Abstract
The need to include both spin eigenfunctions in the wave function in order to consider the hyperfine structure of lithium is demonstrated. An approximation to the orbital functions is first determined using only the two-determinant spin eigenfunction. The energy functional obtained from a linear combination of the two spin eigenfunctions is then minimized only with respect to their relative weighting. This process simultaneously improves the energy and the hyperfine structure over their Hartree-Fock (HF) values. Similar calculations are performed for members of the isoelectronic sequence, up to Z=36. For large Z, the relative amount of the less significant spin eigenfunction appears to behave as Z-12/.

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