Computational methods (COSMOPerm, PerMM) can predict the permeabilities of a wide range of molecules. Their average LogP of around 1.8 and MW of around 260 Da is lower than for experimental methods. This has at least two co-occurring reasons. Firstly, using computational methods, it is possible to calculate small molecules (oxygen, water, carbon dioxide) that are commonly not measured in permeation assays. Secondly, the calculation difficulty scales with molecular weight, and therefore, it is more expensive to calculate molecules with larger MW.