A parameter in a compact description of intra-atomic electron repulsion within a configuration. and control relative d-shell multiplet energies; is not the octahedral splitting or a rotational constant.
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The Racah parameters are a set of coefficients that appear in the theoretical treatment of the interactions among the electrons in a multi-electron atom or ion, particularly when discussing the effects of electron-electron interactions on the energy levels and the spectra of transition metal complexes and rare-earth ions. These parameters are named after the physicist Giovanni Racah.