where R e is the equilibrium distance, and D e is the dissociation energy, and C k is a set of n fitting coefficients. One can choose the order n to fit the potential function. We have chosen n=10 similar to Gomes et al. as we found that at higher orders the function were prone to overfitting. Using the extended Rydberg function we fitted the ground state (X: 0 + g ) and the two lowest lying excited states (A ′ : 2 u and A: 1 u ), which are all bounded. Details on the fitting procedure as well as the obtained fitting coefficients can be found in the SI.