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Carbon X-ray absorption spectra of fluoroethenes and acetone: A study at the coupled cluster, density functional, and static-exchange levels of theory
Linköping University, Department of Physics, Chemistry and Biology, Computational Physics. Linköping University, The Institute of Technology.
University of Trieste, Italy .
Aarhus University, Denmark .
Linköping University, Department of Physics, Chemistry and Biology, Computational Physics. Linköping University, The Institute of Technology.
2013 (English)In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 138, no 12Article in journal (Refereed) Published
Abstract [en]

Near carbon K-edge X-ray absorption fine structure spectra of a series of fluorine-substituted ethenes and acetone have been studied using coupled cluster and density functional theory (DFT) polarization propagator methods, as well as the static-exchange (STEX) approach. With the complex polarization propagator (CPP) implemented in coupled cluster theory, relaxation effects following the excitation of core electrons are accounted for in terms of electron correlation, enabling a systematic convergence of these effects with respect to electron excitations in the cluster operator. Coupled cluster results have been used as benchmarks for the assessment of propagator methods in DFT as well as the state-specific static-exchange approach. Calculations on ethene and 1,1-difluoroethene illustrate the possibility of using nonrelativistic coupled cluster singles and doubles (CCSD) with additional effects of electron correlation and relativity added as scalar shifts in energetics. It has been demonstrated that CPP spectra obtained with coupled cluster singles and approximate doubles (CC2), CCSD, and DFT (with a Coulomb attenuated exchange-correlation functional) yield excellent predictions of chemical shifts for vinylfluoride, 1,1-difluoroethene, trifluoroethene, as well as good spectral features for acetone in the case of CCSD and DFT. Following this, CPP-DFT is considered to be a viable option for the calculation of X-ray absorption spectra of larger pi-conjugated systems, and CC2 is deemed applicable for chemical shifts but not for studies of fine structure features. The CCSD method as well as the more approximate CC2 method are shown to yield spectral features relating to pi*-resonances in good agreement with experiment, not only for the aforementioned molecules but also for ethene, cis-1,2-difluoroethene, and tetrafluoroethene. The STEX approach is shown to underestimate pi*-peak separations due to spectral compressions, a characteristic which is inherent to this method.

Place, publisher, year, edition, pages
American Institute of Physics (AIP) , 2013. Vol. 138, no 12
National Category
Engineering and Technology
Identifiers
URN: urn:nbn:se:liu:diva-92613DOI: 10.1063/1.4795835ISI: 000316969500078OAI: oai:DiVA.org:liu-92613DiVA: diva2:621692
Note

Funding Agencies|Swedish Research Council|621-2010-5014|EU|254326|Supercomputer Centre (NSC), Sweden||

Available from: 2013-05-16 Created: 2013-05-14 Last updated: 2017-12-06
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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