Carbon atoms have sizable dispersion coefficients due to their high polarizability ( C 6,XDM = 22.0±1.3 a.u.). Bonding partners have the largest effect on the dispersion coefficient of carbon atoms. As shown in Figure , the XDM dispersion coefficient of the electron-poor tertiary carbon of t-butylamine is particularly low, with a value of 18. The force fields all overestimate the majority of the carbon dispersion coefficients, with the average values roughly a factor of two higher than the XDM values ( Conversely, within some force-field carbon atom types, the XDM dispersion coefficients show significant variation. For instance, the CGenFF CG331 atom type represents all methyl groups. This type includes the electron-deficient methyl group of N-methylformamide, which has an XDM dispersion coefficient of 20.8 a.u. (Figure ). At the other extreme, the XDMcomputed dispersion coefficient is 23 a.u. for carbons bound to bromine or sulfur, as in dimethyl sulfide or bromoethane. The XDM dispersion coefficients also span a significant range for aromatic carbons (Figure ), such as those represented by the CGenFF CG2R61 atom type. The computed XDM coefficients for this atom type range between 21.5-25.5 a.u. The dispersion coefficients of carbon atoms in electron-rich heteroaromatics, like furan and pyrrole, range between 24-25 a.u., which is incrementally higher than the value of 23.6 a.u. calculated for benzene. There is also a significant variation in the C 6 coefficients of aromatic carbons due to substituent effects. For example, the ipso carbon of anisole has a C 6 coefficient of 21.9 a.u., but the para-carbon has a dispersion coefficient of 23.8 a.u. As force fields use the same dispersion coefficients for all atoms of the same type, the calculated strength of interatomic dispersion interactions could be in error by up to 10% due to the use of atom types.