Application of Relativistic Coupled-cluster Theory to Heavy Atomic   Systems with Strongly Interacting Configurations: Hyperfine Interactions in   $^{207}Pb^+$
  This work presents a first time accurate calculation of the magnetic dipole hyperfine structure constants for the ground state and some low-lying excited states of Pb$^+$. By comparing different levels of approximation with experimental results, we demonstrate the importance of correlation effects which reach beyond lower order relativistic many body perturbation theory. Employing relativistic coupled-cluster theory we obtain a quantitative understanding of the core-polarization and correlation effects inherent in this system and observe completely different trends compared to $Ba^+$. 
Bijaya K. Sahoo, Rajat K. Chaudhuri, B.P. Das, Holger Merlitz,   Debashis Mukherjee