An assessment of different electronic structure approaches for modeling time-resolved x-ray absorption spectroscopy

Abstract

We assess the performance of different protocols for simulating excited-state x-ray absorption spectra. We consider three different protocols based on equation-of-motion coupled-cluster singles and doubles, two of them combined with the maximum overlap method. The three protocols differ in the choice of a reference configuration used to compute target states. Maximum-overlap-method time-dependent density functional theory is also considered. The performance of the different approaches is illustrated using uracil, thymine, and acetylacetone as benchmark systems. The results provide guidance for selecting an electronic structure method for modeling time-resolved x-ray absorption spectroscopy.

Following excited-state chemical shifts in molecular ultrafast x-ray photoelectron spectroscopy

“…To this end, reference CCSD wavefunctions for the different electronic states of the neutral molecule were calculated starting from unrestricted Hartree-Fock wavefunctions, which were optimised using the maximum overlap method, in order to mimic the (singly excited) orbital occupancy of the excited states. A similar strategy has been recently validated by Coriani et al to model pump-probe x-ray absorption spectra of nucleobases at the coupled-cluster level 54 . Spin-orbit coupling, leading to a splitting of the core-ionised states of the order of 1 eV (not resolved due to spectral broadening) is not included in the calculations.…”

Using core-hole reference states for calculating X-ray photoelectron and emission spectra

“…The use of non-Aufbau reference states is also adopted for other property calculations, including doubly coreionized states, 57 resonant inelastic X-ray scattering (RIXS) 24 and transient X-ray absorption spectroscopy (TR-XAS). 25 For RIXS the reference state is coreexcited, and the same issues of near-singularities are thus expected. The approach presented here should thus work for these calculations as well.…”

“…23 In principle, one can exploit this procedure to start from a non-Aufbau reference state also for the computation of other X-ray spectroscopies such as RIXS 24 and transient XAS. 25 However, when using such reference states for response and equation-of-motion theories, the convergence issues related to near-singularities from certain doubly excited configurations with near-zero energy difference, are again present. These leads to numerical instabilities in the solution algorithms and to spurious results for transition energies and oscillator strengths, as will be shown below.…”

Excited state absorption of DNA bases in the gas phase and in chloroform solution: a comparative quantum mechanical study

“…20 MOM-TDDFT (and even MOM-CCSD) is often used to obtain X-ray absorption spectra of valence excited states, i.e. to simulate valence pump-core probe spectra, 21–24 but hardly explored for ESA in the UV-vis region.…”