for fields calculated at the substrate peptide N and C atoms respectively (Fig. ). Correlations between Amber or Charmm fields and ONETEP were far smaller, with R 2 no higher than 0.60 (Fig ). Moreover, regression lines for the additive force fields appeared to have a large positive intercept for fields calculated at the peptide N and C atoms individually. An identical trend was observed when comparing QM and MM results after averaging the N and C atom field components for each frame (Fig. ). Again, AMOEBA fields gave close agreement to those calculated with ONETEP (R 2 = 0.90), with slope close to unity and without the substantial positive y-intercept and broad confidence intervals exhibited by Amber and Charmm fields (Table ).