The index of density is the product of the (2Fobs-Fcal) electron
density values for the group of atoms divided by the average value
for the structure. Low values (less than 1.0) may be problematic for
index = [(D(xi)...D(xi))(1/N)] /<D>_all
D(xi) = (2*Fobs - Fcal)
<D>_all is the averaged value of density for the structure.
The product is for N atoms of group.
Ref: Vaguine, A.A., Richelle, J. & Wodak, S.J. (1999). Acta Cryst. D55,199-205