The molecular C 6 dispersion coefficients for the molecules in the test set were calculated using XDM and the force-field parameters. The correlations between the molecular XDM dispersion coefficients and the molecular dispersion coefficients for the CGenFF, GAFF, and OPLS force fields are plotted in Figure . There is a systematic trend for the force fields to overestimate the molecular dispersion coefficients, with regression coefficients of 1.53, 1.59, and 1.55 for the CGenFF, GAFF, and OPLS force fields, respectively. This suggests that the dispersion interactions in molecular liquids will be overestimated by these molecularmechanical force fields.