Correlation Effects in Complex Spectra. I. Term Energies for the Magnesium Isoelectronic Sequence

Abstract

We present a method for the ab initio calculation of configuration interaction in many‐electron spectra. It is based on the expansion of the total electronic wavefunction in terms of Hartree—Fock—Slater determinants, as calculated by the programs of Herman and Skillman. This procedure has been applied to several of the low‐lying excited configurations of the magnesium isoelectronic sequence which are noted for the presence of violent perturbations causing term values arising from a single configuration to be distorted and/or inverted. Convergence to the observed energy separation between multiplet term values appears to be quite rapid, and the method offers promise as a means of including electronic correlation in the calculation of atomic properties. For example the calculated energy difference between the ¹D and ³D terms arising from the 3s3d configuration differs from the observed energy difference by 24 cm⁻¹ out of 1554 cm⁻¹ for Mg I, 720 cm⁻¹ out of 14 540 cm⁻¹ for Al II, and 1969 cm⁻¹ out of 22 820 cm⁻¹ for Si III. Following a discussion of the nature and degree of configurational mixing with increasing nuclear charge along the magnesium isoelectronic sequence, an examination is made of the significance that can be attached to configurational assignments. An alternative nomenclature is proposed.