text: Some suggest that better metrics for ligand efficiency are 
  percentage/potency efficiency index (PEI), binding efficiency index (BEI) and surface-binding efficiency index (SEI) because they are easier to calculate and take into account the differences between elements in different rows of the periodic table. It is important to note that PEI is a relative measure for comparing compounds tested in the same conditions (e.g. a single-point assay) and are not comparable at different inhibitor concentrations. Also for BEI and SEI, similar measurements must be used (e.g. always using pK).
  :PEI = (% inhibition at a given compound concentration as fraction: 0 – 1.0) / (molecular weight, kDa)
  :BEI = (pK, pK, or pIC) / (molecular weight, kDa)
  :SEI = (pK, pK, or pIC) / (PSA/100 Å)
  where pK, pK and pIC is defined as −log(K), −log(K), or −log(IC
  ), respectively. K and IC in mol/L.
  The authors suggest plotting compounds SEI and BEI on a plane and optimizing compounds towards the diagonal and so optimizing both SEI and BEI which incorporate potency, molecular weight and PSA.
  There are other metrics which can be useful during hit to lead optimization: group efficiency (GE), lipophilic efficiency/lipophilic ligand efficiency (LipE/LLE), ligand lipophilicity index (LLE) ligand efficiency dependent lipophilicity (LELP), fit quality scaled ligand efficiency (LE), size independent ligand efficiency (SILE).