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Abstract. Using density functional molecular dynamics simulations, we analyze the broken chemical order in a GeS 2 glass and its impact on the dynamical properties of the glass through the in-depth study of the vibrational eigenvectors. We find homopolar bonds and the frequencies of the corresponding modes are in agreement with experimental data. Localized S-S modes and 3-fold coordinated sulfur atoms are found to be at the origin of specific Raman peaks whose origin was not previously clear. Through the ring size statistics we find, during the glass formation, a conversion of 3-membered rings into larger units but also into 2-membered rings whose vibrational signature is in agreement with experiments.

Abstract. Using density functional molecular dynamics simulations we study the electronic properties of glassy g-GeS 2 . We compute the electronic density of states, which compares very well with XPS measurements, as well as the partial EDOS and the inverse participation ratio. We show the electronic contour plots corresponding to different structural environments, in order to determine the nature of the covalent bonds between the atoms. We finally study the local atomic charges, and analyze the impact of the local environment on the charge transfers between the atoms. The broken chemical order inherent to amorphous systems leads to locally charged zones when integrating the atomic charges up to nearest-neighbor distances.

We present a study of the structural properties of (x)Na 2 S-(1-x)GeS 2 glasses through DFTbased molecular dynamics simulations, at different sodium concentrations (0 < x < 0.5). We computed the radial pair correlation functions as well as the total and partial structure factors. We also analyzed the evolution of the corner-and edge-sharing intertetrahedral links with the sodium concentration and show that the sodium ions exclusively destroy the former. With the increase of the sodium concentration the "standard" FSDP disappears and a new pre-peak appears in the structure factor which can be traced back in the Na-Na partial structure factor. This self organization of the sodium ions is coherent with Na-rich zones that we find at high modifier concentration. PACS numbers: PACS numbers: 61.43.Bn,61.43.Fs,71.15.Pd,63.50.+x ModelThe mathematical model we have used was developed by Sankey and Niklewski [7]. It is included in a first-principles type molecular-dynamics code, called fireball96, that is based on the DFT [8], within the Local Density Approximation (LDA) [9,10]. A tight-binding-like linear combination of pseudoatomic orbitals, satisfying the atomic self-consistent Hohenberg-Kohn-Sham equations [11], is used to determine the electronic eigenstates of the system. A basis set of one s and three p pseudo-orbitals, slightly excited since they vanish outside a cut-off radius of 5a o (2.645Å), is required.

Using density-functional molecular dynamics simulations we analysed the different steps that permit us to create a glass starting from a crystal in the case of GeS2. Thus we studied, mainly from a structural point of view, α-GeS2 crystals, GeS2 liquid at 2000 K and the evolution of the system during the quench. We found that in the liquid a first sharp diffraction peak exists even though the atoms are in a diffusive state. During the quench we evaluated the lifetime of the different interatomic bonds and we showed that the ‘wrong’ bonds present in the liquid can be destroyed and re-created several times during the length of the quench. Thus the ultra-fast cooling rates used in simulations cannot solely be held responsible for the structural differences that may be found between experimental and simulated glasses.