We first recognize that if we treat the Q I structure as a distorted bilayer, the pore-topore distance in the Q I structure is the sum of the bilayer thickness and the pore size (see the inset in Fig 4). Since the ideal minimal surface lies in the center of the hydrophilic pore between the bilayers, we can then assume that the most likely space group is the one whose normal distances between sections of the minimal surface are most closely aligned with the sum of the bilayer thickness and the pore diameter. Otherwise, it would require significant distortion to create the Q I phase. We, thus, first determined the thickness of the lamellar bilayer using FAT-SLiM 63 on an atomistic simulation in the NPT ensemble and then combined the bilayer thickness with the experimentally determined pore size to arrive at an expected pore-to-pore distance. Our collaborators previously fitted rejection data of uncharged solutes to Ferry's equation to obtain an approximate pore diameter, and found the best fit for the experimental pore diameter to be 0.96 ± 0.05 nm (error obtained through a parametric bootstrap of the experimental Ferry's equation fit, see SI section S2 for details). From the results of atomistic simulations of a lamellar bilayer, we expect the local bilayers to have thicknesses near 3.6 ± 0.2 nm. Therefore, the expected pore-to-pore distance is 4.6 ± 0.2 nm. We tested the compatibility of this bilayer width with each space group with a procedure as follows: