Restricted active space calculations of L-edge X-ray absorption spectra: From molecular orbitals to multiplet states

Abstract

PostprintThis is the accepted version of a paper published in Journal of Chemical Physics. This paper has been peer-reviewed but does not include the final publisher proof-corrections or journal pagination.Citation for the original published paper (version of record):Pinjari, R V., Delcey, M G., Guo, M., Odelius, M., Lundberg, M. (2014) Restricted active space calculations of L-edge X-ray absorption spectra: From molecular orbitals to multiplet states. The metal L-edge (2p → 3d) X-ray absorption spectra are affected by a number of different interactions; electron-electron repulsion, spin-orbit coupling, and charge transfer between metal and ligands, which makes the simulation of spectra challenging. The core restricted active space (RAS) method is an accurate and flexible approach that can be used to calculate X-ray spectra of a wide range of medium-sized systems without any symmetry constraints. Here, the applicability of the method is tested in detail by simulating three ferric (3d 5 ) model systems with well-known electronic structure viz. atomic Fe 3+ , high-spin [FeCl 6 ] 3-with ligand donor bonding, and low-spin [Fe(CN) 6 ] 3-that also has metal backbonding. For these systems, the performance of the core RAS method, which does not require any system-dependent parameters, is comparable to that of the commonly used semi-empirical charge-transfer multiplet model. It handles orbitally degenerate ground states, accurately describes metal-ligand interactions, and includes both single and multiple excitations. The results are sensitive to the choice of orbitals in the active space and this sensitivity can be used to assign spectral features. A method has also been developed to analyze the calculated X-ray spectra using a chemically intuitive molecular orbital picture. Journal of Chemical

Soft x-ray absorption spectroscopy of metalloproteins and high-valent metal-complexes at room temperature using free-electron lasers

“…This analysis, however, neglects the multiplet structure in the spectra 10 . A more detailed interpretation of our spectra will have to await progress in ab-initio theoretical methods 11,39–43 to correlate the multiplet structures with valence electronic spin and charge densities of the systems.…”

“…For future time-resolved experiments assessing different time points in the photocycle, better spectral statistics for the PS II samples and progress in the theoretical interpretation 11,39–43 will be essential, and such effort is underway.…”

Final-State Projection Method in Charge-Transfer Multiplet Calculations: An Analysis of Ti L-Edge Absorption Spectra

“…15-23 Recently, restricted active space and restricted open shell calculations have been used to calculate L-edge XAS spectra including a final state analysis. 24,25 …”

Applications to metal K pre-edges of transition metal dimers illustrate the approximate origin independence for the intensities in the length representation

“…[10,11,13] Recently, the restricted active space (RAS) SCF and PT2 methods have been used to model metal L-edge XAS spectra of a variety of transition metal complexes using the electric dipole approximation. [14][15][16][17][18][19][20]. For metal K-edge XAS, in particular the pre edge, the dipole approximation no longer suffices and electric quadrupole and magnetic dipole contributions are therefore also considered.…”