Parameters for virtual site construction were obtained from an AA CHARMM36 MD trajectory of cholesterol in a DOPC bilayer. The cholesterol ring particles were CG-mapped as described in the supplementary information (Supplementary Table ). The positions of the particles were then averaged after rotationally and translationally fitting the entire molecule to the R1-R2-C1 frame. The placement of the beads relative to bead C1 was then decomposed as a function of vectors R1-R2, R1-C1, and of their cross product. Nonbonded interactions between the virtual site particles and between them and the frame particles were switched off. For each bead, Martini bead masses were used, instead of the underlying atomic masses, for better agreement with the Martini 3 lipid models. All bead masses were then redistributed from the virtual site beads to the remaining non virtual site beads. See the supplementary material for more details on the virtual site construction.