The coupled cluster theory generalized for the description of the high spin ionized states via equation-of-motion philosophy
Monika Musiał1, Artur Lisoń1
1University of Silesia, Institute of Chemistry,
Szkolna 9, 40-006 Katowice, Poland
In this work we focus on the ionization potential (IP) equation-of-motion (EOM) coupled cluster (CC) scheme based on the restricted Hartree-Fock (RHF) reference and the amplitude equations in the formulation suitable for the use in the generalized Davidson diagonalization procedure. The conventional IP-EOM-CC scheme in the RHF formulation and the standard amplitude equations, i.e. set up for the spin z-component equal to ½ (Sz = ½), yields the doublet states. The quartets require explicit treatment of all spin cases.
In the current work we develop a new approach to the IP calculations for the high spin states. We propose an independent set of the amplitude equations applicable to the case of Sz = 3⁄2 (quartets and sextets) and to the case of Sz = 5⁄2 (sextets) as an alternative to the equations solved for the Sz = 1 2 case. The new equations are generally simpler than those for the doublet case but in special cases, e.g. Sz = 5⁄2 , dramatically reduce the number of terms. Out of 62 diagrammatic terms contributing to the R1, R2 and R3 doublet equations in the case of sextets only R3 operator survives with 5 diagrammatic terms present. In addition all terms engaging threeand four-body elements of the similarity transformed Hamiltonian disappear. This indicates a substantial simplification of the theory. The implemented method has been applied to the pilot study of the potential energy curves and their spectroscopic characteristics for the selected diatomic ions. This new strategy was successfully applied in the calculations of the energies and the excited states of Cl and Br atoms.