where C 6,ij , C 8,ij , and C 10,ij are the dispersion coefficients. These coefficients depend on the identity of the pair of interacting atoms. Various methods have been developed to infer these coefficients from experiment or to calculate them from first principles. Ab initioderived dispersion coefficients have been calculated by Cole et al. and Vandenbrande et al. using the Tkatchenko-Scheffler method, where the dispersion coefficients of free atoms were calculated using TDDFT then scaled according to the relative free volume of the atom in the molecule. One straightforward and non-empirical method for calculating the dispersion coefficients of atoms in molecules is the exchange-hole dipole moment (XDM) model. The dispersion coefficients can be defined in terms of expectation values of the square of the instantaneous dipole moments resulting from a reference electron and its exchange hole, d 2 X , and the atom-in-molecule polarizabilities, α.