The propionate binding site is "fully occluded from the extracellular milieu" . We verified with the VMD software that the nearest water molecule is almost 10.9 Å distant from the olfactant; likewise, the nearest lipid molecule is more than 13 Å from C 2 H 5 COO -. Since we expect the relevant vibrational modes to be spatially localized, we remove the solvent, lipid, and counterions from the structure; leaving only OR51E2 and the propionate ligand.