No abstract

A sy-stematicanalysisofthe spectral moments method is presenred anddeveloped to compute the response functions of very large harmonic systems. Convergence of the algorirhms is discussed and solutions are proposed to improve lhe results obtained. New developments are proposed. They concern, on the one hand, thedetermination ofthe Green funcrionsorcorrelarion functions ofthe system, and the localizationofeigenvectors and. on theotherhand. thedeterminationofthespectraldensityofvery large homogeneousmatrices by a very simple and powerful technique. These results are illustrated by several examples taken from the main subjects studied by rhe aurhors: conducting polymers. fractals and quasi-crystals. Then comparison with other merhods is discussed.

We calculate the parallel (VV) and perpendicular (VH) polarized Raman spectra of amorphous silica. Model SiO2 glasses, uncompressed and compressed, were generated by a combination of classical and ab initio molecular-dynamics simulations and their dynamical matrices were computed within the framework of the density functional theory. The Raman scattering intensities were determined using the bond-polarizability model and a good agreement with experimental spectra was found. We confirm that the modes associated to the fourfold and threefold rings produce most of the Raman intensity of the D1 and D2 peaks, respectively, in the VV Raman spectra. Modifications of the Raman spectra upon compression are found to be in agreement with experimental data. We show that the modes associated to the fourfold rings still exist upon compression but do not produce a strong Raman intensity, whereas the ones associated to the threefold rings do. This result strongly suggests that the area under the D1 and D2 peaks is not directly proportional to the concentration of small rings in amorphous SiO2.