The grid spacing is reduced locally around each atom, and the reduction is
given by 1/(1+A), where A is specified by this variable. So, if
A=1/2 (the default), the grid spacing is reduced to two thirds = 1/(1+1/2).
[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.