The first two terms account for the flexibility of a molecule by summing the number of rotatable bonds (rotatable bond count, RBC) (single bonds between non-hydrogen atoms, that change the conformation of the molecule when rotated) and the number of atoms common to more than one rotatable bond (NconnectedRB). The latter term accounts for the rigid units in a molecule by summing the number of rings and by multiplying the "ring multiplicity" (number of rings in which the atom is present) of each atom that is bound in more than one ring by the number of atoms that have that connectivity. From this term, the number of atoms in only one ring divided by the number of rings in the molecule is subtracted. The sign of the resulting descriptor determines whether a molecule is classified as rigid or flexible. As an example, this shall be demonstrated for SMX and CBZ. Figure shows the structures and F/R of all molecules considered so far.