The SPS is based on four molecular parameters that we consider significant, i.e. an atom hybridisation term (ℎ), a stereoisomeric term (𝑠), a non-aromatic ring term (𝑟) and the number of heavy atom neighbours (𝑛). By design, the score does not account for heteroatom diversity, as it is not intended to capture the total molecular complexity. The terms are calculated for each heavy atom in a molecule, and are then summed across the whole structure according to eq. 3, where the symbol 𝑖 denotes the heavy atom index in a molecule. The total score value is intended to increase with the increase in the relative complexity of the skeletal arrangements.