Another example of an application where this kind of treatment can be useful is the region where two atoms get close. In such regions, the potential is very steep, and any analytic fit extrapolated into this region is liable to have wells instead of steep repulsive wall. However, a system with nZ atoms has nZ(nZ -1)/2 internuclear distances. Because this is quadratic in the number of atoms, it is often prohibitive to include data in all regions where two atoms get close.