are least-squares fitted to reproduce the ab initio interaction energies per monomer of dimers and clusters up to hexamer. The ab initio interaction energies were calculated using the cluster geometries from Ref., and relaxing them with the hybrid functional PBE0 extended to include the MBD@rsSCS dispersion correction. All these calculations have been performed using a plane wave representation of valence electrons and the use of projected augmented wave (PAW) representation of the e↵ect of the inner electrons. As proposed by Ambrosetti et al, the value of the adjustable parameter was set to 0.85 in all the PBE0 + MBD@rsSCS calculations. The convergence criteria for the cluster energies were set to 10 6 eV for the SCF and 0.005 eV/ Å for the geometry optimizations. See Figure (and Table in the Supporting Information) for the DFT optimized geometries, and their corresponding interaction energy.