, can then be used to determine heteroatomic dispersion coefficients. The A ij parameter, which describes the short-range repulsion between overlapping electron clouds, cannot be readily calculated directly from the electron density. Instead it is computed by requiring that the minimum in the interatomic Lennard-Jones potential coincides with the estimated van der Waals radius of the atom in the molecule. This non-bonded parameter derivation scheme requires just one fitting parameter per element (corresponding to the van der Waals radius of the free atom in vacuum).