The 2S Rydberg series of the lithium atom. Calculations with all-electron explicitly correlated Gaussian functions

•High-accuracy calculations for the 12 2S Rydberg states of Li.•Finite-nuclear-mass approach is used.•Isotope shifts of the transition energies are calculated.•All-electron explicitly correlated Gaussian functions are used.•The non-linear parameters of the Gaussians are optimized.•Analytical energy gradient is used in the optimization.•For the 10s, 11s, 12s, and 13s states the present calculations are the first ever.