Computer simulations have become an important exploring tool of the properties of a vast class of systems belonging to various areas as physics, chemistry, biology or earth sciences. In the particular case of silicate glasses, intensive efforts have been made also during the last two decades to probe their microscopic properties by means of computer simulations. Most of these materials that man has used and produced for thousands of years, have technological, geophysical or biomedical importance, and hence they are widely studied from experimental and theoretical points of view. In this contribution, we will present a detailed structural characterization of the local environment of atoms in some (Li/Na) single-alkali silicate glasses with different cation content (20% and 33%), and obtained by combined classical and Car–Parrinello molecular dynamics simulations. The attention given to these systems is justified by the fact that they can be used as prototypes for more complicated ternary silicate glasses and/or to obtain a precise microscopic insight into one of the unusual features of oxide glasses, namely the so-called mixed alkali effect. The influence of the nature and proportion of the network modifier cation on the relevant structural parameters (bond length and bond angle distributions, distribution of alkali cations) will be discussed in terms of their correlations to the first-principles NMR spectra and parameters computed for our glassy models using the GIPAW method [1,2]. We will equally show a detailed analysis of the nature of the vibrational modes computed for our models and how they are affected by the alkali content and nature. [1] C. J. Pickard, F. Mauri, Phys. Rev. B 63 (2001) 245101. [2] S. Ispas, T. Charpentier, F. Mauri, D.R. Neuville, Solid State Sci. 12 (2010) 183-192.

Dans cet exposé, je présenterai brièvement les travaux de simulations numériques de systèmes désordonnés, au niveau atomistique, et qui sont menés dans le groupe 'Théorie& Simulation' au LCVN.

Cet exposé comportera 2 parties: - la 1ère partie dédiée a l'étude de la silice amorphe sous pression, dans une approche de dynamique moléculaire classique. Nous avons étudié la densification d'un verre de silice sous pression (caractérisation de l'evolution de la structure du verre: coordinence, topologie, réarrangements). -la 2ème partie présentera propriétés structurales et vibrationnelles de verres borosilicatés via de simulations de DM ab initio, comportant également une analyse de la signature vibrationnelles des BO3 et BO4.

In this work we present the structural properties of lithium tetrasilicate glass (LS4) obtained by combined classical and Car-Parrinello molecular dynamics simulations. The computed features are compared with corresponding experimental results as well as to those obtained for sodium tetrasilicate glass (NS4) models generated using the same approach, and containing the some percentage of alkali oxide. The ab initio description improves the comparison of the structural characteristics with experimental data, in particular concerning the Li-O, Si-Li and Si-Si bond lengths. The influence of the network modifier nature on the relevant structural parameters is further examined in terms of their correlations to the first-principles calculated NMR parameters Of Our glassy models. For both glasses, Si-29 MAS NMR measurements are reported and the obtained experimental spectra are compared to the theoretical ones. (C) 2009 Elsevier Masson SAS. All rights reserved.

We have studied the structural properties of two (Li/Na) single-alkali silicate glasses with the same cation content (i.e. 20\%) by combined classical and Car-Parrinello molecular dynamics simulations. The influence of the network modifier nature on the relevant structural parameters have been further examined in terms on their correlations to the first-principles NMR spectra and parameters computed for our glassy models using the GIPAW method. Hence, we found that the \Si isotropic chemical shift of \Q4 units upon the mean Si-O-T bond angle (where T denotes the Q${}^{(n)}$ connected tetrahedron, $n=4,3,2$). could be well described by means of a {\it single} (i.e. the same for both glasses) simple linear dependence.

Silica glass is a typical example of disordered material. Theoretically, molecular dynamics computer simulation is a common and effective way to investigate structural and dynamic microscopic properties of silica glass, such as pair correlation functions, bond angle distribution, vibrational density of states and so on. In this work, we have prepared several different silica models solely within the framework of first-principles approach of the order of hundred atoms (38 Si and 76 O atoms) and analyze their structural, electronic and vibrational properties. The SiO2 models have been generated by quenching liquid silica (equilibrated at 3600 K and with a density equal to 2.20 g/cm3 ) to room temperature (300 K) using different cooling rates. We have compared the microscopic properties with experimental and previous simulation results. We have found that the cooling rate affects the properties of generated silica samples and that the slowly quenched model gives better agreement with the experimental results. These models have been gradually compressed up to 2.67 g/cm3 density, which corresponds to about 7 GPa: just below the elastic to plastic transition regime (estimated around 8 to 10 GPa) of silica glass. To characterize microscopic properties of compressed silica models below the transition pressure is an important step to investigate the mechanism of the transition with further compressed silica models (above the transition pressure). We show the properties of these compressed silica models and compare them with those of the uncompressed ones. Another type of models are also generated by compressing and equilibrating the liquid silica at 3600 K to 2.67 g/cm3 and then quenched to 300 K. The microscopic properties of these samples will be also presented and discussed.

A Molecular Dynamics (MD) study of static and dynamic properties of moltenand glassy germanium dioxide is presented. The interactions between the atomsare modelled by the classical pair potential proposed by Oeffner and Elliott(OE) [Oeffner R D and Elliott S R 1998, Phys. Rev. B, 58, 14791]. We compareour results to experiments and previous simulations. In addition, an ab initiomethod, the so-called Car-Parrinello Molecular Dynamics (CPMD), is applied tocheck the accuracy of the structural properties, as obtained by the classicalMD simulations with the OE potential. As in a similar study for SiO2, thestructure predicted by CPMD is only slightly softer than that resulting fromthe classical MD. In contrast to earlier simulations, both the static structureand dynamic properties are in very good agreement with pertinent experimentaldata. MD simulations with the OE potential are also used to study therelaxation dynamics. As previously found for SiO2, for high temperatures thedynamics of molten GeO2 is compatible with a description in terms of modecoupling theory.