Towards the "puzzle" of Chromium dimer Cr$_2$: predicting the Born-Oppenheimer rovibrational spectrum

Abstract: The experimentally-observed non-trivial electronic structure of the Cr$2$ dimer has made the calculation of its potential energy curve a theoretical challenge in the last decades. By matching the perturbation theory at small internuclear distances $R$ and the multipole expansion at large distances $R$ (supposedly both of asymptotic nature), and by adding a few Rydberg-Klein-Rees (RKR) turning points, extracted from experimental data by Casey-Leopold (1993), the analytic form of the potential energy curve for the ground state $X^1\Sigma^+$ of the Cr$_2$ dimer is found for the first time for the whole range of internuclear distances $R$. This has the form of a two-point Pad\'e approximant and provides an accuracy of 3-4 decimal digits in 29 experimental vibrational energies. The resulting ground state $X^1\Sigma^+$ potential curve supports 19694 rovibrational states with a maximal vibrational number $\nu\text{max}=104$ at zero angular momentum and with a maximal angular momentum $L_\text{max}=312$ with energies $> 10^{-4}$ { hartree}, and additionally 218 weakly-bound states (close to the dissociation limit) with energies $< 10^{-4}$ { hartree}.