  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.

Atomic Physics