A Study of 15 Singlet Rydberg Series of HeH and Their Correlation with the Rydberg Series of Li

We theoretically study 37 Rydberg states of HeH derivable from the He(1 s 2 ) + H( nl ) configuration with n ≤ 5 along with three states with n = 6. The multireference configuration interaction method is used to calculate their electronic energy curves ( EECs ) for the internuclear distance R from 0 to 28 a 0 , which are regarded as the united‐atom ( UA ) and separated‐atoms ( SA ) limits, respectively. The 15 Rydberg series ( nsσ , npσ , ndσ , nfσ , ngσ , ndδ , nfδ , ngδ , ngγ , npπ , ndπ , nfπ , ngπ , nfϕ , and ngϕ ) are identified. Bingel's perturbation theory is used to explain the role of the core part and the Rydberg orbitals in the behavior of EECs at small R . Smith's theory of diabatization is directly solved to obtain the diabatic quantum defect curves ( QDCs ). From the correlations in diabatic QDCs between the UA and SA limits, the following order of orbital energies in the equilibrium region is found: npσ < nsσ < npπ < ndσ < ndπ < nfσ < nfπ < nfδ < nfϕ < ndδ , except 5 dδ < 5 fϕ . This order is identical to that in one‐electron molecular systems at small R , except for the nsσ and npπ series. The correlation rules are n UA = n SA + 1 for the npσ series and n UA = n SA for other series; additionally, l UA = l SA for all cases except for the 2 pσ − σ 1 s correlation. A reversal of the diabatic dipole moment at avoided crossing points of the nsσ and npσ series is observed, and explained by the behaviors of ns and np wavefunctions of H at small R .