Multireferent superposition-of-configurations calculations of core-correlation effects on energy levels and oscillator strengths: Be andB+

The superposition-of-configurations (SOC) method is used to calculate intershell and core-correlation corrections to energy levels for the low-lying states of Be and ${\mathrm{B}}^{+}$ belonging to the configurations 2${\mathit{s}}^2$, 2s2p, and 2${\mathit{p}}^2$ and the singlets of 2s3s and 2s3d. The wave functions are also used to calculate the oscillator strengths for all allowed transitions among these states. Transition energies are calculated with an accuracy ranging from 10 to 50 ${\mathrm{cm}}^{\mathrm{-}1}$ and the oscillator strengths are estimated to be accurate to 1–2 %. The SOC wave functions are single- and double-substitution calculations based on a multiconfiguration referent, where the reference set is derived from a two-electron natural-orbital transformation of valence-shell-correlated SOC functions. It is found that a fairly substantial reference set is necessary to obtain these results, ranging from three configuration-state functions (CSFs) for 2s2p ${}_{}{}^3{}_{}{}^{}$P and 2${\mathit{p}}^2$ ${}_{}{}^3{}_{}{}^{}$P to 11 CSFs for 2s3s ${}_{}{}^1{}_{}{}^{}$S. Intershell correlations are necessary for both energy differences and oscillator strengths, but core correlations with angular symmetries higher than l=1 (p orbitals) are not.