The index of connectivity is the product of the (2Fobs-Fcal) electron
density values for the backbone atoms (N, CA and C) divided by the
average value for the structure. Low values (less than 1.0) of this
index indicate breaks in the backbone electron density which may be
due to flexibility of the chain or incorrect tracing.
connect = [(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