Preparing Small Gaussian Basis for Highly Accurate Ab Initio Description of Lithium Rydberg States

A new small and highly accurate Gaussian basis set has been developed for the ab initio description of $^2$Li Rydberg excited states up to $n = 7$ and S, P, D, F, and G angular momentum symmetry, and the appropriate optimization protocol is presented. The obtained Rydberg excitation energies are compared with results from other highly accurate approaches. At the EOM-CCSD level of theory the new basis exhibits higher than $10^{-2}$ eV accuracy of the excitation energies and the ionization potential and provides superior results than an analogous universal Gaussian basis set, while utilizing even smaller number of basis functions. Plots of the 1-dimensional Rydberg-orbital cuts reveal a regular nodal structure along the logarithmic scale of the atomic radius reaching tens of angstrom far from the nucleus. The presented Rydberg basis set generation methodology is an important step towards routine ab initio Rydberg-state related investigations of more complex systems, such as large atoms and polyatomic molecules.