On the calculation of multiplet energies of three-open-shell 4f5f6d electron configuration by LFDFT: modeling the optical spectra of 4f core-electron excitation in actinide compounds.

Methodological concepts are reported for the calculation, without empirical parameters, of multiplet energy levels and ligand-field effects associated with three-open-shell 4f5f6d electron configurations, and for the modeling of X-ray absorption spectra in relation to intra-atomic 5f → 4f5f6d electron transitions. A density functional theory (DFT) method is used for the determination of the electronic structure. An effective ligand-field Hamiltonian is also used to incorporate many body effects and corrections via the configuration interaction algorithm within the active space of Kohn-Sham orbitals with dominant actinide 4f, 5f and 6d characters. The theoretical method ensures a parameter-free ligand-field model, which will be implemented in the Amsterdam density functional (ADF) program package as part of the available and automated ligand-field density functional theory (LFDFT) routine. The theoretical method is illustrated with examples for applications: U in the free ion and U in bulk UO by means of the molecular (UO) cluster. The DFT calculations are performed at different levels of the DFT functional, from which the LFDFT parameters such as Slater-Condon integrals, spin-orbit coupling constants and ligand-field potential (represented within the Wybourne formalism) are emulated. The comparison with available experimental data is good. Therefore, a non-empirical ligand-field treatment of the 4f5f6d configuration is established illustrating the spectroscopic details of the 4f core-electron excitation, which can be valuable for further understanding and prediction of the spectral profiles of actinide N-edge X-ray absorption spectroscopy.