For the molecular ions, we tested both CHARMM36 42 and AMBER99SB force fields, as well as scaled-charge variants using the Electronic Continuum Correction (ECC) approach with different charge-scaling factors (0.75 and 0.85). For CHARMM36, we also tested variants with modified CO-Ac or CH-TMA dipole moments. Simulations were performed with the corresponding TIP3P water model. The main text reports results obtained with the force fields CHARMM36-standard (CHARMM36-FULL) and ECC with charge scaling factors of 0.85 (CHARMM36-ECC85) and 0.75 (CHARMM36-ECC75)-and AMBER99SB-standard (AMBER99SB-FULL) and ECC with the charge scaling factor of 0.85 (AMBER99SB-ECC85). The Amber force field with 0.75 (AMBER99SB-ECC85) scaling and the dipole changes (CHARMM36-Low CH dipole, CHARMM36-Low CO dipole, and CHARMM36-High CO dipole) are presented in the SI. The list of all tested combinations and the details of all force fields are provided in Tables S1-3.