The polarized frozen-core approximation: Application to ionization energies and oscillator strengths for beryllium

We have calculated orbital wave-functions and ionization energies for a large number of singlet and triplet s-, p- and d-states of Be(I), all of which are members of series which converge to the Be(II)²S limit. The simple frozen-core Hartree-Fock model yields energy levels which agree with observations to within 5% for most excited states, but there are substantial discrepancies for the low-lying excited states and especially the 2s- and 2p-states. By including an l-dependent core-polarization potential with an adjustable cut-off parameter, we are able to reproduce the observed ionization energy of the lowest member of each series (except the 2snd ¹D series) and obtain improved agreement for the higher members. Both the unpolarized and the polarized frozen-core energies and orbitals have been employed to calculate electric dipole oscillator strengths. Comparisons with results of refined superposition of configuration calculations for a number of ¹P⁰-¹S transitions suggest that the introduction of a polarization potential yields substantial improvement in the derived oscillator strengths.