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Calvo F.,CNRS Laboratory of Ionic and Molecular Spectrometry | Parneix P.,University Paris - Sud
ChemPhysChem | Year: 2012

The influence of one or several infrared laser pulses on the stability of bare and argon-tagged sodium chloride clusters is investigated theoretically by a combination of computational methods involving explicit molecular dynamics and properly calibrated unimolecular rate theories. The fragmentation spectra obtained by varying the laser frequency in the far-IR range is compared to the linear absorption spectrum resulting from the dipole moment autocorrelation function. Under appropriate laser field parameters, the action spectra are found to resemble the absorption spectra quite accurately in terms of positions, line widths, and even relative intensities. However, the action spectra exhibit residual and systematic redshifts of a few percent, which are partly due to the finite spectral bandwidth but are amplified by the progressive heating by the laser. A quantitative analysis suggests that these anharmonicity effects should generally arise upon multiple photon absorption. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Calvo F.,CNRS Laboratory of Ionic and Molecular Spectrometry | Parneix P.,University Paris - Sud | Van-Oanh N.-T.,University Paris - Sud
Journal of Chemical Physics | Year: 2010

The vibrational spectra of the naphthalene, pyrene, and coronene molecules have been computed in the 0-3500 cm-1 infrared range using classical and quantum molecular dynamics simulations based on a dedicated tight-binding potential energy surface. The ring-polymer molecular dynamics (RPMD) and partially adiabatic centroid molecular dynamics (CMD) methods have been employed to account for quantum nuclear effects. The contributions of quantum delocalization to the line shift and broadening are significant in the entire spectral range and of comparable magnitude as pure thermal effects. While the two methods generally produce similar results, the CMD method may converge slower at low temperature with increasing Trotter discretization number. However, and contrary to the CMD method, the RPMD approach suffers from serious resonance problems at high frequencies and low temperatures. © 2010 American Institute of Physics.

Allouche A.-R.,University Claude Bernard Lyon 1 | Allouche A.-R.,CNRS Laboratory of Ionic and Molecular Spectrometry
Journal of Computational Chemistry | Year: 2011

Gabedit is a freeware graphical user interface, offering preprocessing and postprocessing adapted (to date) to nine computational chemistry software packages. It includes tools for editing, displaying, analyzing, converting, and animating molecular systems. A conformational search tool is implemented using a molecular mechanics or a semiempirical potential. Input files can be generated for the computational chemistry software supported by Gabedit. Some molecular properties of interest are processed directly from the output of the computational chemistry programs; others are calculated by Gabedit before display. Molecular orbitals, electron density, electrostatic potential, nuclear magnetic resonance shielding density, and any other volumetric data properties can be displayed. It can display electronic circular dichroism, UV-visible, infrared, and Raman-computed spectra after a convolution. Gabedit can generate a Povray file for geometry, surfaces, contours, and color-coded planes. Output can be exported to a selection of popular image and vector graphics file formats; the program can also generate a series of pictures for animation. Quantum mechanical electrostatic potentials can be calculated using the partial charges on atoms, or by solving the Poisson equation using the multigrid method. The atoms in molecule charges can also be calculated. Gabedit is platform independent. The code is distributed under free open source X11 style license and is available at. © 2010 Wiley Periodicals, Inc.

Linguerri R.,University Paris Est Creteil | Hochlaf M.,University Paris Est Creteil | Bacchus-Montabonel M.-C.,CNRS Laboratory of Ionic and Molecular Spectrometry | Desouter-Lecomte M.,CNRS Laboratory of Chemical Physics
Physical Chemistry Chemical Physics | Year: 2013

Franzreb and Williams at Arizona State University detected recently the MgO2+ molecular species in the gas phase. Here we report a very detailed theoretical investigation of the low-lying electronic states of this dication including their potentials, spin-orbit, rotational and radial couplings. Our results show that the potential energy curves of the dicationic electronic states have deep potential wells. This confirms that this dication does exist in the gas phase; it is a thermodynamically stable molecule in its ground state, and it has several excited long-lived metastable states. The potential energy curves are used then to predict a set of spectroscopic parameters for the bound states of MgO2+. We have also incorporated these potentials, rotational and radial couplings in dynamical calculations to derive the cross sections for the charge transfer Mg2+ + O → Mg+ + O+ reaction in the 1-103 eV collision energy domain via formation-decomposition of the MgO2+ dication. Our work shows the role of MgO2+ in the Earth ionosphere and more generally in atmospheric processes in solar planets, where this reaction efficiently participates in the predominance of Mg+ cations in these media compared to Mg and Mg2+. © 2013 the Owner Societies.

Calvo F.,CNRS Laboratory of Ionic and Molecular Spectrometry
Physical Review E - Statistical, Nonlinear, and Soft Matter Physics | Year: 2010

Several parallel adaptive biasing methods are applied to the calculation of free-energy pathways along reaction coordinates, choosing as a difficult example the double-funnel landscape of the 38-atom Lennard-Jones cluster. In the case of classical statistics, the Wang-Landau and adaptively biased molecular-dynamics (ABMD) methods are both found efficient if multiple walkers and replication and deletion schemes are used. An extension of the ABMD technique to quantum systems, implemented through the path-integral MD framework, is presented and tested on Ne38 against the quantum superposition method. © 2010 The American Physical Society.

