*Article* **Level Structure and Properties of Open** *f***-Shell Elements**

**Stephan Fritzsche 1,2,3**


**Abstract:** Open *f*-shell elements still constitute a great challenge for atomic theory owing to their (very) rich fine-structure and strong correlations among the valence-shell electrons. For these medium and heavy elements, many atomic properties are sensitive to the correlated motion of electrons and, hence, require large-scale computations in order to deal consistently with all relativistic, correlation and rearrangement contributions to the electron density. Often, different concepts and notations need to be combined for just classifying the low-lying level structure of these elements. With JAC, the Jena Atomic Calculator, we here provide a toolbox that helps to explore and deal with such elements with open *d*- and *f*-shell structures. Based on Dirac's equation, JAC is suitable for almost all atoms and ions across the periodic table. As an example, we demonstrate how reasonably accurate computations can be performed for the low-lying level structure, transition probabilities and lifetimes for Th2<sup>+</sup> ions with a 5 *f* 6*d* ground configuration. Other, and more complex, shell structures are supported as well, though often for a trade-off between the size and accuracy of the computations. Owing to its simple use, however, JAC supports both quick estimates and detailed case studies on open *d*- or *f*-shell elements.

**Keywords:** atomic structure; level and excitation energies; hyperfine splitting; isotope-shift; Jena Atomic Calculator; *LSJ* level notation; open *d*- and *f*-shell ion; relativistic; transition probability
