PhD Position in Theoretical Chemistry (75% TVL E13 for three years)

A PhD position is available at the Chemistry Department, School of Natural Sciences, Technical University of Munich. We are seeking a highly motivated candidate with a passion for code development and computational applications.

Research Topic: The research topic involves adapting multi-configurational quantum-chemical methods (such as DMRG, MC-PDFT, etc.) for the computation of highly-lying excited states, with a focus on applications to X-ray spectroscopy. The position also includes a research stay of several months at ETH Zürich. The project is part of a larger endeavor titled “Theoretical X-ray spectroscopy and correlated electron dynamics in highly-excited molecular systems.”

More Information: Further details about the research group can be found at https://molscience.wordpress.com.

Requirements: Applicants should possess an MSc (or equivalent) in theoretical chemistry, physics, or related fields, along with demonstrated excellence in theoretical concepts and scientific programming.

Application: Interested candidates are invited to submit their CV and degree certificates to sergey.bokarev@tum.de.

Teaching Prize for Thies Romig

The member of the group Thies Romig has got a Teaching Prize from the Faculty of Natural Sciences of Rostock University. This prize is awarded every semester to a professor, research assistant, and student assistant for their outstanding teaching activities. Congrats!

Hellmann Prize 2022

I am pleased to write that this year’s Hellmann Prize of the Theoretical Chemistry Division of the German Bunsen Society was awarded to me for “outstanding research in the field of dynamics and spectroscopy of transition metal complexes.” The yearly prize acknowledges young scientists in memory of Hans G.A. Hellmann, the pioneer of quantum mechanics and quantum chemistry whose life was tragically cut short.

The awarding ceremony and the adjacent lecture have taken place on the 20th of September at the 58th Symposium for Theoretical Chemistry at Heidelberg University.

Prof. Dr. Peter Saalfrank (left) – the Chairman of the Division for Theoretical Chemistry – and me at the awarding ceremony.

Professor at the Technical University of Munich

Starting from the summer semester of 2022, I was invited to temporarily occupy the Chair of Theoretical Chemistry at the Chemistry Department of the Technical University of Munich as a substitute professor. The formal contact information is thus changing accordingly.

RhoDyn: a versatile tool to study ultrafast dynamics

Recently, we have made available our code to compute ultrafast electron dynamics in molecules, which has been already in focus of our studies before, see, e.g., 1, 2, 3, or 4.

Vladislav Kochetov and Sergey I. Bokarev RhoDyn: A ρ-TD-RASCI Framework to Study Ultrafast Electron Dynamics in Molecules J. Chem. Theory Comput. 2021, 18, 1, 46–58.

The workflow of the RhoDyn program within OpenMolcas program suite

The program is organized as a module within the open-source OpenMolcas program package and relies on the data computed by other modules, e.g., RASSCF and RASSI as displayed in the Figure. We have tried to make it as universal and versatile as possible. It is intended to address the ultrafast (attosecond or few-femtosecond) dynamics initiated, driven, and stirred by ultrashort light pulses as obtained by modern light sources – free-electron lasers and high harmonic generation setups. It provides a researcher with an easy way to access non-linear spectra, charge, and spin dynamics. The article demonstrates some of the possible applications to linear and non-linear spectra of titanium oxide and high harmonic generation in the hydrogen molecule, ultrafast charge migration in benzene and iodoacetylene, and spin-flip dynamics in core-excited iron complexes. Of course, the applications are not limited to these cases.

More information on how to use the new module can be obtained from the OpenMolcas page or from the supplement to the article where all input files for the above-mentioned examples are collected.

A review of theoretical approaches to 2p X-ray absorption

As a result of the work of the giant community of authors a comprehensive review of different theoretical approaches to the L-edge X-ray spectroscopy of transition metal compounds appeared. It is available under the open-access license:

Frank MF de Groot, Hebatalla Elnaggar, Federica Frati, Ru-pan Wang, Mario U Delgado-Jaime, Michel van Veenendaal, Javier Fernandez-Rodriguez, Maurits W Haverkort, Robert J Green, Gerrit van der Laan, Yaroslav Kvashnin, Atsushi Hariki, Hidekazu Ikeno, Harry Ramanantoanina, Claude Daul, Bernard Delley, Michael Odelius, Marcus Lundberg, Oliver Kühn, Sergey I Bokarev, Eric Shirley, John Vinson, Keith Gilmore, Mauro Stener, Giovanna Fronzoni, Piero Decleva, Peter Kruger, Marius Retegan, Yves Joly, Christian Vorwerk, Claudia Draxl, John Rehr, Arata Tanaka 2p x-ray absorption spectroscopy of 3d transition metal systems J. Electron Spectros. Relat. Phenomena 249 (2021) 147061.

It covers topics from ligand-field multiplet theory to multi-reference methods for the investigation of molecular complexes in the gas phase and solution to solid-state systems.

