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.



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!


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.


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!


Molecular asymmetry and photoelectron spectra

As a result of collaboration between our group and the group of Michael Odelius from Stockholm University an open-access article has been recently published on pages of Physical Chemistry Chemical Physics journal:

Jesper Norell, Gilbert Grell, Oliver Kühn, Michael Odelius, Sergey I. Bokarev Photoelectron shake-ups as a probe of molecular symmetry: 4d XPS analysis of I3 in solution  Phys. Chem. Chem. Phys., 20 (2018) 19916.


The previous work from Michaels group dealt with the influence of the polarity and proticity of the solvent on the photoelectron spectrum of this model system.  For instance, in water I3 represents a quite asymmetric moiety which can be described as I2—I, whereas in less protic solvents the charge is fairly delocalized [I–I–I]. In present work, we joined our forces and applied a protocol derived by us to the analysis of photoelectron intensities. The main focus was on assignment and relative intensities of different transitions. We have considered the so-called main transitions, when one electron is kicked out of a system, and the shake-ups, when in addition to removal of an electron another one is excited to a bound state. Relative intensities of these two types of bands appeared to be a convenient measure of the asymmetry of the structure. Understanding the interaction of I3 with the solvent may serve for better design of redox systems, e.g., dye-sensitized solar cells.

Repeat until successful

I have already told you about out activity in the field of electron dynamics and namely spin-flip dynamics. To wrap it briefly, we have looked what will happen to a system with the strong coupling of spin and orbital motion of electrons if we prepare a pure spin-state. One can imagine, e.g., a Fe2+ ion with four spin-up electrons forming a so-called quintet state. Now if such a system has a hole in the electronic core, which immediately causes strong spin-orbit coupling, an ultrafast spin-flip of an electron will occur leading to triplet final state with effectively only two spin-up electrons.

Well, sounds easy. However, such a situation is so far pure speculation, and we should ask ourselves, how to practically prepare this particular initial state? It is very special because it corresponds not to an eigenstate of a system but rather to a quantum superposition of such “natural” states. One can, of course, absorb light, but this light should also possess somewhat unusual characteristics. First of all, the light pulse should be very short. According to the uncertainty principle, the shorter is the pulse, the broader is it in energy range which it excites. And we exactly need excitation of lots of eigenstates to resemble the situation with an initially prepared core-excited pure-spin state.

Pulse trains

Out of possible light sources, two candidates would potentially fit: free electron lasers and high harmonic generation (HHG) setups. They are able to produce light with energy that is high enough to excite core states and emerged pulses have temporal durations below 1 femtosecond (10-15 seconds). Free electron lasers provide single isolated pulses, whereas HHG usually gives periodic sequences of pulses. We have already discussed in previous publications, how the transition metal complex reacts on the isolated pulse. I have briefly described it in this blog.

In a recent publication:

Huihui Wang, Tobias Möhle, Oliver Kühn, and Sergey I. Bokarev Ultrafast dissipative spin-state dynamics triggered by x-ray pulse trains, PHYSICAL REVIEW A, 98, 013408 (2018).

we have looked how spin of the system will behave if we subject it to repeated sub-femtosecond pulses as resulting from the HHG source. The key difference to isolated pulses is that the yield of spin-fliped states rises in a stepwise manner after every subpulse in a train as can be seen from a figure. This makes this process to occur faster and more complete. Even more important is that due to stimulated emission the population is dumped from core-excited to spin-flipped valence states. Thus, via a core excitation and stimulated emission, and thus mediated by the strong spin-orbit coupling in the core state, an spin-flip which is faster than few fs can be triggered in the manifold of valence electronic state. Usually such a transition requires up to few hundreds of femtoseconds and might be immensely accelerated in this process. Such dumping also decreases the destructive influence of the Auger decay.

Finally, we have looked at the role of the decoherence caused by nuclear motions. Molecular vibrations have been treated at the level of a heat bath. Essentially, the electron dynamics studied in this work is that fast, that relatively slow vibrational motions do not much influence the result.

We envisage that this effect could be used for clocking ultrafast events. In this respect, it is of core-hole clock type but has a different nature. In case of spin flips, the characteristic timescale may be varied by changing the carrier frequency and bandwidth of the incoming radiation, thus adjusting the strength of the coupling and thereby determining essentially the measured time window.

A new member of the group

Starting from July, our subgroup has got a new Ph.D. student – Vladislav Kochetov. Vlad has taken part in the Erasmus program working on his MSc diploma in parallel in Rostov-on-Don in Russia, Rennes in France and Munich in Germany. Before he has focused on the multi-channel scattering for periodic systems and now will try his forces in the field of molecular physics. He will be working in the project “Soft X-ray spectroscopy and correlated many-electron dynamics of molecular systems from first principles theory.” Good luck!

ICQC 2018

The ICQC 2018 conference in Menton on the Cote d’Azur in France is over. This was an excellent event confirming my previous impressions on this congress hold in Beijing in 2015. Thanks to Odile Eisenstein and her team for a selection of speakers and good organization.


This was a pleasure to meet with many colleagues. I hope that the discussion will continue further resulting in some collaboration projects. As you can see from the photo below, our discussions did not stop even when the poster session was over, and the lights were turned off. Special thanks to Matthias Berg, Fabian Weber, and Leon Freitag for having the entertaining time.