Argentina. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas. Oficina de CoordinaciĂłn Administrativa Houssay. Instituto de BiologĂa Celular y Neurociencia "Prof. Eduardo de Robertis". Universidad de Buenos Aires. Facultad de Medicina. Instituto de BiologĂa Celular y Neurociencia; ArgentinaFil: Fiszer de Plazas, Sara. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas. Oficina de CoordinaciĂłn Administrativa Houssay. Instituto de BiologĂa Celular y Neurociencia "Prof. Eduardo de Robertis". Universidad de Buenos Aires. Facultad de Medicina. Instituto de BiologĂa Celular y Neurociencia; Argentin
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