Maintaining physical behavior in regions where one or more interatomic distance is very large or very small is hard because one does not usually saturate such regions of space with training data. When poor physical behavior arises, one often resorts to an iterative scheme: fit the surfaces, run trajectories to discover regions of space where the fit is unphysical, add data in the problematic regions, refit, and iterate the steps until no more unphysical behavior is found. However, this is problematic for maintaining physical behavior at large internuclear separation because the amount of space in asymptotic regions is infinite. And it can be troublesome in regions where two or more atoms get very close because the potential is very steep in those regions.