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Coupled-cluster response theory for near-edge x-ray-absorption fine structure of atoms and molecules
University of Aarhus.
University of Aarhus.
Linköping University, Department of Physics, Chemistry and Biology. Linköping University, The Institute of Technology.
Linköping University, Department of Physics, Chemistry and Biology, Computational Physics. Linköping University, The Institute of Technology.
2012 (English)In: Physical Review A. Atomic, Molecular, and Optical Physics, ISSN 1050-2947, E-ISSN 1094-1622, Vol. 85, no 2, 022507- p.Article in journal (Refereed) Published
Abstract [en]

Based on an asymmetric Lanczos-chain subspace algorithm, damped coupled cluster linear response functions have been implemented for the hierarchy of coupled cluster (CC) models including CC with single excitations (CCS), CC2, CC with single and double excitations (CCSD), and CCSD with noniterative triple corrected excitation energies CCSDR(3). This work is a first step toward the extension of these theoretical electronic structure methods of well-established high accuracy in UV-vis absorption spectroscopies to applications concerned with x-ray radiation. From the imaginary part of the linear response function, the near K-edge x-ray absorption spectra of neon, water, and carbon monoxide are determined and compared with experiment. Results at the CCSD level show relative peak intensities in good agreement with experiment with discrepancies in transition energies due to incomplete treatment of electronic relaxation and correlation that amount to 1-2 eV. With inclusion of triple excitations, errors in energetics are less than 0.9 eV and thereby capturing 90%, 95%, and 98% of the relaxation-correlation energies for C, O, and Ne, respectively.

Place, publisher, year, edition, pages
American Physical Society , 2012. Vol. 85, no 2, 022507- p.
National Category
Engineering and Technology
Identifiers
URN: urn:nbn:se:liu:diva-75466DOI: 10.1103/PhysRevA.85.022507ISI: 000300081800005OAI: oai:DiVA.org:liu-75466DiVA: diva2:507087
Note
Funding Agencies|EU|254326|Swedish Research Council|621-2010-5014|National Supercomputer Centre (NSC), Sweden||Available from: 2012-03-02 Created: 2012-03-02 Last updated: 2017-12-07
In thesis
1. X-ray absorption spectroscopy through damped coupled cluster response theory
Open this publication in new window or tab >>X-ray absorption spectroscopy through damped coupled cluster response theory
2013 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

For a fundamental understanding of the interaction of electromagnetic radiation and molecular materials, experimental measurements are to be combined with theoretical models. With this combination, materials can be characterized in terms of composition, structure, time-resolved chemical reactions, and other properties. This licentiate thesis deals with the development and evaluation of a theoretical method by which X-ray absorption spectra can be interpreted and predicted.

In X-ray absorption spectroscopy the photon energy is tuned such that core electrons are targeted and excited to bound states. Such core excitations exhibit strong relaxation eects, making theoretical considerations of the processes especially challenging. In order to meet these challenges, a damped formalism of the coupled cluster (CC) linear response function has been developed, and the performance of this approach evaluated. Amongst the quantum chemical methods available, CC stands out as perhaps the most accurate, with a systematic manner by which the correct physical description can be approached. Coupled with response theory, we thus have a reliable theoretical method in which relaxation eects are addressed by means of an accurate treatment of electron correlation.

By use of the hierarchy of CC approximations (CCS, CC2, CCSD, CCSDR(3)), it has been shown that the relaxation eects are accounted for by the inclusion of double and triple excitations in the CC excitation manifold. The performance of the methods for K-edge NEXAFS spectra for water, neon, carbon monoxide, ammonia, acetone, and a number of uorine-substituted ethenes has been investigated, and we observe relaxation eects amounting to 7–21 eV. The discrepancy in absolute energy for the most accurate calculations as compared to experiments are reported as 0.4–1.5 eV, and the means by which this can be decreased further are discussed. For relative energies, it has been demonstrated that CCSD yields excellent spectral features, while CC2 yields good agreement to experiments only for the most intense features. Comparisons have also been made to the more computationally viable method of density functional theory, for which spectral features are in excellent agreement with experiment.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2013. 55 p.
Series
Linköping Studies in Science and Technology. Thesis, ISSN 0280-7971 ; 1625
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-102177 (URN)10.3384/lic.diva-102177 (DOI)LIU-TEK-LIC-2013:59 (Local ID)978-91-7519-484-4 (ISBN)LIU-TEK-LIC-2013:59 (Archive number)LIU-TEK-LIC-2013:59 (OAI)
Presentation
2013-12-12, Nobel, B-huset, Campus Valla, Linköpings universitet, Linköping, 10:15 (English)
Opponent
Supervisors
Available from: 2013-12-02 Created: 2013-12-02 Last updated: 2013-12-05Bibliographically approved
2. X-ray spectroscopies through damped linear response theory
Open this publication in new window or tab >>X-ray spectroscopies through damped linear response theory
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

