Origins of NpO$_{2}$${2}$+ XAS features: Hydrated compared to isolated NpO^$_{2}$${2}$+

The principal concern of this paper is to compare theoretical predictions for M5 → 5f x-ray absorption spectroscopy (XAS) obtained between an isolated linear NpO22+ model and an embedded NpO22+ model that takes into account the environment of the solution where XAS measurements are actually conducted. The embedded model is an NpO2[H2O]52+ cluster. Novel methods are used to quantify the role of the hydration both for the molecular orbitals and for the many-body wavefunctions that account for the interaction of the hole in the M5 shell with the two electrons in the open 5f shell of the excited states. It is found that the hydration has only a very minor effect on the XAS excitation energies and intensities, thus validating the previous work based on the properties of an isolated neptunyl cation.