The idea behind this equation is to compare the overlap of a certain site-state pair with the smallest site-state overlap for the given pair of fragments, and to request that the ratio of those two is not lower than a threshold t O . It is reasonable to keep this threshold slightly lower than 1. In the limiting case of t O = 1, each sitestate pair in an ESD would have to have the same overlap as the least-overlapping site-state pair for the respective pair of fragments, in order for the ESD to be accepted in the ECI basis.