120 research outputs found
Ab Initio Simulation of Heat Transport in Silica Glass
No full textThe simulation of heat transport and the estimation of thermal conductivity in glasses is of crucial importance for many technological applications, ranging from thermal insulation to semiconductor fabrication, and for the interpretation of laser damage of optical glasses.
In particular, vitreous silica (a-SiO2) is one the most used and investigated materials, and serves as the basis of multicomponent silica glasses, which are usually characterised by a complex chemistry.
Classical force fields have demonstrated to reproduce quite well all the structural properties of a-SiO2, but lacks a proper description of its vibrational spectrum, that instead requires first-principles simulations.
The methods usually adopted to study heat transport in crystalline solids, such as the Boltzmann transport equation, cannot be applied to glasses, were the disorder makes the phonon picture break down.
Instead, the Green-Kubo (GK) theory of linear response can be straightforwardly applied to obtain the thermal conductivity from the fluctuations of the heat current at equilibrium.
Nonetheless, until very recently, the GK was not deemed compatible with quantum simulation techniques based on density functional theory because the concepts of energy density and current are not well defined at the atomic scale. Besides, the study of transport coefficients using the GK theory is known to require very long molecular dynamics (MD) simulations, thus making ab initio techniques unaffordable.
We discuss how it is possible to overcome these two hurdles thanks to a paradigm shift based on the concept of gauge invariance of transport coefficients, and by using a novel data analysis technique based on the so-called cepstral analysis of stationary time series. These theoretical and methodological advances make the quantum simulation of heat transport in liquids and amorphous solids possible, using equilibrium ab initio molecular dynamics.
By means of classical MD simulations we study the dependence of the thermal conductivity on the sample size and the quenching protocol adopted, and we show that relatively short trajectories are needed to obtain an accuracy of the order of 10% on the thermal conductivity. One sample of a-SiO2 is finally simulated with Car-Parrinello MD at four different temperatures. The resulting thermal conductivities show fairly good agreement with experimental data, and a substantial improvement with respect to classical MD results
The cytotoxic effect of slow Loris (nycticebus) venom, on human cancer cells
No full textWithin the Kingdom Mammalia, venom evolution is rare, occurring in only six orders. Arguably the most cryptic, and academically neglected venom occurs within primates among slow lorises (Nycticebus spp.). Venoms comprise novel biological compounds with a potential plethora of proteins and peptides available for utilisation in bio-medical research. We collected samples of slow loris saliva from eight captive-bred pygmy slow lorises (N. pygmaeus) at Paignton Zoo and Shaldon Wildlife Trust UK, given voluntarily as slow lorises chewed on Salimetrics children’s swabs. From January to March 2017, we employed MTT assays, and microscopy assessments to determine cell survival on human epidermal carcinoma cells (A431 line) after the application of concentrations of slow loris salivary venom. Cell survival from both male and female derived saliva was half that of untreated cells. Cytotoxic action is demonstrated in concentrations as low as 0.01% venom. Results demonstrate a cytotoxic effect with ensuing physiological damage on human cancer cells, demonstrating the cytotoxic action of slow loris saliva only, without the admixture of brachial gland exudate. We show that even captive-bred slow loris saliva harbours potentially dangerous substances, with functional applications towards slow loris husbandry. Knowledge of slow loris salivary venom increases understanding of the novel salivary composition and supports discussions of slow loris conservation by proposing a functional narrative to oppose the illegal pet trade, by contradicting their ‘cute and cuddly’ appeal. Evidence of salivary venom shows that cytotoxic effects can result even in the absence of a bite puncturing the skin, and further demonstrates their inappropriateness as pets
Accurate thermal conductivities from optimally short molecular dynamics simulations
