The density ratio is similar to the density index, but summation of the
density for the group is used for calculation.
The ratio of density is the summation of the (2Fobs-Fcal) electron
density values for the group of atoms divided by the average value
for the structure. Low values (less than 0.4) of this ratio may be
problematic for the group.
index = [Sumi D(xi)]/<D>_all
D(xi) = (2*Fobs - Fcal)
<D>_all is the average value of density for the structure.
The summation is for all the atoms of group.