The Workshop “Theoretical methods in molecular spintronics (TMSpin) was held at the Materials Physics Center of the University of the Basque Country in Donostia-San Sebastian from the 17^{th} to the 20^{th} of September 2018. This workshop welcomed 31 invited speakers and several postgraduate students presenting posters. The event was co-sponsored by Psi-k and the Donostia International Physics Centre (DIPC- http://dipc.ehu.es/).

Molecular spintronics is the study of spin-related phenomena in molecules and atoms and their possible applications for the next generation of data storage and processing devices as well as for the implementation of quantum computers. Electronic structure theory has played a prominent role in molecular spintronics. The comparison of theory and experiments has demonstrated the importance of first-principles calculations, which go beyond model representations of molecular devices as simple “quantum dots” or effective spin Hamiltonians. Nonetheless, standard electronic structure methods based on Density Functional Theory often fail in describing molecular spintronic systems even at a qualitative level. This is because most magnetic phenomena are manifestations of correlation effects, which become extreme at the single molecule scale and which are not captured by standard implementations and approximations of DFT. The goal of TMSpin was to address the question:

“What electronic structure theory to use for molecular spintronics?”

The workshops gathered theoretical physicists and quantum chemists with different areas of expertise. On the one hand there were those researchers that have provided important contributions to the advancement of molecular spintronics since its inception. They were asked to give an overview about the field and moreover to highlight the open questions that to date cannot yet be addressed by theory. On the other hand, the workshop gathered some of the leading researchers in theory and code development, who presented the most recent fundamental and numerical advancements for a number of methods. The organizers promoted an intense discussion to understand whether such methods can be already employed in molecular spintronics.

The main problems discussed during the workshop were:

- Magnetic properties of molecules in the gas phase, in particular spin state energetics and exchange coupling between several magnetic centres;
- Magnetic properties of atoms and molecule on surfaces, in particular the Kondo effect and the surface mediated exchange-coupling between adsorbed atoms;
- Finite-bias transport and magnetic excitations at the atomic scale;
- Description of entangled states from first principles and application to quantum computation in magnetic molecules and atoms.

Among the treated theoretical methods there was, in the first place, the time-dependent DFT, which has been recently employed to study magnetic excitations in few-atom systems. Then a particular focus was laid on many-body physics methods. The so-called DFT+ dynamical mean field theory (DMFT) has provided an established theoretical framework to project low-energy models out of first-principles calculations. These models can then be solved by using accurate numerical techniques and allow for a proper description of correlation effects in molecules as well as in transport experiments. Extension to non-equilibrium problems beyond the linear-response regime are currently a subject of great interest and were covered in detailed during the workshop. DFT+DMFT belongs to the general class of embedding methods, which are now also emerging in the quantum chemistry community. Recent progresses in these methods were discussed to understand whether they may find important applications in molecular spintronics over the coming years. Finally, an alternative to the above mentioned methods is represented by Quantum Monte Carlo techniques in continuum space, by Density Matrix Renormalization Group and by traditional quantum chemistry approaches. In recent years, all of them have been actively developed for the description of magnetic properties of molecules, and were discussed at this workshop in comparison to DFT and DMFT.

**Importance and timing**

Every year there are a few international conferences dedicated to molecular spintronics, which is an important and still growing research area in Europe. There, theoretical talks are often limited to present results supporting experiments, while no room is left to discuss methodological advancements because of the generally broad audience, which comprises researchers with expertise ranging from device fabrication to synthetic chemistry. However, molecular spintronics poses many challenges for electronic structure theory and most problems of current interest encompass molecular magnetism, Kondo physics, transport, molecular adsorption and energy level alignment at hybrid organic-inorganic interfaces. TMSpin gathered only theorists in order to focus on methods, address how to do the calculations and establish what theory has to achieve to move forward the field of molecular spintronics during the next few years.

**Organizers**

- Andrea Droghetti, Universidad del País Vasco, Donostia-San Sebastian
- Ivan Rungger, National Physical Laboratory, Teddington, UK
- Tim Wehling, University of Bremen, Bremen, Germany

**General remarks**

TMSpin provided a friendly and informal atmosphere to foster knowledge transfer and new collaborations between researchers with different backgrounds. All talks were very well-delivered by the speakers, who responded with great enthusiasm to the invitation and contributed to make the best of this workshop. Questions were allowed also during the talks and not just at the end. In this way, all participations stayed very much engaged throughout every session and the resulting discussions were truly stimulating and helped to really advance the field of molecular spintronics.

**Feedback from the participates **

All the participants were enthusiastic about the workshop. Most of the researchers coming from the molecular spintronics community felt that this had been a long-needed and awaited workshop and they were therefore very happy that it finally happened. All researchers really enjoyed the extensive discussions about subjects, which are transversal across physics and chemistry, on issues such as the definition of electron correlation effects in magnetic molecules. All researchers left after the workshop with an increased baggage of knowledge, with new ideas for research projects and with the contact of new potential collaborators. This was the demonstration that the workshop was really successful and it was a very rewarding experience for the organizers.

**Conclusion and perspectives**

TMSpin was very successful and its main goal was fully achieved. On the one hand, the traditional molecular spintronics community became aware of the recent theoretical developments that may find important applications in the field. On the other hand, the workshop showed how most molecular spintronic systems are ideal playgrounds to understand correlation effects and to benchmark new methods. The informal atmosphere encouraged cross-discussions between many-body physicists and quantum chemists to provide complementary points of view on several common problems and set the ground for future collaborations. Given the very enthusiastic response of all participants, with many of them strongly suggesting that a follow-on workshop should be organized in the near future, TMSpin will likely be repeated in 2020 to monitor how the field will evolve, consolidate the research network that was formed this year and open the molecular spintronics field to more people.

