Ab Initio Wave Function-Based Determination of Element Specific Shifts for the Efficient Calculation of X-ray Absorption Spectra of Main Group Elements and First Row Transition Metals

Abstract

In this study, a detailed calibration of the performance of modern ab initio wave function methods in the domain of X-ray absorption spectroscopy (XAS) is presented. It has been known for some time that for a given level of approximation, for example, using time-dependent density functional theory (TD-DFT) in conjunction with a given basis set, there are systematic deviations of the calculated transition energies from their experimental values that depend on the functional, the basis set, and the chosen treatment of scalar relativistic effects. This necessitates a linear correlation for a given element/functional/basis set combination to be established before chemical applications can be performed. This is a laborious undertaking since it involves sourcing trustworthy experimental data, lengthy geometry optimizations, and detailed comparisons between theory and experiment. In this work, reference values for the element-specific shifts of all the first-row transition metal atoms and the main group elements C, N, O, F, Si, P, S, and Cl have been computed by using a protocol that is based on the complete active space configuration interaction in conjunction with second-order N-electron valence state perturbation theory (CASCI/NEVPT2). It is shown that by extrapolating the results to the basis set limit of the method and taking into account scalar relativistic effects via the second-order Douglas-Kroll-Hess (DKH2) corrections, the predicted element shifts are on average less than 0.75 eV across all the absorption edges and a very good agreement between theory and experiment in all the studied XAS cases is observed. The transferability of these errors to molecular systems is thoroughly investigated. The constructed CASCI/NEVPT2 database of element shifts is used to evaluate the performance and to automatically calibrate prior to comparison with the experiment two commonly used methods in X-ray spectroscopy, namely, DFT/Restricted open shell configuration interaction singles (DFT/ROCIS) and TD-DFT methods.

Ab Initio Analysis of Metal–Ligand Bonding in An(COT)2 with An=Th, U in Their Ground‐ and Core‐Excited States

“…The most straightforward access to single excitations is then facilitated by allowing as ingle hole/electron in RAS1/3 while RAS2 remains unrestricted. [13,[40][41][42][43][44][45][46][47][48][49][50][51][52][53] In this setup, for XANES the core orbitals usually span RAS1 while the valence orbitals span RAS2/3. It may be important that the basis orbitals are optimized separately for the valence vs. core-ESs, or for different types of core-ESs.…”

“…Here, the wavefunctions are generated from an active space divided into three segments, RAS1/2/3. The most straightforward access to single excitations is then facilitated by allowing a single hole/electron in RAS1/3 while RAS2 remains unrestricted . In this setup, for XANES the core orbitals usually span RAS1 while the valence orbitals span RAS2/3.…”

Efficient calculations of a large number of highly excited states for multiconfigurational wavefunctions

“…[10,11,19] As these methods can also treat systems with near-degenerate valence states, they have been used extensively to model X-ray spectra of transition-metal and rare-earth systems. [20][21][22][23][24][25][26][27][28][29] A majority of these calculations have taken advantage of the flexible structure of the restricted active space SCF (RASSCF) wavefunction, for which dynamical correlation can be added through second-order perturbation (PT2). [30][31][32] Transition intensities are calculated using the RAS state-interaction (SI) approach, which also includes spin-orbit coupling.…”

Theoretical X‐ray spectroscopy of transition metal compounds

“…Recently, the RASSCF/RASPT2‐based MR approach to X‐ray spectroscopy of metal complexes was shown to be quite efficient in unraveling the nature of different transitions in static XAS, RIXS, and core PES as well as dynamics studies . Selected examples will be presented in Section 4.…”