details of computational strategies can be found in Ref. [@Bieron2009; @Li2012].

In our calculations, [ we adopt the extended optimal level (EOL) scheme to optimize the two states of the \[Xe\]$4f^{13}6s^2$ configuration simultaneously.]{} The electron correlations in the $4f$ and $6s$ valence subshells and the correlations between electrons in the valence and $n=3,4$ core subshells were accounted for by CSFs generated by the SD replacement of the $n \ge 3$ occupied orbitals in the reference configuration with the virtual orbitals. The double replacements were restricted to only a single electron of the core subshells being promoted into the virtual orbitals at a time. The final set of virtual orbitals is composed of five orbitals per each of the $s, p, d, f, g, h, i$ angular momenta. The magnetic octupole and electric hexadecapole hyperfine interaction constants were calculated by an extended version [@JGLiunpublished] of the HFS92 code [@HFS92] based on the GRASP package [@FroeseFischer2018b] Our results, labelled as MCDHF, are presented in Table \[Table:HFS\].

Evaluation of theoretical uncertainties {#Sec:TheoreticalErrors}
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We start with a comparison of our computed values for $A$ and $B$ HFS constants with the previously published results and then assess our theoretical accuracy.

Comparing our computed values 