Sampling error is shown as the standard deviation of the mean free energy estimates across all five repeats. Where appropriate, an analytical correction was included to account for finite size errors as defined by Rocklin et al. and implemented in rocklinc (tempula/rocklinc), using ABPS v3.0 for Poisson-Botlzmann calculations . It should be noted that no additional long range dispersion corrections (e.g. EXP-LR) were employed here.