The ab initio computational basis of the angular overlap model and its phenomenological effectiveness in the 5f ion compounds

Abstract

Both the fundamental assumptions of the angular overlap model (AOM) and its common simplifications are shown to have a sound basis in the ab initio calculations of the crystal-field effect in uranium (III), (IV), (V), neptunium (IV) and plutonium (IV) ions in various crystals. The traditional sigma - pi approach is confirmed as a well-aimed initial step towards an interpretation of the effect. The specific role of the delta -contribution as an important lattice-dependent correction indicates the necessity of its inclusion in the model. The practical two-step interpretation method that naturally emerges from the ab initio calculations is recommended. The effectiveness of the AOM for actinide ionic systems is illustrated using available phenomenological results. The universal character of the AGM parametrization and simple rules concerning the relationships between the parameters-their mutual hierarchy, spectrochemical ordering and the definite dependence on the metal ionization degree-are pointed out and discussed in the light of other phenomenological models including the Newman's twin superposition model.