8 there was no reference at all to any experimental data in the parameterization of this potential, this represents a strong test of the model reliability. The study of pressur- ized germania is among our top priorities for future work. The potential is of a similar form to that introduced to describe silicates $^{32}$; since these potentials have proved to be transferable, we will be able, in the future, to focus on GeO$_{2}$-SiO$_{2}$ mixtures as well. These mixtures, as already mentioned in the introduction, attract great interest as lower part of figure 7, it is clear that it is not the case here. Each spectra consists in the superposition of sev- eral bands, which shows the existence of some important coupling between the symmetry coordinates. It is there- fore difficult to assign the IR features to the different modes. In particular, all of the three bands observed in the IR spectrum appear in the tetrahedral-VDOS of the bending E and F$_{2}$ modes. The other F$_{2}$ modes, which is associated to some kind of stretching of the tetrahedra, only exhibits the medium and high frequency modes. Fi- nally, the A$_{1}$ stretching spectrum exhibits two bands at frequencies centered on the values of 510 and 970 cm − $^{1}$, which do not appear on the IR spectrum. Such a mode of this symmetry is indeed not expected to be IR active for an isolated tetrahedral molecule, since it corresponds to the symmetric breathing mode. Coming back to the total VDOS of each ion, the low frequency vibrations are not associated to any of the tetrahedral modes. In fact these are due to some slow collective bending of two tetrahedra linked by a bridging oxide anion $^{31}$, so that they would not appear in the DOS of a single tetrahedron. FIG. 8: Comparison of the charge-charge current contribu- tions to the infrared spectrum obtained with the three differ- ent potentials (the two OE curves were rescaled with a factor q 2 $_{O}$/ 4, where q O is the partial charge carried by the oxide ion in the corresponding potential. The vibrational properties of the system as obtained with the original and rescaled OE interaction potentials have also been computed. As these potentials do not include any polarization effect, it is only possible to com- pare the charge-charge current contributions to the IR absorption spectrum with the one obtained with the DIP- PIM potential. The obtained spectra are given on fig- ure 8. Unsurprisingly, the original OE potential seem to fail completely in describing the vibrational properties of GeO$_{2}$; this is the reason why the rescaled OE potential had to be developed. For that potential, the agreement is better, in particular for both the small and high fre- quency bands. Concerning the medium frequency band, observed at around 604 cm − 1 for the DIPPIM potential, it does not appear for the rescaled OE potential. Instead there appears a shoulder to the small frequency band, for frequencies ranging from 400 to 500 cm − $^{1}$. As these bands were associated to the F$_{2}$ stretching and bending modes on the tetrahedral-VDOS, this means that this modes are not correctly depicted by the rescaled OE po- tential. they are widely used in optical fibers and waveguides. We thank Philip Salmon for providing his diffraction data. Calculations were carried out at the EaStCHEM computing facility (http://www.eastchem.ac.uk/rcf). DM thanks the EPSRC and the CMPC for his PhD funding. This work was carried out under the HPC- EUROPA++ project (project number: 211437), with the support of the European Community - Research Infrastructure Action of the FP7 ”Coordination and support action” Programme. In conclusion, the obtained potential seem to repro- duce all the studied properties (structural, dynamical and vibrational) to a high degree of precision. Since Rev. B , 27(2):1052–1078, 1983. Acknowledgements VII. CONCLUSION 3 E.I. Kamitsos, Y.D. Yiannopoulos, M.A. Karakassides, G.D. Chryssikos, and H. Jain. Raman and infrared struc- tural investigation of x Rb 2 O.(1 − x )GeO 2 glasses. J. Phys. Chem. , 100(28):11755–11765, 1996. 4 P.V. Teredesai, D.T. Anderson, N. Hauser, K. Lantzky, 1 M. Micoulaut, L. Cormier, and G.S. Henderson. The structure of amorphous, crystalline and liquid GeO 2 . J. Phys.:Condens. Matter , 18:R753–R784, 2006. 2 F.L. Galeener, A.J. Leadbetter, and M.W. Stringfellow. Comparison of the neutron, raman, and infrared vibra- tional spectra of vitreous SiO 2 , GeO 2 and BeF 2 . Phys.