XDM predicts nitrogen atoms to have dispersion interactions of moderate strength, with an average C 6 coefficient of 15.9 a.u. Nitrogen atoms have one of the broadest distributions of C 6 coefficients for this test set; electron-poor amide nitrogens have particularly low dispersion coefficients (e.g. 13.4 a.u. in N-methylformamide), while electron-rich alkyl nitrogens have high dispersion coefficients (e.g. 17.2 a.u. in ethane-1,2-diamine). The XDM dispersion coefficients for these molecules are presented in Figure . The dispersion coefficients for nitrogen atoms are systematically overestimated by the GAFF and OPLS force fields, which give average C 6 values of 58.2 a.u. and 59.6 a.u., respectively. The CGenFF coefficients for amines are somewhat closer to the XDM values (12-34 a.u for CGenFF vs 14-17 a.u. for XDM), but the coefficients for amide nitrogens (i.e., NG2S0, NG2S1, NG2S1, NG2S2, and NG2S3 atom types) are assigned an anomalously large value (74.5 a.u.). The XDM coefficients for these atoms are actually lower than the average for nitrogen atoms, ranging from 12-15 a.u. The CGenFF amide-nitrogen parameters are shared with the protein backbone of the CHARMM36 force field. The Lennard-Jones parameters for these amide nitrogens were adjusted to provide more accurate backbone hydrogen bonding, but this appears to have caused the C 6 dispersion coefficient to be anomalously high.