Triple excitation in relativistic coupled-cluster theory and properties   of one-valence systems Rb and Sr$^+$
  We examine the contributions from triple excitation cluster operators in the relativistic coupled-cluster theory in atoms and ions. For this, we propose a tensor representation of the triple cluster operator. Based on this representaion and using diagrammatic analysis, we derive the linearized coupled-cluster equations for single, double and triple excitation cluster operators. The contributions from the triple cluster operators to the hyper fine structure constants of single-valence systems Rb and Sr$^+$ are analysed using the perturbed triples. 
B. K. Mani and Dilip Angom
physics.atom-ph