The force-field dispersion coefficients for hydrogen atoms are systematically underestimated relative to the XDM values. As shown in Table , the average XDM C 6 coefficient for hydrogen atoms is 2.5 a.u., which is higher than the averages of 1.7 a.u., 1.2 a.u., and 1.7 a.u. for the CGenFF, GAFF, and OPLS force fields, respectively. As hydrogen atoms have relatively weak dispersive and repulsive interactions in comparison to their parent atoms, Lennard-Jones parameters of hydrogen atoms are sometimes assigned somewhat arbitrary parameters or are even neglected in some cases.