Calvo F.,CNRS Laboratory of Ionic and Molecular Spectrometry
Physica D: Nonlinear Phenomena | Year: 2011

The ring-polymer molecular dynamics (RPMD) method was recently shown to provide a powerful framework to address the time evolution of many-particle semiclassical systems. Its underlying Hamiltonian formulation is exploited here to define and compute some measures of deterministic chaos, namely the Lyapunov characteristic exponent. Applications are presented for weakly bound neon clusters, with the aim of assessing the influence of nuclear delocalization on the nonlinear dynamics. It is found that the balance between the additional degrees of freedom in the RPMD description and the harmonic nature of the additional interactions leads to more regular dynamics at high temperature, but more chaotic dynamics at low temperature. Intrinsic features of quantum effects on the isomerization dynamics also have a signature on the Lyapunov exponent. © 2011 Elsevier B.V. All rights reserved.

Calvo F.,CNRS Laboratory of Ionic and Molecular Spectrometry | Douady J.,University of Caen Lower Normandy
Physical Chemistry Chemical Physics | Year: 2010

The structure and finite-temperature properties of hydrated nucleotide anion adenosine 5′-monophosphate (AMP) have been theoretically investigated with a variety of methods. Using a polarizable version of the Amber force field and replica-exchange molecular dynamics simulations, putative lowest-energy structures have been located for the AMP-(H 2O)n cluster anions with n = 0-20. The hydration energies obtained with the molecular mechanics potential slightly overestimate experimental measurements. However, closer values are found after reoptimizing the structures locally at more sophisticated levels, namely semi-empirical (PM6) and density-functional theory (B3LYP/6-31+G*). Upon heating the complexes, various indicators such as the heat capacity, number of hydrogen bonds or surface area provide evidence that the water cluster melts below 200 K but remains bonded to the AMP anion. The sequential loss of water molecules after sudden heating has been studied using a statistical approach in which unimolecular evaporation is described using the orbiting transition state version of phase space theory, together with anharmonic densities of vibrational states. The evaporation rates are calibrated based on the results of molecular dynamics trajectories at high internal energy. Our results indicate that between 4 and 10 water molecules are lost from AMP-(H2O) 20 after one second depending on the initial heating in the 250-350 K range, with a concomitant cooling of the remaining cluster by 75-150 K. © the Owner Societies.

Calvo F.,CNRS Laboratory of Ionic and Molecular Spectrometry
Journal of Chemical Physics | Year: 2012

Using molecular dynamics simulations, an embedded-atom model potential, and the mechanistic route, we have computed the pressure tensor and the surface tension of Ag-Au liquid alloys. Although the model generally underestimates for pure metals, calculations for a bulk planar slab exhibit nonlinear variations of with increasing gold concentration, which agree with experiments and can be accounted for by a perfect solution model. Calculations for various nanoscale droplets containing between 100 and 3200 atoms show a systematic decrease of with increasing droplet radius R. The positive Tolman length of the alloy determined from these size variations is estimated to vary slightly with gold concentration. The effects of temperature in the range 1300-1700 K are discussed. © 2012 American Institute of Physics.

Calvo F.,CNRS Laboratory of Ionic and Molecular Spectrometry
Journal of Physical Chemistry C | Year: 2011

The vibrations of a metallic nanoparticle are strongly affected by its size and shape. In the present work, the respective roles of size, chemical order, and composition have been theoretically examined in the case of gold-silver nanoalloys by means of atomistic simulations. Whereas the vibrational density of states exhibits some qualitative differences between alloyed and segregated (core-shell) particles, the breathing frequency varies smoothly but nonlinearly with composition in all cases considered. Elasticity theory accounts reasonably well for the size dependencies, with finite size corrections scaling as powers of the inverse radius. These deviations are found to vary with composition through a simple quadratic expansion. © 2011 American Chemical Society.

Lerme J.,CNRS Laboratory of Ionic and Molecular Spectrometry
Journal of Physical Chemistry C | Year: 2011

A key parameter for optimizing nanosized optical devices involving small metal particles is the spectral width of their localized surface plasmon resonances (LSPR), which is intrinsically limited by the confinement-induced broadening (quantum finite size effects). I have investigated the size evolution of the LSPR width induced by quantum confinement in silver nanoparticles isolated in vacuum or embedded in transparent matrixes. Calculations have been performed within the time-dependent local density approximation in an extended size range, up to 40000 conduction electrons (diameter D ≈ 11 nm). The slope characterizing the 1/D linear evolution predicted by the simple classical "limited mean free path" model is found to depend noticeably on the surrounding matrix. The confinement-induced damping of the collective LSPR excitation is shown to be intimately related to the departure of the electron-background interaction from a pure harmonic law. In jellium-type models the damping is governed by the electronic spillout tail, which leads to the decay of the coherent excitation of the electronic center-of-mass coordinate into incoherent intrinsic electronic motions, that is, single particle-hole excitations (Landau damping). The computed linear slope is found roughly two times smaller than the one measured in experiment on single silver particles, indicating that part of the size dependence of the fast plasmon damping results from the contribution of the granular ionic structure, especially the electron-phonon contribution in the large size domain. The strong sensitivity of the LSPR damping to the surface profile of the confining potential, which depends on numerous structural surface parameters, in particular those related to the ionic background modeling, is emphasized. A short analysis of the quantum box model and of the classical approach is also provided. © 2011 American Chemical Society.

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