Review on theoretical X-ray spectroscopy

A review of the theoretical developments to predict various kinds of soft X-ray spectra has been published by us recently. It describes how different observables can be extracted from the theoretical calculations: absorption (XAS), photoionization (PES or XPS), resonant inelastic scattering (RIXS), and Auger. The main focus is on L-edge spectra of transition metals and in particular on multi-reference methods as they appear to be essential for this type of systems. However, the review also contains a brief overview of other methods and applications. Enjoy!

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Matsubara-like dynamics for vibronic spectroscopy

I forgot to write about a publication which appeared at the end of the last year. It is a logical continuation of the previous works of Sven Karsten on the semi-classical description of vibronic spectra. That is when both electronic and nuclear motion of a molecule is excited.

 

This time Sven has elaborated on the imaginary time-shifted  correlation functions and applied a Matsubara approximation to a ring polymer which is residing partly on the excited potential energy surface and partly on the ground state one, as has been described in previous publication. This approximation filters out high-frequency vibrations of the ring polymer, leaving only the low-frequency “smooth” distortions. The idea is that the high-frequency counterpart is anyway cut by the Boltzmann distribution at the finite temperature. However, strictly speaking, one cannot call this approximation just Matsubara as there is an abrupt change in the ring due to different potentials it is evolving according to. That is why we call it Matsubara-like.

I hope that this approach upon the proper generalization to more complicated molecules will be a step towards reliable simulation of vibrionic spectra with semi-classical methods.

STC 2019 – Registration is open

Registration is open for the ’55th Symposium on Theoretical Chemistry’ (STC2019) to be held at the University of Rostock (Germany) from September 22 to 26,  2019.  The STC will be one of the highlight events during this year’s celebration of the 600th anniversary of the foundation of the University of Rostock in 1419. With its long tradition, the STC serves as a yearly meeting for theoretical chemists to exchange ideas and discuss the latest results. Being broad in scope, every STC in general features a main topic. The focus of the STC2019 in Rostock will be

Spectroscopy and Photoinduced Dynamics.’

Spectroscopy in time- and frequency-domain provides a primary tool for the detailed investigation of molecular processes ranging from simple gas phase reactions to biomolecular catalysis. With the advent of novel techniques such as two-dimensional spectroscopy and light sources in the X-ray regime, spectroscopy is pushing the borders, not only in terms of applications but also as far as theoretical methods are concerned. The challenge is open for Theoretical Chemistry, and the STC2019 will reflect the current status, covering techniques and applications from the infrared to the X-ray regime.

This Rostock STC  will feature about 11 invited speakers. About 25 oral presentations will be selected from the submissions. Two poster sessions will provide the platform for intense discussions. On September 25th, the Hellmann Prize will be awarded to an early career theoretical chemist. In addition to the scientific program, there will be a joint boat trip and a conference dinner on the Warnemünde beach.

Invited Speakers:

Sonia Coriani, Technical University of Denmark, Lyngby (DEN)

Bernd Engels, Universität Würzburg (GER)

Marco Garavelli, Università degli Studi di Bologna (ITA)

Stefanie Gräfe, Universität Jena (GER)

Stephan Kümmel, Universität Bayreuth (GER)

Christel Marian, Heinrich Heine Universität, Düsseldorf (GER)

Dominik Marx, Ruhr Universität Bochum (GER)

Michael Odelius, Stockholm University (SWE)

Oleg V. Prezhdo, University of Southern California, Los Angeles (USA)

Oriol Vendrell, Universität Heidelberg, (GER)

Regina de Vivie-Riedle, LMU München (GER)

Hans Jakob Wörner,  ETH Zürich (CH)

 

For detailed information on the scientific program, registration fees, location, accommodation, travel, and registration  see the conference webpage http://www.stc2019.de

We look forward to welcoming you at the Baltic Sea.

 

Oliver Kühn, Ralf Ludwig, Sergey Bokarev

(Scientific Organizers)

Optimal tuning and photoelectron spectra

Density functional theory is an indispensable theoretical method to study moderate to large systems due to its computational efficiency. Being physically sound, it relies on approximations since the explicit form of the density functional is not known. Most of the standard functionals available on the market have an inherent problem – electrons in the system experience spurious self-interaction. In many applications, this is not critical, but in some cases, it does matter.

A way to overcome it is to calculate exchange energy exactly. However, the perspective strategy is to add this “exact” correction only at long distances. The smooth function switching between short- and long-range behavior is density-dependent and thus varies for different systems. We use a purely first principles procedure on how to determine the parameters of the switching function for the particular system. As an end effect, we correct the orbital energies such that they become better estimates of ionization potentials which has immediate implication for the accuracy of the computed photoelectron spectra.

 

PES

We have applied this procedure to the prediction of properties of charge-transfer states of photosensitizers and also to photoelectron spectroscopy before. In the recent publication

T. Möhle, O.S. Bokareva, G. Grell, O. Kühn, S.I. Bokarev Tuned Range-Separated Density Functional Theory and Dyson Orbital Formalism for Photoelectron Spectra J. Chem. Theory Comput. 14 (2018), 5870–5880.

we systematically analyze the performance of a combination of optimally-tuned density functionals with Dyson orbital approach. Thus, this approach relies on two cornerstones: reliable prediction of ionization energies and more accurate treatment of intensities using Dyson orbital formalism together with TDDFT.

We critically discuss the advantages and disadvantages of this procedure. It should be advisable in cases when the system studied with photoelectron spectroscopy has a non-singlet ground state. In this case, one has two non-equivalent ionization spin-channels which are otherwise unsatisfactorily reproduced. Moreover, TDDFT with the Dyson approach even levels the error of conventional functionals, and the results are not much different from more accurate range-sepated functional. Another two issues which might be problematic for our approach are the stability of the ground state with respect to orbital variations and spin-contamination. The latter one is unavoidable as either a non-ionized or ionized system has open electronic shells and needs to be treated by the unrestricted variant of DFT which introduces this undesirable spin mixing.

The recommendations formulated in this publication should facilitate the practical application of the protocol. Some of the unsolved issues warrant further research.

55th Symposium on Theoretical Chemistry STC 2019

The year 2019 has a special meaning for the University of Rostock. We are all preparing to celebrate its 600th anniversary. There will be two major events in the field of molecular physics and theoretical chemistry which are organized in connection to this anniversary: 83rd Spring Meeting of the German Physics Society (DPG-Frühjahrstagung) and 55th Symposium on Theoretical Chemistry (STC 2019). As a co-organizer of the STC 2019, I cordially invite you to participate!

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The conference will take place from 22nd to 26th of September 2019 in Rostock, and its headline is “Spectroscopy and Photoinduced Dynamics.” The main topics are:

  • IR, UV-Vis, X-ray spectroscopy
  • photon in/out & photon in/electron out events
  • energy & time-domain methods
  • quantum & trajectory-based dynamics
  • quantum chemistry (DFT & wavefunction)
  • photocatalysis, energy materials, etc.

And here are some views of Rostock to attract your attention. The conference dinner is planned to take place just next to the beach and will be joined with the boat trip.

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Looking forward to seeing you in Rostock!

Huihui Wang is now a doctor

Last Friday Huihui Wang has defended her Ph.D. thesis “Laser-driven electron and spin-state quantum dynamics in transition metal complexes” which has been partially done within the joint project with the King Abdulaziz University in Jeddah and also supported by the DFG grant. I have already briefly written about the details of her work on dynamics triggered by isolated as well as trains of pulses.

Congratulations and we wish her further success in her scientific career!

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