[en] Full text of publication follows: In fusion reactors, helium is one of the main gaseous by-products produced as a result of high energy neutron irradiation. In the case of ferritic steels to be used in future fusion and fission reactors, the formation of helium bubbles at grain boundaries (GBs) leads to drastic modification of the material's mechanical properties such as high temperature embrittlement. A quantitative description of the interaction of He atoms with grain boundaries is therefore essential. First principles calculations provide such information at an atomic scale which is not directly accessible through experiments. In this work, a 'dynamic coupling' of ab initio and MD simulations has been performed to study the properties of representative symmetric tilt grain boundaries in Fe. More specifically, density functional theory within the Generalised Gradient Approximation code is used to calculate the structural, energetic and magnetic properties of certain GBs in bicrystals which contain a few hundred atoms. The lowest energy binding sites of He atoms at these GBs are also investigated. These results are compared to empirical potential calculations of similar and much larger systems. Such a coupling enables an in-depth study of GBs by incorporating predictive ab initio calculations which are essential for the validation of the existing empirical potential or the fitting of new ones, while the empirical simulations are necessary to study the finite temperature effects on these properties as well as providing an understanding of simulation cell size effects. The present study is the first step towards a predictive description of the segregation of He at GBs and their embrittling effects in the steels. (authors)