Ab initio calculation of d-metal L-edge RIXS spectra using many-body quantum chemistry methods

We designed a fully ab initio quantum chemistry scheme for the computation of both d-d excitation energies and intensities as measured by resonant inelastic x-ray scattering (RIXS) in d-electron systems. RIXS has recently emerged as a powerful tool to reliably probe the charge, spin, and orbital degrees of freedom of correlated electrons in solids [1,2]. As a first application we picked up Li$_2$CuO$_2$, a quasi-1D Cu 3$d^9$ oxide with a simple valence configuration in the intermediate state. We use embedded-cluster MCSCF and MRCI techniques [3], including scalar relativistic effects, spin-orbit coupling, and the valence orbital relaxation in the presence of the core hole. The transition matrix elements of the dipole operator are obtained by non-orthogonal configuration interaction. A careful analysis of the RIXS spectra is important for understanding the interplay between local distortions and longer-range lattice anisotropy and its effect on the d-level electronic structure [3,4] and magnetic interactions [4]. [1] L. Ament {\it et al.} Rev. Mod. Phys. {\bf 83}, 705 (2011); [2] J. Schlappa{\it et al.} Nature {\bf 485}, 82 (2012); [3] H.-Y. Huang {\it et al.} Phys. Rev. B {\bf 84}, 235125 (2011); [4] N.~A. Bogdanov {\it et al.} Phys. Rev. Lett. {\bf 110}, 127206 (2013).