To categorize the set of PPCPs into rigid or flexible, a new descriptor (F/R) has been introduced accounting for the balance between flexible and rigid structure motives in the molecule. This new descriptor can be understood as an extension of the rotatable bond count (RBC) as it takes the increase of theoretical degrees of freedom by connected rotatable bonds into account and also reflects the "degree of annulation" as a metric for the rigidness of a molecule. The nconf20 as an existing conformational flexibility descriptor showed to be unsuitable for our classification, since it only captures unconstrained gas phase conformational space. The F/R is expressed as: