Even though the repulsive branch of our model is correct up to about 400 kJ/mol, as they appear for small interatomic distances (intermolecular ∆L O-H for example) our model fails at some point. We observe strong oscillations in the predicted interaction energies (Figure ). This is due to the breakdown of the ECP basis-set used in the ab initio calculations used to fit the electronic densities of the noninteracting molecules. Fitting the electron density from ab initio calculations that use a full electron basis-set cannot be done accurately for most atoms simply using three s-type Gaussian basis-functions. We thus stayed with the simple ECP basis since the work should serve as a proof of concept of our method. Fig. . Interaction energy of a water dimer in the O -H reaction coordinate shown in Figure (θ = -π/4) obtained from CCSD(T)/cc-pVTZ calculations and our calibrated order ten model. The interaction energies are extrapolated to shorter interatomic distances with our model.