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Search Results: 1 - 10 of 219606 matches for " C. Valeriani "
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Free energy calculations for atomic solids through the Einstein crystal/molecule methodology using GROMACS and LAMMPS
J. L. Aragones,C. Valeriani,C. Vega
Physics , 2012, DOI: 10.1063/1.4758700
Abstract: In this work the free energy of solid phases is computed for the Lennard-Jones potential and for a model of NaCl. The free energy is evaluated through the Einstein crystal/molecule methodologies using the Molecular Dynamics programs: GROMACS and LAMMPS. The obtained results are compared with the results obtained from Monte Carlo. Good agreement between the different programs and methodologies was found. The procedure to perform the free energy calculations for the solid phase in the Molecular Dynamic programs is described. Since these programs allow to study any continuous intermolecular potential (when given in a tabulated form) this work shows that for isotropic potentials (describing for instance atomic solids or colloidal particles) free energy calculations can be performed on a routinely basis using GROMACS and/or LAMMPS.
Rate of Homogeneous Crystal Nucleation in molten NaCl
C. Valeriani,E. Sanz,D. Frenkel
Physics , 2009, DOI: 10.1063/1.1896348
Abstract: We report a numerical simulation of the rate of crystal nucleation of sodium chloride from its melt at moderate supercooling. In this regime nucleation is too slow to be studied with "brute-force" Molecular Dynamics simulations. The melting temperature of ("Tosi-Fumi") NaCl is $\sim 1060$K. We studied crystal nucleation at $T$=800K and 825K. We observe that the critical nucleus formed during the nucleation process has the crystal structure of bulk NaCl. Interestingly, the critical nucleus is clearly faceted: the nuclei have a cubical shape. We have computed the crystal-nucleation rate using two completely different approaches, one based on an estimate of the rate of diffusive crossing of the nucleation barrier, the other based on the Forward Flux Sampling and Transition Interface Sampling (FFS-TIS) methods. We find that the two methods yield the same result to within an order of magnitude. However, when we compare the extrapolated simulation data with the only available experimental results for NaCl nucleation, we observe a discrepancy of nearly 5 orders of magnitude. We discuss the possible causes for this discrepancy.
Non-equilibrium dynamics of an active colloidal "chucker"
C. Valeriani,R. J. Allen,D. Marenduzzo
Physics , 2010, DOI: 10.1063/1.3428663
Abstract: We report Monte Carlo simulations of the dynamics of a "chucker": a colloidal particle which emits smaller solute particles from its surface, isotropically and at a constant rate k_c. We find that the diffusion constant of the chucker increases for small k_c, as recently predicted theoretically. At large k_c the chucker diffuses more slowly due to crowding effects. We compare our simulation results to those of a "point particle" Langevin dynamics scheme in which the solute concentration field is calculated analytically, and in which hydrodynamic effects can be included albeit in an approximate way. By simulating the dragging of a chucker, we obtain an estimate of its apparent mobility coefficient which violates the fluctuation-dissipation theorem. We also characterise the probability density profile for a chucker which sediments onto a surface which either repels or absorbs the solute particles, and find that the steady state distributions are very different in the two cases. Our simulations are inspired by the biological example of exopolysaccharide-producing bacteria, as well as by recent experimental, simulation and theoretical work on phoretic colloidal "swimmers".
Curvature Induced Activation of a Passive Tracer in an Active Bath
S. A. Mallory,C. Valeriani,A. Cacciuto
Physics , 2014, DOI: 10.1103/PhysRevE.90.032309
Abstract: We use numerical simulations to study the motion of a large asymmetric tracer immersed in a low density suspension of self-propelled nanoparticles in two dimensions. Specifically, we analyze how the curvature of the tracer affects its translational and rotational motion in an active environment. We find that even very small amounts of curvature are sufficient for the active bath to impart directed motion to the tracer which results in its effective activation. We propose simple scaling arguments to characterize this induced activity in terms of the curvature of the tracer and the strength of the self-propelling force. Our results suggest new ways of controlling the transport properties of passive tracers in an active medium by carefully tailoring their geometry.
Anomalous dynamics of an elastic membrane in an active fluid
S. A. Mallory,C. Valeriani,A. Cacciuto
Physics , 2015, DOI: 10.1103/PhysRevE.92.012314
Abstract: Using numerical simulations, we characterized the behavior of an elastic membrane immersed in an active fluid. Our findings reveal a nontrivial folding and re-expansion of the membrane that is controlled by the interplay of its resistance to bending and the self-propulsion strength of the active components in solution. We show how flexible membranes tend to collapse into multi-folded states, whereas stiff membranes oscillates between an extended configuration and a singly folded state. This study provides a simple example of how to exploit the random motion of active particles to perform mechanical work at the micro-scale.
Calculation of the melting point of alkali halides by means of computer simulations
J. L. Aragones,E. Sanz,C. Valeriani,C. Vega
Physics , 2012, DOI: 10.1063/1.4745205
Abstract: In this manuscript we study the liquid-solid coexistence of NaCl-type alkali halides, described by interaction potentials such as Tosi-Fumi (TF), Smith-Dang (SD) and Joung-Cheatham (JC), and compute their melting temperature (Tm) at 1 bar via three independent routes: 1) liquid/solid direct coexistence, 2) free-energy calculations and 3) Hamiltonian Gibbs-Duhem integration. The melting points obtained by the three routes are consistent with each other. The calculated Tm of the Tosi-Fumi model of NaCl is in good agreement with the experimental value as well as with other numerical calculations. However, the other two models considered for NaCl, SD and JC, overestimate the melting temperature of NaCl by more than 200 K. We have also computed the melting temperature of other alkali halides using the Tosi-Fumi interaction potential and observed that the predictions are not always as close to the experimental values as they are for NaCl. It seems that there is still room for improvement in the area of force-fields for alkaline halides, given that so far most models are still unable to describe a simple yet important property such as the melting point.
