Fig. shows J90 plotted as a function of this FOM. There is a stronger correlation (ρ = 0.680) between J90 and this FOM than for any other parameters considered so far. Higher 𝑘 ! " kRISC/kISC leads to higher J90. Accordingly, maximising 𝑘 ! " kRISC/kISC is a better strategy for improving efficiency roll-off than any other options discussed so far. There is still some spread, but it is significantly reduced, and we discuss reasons for this later. It is interesting to apply this FOM to recent attempts to increase kRISC by incorporating heavy atoms into the molecule to increase spin-orbit coupling .