The structural properties of As2Se3 and AsSe4 glasses are investigated using the First Principles Molecular Dynamics. The results show an excellent agreement with measured structural properties in real and reciprocal space (total structure factors and total pair distribution functions, differential pair correlation functions). The analysis of the detail of the structure shows that homopolar As - As not only exist in the stochiometric compound but also for chalcogen rich compositions. Furthermore, coordination defects consisting of four-fold As and 1-fold Se are found to be of about 5-9%. Their topological nature, their evolution with temperature, as well as their contribution to the understanding of floppy to rigid transitions are discussed.