Molecular-mechanical force fields use the same dispersion interaction parameters for all atoms of the same type. In some cases, the XDM dispersion coefficients spanned a significant range of values for atoms of the same type, indicating that the Lennard-Jones parameters are not optimal for some atoms. This is particularly true for the GAFF and OPLS force fields, which have fewer variants of Lennard-Jones parameters than the CGenFF force field. One example where molecular-mechanical atom typing breaks down are methyl carbon atoms in electron-rich versus electron-poor environments, which XDM predicts to have significantly different dispersion coefficients. The introduction of new atom types motivated by groupings of C 6 values may improve the accuracy of force-field models.