This suggests that these two force fields properly capture the amount of TMA-Ac ion pairing in the solution, and that other tested force fields over-or under-estimate it. What can explain, then, the distinguished feature in the H TMA H Ac signal that could not be captured by our force fields? If the overall amount of association between the ions is correctly described in our simulations, then this would suggest that it is the internal geometry of the ion pair (relative orientation of the two ions) that is not fully captured by simulations. However, characterizing how the orientation of the ions should change to give a better fit to experimental data is very challenging. To investigate in more detail the structure of contact ion pairs, we thus performed multiple ab initio MD simulations, at the revPBE DFT level of theory. The large computational cost of such simulations limits the size of the simulated system to a single TMA-Ac ion pair and the length of the simulations to 100 ps. However, starting from 20 different geometries of the ion pairs, these simulations offer enough sampling to yield the H TMA H Ac pair correlation function, and obtain, after Fourier-transform, the double difference in Q-space, ∆∆S H TMA H Ac (Q) (Figure ).