where C free 6,a is the isolated-atom C 6 coefficient, which can be determined using time-dependent DFT calculations. The scaling ratio in Eq. ( ), which measures how the volume of atom a is compressed by its molecular environment, is determined by Hirshfeld partition. Pairwise coefficients C 6,ab can be determined using a combination rule, exploiting a volume scaling analogous to Eq. ( ) for the atoms-inmolecules polarizabilties ᾱeff a . 1 It should also be noted that the relationship in Eq. ( ) is simply a model, based on a long-presumed relationship between polarizability and volume. Recent work suggests that ᾱTS a = r 4 a r 4 free a ᾱfree a (30) may be the more fundamental relationship, leading to improvements in the vdW-TS method. 144 Thus, the model may be improvable.