Photocatalytic reaction through the lens of X-rays

As a logical continuation of our activities in the field of joint experimental and theoretical study of photocatalytic hydrogen water splitting in the optical regime, we have recently published an extension, where the oxidation state of the central metal atom of a photosensitizer is monitored directly by the ultrafast X-ray absorption spectroscopy.

Alexander Britz, Sergey I. Bokarev, Tadesse A. Assefa, Èva G. Bajnóczi, Zoltán Németh, György Vankó, Nils Rockstroh, Henrik Junge, Matthias Beller, Gilles Doumy, Anne Marie March, Stephen H. Southworth, Stefan Lochbrunner, Oliver Kühn, Christian Bressler, Wojciech Gawelda Site‐Selective Real‐Time Observation of Bimolecular Electron Transfer in a Photocatalytic System Using L‐Edge X‐Ray Absorption Spectroscopy ChemPhysChem 22 (2021) 693-700.

The question of the localization of an additional electron received by the photosensitizer from the reductant has already caused debates. The reason for it was that the EPR method predicts an unusual signal for the ligand-localized unpaired electron suggesting an interpretation where the central iridium ion is reduced. However, other experimental methods and theoretical calculations strongly suggested that iridium stays untouched whereas the redox chemistry happens on the bipyridine-like ligand. X-ray spectroscopy has the advantage that it provides direct access to the local atomic properties of a specific type of atoms in a molecule. Together with proper theoretical treatment and interpretation, it represents one of the most powerful methods to monitor oxidation states.

In photosensitizer, the excitation is followed by relaxation and after about 100 femtoseconds the population resides in a long-living triplet metal-to-ligand charge-transfer state. In this state, there is a hole in the 5d orbital of iridium and an additional electron on the bipyridine ligand. This hole has a distinct signature in the L-edge X-ray spectrum; it is filled in course of the catalytic reaction by another electron. Hence, monitoring the spectral changes allowed to measure the rate of triplet state quenching.

This study is, I hope, a final chord in disentangling the photochemistry of this prototypical photosensitizer. Moreover, it should have more significance in establishing techniques how to perform operando measurements on real photocatalytic reactions with the emergent ultrafast X-ray spectroscopic methods.

New website of AGTC

The Workgroup on theoretical chemistry (AGTC), a part of Bunsen Society for Physical Chemistry, has renewed its website. Please visit www.theochem.de

Workshop “Molecular quantum dynamics beyond bound states” will occur in the online format

Due to tendencies to prolong and tighten Corona restrictions in Germany, the workshop “Molecular quantum dynamics beyond bound states” will occur on March 10-12 2021 purely in the online format. Further informaion can be found on the webpage of the workshop. Registration proceeds via e-mail.

Highly excited states and spin dynamics

Nowadays, the ultrafast sciences paradigm is changing from working on the femtosecond to a shorter sub-femtosecond or attosecond timescale. Such research attracts scientists’ attention as it allows studying different atomic and molecular processes on the scale of the fastest electronic motion. I have already touched on the subfemtosecond spin-flip dynamics simulations in the highly excited states in my blog. In a new article, we continued research in this direction and have asked a question: are the ultrafast spin dynamics in the 2p-core-excited states a predominantly atomic process or the chemical environment plays a crucial role?

Vladislav Kochetov, Huihui Wang, Sergey I. Bokarev Effect of chemical structure on the ultrafast spin dynamics in core-excited states J. Chem. Phys. (2020) 153, 044304.

From a general viewpoint, such dynamics are rooted in preparing the superposition of the spin-orbit split states with 2p_3/2 and 2p_1/2 core holes. Thus, it should be nearly an atomic process, and the strength of the spin-orbit coupling of the hole-bearing atom is decisive. However, the systematic theoretical study of several transition metal complexes of titanium, chromium, iron, and nickel has demonstrated that these simple considerations do not hold.

The ligands attached to the same ion appeared to play only a minor role; of course, if one does not change, e.g., all weak-field ligands to strong-field ones. (In the latter case, the dynamics may change qualitatively.) In turn, the central metal ion’s influence is notably more pronounced but does not correlate with the strength of spin-orbit coupling. For instance, the titanium complex demonstrates an efficient spin-flip, whereas the nickel one does not. It is despite a three times larger coupling constant for nickel.

The dynamics are susceptible to the energetic distribution of states with different multiplicity and their “availability” for the excitation by an ultrashort pulse. The figure above shows that singlet (red) and triplet (blue) states cluster according to spin-orbit interaction and dipole absorption strength. Those nodes which are larger correspond to states which are involved in the dynamics. Those with small nodes are unaffected. Decisive for the efficiency is the ratio between the number of involved states with flipped spin (big blue nodes) and the number of involved states with the ground state spin (big red nodes). The influence of vibrations, which are also inherent to the chemical structure of the complex, was found to be negligible.

This work adds to the understanding of the spin-flip dynamics mechanism and suggests some ways to decrease the computational effort and thus include more states in future simulations.