  In this article we study the effects of higher body excitations in the
relativistic CC calculations for atoms and ions with one valence electron using
Fock-space CCSD, CCSD(T) and its unitary variants. The present study
demonstrates that CCSD(T) estimates the ionization potentials (IPs) and the
valence electron removal energies quite accurately for alkali atoms and singly
ionized alkaline earth ions, but yields unphysical energy levels for atoms
and/or ions with partially filled sub-shell like C II. We further demonstrate
that the higher body excitation effects can be incorporated more effectively
through the unitary coupled cluster theory (UCC) compared to the CCSD(T)
method.
