The B f ree i coefficients are computed using time-dependent density functional theory (TDDFT) calculations on free atoms in vaccum. V f ree i is the reference volume of the atom calculated using the MP4(SDQ)/aug-cc-pVQZ method in Gaussian 09 and the chargemol code for each of the elements in our model (Table ). To ensure that the dispersion and repulsion coefficients result in a minimum in the L-J potential close to the van der Waals radius of the atom, it can be shown that the A i coefficient can be approximated by: