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## Triggering out-of-equilibrium dynamics in molecular systems

#### CECAM-HQ-EPFL, Lausanne, Switzerland

#### Organisers

Describing the out-of-equilibrium dynamics of molecular systems with numerical simulations is a challenging task, whether it happens in the gas phase or in the condensed phase. While the quantum-mechanical evolution equations are essentially known, they can be employed to describe the dynamics of systems of “experimental complexity” only by invoking approximations [1-8], which make them treatable on a computer. Phenomena initiated or driven by different light sources [9-15] are of interest nowadays for potential applications in the control of chemical reactions, or in domains like energy conversion and phase transitions. High-energy charged particles interacting with solids [16] or biological matter [17] are capable of inducing strong non-equilibrium responses and damages in the target systems. Exciton formation followed by charge recombination and photoluminescence [18] or charge separation [19, 20] for the production of electric current are currently of major interest for the development of organic electronic devices. Those phenomena happen at the frontier between physics and chemistry, and as a result a complete understanding of their microscopic details requires an intense knowledge exchange between the communities. In addition, the complexity of the developed equations requires a strong support from mathematics and computer-science experts to facilitate the development of efficient theoretical and computational tools for the simulations [21, 22].

The purpose of this alternative workshop is not to discuss applications *per se*, but rather to focus on how theory and simulations can achieve a realistic description of the fundamental physics underlying out-of-equilibrium (quantum) dynamics in complex molecular systems while preserving as much as possible a broad perspective of the field. We are interested in how light-matter [5, 9, 10, 12-14] and system-environment [23, 24] interactions are modelled, how to account for quantum (nuclear and electronic) effects [1-5] at a low computational cost, how to obtain multi-scale information (in time and in space) at a reasonable computational cost, and how the out-of-equilibrium process is slowly reaching equilibrium again. Discussions on current theoretical and software developments are essential as well for advancements in the field. Quantum-mechanical approaches that treat microscopic systems out of equilibrium can be based on the time-dependent Schrödinger equation, where the interaction with light is treated within perturbation theory, or explicitly in the classical limit, or in the strong coupling regime; QM/MM approaches are often employed to model the effect of the environment, as well as generalized master equations based on a density-matrix formalism. These application-oriented questions and theory-related efforts are very general and of concern for any of the examples listed above. Therefore, we aim at bringing together during this alternative workshop a broad variety of expertise and background, with personalities willing to share not only their knowledge and recent developments, but also their failures. As such, participants should be ready for open and constructive discussions to advance the fields in different directions.

## References

**France**

Federica Agostini (University Paris-Saclay) - Organiser

**United Kingdom**

Basile Curchod (Durham University) - Organiser

Antonio Prlj (Durham University) - Organiser

Graham Worth (University College London) - Organiser