Case studies of atomic properties using coupled-cluster and unitary   coupled-cluster methods
  The magnetic dipole and electric quadrupole hyperfine constants of Aluminium ($^{27}Al$) atom are computed using the relativistic coupled cluster (CC) and unitary coupled cluster (UCC) methods. Effects of electron correlations are investigated using different levels of CC approximations and truncation schemes. The ionization potentials, excitation energies, transition probabilities, oscillator strengths and nuclear quadrupole moment are computed to assess the accuracy of these schemes. The nuclear quadrupole moment obtained from the present CC and UCC calculations in the singles and doubles approximations are 142.5 mbarn and 141.5 mbarn respectively. The discrepancies between our calculated IPs and EEs and their measured values are better than 0.3%. The other one-electron properties reported here are also in excellent agreement with the measurements. 
Chiranjib Sur, Rajat K. Chaudhuri, B. P. Das and D. Mukherjee