Quasi-periodic ab-initio models in material science: the case of oxygen deficient centres in optical fibers

Ab initio methods for the study of multielectronic systems have become an essential tool also in the field of material science. They can give an invaluable contribution for the analysis and interpretation of experimental results or for the prediction of the properties of new materials. Drastic simplifications must be adopted anyway, owing to the inherent complexity of the systems that are the subjects of these studies. A proper choice of the model (both concerning geometry and level of approximation) is often the most critical step to be taken as concerns the reliability and usefulness of the results, given the variety of techniques available, whose merits and drawbacks are often difficult to assess a priori. The case study of oxygen vacancies in optical fibers serves the purpose of discussing advantages and limitations of one such approaches, based on the perturbed-cluster theory. The perturbed-cluster method consists in first providing a periodic model of the glassy fiber, whose quantum mechanical description can be obtained rather easily, then simulating locally the defective region and studying the processes that occur there through the use of an embedding ab initio technique. The problem is presented, the method adopted and some results obtained are critically analyzed.