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Search Results: 1 - 10 of 32809 matches for " Daniel Sebastiani "
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Polyhedral Phenylacetylenes: The Interplay of Aromaticity and Antiaromaticity in Convex Graphyne Substructures
Daniel Sebastiani,Matt A. Parker
Symmetry , 2009, DOI: 10.3390/sym1020226
Abstract: We have studied a series of bridged phenylacetylene macrocycles with topologies based on Platonic and Archimedean polyhedra, using density functional calculations to determine both their molecular structure and their electronic response to external magnetic fields (NICS maps). We are able to elucidate the interplay of aromaticity and anti-aromaticity as a function of structural parameters, in particular the symmetry properties of the intramolecular bond connectivities, in these compounds.
The isotope-effect in the phase transition of KDP: New insights from ab initio path-integral simulations
Varadharajan Srinivasan,Roberto Car,Daniel Sebastiani
Physics , 2010,
Abstract: We investigate the quantum-mechanical localization of protonated and deterated isotopes in the symmetric low-barrier hydrogen-bonds of potassium dihydrogen phosphate (KDP) crystals in the paraelectric phase. The spatial density distributions of these hydrogen atoms are suspected to be responsible for the surprisingly large isotope effect observed for the ferroelectric phase transition in KDP. We employ ab initio path integral molecular dynamics simulations to obtain the nuclear real-space and momentum-space densities n(R) and n(k) of protons and deuterons, which are compared to experimental Neutron Compton Scattering data. Our results suggest a qualitative difference in the nature of the paraelectic phase in KDP between the two isotopes. We are able to discriminate between real quantum delocalization and vibration-assisted hopping and thus provide evidence for two distinct mechanisms of the ferroelectric phase transition in this class of materials.
Time and space analysis of two earthquakes in the Appennines (Italy)  [PDF]
Michele Caputo, Giovanni Sebastiani
Natural Science (NS) , 2011, DOI: 10.4236/ns.2011.39101
Abstract: In this paper, we study two earthquakes: the April 6th 2009 earthquake of L’Aquila in the region of Abruzzo (Italy) and the 1997 Colfiorito earthquake in the regions of Umbria and Marche (Italy). The data sets of these two earthquakes were analysed in both time and space domains. For time domain we used statistical methods and models both parametric and non-parametric. Concerning the space domain, we used Mathematical Morphology filters. The time domain analysis provides evidence of a possible correlation between seismic activities and the tides of the crust of the Earth. The results obtained show evidence that the daily number of earthquakes of the sequences proceeding and following the April 6th 2009 earthquake of L’Aquila and that of the sequence following the 1997 Colfiorito earthquake have a periodic component of occurrence with period of about 7 days. It seems that the maxima of this component occur at a position of the Moon with respect to the Earth and the Sun corresponding to approximately 3 days before the four main Moon phases. The space domain analysis indicates that the foreshock activity in both earthquakes is clustered and concentrated. Furthermore, in each of the two earthquakes the clusters are located at about 3 kilometers from the epicentre of the main shock.
Nuclear velocity perturbation theory for vibrational circular dichroism: An approach based on the exact factorization of the electron-nuclear wave function
Arne Scherrer,Federica Agostini,Daniel Sebastiani,E. K. U. Gross,Rodolphe Vuilleumier
Physics , 2015, DOI: 10.1063/1.4928578
Abstract: The nuclear velocity perturbation theory (NVPT) for vibrational circular dichroism (VCD) is derived from the exact factorization of the electron-nuclear wave function. This new formalism offers an exact starting point to include correction terms to the Born-Oppenheimer (BO) form of the molecular wave function, similarly to the complete-adiabatic approximation. The corrections depend on a small parameter that, in a classical treatment of the nuclei, is identified as the nuclear velocity. Apart from proposing a rigorous basis for the NVPT, we show that the rotational strengths, related to the intensity of the VCD signal, contain a new contribution beyond-BO that can be evaluated with the NVPT and that only arises when the exact factorization approach is employed. Numerical results are presented for chiral and non-chiral systems to test the validity of the approach.