Homogeneous ice nucleation evaluated for several water models
J. R. Espinosa,E. Sanz,C. Valeriani,C. Vega
Physics , 2014, DOI: 10.1063/1.4897524
Abstract: In this work, we evaluate by means of computer simulations the rate for ice homogeneous nucleation for several water models such as TIP4P, TIP4P/2005,TIP4P/ICE, and mW (following the same procedure as in Sanz et al. [J. Am. Chem. Soc.135, 15008 (2013)]) in a broad temperature range. We estimate the ice-liquid interfacial free-energy, and conclude that for all water models {\gamma} decreases as the temperature decreases. Extrapolating our results to the melting temperature, we obtain a value of the interfacial free-energy between 25 and 32 mN/m in reasonable agreement with the reported experimental values. Moreover, we observe that the values of {\gamma} depend on the chosen water model and this is a key factor when numerically evaluating nucleation rates, given that the kinetic prefactor is quite similar for all water models with the exception of the mW (due to the absence of hydrogens). Somewhat surprisingly the estimates of the nucleation rates found in this work for TIP4P/2005 are slightly higher than those of the mW model, even though the former has explicit hydrogens. Our results suggest that it may be possible to observe in computer simulations spontaneous crystallization of TIP4P/2005 at about 60 K below the melting point.
Local structure of liquid carbon controls diamond nucleation
L. M. Ghiringhelli,C. Valeriani,E. J. Meijer,D. Frenkel
Physics , 2009, DOI: 10.1103/PhysRevLett.99.055702
Abstract: Diamonds melt at temperatures above 4000 K. There are no measurements of the steady-state rate of the reverse process: diamond nucleation from the melt, because experiments are difficult at these extreme temperatures and pressures. Using numerical simulations, we estimate the diamond nucleation rate and find that it increases by many orders of magnitude when the pressure is increased at constant supersaturation. The reason is that an increase in pressure changes the local coordination of carbon atoms from three-fold to four-fold. It turns out to be much easier to nucleate diamond in a four-fold coordinated liquid than in a liquid with three-fold coordination, because in the latter case the free-energy cost to create a diamond-liquid interface is higher. We speculate that this mechanism for nucleation control is relevant for crystallization in many network-forming liquids. On the basis of our calculations, we conclude that homogeneous diamond nucleation is likely in carbon-rich stars and unlikely in gaseous planets.
Anomalous Thermomechanical Properties of a Self-propelled Colloidal Fluid
S. A. Mallory,A. ?ari?,C. Valeriani,A. Cacciuto
Physics , 2013, DOI: 10.1103/PhysRevE.89.052303
Abstract: We use numerical simulations to compute the equation of state of a suspension of spherical, self-propelled nanoparticles. We study in detail the effect of excluded volume interactions and confinement as a function of the system temperature, concentration and strength of the propulsion. We find a striking non-monotonic dependence of the pressure with the temperature, and provide simple scaling arguments to predict and explain the occurrence of such an anomalous behavior. We conclude the paper by explicitly showing how our results have an important implications for the effective forces exerted by fluids of self-propelled particles on passive, larger components.
Management of Demented Patients in Emergency Department
Lavinia Valeriani
International Journal of Alzheimer's Disease , 2011, DOI: 10.4061/2011/840312
Abstract: The hospitalization of the elderly with acute illness is one of the most discussed in the organization of health services, it is not yet clear whether the hospital is really the best response to the needs of the elderly, especially those with cognitive impairment. Despite evidence of possible adverse effects of hospitalization (immobilization, acute confusional state resulting in sedation, risk of falls, intestinal sub-ileus), there has been an increasing use of the hospital, particularly to specialist services. Regardless of the benefits from the shelter (instrumental diagnosis and prompt treatment of acute somatic disease), in people with dementia it needs to identify the characteristics of the person (cognitive impairment, functional status, somatic comorbidity, social and familial status), the personal needs and, therefore, diagnostic and therapeutic targets which must be assumed for that sick person during hospitalization. To this end, it is fundamental the role of assessment and diagnostic orientation that takes place in the Department of Emergency and Acceptance (DEA), which mainly receives patients at the hospital. Even before the hospital recovery it is therefore essential to check how many elderly patients with cognitive impairment that belong to the DEA, and what are their needs. 1. Epidemiological Data Over the past 10 years an increasing proportion of elderly people, especially those with dementia, had recourse to the Department of Emergency and Acceptance (DEA): the elderly, in respect to young and adult people, on average, have more urgent requests, stay longer in the department (both for diagnosis and for therapy), use more resources and staff time [1]. Despite a great number of tests and procedures, diagnoses tend to be less accurate: this condition is explained by the atypical presentation of many diseases, the clinical and pharmacological comorbidity, which tend to complicate the presentation, the diagnosis, and the treatment of acute or chronic disease. In addition, older people often belong to the DEA for diseases that require intensive care: a study carried out for 5 years in 352 hospitals in the USA and England, it was found that the percentage of admission in polyfunctional intensive therapy (PIT) of patients over 65 years is very high (above 50%). In the total group of patients belonging to the DEA, the subsequent admission to the PIT was mainly for medical conditions (53% in England, 63% in USA), and less for acute surgical diseases (48 vesus 41%, resp.) [2]. The data shows that elderly patients who belong to the DEA, and among
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