On the performance of the intermediate Hamiltonian Fock-space coupled-cluster method on linear triatomic molecules: The electronic spectra of NpO2+, NpO22+, and PuO22+

Abstract

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 ͑NpO 2 + , NpO 2 2+ , and PuO 2 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.

Theoretical study of the electronic spectra of neutral and cationic NpO and NpO2

“…[16][17][18][19][20][23][24][25][30][31][32][33][34][35] Due to the environment influencing considerably the spectral intensities in solutions and solids, our Einstein coefficients computed for the isolated molecules may not be suited for comparison. In contrast, the transition energies suffer less from the environment.…”

“…We neglected the Np 7p orbitals due to their relatively high energy with respect to the 5f, 6d, and 7s orbitals, 30 which makes them less important in low-energy excitations. Accordingly, previous studies of the electronic spectra of NpO 2 + and NpO 2 2+ did not find any significant excitations to Np 7p orbitals.…”

“…23,24,26 The electronic spectra of the solutions were first assigned using semi-empirical ligand-field theory (NpO 2 +27 and NpO 2 2+ 28 ) and were revised later by quantum chemical calculations. 19,25,[29][30][31][32][33][34][35] Matsika et al 19,25,29 used configuration interaction methods in conjunction with relativistic effective core potentials that incorporate also spin-orbit coupling 36 (SO-MRCI). In one of these studies, 19 coordination models with five explicit coordinating water molecules and chloride ions, respectively, were tested.…”

“…12,19,[23][24][25][26][29][30][31][32][33][34][35]38,39 We computed the excited electronic states up to ca. 35 000 cm −1 using the SO-CASPT2 method.…”

“…In one of these studies, 19 coordination models with five explicit coordinating water molecules and chloride ions, respectively, were tested. Infante et al 30 used for the free ions the more advanced intermediate Hamiltonian Fock-space coupled-cluster (IHFSCC) method in conjunction with the Dirac-Coulomb Hamiltonian which accounted for relativistic effects including spin-orbit coupling as well. Recently, Danilo et al applied SO-CASPT2 and SO-MRCI calculations using a small active space for modelling the free and hydrated NpO + 2 (the latter as [NpO 2 (H 2 O) 5 ] + ) ions.…”