On the performance of the intermediate Hamiltonian Fock-space coupled-cluster method on linear triatomic molecules: The electronic spectra of NpO2+, NpO2(2+), and PuO2(2+).

In this paper we explore the use of the novel relativistic intermediate Hamiltonian Fock-space coupled-cluster method in the calculation of the electronic spectrum for small actinyl ions (NpO2+, NpO2(2+), and PuO2(2+)). It is established that the method, in combination with uncontracted double-zeta quality basis sets, yields excitation energies in good agreement with experimental values, and better than those obtained previously with other theoretical methods. We propose the reassignment of some of the peaks that were observed experimentally, and confirm other assignments.