No full textThe evaluation of transport coefficients in extended systems, such as thermal conductivity or shear viscosity, is known to require impractically long simulations, thus calling for a paradigm shift that would allow to deploy state-of-the-art quantum simulation methods. We introduce a new method to compute these coefficients from optimally short molecular dynamics simulations, based on the Green-Kubo theory of linear response and the cepstral analysis of time series. Information from the full sample power spectrum of the relevant current for a single and relatively short trajectory is leveraged to evaluate and optimally reduce the noise affecting its zero-frequency value, whose expectation is proportional to the corresponding conductivity. Our method is unbiased and consistent, in that both the resulting bias and statistical error can be made arbitrarily small in the long-time limit. A simple data-analysis protocol is proposed and validated with the calculation of thermal conductivities in the paradigmatic cases of elemental and molecular fluids (liquid Ar and H2O) and of crystalline and glassy solids (MgO and a-SiO2). We find that simulation times of one to a few hundred picoseconds are sufficient in these systems to achieve an accuracy of the order of 10% on the estimated thermal conductivities.THEO
«Mandalo incontinente a pigliare et fagli dare quattro tracti di corda». Qualche osservazione sui rapporti tra i duchi e l’amministrazione estense nel tardo Quattrocento, a partire dal Caleffini
No full textNel XV secolo i funzionari impegnati nell’amministrazione dello Stato estense erano numerose decine: già nel 1456 i soli impiegati nella Camera Ducale erano più di trenta, ma erano destinati ad aumentare significativamente. È stato già notato che, per quanto le informazioni lacunose consentano di sapere, all’interno della Corte gli ufficiali erano di preferenza cittadini ferraresi, spesso dotati di formazione giuridica, e mantenevano i propri incarichi all’interno della famiglia, di padre in figlio o di fratello in fratello. Non è stato però mai tentato uno studio della corposa documentazione amministrativa, in primo luogo dei carteggi, con l’obiettivo di mettere in luce le dinamiche tra gli Este e i loro sudditi collaboratori. I tempi e i modi della retribuzione degli ufficiali, le concessioni di incarichi o gli allontanamenti da essi, gli abusi nel richiedere oltre il dovuto o, al contrario, l’incapacità di ottenere i legittimi pagamenti delle gabelle, sono tutti elementi che possono concorrere a una conoscenza meno superficiale dello Stato estense nella tarda età medievale: delle strategie di controllo e di fidelizzazione come delle dimensioni e caratteristiche delle reti clientelari. Il contributo propone brani da numerose lettere scritte da Borso e Ercole ai loro ufficiali, o viceversa, allo scopo di sollecitare la ricerca
Iscrizione nei libri contabili obbligatori e circolazione dei debiti aziendali (con alcune considerazioni sui criteri di imputazione e di responsabilità tra diritto civile e diritto commerciale)
No full textThe author, inspired by a recent jurisprudential conflict emerged in the judgments passed by the Italian Supreme Court about the legal interpretation of the section 2560, paragraph 2 c.c. (which states the assumption of the company debts), examines the most important and recent Italian theories about the legal framework of the company and shows how the different solutions to the issue are dictated by a different concept of the subjective imputation criteria and the liability for breach of contract
Gauge Fixing for Heat-Transport Simulations
No full textThermal and other transport coefficients were recently shown to be largely independent of the microscopic representation of the energy (current) densities or, more generally, of the relevant conserved densities/currents. In this Article, we show how this gauge invariance, which is intimately related to the intrinsic indeterminacy of the energy of individual atoms in interacting systems, can be exploited to optimize the statistical properties of the current time series from which the transport coefficients are evaluated. To this end, we introduce and exploit a variational principle that relies on the metric properties of the conserved currents, treated as elements of an abstract linear space. Different metrics would result in different variational principles. In particular, we show that a recently proposed data-analysis technique based on the theory of transport in multicomponent systems can be recovered by a suitable choice of this metric
Heat Transport in Insulators from Ab Initio Green-Kubo Theory
