Papers by M. Monteferrante
Molecular Simulation, 2009
Two modifications of the recently developed single sweep method to efficiently reconstruct free e... more Two modifications of the recently developed single sweep method to efficiently reconstruct free energy landscapes in several dimensions are proposed. In single sweep, the free energy is estimated as a linear combination of radial basis functions whose parameters are optimized by miniminzing an appropriate objective function. In this work, the single variance Gaussian radial basis usually employed in single sweep when non-periodic collective variables are used is generalized to a basis of multivariate Gaussians whose covariance matrix can change with the location of the basis function on the landscape. A new objective function, aimed at a more accurate reconstruction of the landscape in the vicinity of stationary points, is also introduced. The performance of the modified single sweep is compared to that of the standard method in the reconstruction of a model benchmark potential and of the free energy profile of a physically relevant example. The results of these comparisons show that employing the new objective function can result in an improvement of the efficiency and accuracy of the reconstruction.
Short range hydrogen diffusion in Na3AlH6
Physical Chemistry Chemical Physics, 2011
Ab initio free energy and rate calculations are performed to investigate two activated mobility p... more Ab initio free energy and rate calculations are performed to investigate two activated mobility processes observed, respectively, in neutron scattering and anelastic spectroscopy experiments on sodium alanates. The system is modeled as a Na(3)AlH(6) crystal hosting one hydrogen vacancy. We identify the process observed via neutron scattering with a positively charged hydrogen vacancy diffusing from the AlH to one of the AlH groups. As for the anelastic spectroscopy experiments, our calculations negate the current hypothesis on the process, i.e. local rearrangement of the H vacancy around the pentacoordinated Al group.
Motivated by recent experiments, we present a theoretical investigation of how the electro-osmoti... more Motivated by recent experiments, we present a theoretical investigation of how the electro-osmotic flow occurring in a capillary is modified when its charged surfaces are coated by charged polymers. The theoretical treatment is based on a three dimensional model consisting of a ternary fluid-mixture, representing the solvent and two species for the ions, confined between two parallel charged plates decorated by a fixed array of scatterers representing the polymer coating. The electro-osmotic flow, generated by a constant electric field applied in a direction parallel to the plates, is studied numerically by means of Lattice Boltzmann simulations. In order to gain further understanding we performed a simple theoretical analysis by extending the Stokes-Smoluchowski equation to take into account the porosity induced by the polymers in the region adjacent the walls. We discuss the nature of the velocity profiles by focusing on the competing effects of the polymer charges and the frictional forces they exert. We show evidence of the flow reduction and of the flow inversion phenomenon when the polymer charge is opposite to the surface charge. By using the density of polymers and the surface charge as control variables, we propose a phase diagram that discriminates the direct and the reversed flow regimes and determine its dependence on the ionic concentration.
Calculations of free energy barriers for local mechanisms of hydrogen diffusion in alanates
Brute force histogram calculation and a recently developed method to efficiently reconstruct the ... more Brute force histogram calculation and a recently developed method to efficiently reconstruct the free energy profile of complex systems (the single-sweep method) are combined with ab initio molecular dynamics to study possible local mechanisms for the diffusion of hydrogen in sodium alanates. These compounds may help to understand key properties of solid state hydrogen storage materials. In this work, the identity of a mobile species observed in experiments characterizing the first dissociation reaction of sodium alanates is investigated. The activation barrier of two suggested processes for hydrogen diffusion in Na3AlH6 is evaluated and, by comparing our results with available experimental information, we are able to discriminate among them and to show that one is compatible with the observed signal while the other is not.
The Journal of chemical physics, Jan 7, 2014
When simulating multicomponent mixtures via the Lattice Boltzmann Method, it is desirable to cont... more When simulating multicomponent mixtures via the Lattice Boltzmann Method, it is desirable to control the mutual diffusivity between species while maintaining the viscosity of the solution fixed. This goal is herein achieved by a modification of the multicomponent Bhatnagar-Gross-Krook evolution equations by introducing two different timescales for mass and momentum diffusion. Diffusivity is thus controlled by an effective drag force acting between species. Numerical simulations confirm the accuracy of the method for neutral binary and charged ternary mixtures in bulk conditions. The simulation of a charged mixture in a charged slit channel show that the conductivity and electro-osmotic mobility exhibit a departure from the Helmholtz-Smoluchowski prediction at high diffusivity.
