The final goal of our work is to parameterize a DFTB model that allows the simulation of silicon anode lithiation. Note that this is a very complex process that involves different chemical environments, with a wide range of Li x Si compositions. However, it is important that the new parameterization maintains its accuracy for the widest possible range of concentrations, in order to avoid the need to change the model "on-the-fly" during a simulation. This could be particularly disturbing for approaches like grand-canonical MD simulations, where the relative composition is constantly changing. Thus, we require the parameterization to be as transferable as possible between different stoichiometries. In this sense, to predict relative formation energies, F (ř s ):