Fonction de distribution radiale

Résumé

En mécanique statistique, la fonction de distribution radiale (ou fonction de corrélation de paires) notée g (r), décrit, dans un système de particules (atomes, molécules, colloïdes, etc.), comment la densité varie en fonction de la distance par rapport à une particule de référence. Cette fonction est également utilisée pour décrire la . Présentation Si une particule donnée est considérée comme étant à l'origine O, et si ρ est la densité moyenne en nombre des particules, alors la densité moyenne locale en fonction du temps à une distance r de O est \rho g(r). Cette définition simplifiée est valable pour un système homogène et isotrope. Un cas plus général sera examiné ci-dessous. En termes plus simples, il s'agit d'une mesure de la probabilité de trouver une particule à une distance r d'une particule de référence donnée, par rapport à celle d'un gaz idéal. L'algorithme général implique de déterminer combien de particules se trouvent

Source officielle

https://fr.wikipedia.org/wiki/Fonction_de_distribution_radiale

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, ,

The parameterisation of accurate and transferable repulsive potentials is a key ingredient for the self-consistent-charge density functional tight-binding method (SCC-DFTB). In the conventional parameterisation scheme the balanced description of different chemical environments involves significant human effort and chemical intuition. In this work, we propose an in situ parameterisation method with reduced transferability but maximal accuracy for the chemical and physical environment under investigation. Starting from an initial guess, we used iterative Boltzmann inversion to successively improve the repulsive potentials. The corrections were extracted iteratively from the differences in the radial distribution functions with respect to a density functional reference calculation. With this protocol convergence was reached within a few iterations involving only minimal human input. We applied this new scheme to liquid water at ambient conditions, a particularly challenging case for conventional SCC-DFTB. The newly determined parameters lead to a clear improvement of both the structural and dynamical properties of liquid water.

Taylor & Francis2013

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Electronic solvation dynamics in non-polar supercritical fluids

Majed Chergui

The excitation of the Rydberg states is used to study the nonpolar solvation dynamics in supercrit. fluids (SCFs). The system NO in supercrit. Ar serves as the model system. First, the steady-state, absorption and fluorescence spectra of the NO X 2P-A 2S+(3ss) Rydberg transition were recorded in the supercrit. domain of Ar. Then, mol.-dynamics simulations were carried out to explore the details of the structural rearrangements and solvation dynamics upon the excitation of the Rydberg state. The validity of the MD simulations was checked by reproducing the steady-state spectroscopical observables. Esp., the influence of the near crit., local d. fluctuations on the solvation dynamics was analyzed. The differences between the time-dependent Stokes shift (TDSS) and the equil. transition energy correlation function (ECF) are used for the anal. [on SciFinder (R)]

2004

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First-principles investigation of the structural and vibrational properties of vitreous GeSe2

Luigi Giacomazzi, Alfredo Pasquarello

Using a density-functional scheme, we study the structural and vibrational properties of vitreous germanium diselenide (v-GeSe2). Through the use of classical and first-principles molecular-dynamics methods, we generate a set of structural models showing a varying degree of chemical disorder. In particular, two types of structural concepts are represented: one in which the tetrahedral order is preserved to a very large extent, and one which reproduces the high degree of disorder in first-neighbor shells found in first-principles molecular-dynamics simulations of the liquid. The investigated structural properties include the angular distributions, the atomic arrangements in the first-neighbor shells, and the pair-correlation functions. In reciprocal space, we have calculated the x-ray and neutron total structure factors and the partial structure factors. Comparison with experiment gives overall good agreement for the models of either structural conception. We then investigate the vibrational properties via the vibrational density of states and the inelastic neutron spectrum. The considered models yield similar spectra and agree with experimental data. We also obtain infrared and Raman spectra through a density-functional scheme based on the application of finite electric fields. For these spectra, significant differences appear among the models. The comparison with experiment favors a model showing a high degree of chemical order. The Raman intensity is analyzed in terms of the underlying atomic vibrations. The assignment of the Raman companion line to Se motions in edge-sharing tetrahedra is supported.

2007

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