We now define FB-ENM for multiple reference structures by merging multiple FB-ENMs defined for each reference structure. In ENM simulations of biomolecules, it is common to merge two potential energies by taking the smaller value and smoothing it, allowing both reference structures to become minimum points of the merged potential energy and enabling the simulation of structural transitions between them. However, in the present FB-ENM, we employ a different merging approach to allow barrier-free transitions between reference structures within a flat bottom.