
The region defined by a sphere of the given radius around the excited atom defining
which RI_XAS basis elements are considered for the RI projection of the density. Each
basis element which center is in this region is taken. The density for a given excited
atom is expressed as : sum_ijkl P_ij (ijk) S_kl^1 xi_l, where P_ij is the density
matrix, i,j span the orbital basis and k,l the RI_XAS basis in the region. The larger
the radius, the more basis functions to expand the density. However, it is assumed
that it is a small number and the code does not scale well as the number of RI basis
elements gets too large. Expressed in Angstrom. If the radius is set to 0.0, only
the RI basis elements centered on the excited atom are used. 