4,013 research outputs found
High-order density-matrix perturbation theory
We present a simple formalism for the calculation of the derivatives of the
electronic density matrix at any order, within density functional theory. Our
approach, contrary to previous ones, is not based on the perturbative expansion
of the Kohn-Sham wavefunctions. It has the following advantages: (i) it allows
a simple derivation for the expression for the high order derivatives of the
density matrix; (ii) in extended insulators, the treatment of
uniform-electric-field perturbations and of the polarization derivatives is
straightforward.Comment: 4 page
Random Graph-Homomorphisms and Logarithmic Degree
A graph homomorphism between two graphs is a map from the vertex set of one
graph to the vertex set of the other graph, that maps edges to edges. In this
note we study the range of a uniformly chosen homomorphism from a graph G to
the infinite line Z. It is shown that if the maximal degree of G is
`sub-logarithmic', then the range of such a homomorphism is super-constant.
Furthermore, some examples are provided, suggesting that perhaps for graphs
with super-logarithmic degree, the range of a typical homomorphism is bounded.
In particular, a sharp transition is shown for a specific family of graphs
C_{n,k} (which is the tensor product of the n-cycle and a complete graph, with
self-loops, of size k). That is, given any function psi(n) tending to infinity,
the range of a typical homomorphism of C_{n,k} is super-constant for k = 2
log(n) - psi(n), and is 3 for k = 2 log(n) + psi(n)
The OPERA magnetic spectrometer
The OPERA neutrino oscillation experiment foresees the construction of two
magnetized iron spectrometers located after the lead-nuclear emulsion targets.
The magnet is made up of two vertical walls of rectangular cross section
connected by return yokes. The particle trajectories are measured by high
precision drift tubes located before and after the arms of the magnet.
Moreover, the magnet steel is instrumented with Resistive Plate Chambers that
ease pattern recognition and allow a calorimetric measurement of the hadronic
showers. In this paper we review the construction of the spectrometers. In
particular, we describe the results obtained from the magnet and RPC prototypes
and the installation of the final apparatus at the Gran Sasso laboratories. We
discuss the mechanical and magnetic properties of the steel and the techniques
employed to calibrate the field in the bulk of the magnet. Moreover, results of
the tests and issues concerning the mass production of the Resistive Plate
Chambers are reported. Finally, the expected physics performance of the
detector is described; estimates rely on numerical simulations and the outcome
of the tests described above.Comment: 6 pages, 10 figures, presented at the 2003 IEEE-NSS conference,
Portland, OR, USA, October 20-24, 200
Electronic structure and excited state dynamics in a dicyanovinyl-substituted oligothiophene on Au(111)
Dicyanovinyl (DCV)-substituted oligothiophenes are promising donor materials
in vacuum-processed small-molecule organic solar cells. Here, we studied the
structural and the electronic properties of DCV-dimethyl-pentathiophene
(DCV5T-Me2) adsorbed on Au(111) from submonolayer to multilayer coverages.
Using a multi-technique experimental approach (low-temperature scanning
tunneling microscopy/spectroscopy (STM/STS), atomic force microscopy (AFM),
and two-photon photoemission (2PPE) spectroscopy), we determined the energetic
position of several affinity levels as well as ionization potentials
originating from the lowest unoccupied molecular orbitals (LUMO) and the
highest occupied molecular orbitals (HOMO), evidencing a transport gap of 1.4
eV. Proof of an excitonic state was found to be a spectroscopic feature
located at 0.6 eV below the LUMO affinity level. With increasing coverage
photoemission from excitonic states gains importance. We were able to track
the dynamics of several electronically excited states of multilayers by means
of femtosecond time-resolved 2PPE. We resolved an intriguing relaxation
dynamics involving four processes, ranging from sub-picosecond (ps) to several
hundred ps time spans. These show a tendency to increase with increasing
coverage. The present study provides important parameters such as energetic
positions of transport levels as well as lifetimes of electronically excited
states, which are essential for designing organic-molecule-based
optoelectronic devices
Total energy global optimizations using non orthogonal localized orbitals
An energy functional for orbital based calculations is proposed, which
depends on a number of non orthogonal, localized orbitals larger than the
number of occupied states in the system, and on a parameter, the electronic
chemical potential, determining the number of electrons. We show that the
minimization of the functional with respect to overlapping localized orbitals
can be performed so as to attain directly the ground state energy, without
being trapped at local minima. The present approach overcomes the multiple
minima problem present within the original formulation of orbital based
methods; it therefore makes it possible to perform calculations for an
arbitrary system, without including any information about the system bonding
properties in the construction of the input wavefunctions. Furthermore, while
retaining the same computational cost as the original approach, our formulation
allows one to improve the variational estimate of the ground state energy, and
the energy conservation during a molecular dynamics run. Several numerical
examples for surfaces, bulk systems and clusters are presented and discussed.Comment: 24 pages, RevTex file, 5 figures available upon reques
Dual targeting of ptp1b and aldose reductase with marine drug phosphoeleganin: A promising strategy for treatment of type 2 diabetes
An in-depth study on the inhibitory mechanism on protein tyrosine phosphatase 1B (PTP1B) and aldose reductase (AR) enzymes, including analysis of the insulin signalling pathway, of phosphoeleganin, a marine-derived phosphorylated polyketide, was achieved. Phosphoeleganin was demonstrated to inhibit both enzymes, acting respectively as a pure non-competitive inhibitor of PTP1B and a mixed-type inhibitor of AR. In addition, in silico docking analyses to evaluate the interaction mode of phosphoeleganin with both enzymes were performed. Interestingly, this study showed that phosphoeleganin is the first example of a dual inhibitor polyketide extracted from a marine invertebrate, and it could be used as a versatile scaffold structure for the synthesis of new designed multiple ligands
Plasma levels of conjugated bile acids in newborns after a short period of parenteral nutrition.
