Benchmark calculations of the energy spectrum and oscillator strengths of the beryllium atom

We have performed a comprehensive set of benchmark variational calculations for the ground and 19 lowest bound excited singlet S and P states of the beryllium atom. The nonrelativistic wave functions of the states that represent the motion of the nucleus and the four electrons around the center of mass of the atom have been expanded in terms of up to 17000 all-particle explicitly correlated Gaussians that were extensively optimized for each state independently. The leading relativistic and QED corrections to the energy levels have been computed in the framework of the perturbation theory and they explicitly include the nuclear recoil effects. This has allowed us to reach sub-wavenumber accuracy for all 20 states. Using the obtained energy levels and the corresponding wave functions, we have computed the transition frequencies, transition dipole moments, and oscillator strengths. A comparison with the experimental data aggregated in NIST ASD database shows very good agreement, except for the 7-th excited singlet P state, for which the available experimental data is likely to contain an error.