1 Electronic structure methods development

When tackling the problem of interacting many-electron systems, Density Functional Theory (DFT) [1,2] has consistently proven to be the most widely used tool [3] due to its computational feasibility and great success in describing ground-state properties.

Despite being an exact theory, DFT relies on approximations for the exchange and correlation functional, which is the key element both in the success and pitfalls of DFT. On one hand, simple local and semi-local approximations [4] have been shown to accurately reproduce structural and geometrical properties. However, on the other hand, DFT drastically underestimates the energy barriers in chemical reactions, band gaps of solids and dissociation energies of molecular ions, to mention a few. Moreover, DFT can be used to compute properties and dynamics of the many-body systems in presence of time-dependent potentials such as electric or magnetic fields [5] by means of the so-called Time-dependent density functional theory (TD-DFT).

In practice, DFT and post-Hartree-Fock (Post-HF) approaches and their numerous variants rely on solving large linear systems using iterative algorithms, where the finite dimension of the eigenvectors may become very large and is limited in practice to a few billion of components due to the finite aspects of hardware. Instead, Quantum Monte Carlo (QMC) represents the solution of a mathematical/physical/chemical problem as a parameter of a hypothetical distribution. In particular, QMC consists in simulating the probabilities of quantum mechanics by using the probabilities of random walks (Brownian motion and its generalizations) [6].

This section will be devoted to new developments in DFT, Post-HF, QMC and other electronic structure methods which allow us to overcome these and other shortcomings.

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[2] W. Kohn, and L. J. Sham, Phys. Rev. 140, A1133 (1965).

[3] R. Van Noorden, B. Maher, and R. Nuzzo, Nat. News 514, 7524 (2014).

[4] J. Perdew, K. Burke, and M. Ernzerhof, Phys. Rev. Lett. 77, 18 (1996).

[5] Runge, Erich; Gross, E. K. U. (1984). Phys. Rev. Lett. 52, (1984).

[6] Nightingale, M.P.; Umrigar, Cyrus J., eds. (1999). Quantum Monte Carlo Methods in Physics and Chemistry. Springer.

2 Strongly correlated systems and magnetism

The strongly correlated systems exhibit phenomena that cannot be explained using the standard band theory. In these compounds, electronic interactions play a crucial role in defining the properties of the system. Therefore, the correct description of the electronic correlations requires adequate methods such as the dynamical mean field theory (DMFT) [1], slave particle approaches [2], Quantum Monte Carlo [3], and others. The strongly correlated systems are a wide class of materials with the most characteristic ones being the Mott insulators [4], geometrically frustrated pyrochlore magnets (spin ices [5]), 4f-electron systems (heavy fermions [6], Kondo insulators [7]), and unconventional superconductors (cuprates, pnictides). In particular, the last ones have been under large debate and extensive study, with the most prominent compounds being the cuprates [8] and the iron-based superconductors [9]. Overall, the investigation of such systems attracts a large research interest due to the related novel unusual phenomena and the fact that they constitute a great playground for the study of complicated many-body processes.

This session will be devoted not only to new developments in the treatment of strongly correlated systems but also to new results obtained in studying such systems.

[1] A. Georges, G. Kotliar, W. Krauth, and M. J. Rozenberg, Rev. Mod. Phys. 68, 13 (1996)

[2] G. Kotliar, A. Ruckenstein. Phys. Rev. Lett. 57:1362 (1986), L. de’ Medici, A. Georges, S. Biermann Phys. Rev. B 72:205124, (2005), R. Yu, Q. Si Phys. Rev. B 86:085104 (2012)

[3] G. Kotliar, S. Y. Savrasov, K. Haule, V. S. Oudovenko, O. Parcollet, and C. A. Marianetti, Rev. Mod. Phys. 78, 865 (2006)

[4] N.F. Mott Proc. Phys. Soc. (London) A62:416–422 (1949)

[5] S. T Bramwell and M. J Harris J. Phys.: Condens. Matter 32 374010 (2020)

[6] S. Wirth and F. Steglich. Nat Rev Mater 1, 16051 (2016)

[7] M. Dzero, J. Xia, V. Galitski, P. Coleman Annual Review of Condensed Matter Physics 2016 7:1, 249-280

[8] J.G. Bednorz, K.A. Muller, Zeitschrift fur Physik B Condensed Matter 64:189–193 (1986), P. W. Anderson, The theory of superconductivity in the high-Tc cuprate superconductors Vol. 446, Princeton University Press, Princeton (1997)

[9] Y. Kamihara, H. Hiramatsu, M. Hirano, R. Kawamura, H. Yanagi, T. Kamiya, H. Hosono J. Am. Chem. Soc. 128:10012 (2006)

3 Vibrational properties of materials and transport

Most ab initio approaches assume the frozen lattice approximation [1], where one supposes that ions keep their positions fixed. However, the presence of vibrational degrees of freedom, gives often a fundamental contribution to many physical quantities, like thermal conductivity, electrical resistivity/conductance, and optical absorption. Therefore, one has to find a way to properly account for the nuclear motion in the calculation. This can be done, at a first level of accuracy, with the harmonic approximation for phonons. However, this approach is valid only in the approximations of small nuclear displacements, and when strongly anharmonic systems are considered (like for example in the case of structural phase transitions) , one must search for more sophisticated approaches [2] [3].

