Résumé
The structure and the electronic properties of a sodium tetrasilicate (Na2Si4O9) glass were studied by combined Car-Parrinello and classical molecular dynamics simulations. The glass sample was prepared using a method recently employed in a study of a silica glass [M. Benoit et al., Euro. Phys. J. B 13, 631 (2000)]. First we generated a NS4 glass by classical molecular dynamics and then we took it as the initial configuration of a first-principles molecular dynamics simulation. In the ab initio molecular dynamics simulation, the electronic structure was computed in the framework of the Kohn-Sham density functional theory within the generalized gradient approximation using a B-LYP functional. The Car-Parrinello dynamics is remarkably stable during the considered trajectory and, as soon as it is switched on, some significant structural changes occur. The ab initio description improves the comparison of the structural characteristics with experimental data, in particular concerning the Si-O and Na-O bond lengths. From an electronic point of view, we find that the introduction of the sodium oxide in the silica network lowers the band gap and leads to a highly nonlocalized effect on the charges of the network atoms.