LAUSR

The Fock-space coupled-cluster (FSCC) method appears as a robust computational scheme if reformulated within the eigenvalue independent partitioning framework (EIP-FSCC) which efficiently deals with the intruder states that cause convergence problems in its standard effective Hamiltonian format. Within such a reformulation not only the so-called main (principal) root of the effective Hamiltonian version are attainable, but also the alternative satellite roots appear from the higher-sectors of the corresponding matrix (that is properly constructed) upon its direct diagonalization. Selective spectroscopic constants of , (main and satellite), and states of N ion have here been estimated using the EIP-FSCC by exploiting various truncation schemes of cluster operators (EIP-FSCCSD and EIP-FSCCSDτ). EIP-FSCCSDτ includes the set of full connected triple excitation operators for the ionized states where as perturbative is considered by CCSDT1-a approximation. This work indicates that the EIP-FSCCSDτ, compared to EIP-FSCCSD scheme, is capable of providing nice agreement with experiment provided an adequate basis set is used. GRAPHICAL ABSTRACT