Overall, the scaled charge CHARMM36 with a 0.85 scaling factor (CHARMM36-ECC85) and the full charge AMBER99SB force fields provide the best fit of the experimental data, especially in the low Q region with the location of the initial sharp rise of the signal, which is a very robust experimental feature. The computed density maps of H TMA atoms around acetate (Figure ) are very similar for all force fields employed and show that in these simulations TMA interacts with acetate from above and below the -COO -plane, rather than along the direction of the carboxylate group. However, none of the present force fields could properly capture the height of the first peak at 1.2 Å-1 in Q-space (Figure ,b), nor, in real space, the features below 5 Å. Neither did additional tests of force field variants with decreased or increased dipole moments on both CH-TMA and CO-Ac bonds (see Figure ). While none of the force fields properly capture all the experimental features they, however, recover a range of different ion pairing behaviors with varying ion pair structures. For instance, simulations performed with CHARMM36 (both the standard version and scaled charges) present a higher proportion of solvent-shared ion pairs and less direct contacts between the two ions, while AMBER99SB-ECC85 is the model with the highest peak in g(r)