In order to reach a fundamental understanding of interactions between electromagnetic radiation and molecular materials, experimental measurements need to be supplemented with theoretical models and simulations. With the use of this combination, it is possible to characterize materials in terms of, e.g., chemical composition and molecular structure, as well as achieve time-resolution in studies of chemical reactions. This doctoral thesis focuses on the development and evaluation of theoretical methods with which, amongst others, X-ray absorption and X-ray emission spectroscopies can be interpreted and predicted.  In X-ray absorption spectroscopy the photon energy is tuned such that core electrons are targeted and excited to either bound or continuum states, and X-ray emission spectroscopy measures the subsequent decay from such an excited state. These core excitations/de-excitations exhibit strong relaxation effects, making theoretical considerations of the processes particularly challenging. While the removal of a valence electron leaves the remaining electrons relatively unaffected, removing core electrons has a substantial effect on the other electrons due to the significant change in the screening of the nucleus. Additionally, the core-excited states are embedded in a manifold of valence-excited states that needs to be considered by some computationally feasible method. In this thesis, a damped formalism of linear response theory, which is a perturbative manner of considering the interactions of (weak) external or internal fields with molecular systems, has been utilized to investigate mainly the X-ray absorption spectra of small- to medium-sized molecular systems.

Amongst the standard quantum chemical methods available, coupled cluster is perhaps the most accurate, with a well-defined, hierarchical manner of approaching the correct electronic wave function. Combined with response theory, it provides a reliable theoretical method in which relaxation effects are addressed by means of an accurate treatment of electron correlation. The first part of this thesis deals with the development and evaluation of such an approach, and it is shown that the relaxation effects can be addressed by the inclusion of double excitations in the coupled cluster manifold.  However, these calculations are computationally very demanding, and in order to treat larger systems the performance of the coupled cluster approach has been compared to that of the less demanding method of time-density dependent functional theory (TDDFT). Both methods have been used to investigate the X-ray absorption spectrum of water, which has been extensively debated in the scientific community following a relatively recent hypothesis concerning the underlying structure of liquid water. Water exhibits a great number of anomalous properties that stand out from those of most compounds, and the importance of reaching a fundamental understanding of this substance cannot be overstated. It has been demonstrated that TDDFT yields excellent results for liquid water, opening up possibilities of investigating the correlation between spectral features and local structures.

Furthermore, recent developments in damped linear response TDDFT in the four-component relativistic regime have enabled the inclusion of spin-orbit coupling in damped linear response calculations, making black-box calculations of absorption spectra in a relativistic setting practical. With this approach, it is possible to address the spin-orbit splitting in L2,3-edge X-ray absorption spectra, and the performance of such a method has been demonstrated for a set of small molecules. Excellent agreement with experiment is obtained in terms of relative features, but an anomalous error in absolute energy has been observed for silane derivatives featuring fluorine-substitutions. This is likely a result of the strong influence of the very electronegative fluorine atoms on the electron density of the core-excited atom.

Finally, the treatment of non-resonant X-ray emission spectroscopy using damped linear response theory is discussed. The expansion needed for the development of a simple method by which this spectroscopy can be treated using damped linear response theory at the TDDFT level of theory has been identified, and proof of principle calculations at the time-dependent Hartree-Fock level of theory are presented.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2016. 122 p.
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1719
National Category
Chemical Sciences
Identifiers
urn:nbn:se:liu:diva-124272 (URN)10.3384/diss.diva-124272 (DOI)978-91-7685-908-7 (ISBN)
Public defence
2016-02-19, Planck, Fysikhuset, Campus Valla, Linköping, 10:15 (English)
Opponent
Supervisors
Available from: 2016-01-25 Created: 2016-01-25 Last updated: 2016-01-26Bibliographically approved

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