Explicitly correlated Gaussian calculations of the ²P ^o Rydberg spectrum of the lithium atom

Accurate quantum-mechanical nonrelativistic variational calculations are performed for the nine lowest members of the ²P ^o Rydberg series (1s ²np ¹, n 2, , 10) of the lithium atom. The effect of the finite nuclear mass is included in the calculations allowing for determining the isotopic shifts of the energy levels. The wave functions of the states are expanded in terms of all-electron explicitly correlated Gaussian functions. The exponential parameters of the Gaussians are variationally optimized with the aid of the analytical energy gradient determined with respect to those parameters. The calculated state energies are compared with the available experimental data.