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X-ray absorption spectroscopy (XAS) is a powerful experimental technique to probe the local order in materials with core electron excitations. Experimental interpretation requires supporting theoretical calculations. For water, these calculations are very demanding and, to date, could only be done with major approximations that limited the accuracy of the calculated spectra. This prompted an intense debate on whether a substantial revision of the standard picture of tetrahedrally bonded water was necessary to improve the agreement of theory and experiment. Here, we report a first-principles calculation of the XAS of water that avoids the approximations of prior work, thanks to recent advances in electron excitation theory. The calculated XAS spectra, and their variation with changes of temperature and/or with isotope substitution, are in good quantitative agreement with experiments. The approach requires accurate quasiparticle wave functions beyond density functional theory approximations, accounts for the dynamics of quasiparticles, and includes dynamic screening as well as renormalization effects due to the continuum of valence-level excitations. The three features observed in the experimental spectra are unambiguously attributed to excitonic effects. The preedge feature is associated with a bound intramolecular exciton, the main-edge feature is associated with an exciton localized within the coordination shell of the excited molecule, and the postedge feature is delocalized over more distant neighbors, as expected for a resonant state. The three features probe the local order at short, intermediate, and longer range relative to the excited molecule. The calculated spectra are fully consistent with a standard tetrahedral picture of water.
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Chapter Two - Localized Relativistic Two-Component Methods for Ground and Excited State Calculations
Scientists are extending the computational application of relativistic methods to ever-increasing sizes of molecular systems. To this end, reduction of the computational cost of relativistic methods through modest approximations is a welcome effort. In this work, we review several localized two-component approximations and introduce a maximally localized variant. We also extend the focus of local relativistic approximations from the ground state to excited states. Benchmark calculations on both valence and core electron absorption spectra are carried out to analyze the error incurred by using the relativistic local approximations for excited state computations.
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- PAR ID:
- 10253112
- Date Published:
- Journal Name:
- Annual reports in computational chemistry
- Volume:
- 16
- ISSN:
- 1875-5232
- Page Range / eLocation ID:
- 17-37
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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