To obtain a better match between the CG and all-atom models, we made several adjustments to the force field. These changes include 1) optimizing the choice of bead from the Martini force field for the moieties of interest, 2) adjusting bonded interaction parameters to improve geometries compared to atomistic simulations, 3) adjusting interactions between the glycerol and the bromide ion, 4) adjusting glycerol-glycerol interactions, and 5) lowering the dielectric constant to increase the total effect of electrostatic potentials. We initially selected bead types based on previous studies on similar monomers. However, we changed some of the beads (such as using a more polar bead for the head group) to match the CG bilayer with the all-atom bilayer. These changes and justifications of our choices are documented in SI Table . Adjustments were also necessary for bonded interactions. We tuned the parameters for bonds, angles, and dihedrals until the CG model had similar bond and angle distributions as the corresponding groups of atoms in the all-atom model (See SI Tables S2-S4 for the details). The changes to glycerol-glycerol and glycerol-bromide interactions are also documented in detail in SI (See SI Tables and). With these adjustments, we were able to show significant agreement between the atomistic and CG bilayers in MD, as described in the results. The final parameters used for the monomer are also shown in Tables and in the SI. The average number densities along the z-direction of selected components for both resolutions of membranes are shown in Fig 2 . Motivating the choice of space group of the Q I phase