Sampling of free energies agrees well between replicas with a mean error of 0.79 ± 0.38 kcal/mol, although approximately 22% of the estimates show an error bar greater than 1 kcal/mol, with the highest value being 1.93 kcal/mol. This is generally on a par with sampling errors shown by other absolute alchemical free energy studies and matching a similar level of uncertainty as the cycle closure errors shown in the Steinbrecher et al. 2015 fragment FEP+ study . Whilst the error in experimental measurements was not provided for all our systems, our sampling errors are for the most part approximately twice as large as the ~0.5 kcal/mol limit which we might expect from experimental measurements .