Background: Patients receiving parenteral nutrition (PN) frequently exhibit liver dysfunction. The authors previously reported that plant sterols of lipid emulsions added to the nutritional solution of newborns receiving PN accumulate in plasma and cell membranes and may contribute to the development of cholestasis. Conjugated bile acids (BA) have been shown to be useful markers of cholestasis. Plasma levels of several BA in newborns were quantified after administration of PN for less than 2 weeks. Methods: Plasma samples from 15 healthy control infants (CN), 22 patients who had received PN for 3-15 days (T1), and 9 patients scheduled to receive PN (T0) were analyzed. After a simple extraction procedure, plasma BA were analyzed by liquid chromatography-tandem mass spectrometry using a quantitative isotope dilution method. Results: The concentrations of BA did not differ significantly between controls and patients before PN (CN vs T0), with the exception of glycocholic acid (GCA; 2.30 ± 2.60 ??M vs 7.29 ± 5.39 ??M, respectively). There was a significant difference in several BA between controls and patients after PN (2.30 ± 2.60 ??M vs 7.61 ± 6.46 ??M for GCA, respectively; 4.02 ± 3.49 ??M vs 11.88 ± 11.05 ??M for taurocholic acid [TCA], respectively; and 4.81 ± 3.49 ??M vs 13.58 ± 12.22 ??M for taurochenodeoxycholic + taurodeoxycholic + tauroursodeoxycholic acids [TCDCA+TDCA+TUDCA], respectively). Conclusions: In newborns receiving PN, a short period of PN is associated with an early increase of some conjugated BA. These results suggest that GCA, TCA, and TCDCA+TDCA+TUDCA levels could be used as early markers of PN-related cholestasis
Quantum ESPRESSO: a modular and open-source software project for quantum simulations of materials
Quantum ESPRESSO is an integrated suite of computer codes for
electronic-structure calculations and materials modeling, based on
density-functional theory, plane waves, and pseudopotentials (norm-conserving,
ultrasoft, and projector-augmented wave). Quantum ESPRESSO stands for "opEn
Source Package for Research in Electronic Structure, Simulation, and
Optimization". It is freely available to researchers around the world under the
terms of the GNU General Public License. Quantum ESPRESSO builds upon
newly-restructured electronic-structure codes that have been developed and
tested by some of the original authors of novel electronic-structure algorithms
and applied in the last twenty years by some of the leading materials modeling
groups worldwide. Innovation and efficiency are still its main focus, with
special attention paid to massively-parallel architectures, and a great effort
being devoted to user friendliness. Quantum ESPRESSO is evolving towards a
distribution of independent and inter-operable codes in the spirit of an
open-source project, where researchers active in the field of
electronic-structure calculations are encouraged to participate in the project
by contributing their own codes or by implementing their own ideas into
existing codes.Comment: 36 pages, 5 figures, resubmitted to J.Phys.: Condens. Matte
p190 Rho-GTPase activating protein associates with plexins and it is required for semaphorin signalling
Plexins are transmembrane receptors for semaphorins, guiding cell migration and axon extension. Plexin activation leads to the disassembly of integrin-based focal adhesive structures and to actin cytoskeleton remodelling and inhibition of cell migration; however, the underlying molecular mechanisms are unclear. We consistently observe a transient decrease of cellular RhoA-GTP levels upon plexin activation in adherent cells. One of the main effectors of RhoA downregulation is p190, a ubiquitously expressed GTPase activating protein (GAP). We show that, in p190-deficient fibroblasts, the typical functional activities mediated by plexins (such as cell collapse and inhibition of integrin-based adhesion) are blocked or greatly impaired. Notably, the functional response can be rescued in these cells by re-expressing exogenous p190, but not a mutant form specifically lacking RhoGAP activity. We furthermore demonstrate that semaphorin function is blocked in epithelial cells, primary endothelial cells and neuroblasts upon treatment with small interfering RNAs that knockdown p190 expression. Finally, we show that p190 transiently associates with plexins, and its RhoGAP activity is increased in response to semaphorin stimulation. We conclude that p190-RhoGAP is crucially involved in semaphorin signalling to the actin cytoskeleton, via interaction with plexins
Non-linear optical susceptibilities, Raman efficiencies and electrooptic tensors from first-principles density functional perturbation theory
The non-linear response of infinite periodic solids to homogenous electric
fields and collective atomic displacements is discussed in the framework of
density functional perturbation theory. The approach is based on the 2n + 1
theorem applied to an electric-field-dependent energy functional. We report the
expressions for the calculation of the non-linear optical susceptibilities,
Raman scattering efficiencies and electrooptic coefficients. Different
formulations of third-order energy derivatives are examined and their
convergence with respect to the k-point sampling is discussed. We apply our
method to a few simple cases and compare our results to those obtained with
distinct techniques. Finally, we discuss the effect of a scissors correction on
the EO coefficients and non-linear optical susceptibilities
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