Including of triple excitations in the relativistic coupled-cluster   formalism and calculation of Na properties
  A practical high-accuracy relativistic method of atomic structure calculations for univalent atoms is presented. The method is rooted in the coupled-cluster formalism and includes non-perturbative treatment of single and double excitations from the core and single, double and triple excitations involving valence electron. Triple excitations of core electrons are included in the fourth-order of many-body perturbation theory. In addition, contributions from the disconnected excitations are incorporated. Evaluation of matrix elements includes all-order dressing of lines and vertices of the diagrams. The resulting formalism for matrix elements is complete through the fourth order and sums certain chains of diagrams to all orders. With the developed method we compute removal energies, magnetic-dipole hyperfine-structure constants A and electric-dipole amplitudes. We find that the removal energies are reproduced within 0.01-0.03% and the hyperfine constants of the 3s_1/2 and 3p_1/2 states with a better than 0.1% accuracy. The computed dipole amplitudes for the principal 3s_1/2 - 3p_1/2;3/2 transitions are in an agreement with 0.05%-accurate experimental data. 
Sergey G. Porsev and Andrei Derevianko