Effects of partial triple excitations in atomic coupled cluster calculations

In this Letter, we study the effects of higher order excitations in the relativistic CC calculations for atoms and ions with one valence electron using 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, Al I, etc. We further demonstrate that the higher order excitation (as well as electron correlation) effects can be incorporated more effectively through the unitary coupled cluster theory (UCC) compared to the CCSD(T) method.