In contrast with all the empirical force fields we tested, the signal computed from our AIMD simulation provides an excellent fit of the experimental signal in Q-space. Remarkably, it captures perfectly the height of the peak at 1.2 Å-1 , which remained out-of-reach of empirical force fields. This success suggests that the internal geometry of the ion pair-i.e., the relative orientation of the two ions-is properly described by the AIMD simulations. If we now compare (Figure ) the distribution of angle θ that quantifies the orientation of the acetate relative to the TMA ion, it seems distinct in AIMD simulations (despite the higher numerical noise due to limited sampling) from all the empirical force field simulations, with a preference for larger angles. This means that the carboxylate group, instead of lying flat against TMA as in force field simulations, would have a larger tendency to point towards the TMA cation. In other words, in AIMD, TMA behaves less "hydrophobic" in ion pairing than what is depicted in force field simulations.