corresponding to direct contact ion pairs (see Figure ). The two models that best fit the experimental data (CHARMM36-ECC85 and AMBER99SB-FULL) exhibit a very similar propensity to form contact ion pairs. The internal structures of the ion pairs are also very similar to each other, with the dominant interaction mode being the acetate lying flat against the faces of TMA. Nevertheless, we see a broad distribution of relative orientations of the two ions from the carboxylic group pointing towards the TMA to an interaction through the acetate hydrophobic tail (see Figure ). ). This plateau and, overall, the ∆∆S H Ac H W (Q), is very well described by the two force fields that so far gave the best description of the TMA-Ac solution (i.e., AMBER99SB-FULL and CHARMM36-ECC85).