A relativistic crystal field for S-state f electron ions

In order to improve the theoretical reproduction of the splitting of the en ergy levels for S-state f electron ions, a model in which the relativistic effects are included in an effective way is used. Although the effective op erators act within the spin-orbital space, and the radial integrals involve the large and small components of the relativistic theory, all the require d matrix elements are evaluated within the intermediate coupling scheme. Th e approach is illustrated by the results of numerical calculations performe d for the representative ions: Gd3+, Cm3+, and Bk4+. The results of the ana lysis support the expectation that for such large systems the relativistic effects indeed play an important role. In addition, it is concluded that th e fitting procedure applied for the determination of the crystal field para meters has to be performed within the parametrization scheme that includes the relativistic weights for the various parameters.