This function serves as a valuable internal consistency check, verifying the accuracy of the solutions as well as the multiple scattering and absorption corrections applied to the data. Due to the significant inelastic scattering of 1 H and the Placzek effect, samples containing hydrogen always exhibit a strong background. The higher the atomic concentration of 1 H, the more pronounced this effect becomes, as observed in light water samples (e.g., Fig. ). For heavy water samples, this effect is substantially reduced (see Fig. ). The extent of inelastic scattering is primarily determined by the density of 1 H nuclei, so the first-order differences (Fig. ) should contain the same Placzek background. If the resulting second-order difference is constant, as shown in Fig. , we can conclude that there is no detectable background, indicating that the four solutions were prepared with identical chemical compositions. Although NDIS can, in principle, measure correlations between any pair of substitutable hydrogens, the feasibility is strongly dependent on sample contrast, which is largely determined by the product of the atomic concentrations. Consequently, H W -H W measurements are relatively straightforward, whereas H W -H TMA and especially H TMA -H Py are more challenging. Notably, the H TMA -H Py correlation would provide the most direct insight into cation-π interactions, yet these data were not acquired in earlier experiments. Because neutron beamtime is a scarce resource, the H TMA -H W measurement, although less sensitive to cation-π effects, remains the primary experimental dataset available for this work. Nonetheless, it still provides valuable structural information for validating and refining force field models of TMA-pyridine interactions, which shall be a strong foundation for further parametrization of cation-π interaction in FFMD simulations. Systems were run using GROMACS 2022, employing either CHARMM36 28 or prosECCo75 force fields. The primary difference between these force fields is that prosECCo75 incorporates an electronic continuum correction (ECC) by scaling the charges of all charged species by a factor of 0.75. The parameters for pyridine, phenol, and N(Me) + 4 in CHARMM36 were generated with CGenFF, while prosECCo75 parameters for TMA were taken from Mason et al. For clarity distribution of partial charges for TMA is listed in Table . Chloride ions were modeled using the CL type (full charge) for CHARMM36 and CL 2s type 31 (-0.75 charge) for prosECCo75. Simulations were conducted in the isothermal-isobaric (NpT) ensemble using GROMACS 2022. The system temperature was maintained at 298 K using a V-rescale thermostat with a 1 ps coupling constant, while pressure was kept at 1 bar with a C-rescale barostat and a 5 ps coupling constant. Van der Waals interactions were treated with a cutoff of 1.2 nm, employing a force-switch from 1.0 nm, and the Verlet cutoff scheme was used for neighbor searching. Longrange Coulomb interactions were accounted for using the particle mesh Ewald (PME) method, using a cutoff of 1.2 nm. The trajectories obtained from the FFMD simulations were analyzed using an in-house developed software designed for unbiased alignment and density mapping. The methodology employed closely followed the approach described previously, with the key difference being that the present analysis specifically targeted the density distributions around pyridine instead of other molecules.