Molecular flexibility change at bio macro-molecular complex formation is an important factor in molecular recognition and function analysis. We propose an index to compute flexibility loss/gain at complex formation/separation. The algorithm performs a rigidity and/or flexibility evaluation of every node (atom) on the network constituted by all the intra and inter-molecular inter-atomic interactions. Comparison of flexible or rigid molecular regions or domains within the complex with those in the respective isolated monomers leads to the index, quantifying the loss (or gain) in the number of degrees of freedom at complex formation. The index can also be applied to ranking decoys output by docking algorithms.