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Density functional theory is used to calculate the vibrational properties of pure silica and sodo-silicateglasses with 20, 25, and 33 mol% of Na2O. The infrared and Raman spectra are calculated and the full responses aredecomposed into principal structural components (PSC). Those are for example the SiO4n--tetrahedra with n nonbrigdingoxygens defining the Qn-species at the origin of the structured feature at high frequency, and the Si-O-Sibridges leading the broad Raman R-band at intermediate frequencies. Our results confirm that Si-O-Si bending inbridges with large angle vibrate preferentially at lower frequencies than those with low angle. In addition, the spectralresponse of the Q2-species is bimodal and overlaps with that of the Q3, while the Q4 response covers almost all ofthe spectral range of the Qn-band. The ab-initio individual spectral responses of the PSC are used to reconstruct theexperimental Raman responses. Contrary to the commonly used multi-Gaussian decomposition, this approachprovides unambiguous band-assignments and hence a more accurate way to probe the structural and chemicalproperties of glasses from their spectroscopic signature.