strong constraints are imposed on atom pairs linked through one or two bonds, while weak constraints are imposed on all other pairs. By defining D min and D max in this way, the bottom of this FB-ENM, B(D min , D max ), becomes a set of structures with bond lengths and angles equal to those of the reference structure, without excessively small or large interatomic distances. This can be explained as follows. Atom pairs linked through one bond are nothing but directly bonded atom pairs; therefore, their interatomic distances (bond lengths) remain constant at any point in B(D min , D max ) due to strong constraints. When strong constraints are applied to an atom pair linked through two bonds, the lengths of the three sides of the triangle formed by the atom pair and the linking atom remain constant at any point in B(D min , D max ).