No full textThe Green-Kubo theory of thermal transport has long been considered incompatible with modern simulation methods based on electronic-structure theory, because it is based on such concepts as energy density and current, which are ill-defined at the quantum-mechanical level. Besides, experience with classical simulations indicates that the estimate of heat-transport coefficients requires analyzing molecular trajectories that aremore than one order of magnitude longer than deemed feasible using ab initio molecular dynamics. In this paper we report on recent theoretical advances that are allowing one to overcome these two obstacles. First, a general gauge invariance principle has been established, stating that thermal conductivity is insensitive to many details of the microscopic expression for the energy density and current from which it is derived, thus permitting to establish a rigorous expression for the energy flux from Density-Functional Theory, from which the conductivity can be computed in practice. Second, a novel data analysis method based on the statistical theory of time series has been proposed, which allows one to considerably reduce the simulation time required to achieve a target accuracy on the computed conductivity. These concepts are illustrated in detail, starting from a pedagogical introduction to the Green-Kubo theory of linear response and transport, and demonstrated with a few applications done with both classical and quantum-mechanical simulation methods
Theory and Numerical Simulation of Heat Transport in Multicomponent Systems
No full textThe thermal conductivity of classical multicomponent fluids is seemingly affected by the intrinsic arbitrariness in the definition of the atomic energies, and it is ill conditioned numerically, when evaluated from the Green-Kubo theory of linear response. To cope with these two problems, we introduce two new concepts: a convective invariance principle for transport coefficients, in the first case, and multivariate cepstral analysis, in the second. A combination of these two concepts allows one to substantially reduce the noise affecting the estimate of the thermal conductivity from equilibrium molecular dynamics, even for one-component systems.THEO
Theory and Numerical Simulation of Heat Transport in Multicomponent Systems
No full textThe thermal conductivity of classical multicomponent fluids is seemingly affected by the intrinsic arbitrariness in the definition of the atomic energies, and it is ill conditioned numerically, when evaluated from the Green-Kubo theory of linear response. To cope with these two problems, we introduce two new concepts: a convective invariance principle for transport coefficients, in the first case, and multivariate cepstral analysis, in the second. A combination of these two concepts allows one to substantially reduce the noise affecting the estimate of the thermal conductivity from equilibrium molecular dynamics, even for one-component systems
The sticky tasty: the nutritional content of the exudativorous diet of the Javan slow loris in a lowland forest
No full textPlant exudates are an important food source for many primates. The Critically Endangered Javan slow loris (Nycticebus javanicus) was previously found to prefer Acacia decurrens exudate in an anthropogenically disturbed site, while its feeding habits in secondary natural forest remain unknown. Knowledge of the chemical characteristics of the plant exudates that Javan slow lorises consume is limited, especially with respect to those that they feed on in natural forests. As plant exudates may contain plant secondary metabolites (PSM), which are considered unpalatable in high concentrations, differences in PSM composition may drive feeding preferences. This research aims firstly to confirm exudate consumption by the Javan slow loris in a lowland tropical forest in Central Java, and secondly to identify the chemical characteristics of the exudates consumed. We followed wild slow lorises in Kemuning Forest, Central Java and observed their behaviour. We investigated the gum-producing trees that were utilized by the slow lorises by tapping the exudates and examining their nutritional and PSM contents. We found that exudates are the predominant food source for the Javan slow loris in this lowland forest, and that their nutritional contents are similar to those of exudates consumed by lorises in anthropogenically disturbed areas. Significant differences in polysaccharide and flavonoid contents were found between consumed and unconsumed exudates. Knowledge of the diet of the Javan slow loris is crucial to its conservation, and our findings confirm the importance of exudates in its diet. We also highlight the need to preserve natural slow loris habitat, and to manage the diets of these species in captivity. The results of this study indicate that plant exudates should constitute a significant portion of the diet of captive slow lorises, and that the presence of exudate-producing trees is vital in areas into which slow lorises are to be translocated. © 2021, The Author(s), under exclusive licence to Japan Monkey Centre
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