Assessment of the accuracy the experimental energies of the ¹P ^o 1s ²2s6p and 1s ²2s7p states of ⁹Be based on variational calculations with explicitly correlated Gaussians

Benchmark variational calculations are performed for the six lowest states of the ¹P ^o 1s ²2snp state series of the ⁹Be atom. The wave functions of the states are expanded in terms of all-particle, explicitly correlated Gaussian basis functions and the effect of the finite nuclear mass is directly included in the calculations. The exponential parameters of the Gaussians are variationally optimized using the analytical energy gradient determined with respect to those parameters. Besides providing reference non-relativistic energies for the considered states, the calculations also allow to assess the accuracy of the experimental energies of the ¹P ^o 1s ²2s6p and 1s ²2s7p states and suggest their refinement.