Comparison of the MM field components to their DFT equivalents (Fig ) highlighted identical trends to those observed for the WT systems -AMOEBA consistently showed greater correlation and lower systematic errors to DFT fields. Across the four CypA systems individually, no clear trend in terms of overor underestimation was observed with particular force fields. Slopes, intercepts and R 2 varied (Table The observation that additive force fields slightly underestimated the magnitude of the electric field components compared to DFT again appeared to contradict the fact that, when fields were projected perpendicular to the proline ring, field strengths were almost always larger with Amber and Charmm than with AMOEBA (Fig. , Fig. , Fig. ). When the appropriate projections were compared to DFT fields, however, the additive force fields demonstrated a clear systematic error, with y-intercepts of approximately -15 MV cm -1 , and slopes much greater than 1 (Fig. , Table ). Hence the arbitrary projection of the total electric field along a given vector may not assess the general accuracy of an MM force field; instead, our conclusions are better drawn taking into account both magnitude and directionality of the total field, by comparing individual field components. With this in mind, Cartesian field components calculated with AMOEBA correlated well with those of DFT (R 2 = 0.92) and with slope close to unity and intercept close to zero (Table ).