  Understanding the binding mechanism in neptunyl clusters formed due to
cation--cation interactions is of crucial importance in nuclear waste
reprocessing and related areas of research. Since experimental manipulations
with such species are often rather limited, we have to rely on quantum-chemical
predictions of their electronic structures and spectroscopic parameters. In
this work, we present a state-of-the-art quantum chemical study of the T-shaped
and diamond-shaped neptunyl(V) and neptunyl(VI) dimers. Specifically, we
scrutinize their molecular structures, (implicit and explicit) solvation
effects, the interplay of static and dynamical correlation, and the influence
of spin-orbit coupling on the ground state and lowest-lying excited states for
different total spin states and total charges of the neptunyl dications.
Furthermore, we use the picture of interacting orbitals (quantum entanglement
and correlation analysis) to identify strongly correlated orbitals in the
cation--cation complexes that should be included in complete active space
calculations. Most importantly, our study highlights the complex interplay of
correlation effects and relativistic corrections in the description of the
ground and lowest-lying excited states of neptunyl dications.
