Large-scale calculations for Os¹⁶⁺ ions near the 5s-4f level crossing

Highly charged ions exhibit enhanced sensitivity to physics beyond the Standard Model, with those near the 5s-4f level crossing being especially promising, but also particularly challenging to treat theoretically. We present large-scale configuration interaction calculations for Os¹⁶⁺ ions to support recent electron beam ion trap measurements. We systematically investigate correlation effects, including inner-shell excitations, basis set convergence, and the role of higher partial waves, and discuss the large uncertainties arising from the complexity of the atomic structure. Our calculations demonstrate that neglecting sufficient inner-shell excitations leads to a substantial overestimation of inter-configuration transition rates, explaining why several previously predicted lines could not be found.