Density Functional Theory Based Methods for the Calculation of X-ray Spectroscopy.

Nicholas A Besley

doi:10.1021/acs.accounts.0c00171

Abstract

The availability of new light sources combined with the realization of the unique capabilities of spectroscopy in the X-ray region has driven tremendous advances in the field of X-ray spectroscopy. Currently, these techniques are emerging as powerful analytical tools for the study of a wide range of problems encompassing liquids, materials, and biological systems. Time-resolved measurements add a further dimension to X-ray spectroscopy, opening up the potential to resolve ultrafast chemical processes at an atomic level. X-ray spectroscopy encompasses a range of techniques which provide complementary information, and these include X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), X-ray emission spectroscopy (XES), and resonant inelastic X-ray scattering (RIXS). In many studies, the interpretation of the experimental data relies upon calculations to enable the nature of the underlying molecular structure, electronic structure, and bonding to be revealed. Density functional theory (DFT) based methods are some of the most widely used methods for the simulation of X-ray spectra. In this Account, we focus on our recent contributions to the simulation of a range of X-ray spectroscopic techniques using DFT and linear-response time-dependent density functional theory (TDDFT) and show how these methods can provide a computational toolkit for the simulation of X-ray spectroscopy. The importance of the exchange-correlation functional for the calculation of XAS is discussed, and the introduction of short-range corrected functionals is described. The application of these calculations to study large systems through the use of efficient implementations of TDDFT will be highlighted, with the use of these methods illustrated through studies of ionic liquids and transition metal complexes. The extension of TDDFT to calculate XES through the use of a reference determinant for the core-ionized state will be described, and the factors that affect the accuracy of the computed spectra discussed. The application of these approaches will be illustrated through the study of a range of organic molecules and transition metal complexes, which also show how going beyond the dipole approximation in determining the transition intensities can be critical. The application of these approaches to the simulation of the RIXS spectrum of water will also be described, highlighting how ultrafast dynamics on the femtoscale time scale are evident in the measured spectra. In these calculations, the description of the core-ionized and core-excited states becomes increasingly important, and the role of the basis set in accurately describing these states will be explored.

Highlights

In recent years the advances in the intensity and resolution that can be achieved with synchrotron radiation has resulted in a surge in interest in spectroscopic techniques in the X-ray region
The development of computational methods that aid the detailed analysis of experimental data has emerged as an important component for the continued growth of the field of X-ray spectroscopy
In this Account our work on the use of DFT and TDDFT based methods to simulate X-ray photoelectron spectroscopy (XPS), nearedge X-ray absorption fine structure (NEXAFS), X-ray emission spectroscopy (XES) and RIXS spectroscopies has been summarised. These methods have been developed for the treatment of the spectroscopy of valence electrons

Summary

Introduction

In recent years the advances in the intensity and resolution that can be achieved with synchrotron radiation has resulted in a surge in interest in spectroscopic techniques in the X-ray region. The critical benefit from this approach is that only a calculation for the ground state is required, as a consequence no account of the orbital relaxation for the core-ionised state is included Despite its simplicity this approach provides remarkably accurate spectra and has been applied successfully in a variety of applications, including the valence-to-core XES of transition metal complexes by Neese and co-workers.[41] Our recent work[42] has shown that using this approach in combination with the short-range corrected functionals described earlier provides accurate non-resonant X-ray emission spectra with transition energies close to experiment. Our work has shown how XES can be determined using TDDFT through 15

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Summary and Outlook