Conjectural history vs. the Bible: eighteenth-century Scottish historians and the idea of history in the Encyclopaedia Britannica
Silvia Sebastiani
Cromohs (Firenze) , 2001,
Abstract:
Non-invasive assessment of liver fibrosis in chronic liver diseases: Implementation in clinical practice and decisional algorithms
Giada Sebastiani
World Journal of Gastroenterology , 2009,
Abstract: Chronic hepatitis B and C together with alcoholic and non-alcoholic fatty liver diseases represent the major causes of progressive liver disease that can eventually evolve into cirrhosis and its end-stage complications, including decompensation, bleeding and liver cancer. Formation and accumulation of fibrosis in the liver is the common pathway that leads to an evolutive liver disease. Precise definition of liver fibrosis stage is essential for management of the patient in clinical practice since the presence of bridging fibrosis represents a strong indication for antiviral therapy for chronic viral hepatitis, while cirrhosis requires a specific follow-up including screening for esophageal varices and hepatocellular carcinoma. Liver biopsy has always represented the standard of reference for assessment of hepatic fibrosis but it has some limitations being invasive, costly and prone to sampling errors. Recently, blood markers and instrumental methods have been proposed for the non-invasive assessment of liver fibrosis. However, there are still some doubts as to their implementation in clinical practice and a real consensus on how and when to use them is not still available. This is due to an unsatisfactory accuracy for some of them, and to an incomplete validation for others. Some studies suggest that performance of non-invasive methods for liver fibrosis assessment may increase when they are combined. Combination algorithms of non-invasive methods for assessing liver fibrosis may represent a rational and reliable approach to implement non-invasive assessment of liver fibrosis in clinical practice and to reduce rather than abolish liver biopsies.
Na?ve Bayesian Classifier and Genetic Risk Score for Genetic Risk Prediction of a Categorical Trait: Not so Different after all!
Paola Sebastiani
Frontiers in Genetics , 2012, DOI: 10.3389/fgene.2012.00026
Abstract: One of the most popular modeling approaches to genetic risk prediction is to use a summary of risk alleles in the form of an unweighted or a weighted genetic risk score, with weights that relate to the odds for the phenotype in carriers of the individual alleles. Recent contributions have proposed the use of Bayesian classification rules using Na?ve Bayes classifiers. We examine the relation between the two approaches for genetic risk prediction and show that the methods are mathematically related. In addition, we study the properties of the two approaches and describe how they can be generalized to include various models of inheritance.
Finite-time singularities in modified $\mathcal{F}(R,G)$-gravity and singularity avoidance
Lorenzo Sebastiani
Physics , 2010, DOI: 10.1007/978-3-642-19760-4_24
Abstract: We study finite-time future singularities in $\mathcal{F}(R,G)$-gravity, where $R$ and $G$ are the Ricci scalar and the Gauss-Bonnet invariant, respectively. In particular, we reconstruct the $F(G)$-gravity and $\mathcal{F}(R,G)$-gravity models realizing the finite-time future singularities. We discuss a possible way to cure the finite-time future singularities in $\mathcal{F}(R,G)$-gravity by taking into account higher-order curvature corrections or effects of viscous fluids.
Dark Viscous Fluid coupled with Dark Matter and future singularity
Lorenzo Sebastiani
Physics , 2010, DOI: 10.1140/epjc/s10052-010-1398-z
Abstract: We study effects of viscous fluid coupled with dark matter in our universe. We consider bulk viscosity in the cosmic fluid and we suppose the existence of a coupling between fluid and dark matter, in order to reproduce a stable de Sitter universe protected against future-time singularities. More general inhomogeneous fluids are studied related to future singularities.
Applying GSAT to Non-Clausal Formulas
R. Sebastiani
Computer Science , 1994,
Abstract: In this paper we describe how to modify GSAT so that it can be applied to non-clausal formulas. The idea is to use a particular ``score'' function which gives the number of clauses of the CNF conversion of a formula which are false under a given truth assignment. Its value is computed in linear time, without constructing the CNF conversion itself. The proposed methodology applies to most of the variants of GSAT proposed so far.
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