The XDM C 6 coefficients for oxygen are generally smaller than those of carbon and nitrogen ( C 6,O = 12.6 a.u.) and span a fairly narrow range of values (σ O = 1.4 a.u.). Electronrich carbonyl oxygens have coefficients in the 13-15 a.u range, while alcohols have smaller coefficients, between 12-12.5 a.u. The oxygen C 6 parameters from the three force fields are considerably larger than the XDM values. CGenFF has the smallest oxygen dispersion coefficients, with an average of 34.6 a.u. Carbonyl and ether oxygens are only moderately overestimated (C 6 < 30 a.u.), but the coefficients for alcohols are 4-5 times larger than the XDM values (e.g., 53.9 a.u. for OG311). The average C 6 coefficients of the GAFF and OPLS force fields are systematically higher than the XDM values across most oxygen atom types, by factors of 3.5 and 3, respectively. For these force fields, the strengths of dispersion interactions for oxygen atoms are similar to those for carbon atoms, despite oxygen's much lower polarizability.