First, each pair potential in FB-ENM is a piecewise quadratic function, resolving (iii) and (iv). Thus, FB-ENM is expected to be more robust against undesired bond breaking caused by these factors. IDPP can also resolve these problems by making the weight function w in eq 2 a decreasing function of d ij instead of |r ij |, while keeping the original purpose of imposing stronger constraints on pairs with shorter interatomic distances. FB-ENM resolves (i) by imposing strong constraints only on atom pairs that determine the molecular structure. IDPP also introduces the distance-dependent strength of constraints, making constraints on insignificant atom pairs weaker. However, IDPP essentially constrains interatomic distances to a single point, whereas the weak constraints in FB-ENM only prevent excessively small or large interatomic distances and are much weaker than those in IDPP. FB-ENM is designed to link the reactant and product through the flat bottom of the "image-independent" total potential energy. Optimizing the path using this potential energy allows the distances of all atom pairs to converge to sterically favorable, generally nonlinear changes, avoiding the sterically unfavorable linear changes in (ii). Consequently, FB-ENM is expected to be more robust against undesired molecular distortions caused by (i) and (ii).