This number determines the region over which the grid is enhanced (range of
enhancement of the resolution). That is, the grid is enhanced on a sphere
around each atom, whose radius is given by this variable. [This is the $a_{\alpha}$
variable in Eq. 2 of F. Gygi and G. Galli, Phys. Rev. B52, R2229 (1995)].
It must be larger than zero.