As previously explained, only atom pairs strongly constrained in both structures are strongly constrained by the merged FB-ENM (Figure ). In this example, only atom pairs related to the gray molecule are strongly constrained. Conversely, the green molecule is weakly constrained in the merged FB-ENM because its bond is broken in the fictitious addition reaction. Consequently, within B( Dmin , Dmax ), the two green atoms can move freely, enabling the description of the fictitious addition reaction.