Phys. Chem. Chem. Phys., 2014
Motivated by recent experiments, we present a theoretical investigation of how the electro-osmoti... more Motivated by recent experiments, we present a theoretical investigation of how the electro-osmotic flow occurring in a capillary is modified when its charged surfaces are coated with charged polymers.
Gas phase infrared spectra via the phase integration quasi-classical method
The ability to manipulate matter to create non-conventional structures is one of the key issues o... more The ability to manipulate matter to create non-conventional structures is one of the key issues of material science. The understanding of assembling mechanism at the nanoscale allows us to engineer new nanomaterials, with physical properties intimately depending on their structure. This paper describes new strategies to obtain and characterise metal nanostructures via the combination of a top-down method, such as electron beam lithography, and a bottom-up technique, such as the chemical electroless deposition. We realised silver nanoparticle aggregates within well-defined patterned holes created by electron beam lithography on silicon substrates. The quality characteristics of the nanoaggregates were verified by using scanning electron microscopy and atomic force microscopy imaging. Moreover, we compared the experimental findings to molecular dynamics simulations of nanoparticles growth. We observed a very high dependence of the structure characteristics on the pattern nanowell aspect ratio. We found that high-quality metal nanostructures may be obtained in patterns with well aspect ratio close to one, corresponding to a maximum diameter of 50 nm, a limit above which the fabricated structures become less regular and discontinuous. When regular shapes and sizes are necessary, as in nanophotonics, these results suggest the pattern characteristics to obtain isolated, uniform and reproducible metal nanospheres.
Microfluidics and Nanofluidics, 2014
The quality of separation in capillaries electrophoresis is strongly affected by the magnitude of... more The quality of separation in capillaries electrophoresis is strongly affected by the magnitude of the electroosmotic flow (EOF). The EOF can be efficiently suppressed by coating the capillary wall with hydrophilic polymers. In this paper, experimental data are presented to show the effect of coating thickness and charge on mass transport. Simulations performed with the lattice Boltzmann technique quantitatively reproduce the EOF with and without the coating and with either a neutral or charged coating layer. Experimental, simulation and theoretical analyses converge toward the interpretation that EO suppression arises from the frictional forces acting on the ionic currents and that a detailed representation of the polymeric coating is not needed in order to capture the phenomenon.
Linearized symmetrized quantum time correlation functions calculation via phase pre-averaging
Molecular Physics, 2011
We recently introduced an iterative method to compute quantum time correlation functions [Bonella... more We recently introduced an iterative method to compute quantum time correlation functions [Bonella et al., J. Chem. Phys 133 (16), 164105 (2010)]. There, the thermal part of the correlation function is treated exactly and, similar to the linearization techniques, at zero order of iteration only classical dynamics is required. In this work, we propose a new scheme for the zero-order iteration of the method which significantly improves the efficiency of the calculations for high dimensional model systems.
Path integral based calculations of symmetrized time correlation functions. I
The Journal of Chemical Physics, 2010
In this paper, we examine how and when quantum evolution can be approximated in terms of (general... more In this paper, we examine how and when quantum evolution can be approximated in terms of (generalized) classical dynamics in calculations of correlation functions, with a focus on the symmetrized time correlation function introduced by Schofield. To that end, this function is expressed as a path integral in complex time and written in terms of sum and difference path variables. Taylor series expansion of the path integral's exponent to first and second order in the difference variables leads to two original developments. The first order expansion is used to obtain a simple, path integral based, derivation of the so-called Schofield's quantum correction factor. The second order result is employed to show how quantum mechanical delocalization manifests itself in the approximation of the correlation function and hinders, even in the semiclassical limit, the interpretation of the propagators in terms of sets of guiding classical trajectories dressed with appropriate weights.
Two modifications of the recently developed single sweep method to efficiently reconstruct free e... more Two modifications of the recently developed single sweep method to efficiently reconstruct free energy landscapes in several dimensions are proposed. In single sweep, the free energy is estimated as a linear combination of radial basis functions whose parameters are optimized by miniminzing an appropriate objective function. In this work, the single variance Gaussian radial basis usually employed in single sweep when non-periodic collective variables are used is generalized to a basis of multivariate Gaussians whose covariance matrix can change with the location of the basis function on the landscape. A new objective function, aimed at a more accurate reconstruction of the landscape in the vicinity of stationary points, is also introduced. The performance of the modified single sweep is compared to that of the standard method in the reconstruction of a model benchmark potential and of the free energy profile of a physically relevant example. The results of these comparisons show that employing the new objective function can result in an improvement of the efficiency and accuracy of the reconstruction.
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Papers by M. Monteferrante