The correlation between the XDM and Drude molecular C 6 coefficients is poorer than for the non-polarizable models; the regression coefficient is 1.67 and the coefficient of determination is 0.81 (Figure ). The distribution of atomic dispersion coefficients for the Drude force field is extremely wide, with values ranging from 20 a.u. to more than 100 a.u. for some C, N, and O atom types (Figure (a)). The dispersion coefficients for sulfur, spanning between 200 and 300 a.u., are also overestimated by a large margin. As with the nonpolarizable force fields, the systematic overestimation of the dispersion coefficients is likely due to neglect of higher-order C 8 terms and premature truncation of the dispersion-energy summation.