To jugde the accuracy of the potential reference methods two measures are used, the electronic dissociation energy and the RMSD. The RMSD value is an obvious choice when comparing the geometry of small molecules or clusters. The electronic dissociation energy gives a decent estimate of the electronic potential without requiring specific information about "shape" of the potential. the electronic potential, D e , computed as the optimized energy for the cluster from which the optimized energy of the isolated monomers is subtracted: