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 Where: 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}