In order to examine which FF describes the TDM-derived electric �ields more accurately, we determined fractionweighted FC≡N,MD's (Table ; values with a 3.5 AÅ H-bond cutoff are nearly identical, see Table ). The electric �ield magnitudes have the qualitative trend of F96oCNF ≈ F62oCNF < F28oCNF < F92oCNF for both FFs, which is similar to the ordering of the FC≡N,TDM's (Tables ). However, the FC≡N,POL MD's are consistently larger than the FC≡N,FC MD's by up to ~ 40 MV/cm and by factors as big as 5 (a much smaller change in FC≡N,FC MD's and FC≡N,POL MD's was observed for oTN in solvents, see Figure and). As such, when we quantitatively compare the correlations between FC≡N,FC TDM's/FC≡N,FC MD's and FC≡N,POL TDM's/FC≡N,POL MD's, substantial differences are observed depending on the FFs (Figure ). TDM-derived and MDderived electric �ields for F62oCNF and F96oCNF are in reasonable agreement with both FFs. In contrast, an analogous comparison for F28oCNF and F92oCNF, the two nitriles with signi�icant H-bonding in both FC and POL MD, suggests the TDM-derived �ields are better recapitulated with POL MD. Weighted linear regressions for comparisons using FC MD and POL MD have slopes of 0.56 ± 0.04 and 1.17 ± 0.09, respectively (Figure ), a signi�icant approximately twofold difference; the regressions have intercepts of -3.5 ± 0.8 and -5.3 ± 2.7, an insigni�icant difference. In considering the 2σ con�idence intervals (CIs) for the �its (blue-shaded regions in Figure ), the correlation of FC≡N,FC TDM's and FC≡N,FC MD's falls markedly outside the ideal correlation with a slope of one (black lines in Figure ), whereas the CI for FC≡N,POL TDM's and FC≡N,POL MD's indicates a good match between experiments and MD simulations (Figure ). The best �it line and CI for FC≡N,FC TDM's and FC≡N,FC MD's suggest the FC FF substantially underestimates large electric �ields. A related result was documented in calculations of the electric �ield experienced by a functionally important carbonyl group at the active site of the enzyme ketosteroid isomerase, where the �ield is < -100 MV/cm. Our results suggest that such underestimation can already occur at moderate electric �ield strengths of < -10 MV/cm; thus, POL MD is more capable of properly sampling the variants' diverse noncovalent interactions and environments. ). Additionally, the width of the θCNX distributions is 1.4 times wider in FC MD for F28oCNF and 2.5 times wider for F92oCNF (Table ). The �its indicate the H-bond angle sampling is particularly sensitive to the FF. Furthermore, nitriles in POL MD adopt closer, more head-on H-bonds, which aligns with chemical intuition given that the nitrile's lone pair of electrons reside along the -C≡N axis. ). Counts in each panel are normalized. Snapshots of F28oCNF H-bonding with water at geometries with the average θCNX and dNX values from FC and POL MD are shown in (E) and (F), respectively. In analogy, representative snapshots at the average H-bond distance and angle for F92oCNF-T90 H-bonds from FC and POL MD are shown in (G) and (H), respectively (Table ). The snapshots illustrate the FFs' impacts on H-bonding. Note that an alternate T90 rotamer was also observed in FC MD which adopts very similar average θCNX and dNX values (Figure ; Table ). In (E)-(H), carbon is shown in gold (F28oCNF PYP) or red (F92oCNF PYP), nitrogen is shown in blue, oxygen is shown in bright red, and protons are shown in white. To evaluate the origin(s) of the changes to the contour plots, we identi�ied whether the nitrile H-bonds were with the protein or the solvent (Table ) and assessed whether the FF affected this. We found F28oCNF's predicted H-bond donor identity is not sensitive to the FF while F92oCNF's is. F28oCNF H-bonds with the solvent 91 -95% of the time in both FFs, indicating the changes to the contour plots are due to altered water geometries around the nitrile (representative snapshots are shown in Figure ). In contrast, F92oCNF H-bonds with the protein 50% of the time in FC MD but 95% of the time in POL MD (Table ). More speci�ically, F92oCNF H-bonds with T90's hydroxyl, the closest crystallographic H-bond donor (Figure ), in 41% of the FC MD H-bonding frames compared to 90% for POL MD, a larger than twofold difference (Table ). Since F28oCNF-water H-bond sampling was FF-dependent, we investigated the geometries of F92oCNF's H-bonds with either water or T90 to see if altered sampling contributes to the differences in Figure ,D in addition to the changes in H-bond donor identities. The average H-bond distance and angle for F92oCNF H-bonding with water in FC MD (Figure ) are 3.07 AÅ and 129°, quite similar to the values for F28oCNF (Table ). Unfortunately, we cannot provide a comparison with POL MD due to the sparsity of H-bonding. Focusing on the F92oCNF-T90 H-bond, this interaction in FC MD has an average θCNX value of 154° (and average dNX values of 2.91 -2.95 AÅ for two observed rotamers, shown in Figure and Figure ; see Figure for contour plots; Table ). The �it to the contour plot for F92oCNF-T90 H-bonds in POL MD (Figure ) shows that the average H-bond distance is the same (2.93 AÅ ) as in FC MD but the average angle is larger (170°; Figure ), indicating an analogous change to the H-bond angle as for F28oCNF (Table ). However, F28oCNF predominately H-bonds with the labile solvent, while F92oCNF and T90 are covalently linked, making the change in the F92oCNF-T90 H-bond angle particularly striking. This analysis indicates that both H-bond partner fractions and H-bond geometries can be altered by the FF, as in the case of F92oCNF, and that in silico descriptions of nitrile H-bonding can be affected regardless of donor identity.