For the rm-MRF, described in sec. 2.3, an average fluctuation function σ has to be determined for a given input energy density w λ (r) at each grid point. To achieve this, we have adopted an approach similar to the calculation of the inverse electron number function described in subsection 6.2: at each grid point, the energy density is calculated for a range of σ-values. The correct values for a given energy density are again obtained from a straightline interpolation. In this fashion, we have obtained σx (r) from w x (r) = w λ=0 (r), but in principle, any input energy density (in the gauge of Eq. 7) can be used.