Relativistic multiconfigurational ab initio calculations of X-ray spectra of heavy radio nuclides

X-Ray Absorption Near Edge Structure (XANES) spectroscopy and Resonant Inelastic X-ray
Scattering (RIXS) together with relativistic multiconfigurational ab initio calculations including
spin-orbit (SO) interactions are a preferred combination of experimental and theoretical tools
to explore the electronic structure of radionuclide compounds (electron correlation,
multiconfiguration effects and relativistic effects hace to be described accurately).
This requirement stems from the fact that the inner-shell orbitals involved in XAS processes are
most affected by relativity, manifestations of which are frequency shifts of spectral lines due
to the scalar relativistic effects as well as spectral fine structure splitting arising from the SO
coupling. K-edge spectra originating from excitations from 1s1/2 orbitals. The scalar relativistic corrections
have nonzero contributions and result in a constant shift. XAS spectra near L- and M-edges originate from excitations from inner-shell atomic-like p and d
orbitals split by the SO interaction into p1/2 and p3/2 or d3/2 and d5/2 levels, respectively. As a
consequence of the SO splitting, the use of multicomponent relativistic Hamiltonians is mandatory
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to correctly assign L- and M-edge spectra.

The complete and restricted active space multireference wave function method (CASSCF, RASSCF),
is the state of the art in theoretical actinide chemistry. CASSCF/RASSCF deal with the multiconfiguration problem.
The dynamic correlation is usually approximated by multiconfigurational perturbation theory at
second order (CASPT2/RASPT2). For XANES calculations, the RASSCF/RASPT2 approach is used.
In the employed RASSCF approach, the SO coupling is treated by the interaction of scalar
relativistic spin states via an atomic mean-field SO operator (AMFI).