However, counting based permeation methods depend on observing crossing events during unbiased simulations, whereas we see very few transition events, creating large uncertainties as to the potential flux within our simulations. Furthermore, we do not see a single case where these small molecules come out of the membrane, as would be expected during the process of forming root exudates. The REUS approach uses biased simulations to circumvent these limitations, and allow for robust permeability estimates.