The distance,  at which  can be considered as the distance at which the atom will come across a troublesome discontinuity in the potential. This is the point at which the WKB method truly becomes nonsense. The condition for efficient quantum reflection can be written as .
The distance at which an atom will encounter a problematic barrier in the potential energy landscape.  This marks the boundary where the Wentzel-Kramers-Brillouin (WKB) approximation breaks down. The condition for efficient quantum reflection is given by .