Evaluating the charged small molecule binding to membranes in MD simulations using lipid headgroup order parameters Partition coefficients have been typically used to compare binding affinities of small molecules between MD simulations and experiments. 12,13,60 Such values are available also for some charged small molecules, but not for simple ions such as sodium or calcium. On the other hand, the affinities of ions to membranes have been reported using binding constants based on models assuming a certain binding stoichiometry for lipid-ion interactions, but these values strongly depend on the model used to interpret the experimental data. For example, the reported binding constants of calcium to neutral phosphatidylcholine membranes vary between 7 and 441 M -1 . Furthermore, such binding constants may not even reflect the relative binding affinities of different molecules to membranes. This is exemplified in Fig. where the number of bound tetraphenylphosphonium (reported binding constant of 21 M -1 ) and calcium (reported binding constant of 14 M -1 ) ions per lipid molecule are similar, while the number of bound etidocaine molecules (reported binding constant 11 M -1 ) is larger at similar bulk concentrations.