where K is the partition coefficient between the membrane and water phase (considered equal to 1, if there is no extra partitioning into the slice due to no energy barrie), D is the diffusion coefficient (3×10 -9 m 2 /s for water self-diffusion, taken from ref. ), and L is the thickness of the membrane (the thinnest experimentally possible membrane with a thickness around 4 nm). Then, P is 75 cm/s, and logPerm is +1.9 log units. Since drugs are bigger molecules than water, they usually have diffusion constant lower by order of magnitude or more (e.g., ibuprofen has D = 5.5×10 -10 m 2 /s) , we have set this simplified limit to 0 log unit.