Again, AMOEBA showed a narrow distribution of projected field strengths approximately 15-20 MV cm -1 more positive than Amber or Charmm fields calculated from the same structures (Fig. ). In terms of the absolute x, y, and z field components, AMOEBA also exhibited strong correlation with the DFT fields, achieving an R 2 of 0.95 across the ten frames extracted from the WT trans-proline trajectory. In contrast, Amber and Charmm fields for the same structural ensemble showed far lower correlations (R 2 = 0.82 and 0.73 respectively), and again, 95% confidence intervals in R 2 for the AMOEBA and additive model comparisons did not overlap, despite the limited dataset size (Fig. ). Next, we performed an identical comparison of electric fields in two mutant CypA systems to add further structural diversity to the dataset and evaluate the transferability of the observed trends to the functionally relevant R55A mutation. We evaluated R55A systems with either cis-proline or trans-proline substrates in order to be equivalent to the WT systems, and fields from three 25 ns AMOEBA simulation of each mutant system were calculated and compared across the AMOEBA, Amber ff14SB and Charmm C36m force fields (Fig. ). For each simulated system the first trajectory was taken to be representative, and a subset of ten equally spaced frames were again used to correlate Cartesian field components with ONETEP. Fields calculated as the mean of those at the C and N peptide atoms (see Fig. ,F for the regression lines). Uncertainty estimated as the 95% confidence interval in regression statistics calculated by resampling the data with replacement (n = 2000)