Crystal field parameters were calculated, using the ab initio density functional electron theory with constraints for the 4f charge and spin density, for isolated rare-earth impurities that substituted for Mg atoms in the superconductor. The crystal field parameter, A66, was extremely small, due to the structure and bonding properties of MgB2 and the crystal field levels were therefore determined almost exclusively by one magnetic quantum number, M.

Theory of Crystal Field States for Heavy Rare-Earth Impurities in MgB2. F.Welsch, R.Kremer, M.Fähnle: Journal of Physics - Condensed Matter, 2002, 14, 10273-9