The FEP result is shown in figure alongside the TI result and these non-Gaussian results are mirrored in TI and FEP. Whilst this result seems to contradict previous work by Paliwal et al. which found free energy distributions to be Gaussian, that work examined much simpler systems of small molecule hydration. In this work we consider "real world" molecular systems which are strongly influenced by anharmonic terms (e.g. van der Waals interactions), not performed in a homogeneous environment (e.g. in a protein) and exhibit more than one dominant conformational substrate . The underlying nonlinearities in the dynamics are what accounts for both the presence of chaos and non-Gaussian statistics. In general a Gaussian distribution of free energy results can only be assumed for harmonic systems or transformations that can be approximated by linear response theory .