In this subsection, we propose a potential energy that addresses the limitations of IDPP discussed above. The proposed potential energy imposes constraints on interatomic distances using piecewise quadratic functions. In the field of biomolecular simulations, such structure-based potential energies are well known as Elastic Network Models (ENMs) and are frequently applied to coarse-grained models of large molecules. The ENM proposed here is unique in that each pair potential has a flat bottom. In this study, we refer to this ENM as the Flat Bottom ENM (FB-ENM). As explained in Section 2.3, the potential energy with a flat bottom is well-suited for path optimization using the direct MaxFlux method.