A comparison of the force-field and XDM homoatomic C 6 dispersion coefficients allows the validity of force-field atom typing to be assessed and reveals whether the systematic overestimation of the dispersion coefficients can be traced to particular elements. The average of the homoatomic dispersion coefficients for each element, and the accompanying standard deviation, were calculated for the full test set and are reported in Table . The force-field dispersion coefficients for each chemically-unique atom in the test set are plotted against the equivalent XDM dispersion coefficients in Figure . A more narrow distribution that is restricted to H, C, N, and O atoms is presented in Figure .