Il s'agit d'un cours (2h) sur les concepts de la dynamique moléculaire classique. Il a été dispensé dans le cadre d'une école thématique CNRS organisée par le GDR Verres, afin de permettre aux différentes équipes participantes soit de s'initier aux méthodes de modélisation, soit de se perfectionner, ou encore de comparer les démarches propres à celles développées dans d’autres sites ou communautés de recherche.

Short and medium range order of silica and sodium silicate glasses have been investigated from a quantitative analysis of 29Si MAS NMR and 23Na, 17O MQMAS NMR spectra. The method described enables the extraction of the underlying 17O NMR parameter distributions of bridging oxygens (BOs) and non-bridging oxygens (NBOs), and yields site populations which are confirmed by 29Si NMR data. The extracted NMR parameter distributions and their variations with respect to the glass chemical composition can then be analyzed in terms of local structural features (bond angles and bond lengths, coordination numbers) with the help of molecular dynamics simulations combined with first-principles calculations of NMR parameters. Correlations of relevant structural parameters with 23Na, 29Si and 17O NMR interactions (isotropic chemical shift δiso, quadrupolar coupling constant CQ and quadrupolar asymmetry parameter ηQ) are re-examined and their applicability is discussed. These data offer better insights into the structural organization of the glass network, including both chemical and topological disorder. Adding sodium to pure silica significantly diminishes the Si-O-Si bond angles and leads to a longer mean Si-O bond length with a slight decrease of the mean Na-O bond length. Moreover, the present data are in favor of a homogeneous distribution of Na around both oxygen species in the silicate network. Finally, our approach was found to be sensitive enough to investigate the effect of addition of a small quantity of molybdenum oxide (about 1 mol%) on the 17O MAS spectrum, opening new possibilities for investigating the Mo environment in silicate glasses.

We present the study of the behaviour of amorphous silica under compression within the framework of computer simulations. In contrast to previous similar numerical works, our investigation is an exploratory study using finite-range pair-potential in order to get insight into the structural modications on both local and intermediate length scales when the pressure/density is increased. We have hence identified the characteristic bond distances and angles of Si-coordination polyhedra (SiO5 and SiO6) formed under high pressure. We wiil equally consider the plastic deformation upon applied stress and discuss the nature of this deformation upon loading and unloading.

We have carried out first-principles molecular dynamics simulations of a sodium borosilicate (NBS) glass within the framework of DFT. The composition is 6SiO2-3Na2O-B2O3 and the density was fixed to 2.51g.cm-3. The system size is 320 atoms (i.e. 60 silicon, 180 oxygen, 60 sodium and 20 boron atoms) which is rather demanding for ab-initio simulations and has been chosen to obtain a good statistics, in particular for quantities associated to the lowest concentrated element (boron). The simulations have been started at 4500K in the canonical ensemble (NVT) and followed by a simulation in the microcanonical ensemble (NVE). Subsequently, the system is progressively cooled to 300K within 60ps. In order to characterized the structure of the system we have calculated the pair correlation functions, the angular distributions, the Qn distributions, the non bridging oxygen's concentration as well as the partial structure factors. Moreover, we have studied the environment of 3- and 4-fold (BIII and BIV) coordinated boron atoms. Furthermore, the vibrational density of states has been studied in details. Indeed, both BIII and BIV atoms have specific frequencies, and the partial vibrational density of the 3-fold coordinated B atoms has been found to be a weighted sum of two specific contributions: 3-fold symmetric coordinated B (BIIIs) atoms and asymmetric coordinated B (BIIIa) atoms. Identically, the partial vibrational density of the 4-fold coordinated B atoms has been found to depend on the nature of their second neighbor namely so-called BIV(4Si-0B), BIV(3Si-1B), BIV(2Si-2B), BIV(1Si-3B), and BIV(0Si-4B) respectively with 4 Si and 0 B atoms as second neighbor and so on. The infrared spectra of our glassy models is also presented.

First-principles techniques have been employed to study the reactivity of water into a calcium aluminosilicate glass. In addition to the well known hydrolysis reactions Si-O-Si+h2O->Si-OH + Si-OH and Si-O-Al+h2O-> Si-OH+Al-OH, a peculiar mechanism is found, leading to the formation of an AlO3–H2O entity and the breaking of Al–O–Si bond. In the glass bulk,most of the hydrolysis reactions are endothermic. Only a few regular sites are found reactive (i.e. in association with an exothermic reaction), and in that case, the hydrolysis reaction leads to a decrease of the local disorder in the amorphous vitreous network. Afterwards, we suggest that ionic charge compensators transform into network modifiers when hydrolysis occurs, according to a global process firstly suggested by Burnham in 1975. Our theoretical computations provide a more general model of the first hydrolysis steps that could help to understand experimental data and water speciation in glasses.

The vibrational properties of silica glass have been intensively studied experimentally and theoretically during the last four decades. However, only few investigations have explored the evolution of the vibrational features of simple alkali glasses or silica under compression. In this contribution, we will present a detailed mode analysis of two simple (Li/Na) alkali glass models as well as for silica glass models obtained by combined classical and Car-Parrinello molecular dynamics. In particular we will discuss how the presence of the network modifier influences the relevant vibrational parameters: positions, shapes and intensities of the main peaks in the vibrational density of states (VDOS) and IR spectra. We will also show the decomposition of the VDOS on symmetry-adapted modes of the basic structural units of our models networks: SiO4 tetrahedra and SiOSi bridges.  The same analysis will be carried out in the case of few silica glass models at normal density as well as at higher density.