The description of the transport properties for small-bias coherent transport in nanojunction is normally computed using a combination of DFT and non-equilibrium Green’s functions (NEGF), together with Landauer-Buttiker equation [4]. In this formalism, the electron transmission between two semi-infinite reservoirs is normally treated as a time-independent one-electron scattering problem, where each injected electron travels from one lead to the junction, where it is scattered and transmitted with a certain probability [5] [6]. Although this approach allows to describe successfully contact resistance and conductance, it has limitations: 1) it cannot account explicitly for mechanisms such as electron-electron and electron-phonon scattering, 2) transport calculations for voltages far from equilibrium, in the non-linear regime, are not accesibles. Therefore, in those cases different approaches must be used.

This session will be devoted to researchers who want to present their work on quantum transport and vibrational properties of materials, specially those works focused on the influence of vibrational degrees of freedom on spectroscopic quantities, as well as on quantum transport.

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[2] A. Dewandre, et al, Phys. Rev. Lett. 117 (27), 276601 (2016)

[3] G.A. Ribeiro, et al, Phys. Rev. Lett. Phys. Rev. B 97 (1), 014306 (2018)

[4] R. Landauer, Phil. Mag. 21, 863 (1970)

[5] S. Datta, Cambridge University Press (1997)

[6] S. Sanvito, et. al., Phys. Rev. B, 59 (1999), pp. 11936-11948

4 Optical properties and non-linear spectroscopy

The study of light and matter interaction applied to solids, molecules and biological systems provide valuable information regarding their electronic, structural and vibronic properties. Currently, theoreticians face many challenges and opportunities thanks to the continuous improvement of the experimental capabilities. Indeed, thanks to important investment made by the European Union and European countries, large scale facilities have been developed (FLASH, EU XFEL, SwissFEL, ELI-ALPS etc.) providing light sources with properties in term of spectral region, intensities and duration of pulses inaccessible before. The optimal uses of these resources require a theoretical support able to interpret and guide experiments. Despite the challenges, the theoretical works have proven to bridge the differences between the calculated and experimental results in multiple research areas like the use of lasers for biomedical applications [1], 4D imaging [2] and the development of novel photovoltaic materials [3,4] to name a few. Hence, an exposure to the past, present and future of the field of light and matter interaction will help young researchers for its conceptual understanding.

In this session, we will host researchers working both on the development of new methods and phenomenological aspects of the subjects.

[1] M. O. Senge et al., Advanced Materials 19.19 (2007): 2737-2774.

[2] M. N. Slipchenko et al., Proceedings of the Combustion Institute 38.1 (2021): 1533-1560.

[3] Z. Xiao, and Y. Yan, Advanced Energy Materials 7.22 (2017): 1701136.

[4] F. Flory et al., Journal of Nanophotonics 5.1 (2011): 052502.

5 Multiscale modeling

To simulate the electronic structure of most materials, theoreticians most often use Density Functional Theory. Although it is applicable for most materials, molecules or biological systems, the Kohn-Sham theory becomes intractable for systems with a large number of atoms, or for long time-scale dynamic processes. To overcome those limitations, one can resort to a variety of methods. Among them, hybrid quantum mechanical/molecular mechanical [1] is a way to obtain the electronic properties of a local portion of the system with the precision of DFT, and then embed it in an environment treated as a mean-field, simulated with classical dynamics. Alternatively, recent development in machine learning made it possible to deal with more complex systems, by the use of interpolated potentials [2]. Other methods include rate theory [3], that describes the occurrence of rare events unreachable with usual time-scales, or even quantum computing, that is intrinsically able to treat problems that would be physically impossible to solve on classical computers.

This session will be devoted to these methods and even others not detailed here, that go beyond the limitations of the usual methods of theoretical spectroscopy.

[1] A. Warshel, M. Levitt, J. Mol. Biol.,103, 227−249, 1976.

[2] A. P. Bartók, M. C. Payne, R. Kondor, and G. Csányi, Phys. Rev. Lett., 104, 136403, 2010

[3] B. Peters, Reaction Rate Theory and Rare Events Simulations, 2017.

[4] need of 4th ref on Quantum computing