In figure (a) we observed a rapid change for the gradient of the potential with respect to the lambda parameter which controls the LJ interactions of the disappearing alchemical region. Rapid changes such as this may result in poor accuracy for numerical integration and, without due care, this is known to be a weakness of the TI method . This rapid change of the gradient is characteristic of all the transformations where we observe differences in the TI and FEP result. Moreover, these rapid changes are lessened or do not exist in the OpenMM case, explaining why no differences are observed.