Prediction of ²D Rydberg energy levels of ⁶Li and ⁷Li based on very accurate quantum mechanical calculations performed with explicitly correlated Gaussian functions

Very accurate variational nonrelativistic finite-nuclear-mass calculations employing all-electron explicitly correlated Gaussian basis functions are carried out for six Rydberg ²D states (1s²nd, n 6, ⋯, 11) of the ⁷Li and ⁶Li isotopes. The exponential parameters of the Gaussian functions are optimized using the variational method with the aid of the analytical energy gradient determined with respect to these parameters. The experimental results for the lower states (n 3,⋯, 6) and the calculated results for the higher states (n 7, ⋯, 11) fitted with quantum-defect-like formulas are used to predict the energies of ²D 1s²nd states for ⁷Li and ⁶Li with n up to 30.