Recombination rates for 5f^N uranium ions using the analogous 4f⁴ Nd III as a guideline and their effect on Kilonova nebular spectra

Dielectronic recombination (DR) is expected to be the dominant recombination process during the non–local thermodynamic equilibrium (NLTE) phase of kilonovae, yet reliable DR data remain unavailable for most heavy ions. Current spectral models therefore rely on simplified recombination prescriptions, introducing significant uncertainties into predicted spectra. We present an optimization strategy for open f-shell ions using AUTOSTRUCTURE, targeting uranium ions U II–IV relevant to actinide-producing kilonovae. As a benchmark case, calculations are performed for Nd III to validate the treatment of the f-shell structure and its impact on DR. The resulting DR rate coefficients are of order 10 ⁻¹⁰–10 ⁻¹² cm ³ s ⁻¹ over temperatures relevant to kilonova plasmas. The optimized rates are implemented into the radiative-transfer code SUMO, and their impact on kilonova spectra is studied. The Nd III benchmark demonstrates that refinements to the atomic structure can produce measurable changes in spectral features supporting the atomic structure optimization performed for U II–IV.