MD simulations. Membrane simulations were carried out according to literature precedents using POPC bilayers with the Stockholm lipid parameters in TIP3P-FB water. To capture the anisotropy of halogen bond donor atoms and allow anion-bound poses, we used an extra particle (EP) point charge during the RESP charge fitting procedure (See §8 SI for details, Table ). During the RESP procedure, the charge of the ion was also fitted, accounting for the charge transfer from ion to the transporter upon binding. The bonded parameters for the transporter were taken from the general amber force field (GAFF), with the Lennard-Jones parameters for iodine adjusted to better reproduce DFT binding distances (Table ). Chloride atom parameters compatible with TIP3P-FB from Sengupta et al. were chosen, and hydroxide parameters were adapted from Han et al.