In this example, because the dihedral angle around the central C-C single bond differs between the two structures, the planes containing the two atomic groups enclosed by different background colors in Figure are distinguished. All atom pairs within each atomic group included in each plane are then strongly constrained. In this case, the pairs connected by blue lines in Figure are additionally strongly constrained. Consequently, within the bottom of the FB-ENM for this isomerization reaction, only rotation around the C-C single bond is allowed, preserving the two planar structures.