Core level binding energies (BE) were calculated employing the implementation in VASP developed by Köhler and Kresse. According to this approach, the BE is related to the total energy difference between a final state in which the electron of the respective core level is excited into the valence states and the initial ground state. For the calculation of the final states the remaining core electrons are kept frozen within the pseudopotential, whilst the valence electrons are allowed to relax. For direct comparison to the experiment, calculated BEs of the different core levels X (BEcalc(X)) were calibrated, using the experimental and calculated values of core level BEs of intact TFSI adsorbed on HOPG at a monolayer coverage of BMP-TFSI as reference: