Triple excitations in perturbed relativistic coupled-cluster theory and   Electric dipole polarizability of groupIIB elements
  We use perturbed relativistic coupled-cluster (PRCC) theory to compute the electric dipole polarizabilities $\alpha$ of Zn, Cd and Hg. The computations are done using the Dirac-Coulomb-Breit Hamiltonian with Uehling potential to incorporate vacuum polarization corrections. The triple excitations are included perturbatively in the PRCC theory, and in the unperturbed sector, it is included non-perturbatively. Our results of $\alpha$, for all the three elements, are in excellent agreement with the experimental data. The other highlight of the results is the orbital energy corrections from Breit interactions. In the literature we could only get the data of Hg {E. Lindroth et al., J. Phys. B 22, 2447 (1989)} and are near perfect match with our results. We also present the linearized equations of the cluster amplitudes, including the triple excitations, with the angular factors. 
S. Chattopadhyay, B. K. Mani, and D. Angom
physics.atom-ph