The XDM method was used to calculate the C 6 dispersion coefficients of 88 molecules from the Virtual Chemistry test set. These density-functional-theory-derived dispersion coefficients were compared to dispersion coefficients defined through the Lennard-Jones potential parameters of the CGenFF, GAFF, and OPLS molecular-mechanical force fields. All three force fields systematically overestimate the molecular dispersion coefficients relative to XDM. The empirical parameterization process likely led to anomalously high C 6 dispersion coefficients due to the neglect of long-range dispersion interactions, higherorder dispersion terms, and induced electronic polarization. Next-generation force fields that account for these terms may return more realistic C 6 dispersion interactions. Improved model for five organic liquids were successfully developed, demonstrating that it is possible to define accurate force fields with physically-realistic C 6 dispersion coefficients while using the standard form of the force field. This procedure can also dramatically reduce the cost of the parameterization by reducing the number of putative parameters to the small subset that are consistent with the XDM-derived dispersion coefficients.