**Further details
**

Workshop website: http://tmspin.dipc.org/

List of abstracts: Abstracts_TMSpin

**Scientific Program ** **Monday 17 ^{th }September**

8.45 – 9.00 A. Droghetti: *Welcome and introduction*

__Morning session 1: General phenomenology, combined theory-experiment investigations__

9.00 – 9.45 N. Atodiresei: *Interaction of **π**-electron systems with magnetic surfaces*

9.45 -10.30 E. Ruiz: *Room Temperature Magnetoresistance **in Single-Molecule Devices*

10.30 – 11.15 M. Persson: *Charging in a single-molecule junction: **Tunable magnetoresistance and reorganisation energy*

__Morning session 2: Transition metal molecules__

11.45 – 12.30 C. DeGraa*f: Multiconfigurational electronic structure methods **applied to spin anisotropy and excited state deactivation*

12.30 – 13.15 H. Kulik*: Accelerating discovery of new spin crossover materials **with machine learning and alternative DFT approaches*

* **Afternoon session 1*: Many-body effects in molecules I

15.00 – 15.45 C. Weber: *Many body effects in transition metal molecular systems*

15.45 – 16.30 L.K. Wagner: *Diffusion quantum Monte Carlo for systems with spins*

*Afternoon session 2*: Many-Body effects in molecules II

17.00 – 17.45 G. Booth: *Novel Wave Function Approaches **to both Extended and Finite Correlated Molecular Systems*

17.45 – 18.30 L. Freitag*: DMRG-based methods for strong correlation and spin-state problems*

18.30 – 19.00 Round table: How to tackle correlations in magnetic molecules?

**Tuesday 18 ^{th }September**

*Morning session 1*: Transport I

9.00 – 9.45 D. Ryndyk*: STM investigation of molecules on surfaces: **many-body quantum transport from level spectroscopy **to Kondo*

9.45 – 10.30 M. Grifoni*: SU(2) x SU(2) Kondo effect in carbon nanotubes*

10.30 – 11.15 J. Fransson*: The relation between the electric current** and magnetic interactions in molecular junctions*

__ Morning session 2: Nano-DMFT I__ 11.45 – 12.30

D. Jacob*: Transport signatures of nanoscale magnets from NanoDMFT* 12.30 – 13.15

G. Sangiovanni*: How to control the many-body effects of high-spin molecules** *

*Afternoon session *1: Transport and Spin-Crossover and Orbital Crossover

15.00 – 15.45 V. Robert*: Electron transport through spin crossover molecules. **Inspections from wavefunction theory*

15.45 – 16.30 A. Donarini: *Apparent Reversal of **Molecular Orbitals Reveals Entanglement*

16.30 – 16.45 M. Camarasa-Gomez: *Mechanically-tuneable charge transport **in ferrocene-based molecular junctions*

*Afternoon session 2*: Nano-DMFT II

17.15 – 18.00 C. Hermann: *Chemical approaches to the Kondo effect*

18.00 – 18.45 N. Lorente*: Spin-flip scattering off spin chains, **molecules and single adsorbates*

**Wednesday 19 ^{th }September**

*Morning session 1*: Real-time dynamics

9.00 – 9.45 S. Sanvito*: Phonon-induced spin relaxation in molecular magnets*

9.45 – 10.30

J. Perlata: *Spin Dynamics and Electron Transport **from Time-Dependent Density Functional Calculations*

10.30 – 11.15 G. Lefkidis: *Nanospintronics and quantum thermodynamics **in molecular magnets*

*Morning session 2*: Adatoms on surfaces

11.45 – 12.30 R. Korytar: *Manifestations of a coherent Kondo lattice in adatoms*

12.30 – 13.15 M. dos Santos Dias: *Spin interactions, excitations and fluctuations **in magnetic adatoms and small clusters **from first-principles*

*Afternoon session *1: GW and screening at molecule metal interfaces

15.00 – 15.45 G. Stefanucci: *First-principles NEGF approach to ultrafast carrier dynamics*

15.45 – 16.30 J. Lischner: *Modelling quasiparticle excitations of adsorbate in 2d materials*

*Afternoon session 2*: Transport II

17.00 – 17.45 F. Evers: *Charge transport in graphene nanoribbons: **ab-initio simulations & model studies of strong correlations*

17.45 – 18.30 P. Hedegard: *Theory of Chiral induced spin Selectivity*

*18.30 – 19.00 *Discussion: Where is molecular spintronics going?

**Thursday 20 ^{th }September**

*Morning session 1: K*ondo effect (impurity solvers) I

9.00 – 9.45 E. Gull: *Inchworm Quantum Monte Carlo for real-time dynamics*

9.45 – 10.30 S. Kirchner: *The rSPT approach to Kondo-correlated molecular junctions*

10.30 – 11.15 R. Zitko: *Numerical renormalization group investigations of **magnetic impurities coupled to superconductors*

*Morning session 2*: Kondo effect (impurity solvers) II

11.45 – 12.30 S. Kurth*: Steady-State Density Functional Theory*

12.30 – 13.30 Round table: Defining the state-of-the-art for transport including strong electronic correlations.

13.30 – 13.40* Concluding Remarks*