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## Spectroscopy and Quantum Phenomena in Large Molecular Aggregates

#### Location: University of Bremen, Germany

#### Organisers

Quantum coherences and their effects on spectroscopy of large molecular aggregates are a very exciting issue these days due to recent experimental progress on both biological and synthetic samples showing unexpected long coherence times in large disordered systems. An ab initio calculation of the optical and energy/charge transfer processes in these large molecular aggregates is still a very complicated issue and theoreticians are facing several severe problems when trying to model quantum and optical effects in these systems. This workshop aims on bringing those people together who work on biological and/or synthetic systems to discuss and exchange ideas how to most efficiently and accurately solve the above mentioned problems. An important application in the described field of research are biological as well as artificial light-harvesting systems and solar cells.

Recent technological breakthroughs, such as ultra-fast pulsed lasers, high-performance computing and crystallography of macromolecules, helped to improve the experimental tools of physicists, biologists, and chemists working, in particular, in the area of chemical and biological physics. Nowadays, for instance, molecular dynamics simulations and experimental photophysical methodologies reach similar time scales, and spatial resolution is being added to many spectroscopic apparatus, turning them into microscopes. This has helped to boost, for example, the fields of femtochemistry, multi-dimensional spectroscopy [1], and molecular biophysics [2,3]. The experimental detection of long-lived coherent motion in light-harvesting systems [4,5] as well as in polymers [6] both at low and at room temperature has steered a new interest in these systems and possible origins of the phenomena [7].

When modeling large and complex molecular aggregates it is impossible to treat all degrees of freedom quantum mechanically, especially in complicated environments. In many cases one possible solution is to set up a model with a few relevant modes and to treat the rest of the system as a thermal bath [8]. For an accurate description of an experiment all modes which can be probed experimentally, e.g., via spectroscopic techniques, have to be treated explicitly. The remaining vast majority of modes are of interest only to the extent that these modes affect the dynamics of the system. There exists a huge variety of different formalisms on how to treat such system-bath models. Because of their complexity, there is a whole scientific community treating these problems in open quantum systems. In recent years, coherent control with ultrafast lasers became also applicable to reactions in the condensed phase experimentally and to some extent theoretically. Even first experiments on laser control in biological light-harvesting complexes have been performed [9].

Many of the aforementioned system-bath theories have a model character and are most often not directly connected to an atomic-level description of the system of interest. Often atomic-level details are known from experiments in chemical and biological physics these days. For example, in biological physics these details are mainly obtained by x-ray crystallography of a crystallized version of the macromolecules. As detailed above a fully quantum-mechanical treatment of static and/or dynamical properties is out of reach for bigger molecules within an environment such as a surrounding liquid or protein structure.

Therefore one often restricts calculations to classical molecular dynamics simulations for an atomic level description. Force-field-based simulations can handle large systems and are several orders of magnitude faster (and computational cheaper) than quantum-based calculations. But applications beyond the capability of classical force-field methods include such fundamental processes as electronic transitions (photon absorption, non-linear spectroscopy), electron transport phenomena and proton transfer. Therefore an extension of classical molecular-dynamics simulations is needed to treat molecules which behave quantum mechanically within the context of their protein environment which itself can be treated classically. Another route is to use classical molecular dynamics simulations at an atomic level to derive the parameter values for a more simplified system-bath model which then can be treated fully quantum mechanically [10-13].

Understanding life from its molecular foundation has become a very important and active field of research these days and even full photosynthetic vesicles can be modeled [14]. Most calculations on large chemical and/or biological molecules use classical molecular dynamics up to now. But many fundamental biological processes that involve, for example, the conversion of light into energy forms that are usable for chemical transformations are quantum mechanical in their nature. The description of quantum processes in biological systems is usually focused on specialized molecules, e.g., chlorophylls, which are embedded in a larger protein environment. The extremely challenging task of describing the dynamics of such systems involves several open problems: quantum mechanical description of large molecules, combined quantum/classical descriptions, stochastic quantum mechanics combined with molecular dynamics, etc.

Despite the apparent differences in the methods of investigation (classical, quantum mechanical, hybrid), the systems of interest are quite often very similar. Nevertheless, the research community remains strongly divided by traditional boundaries. The main goal of the proposed interdisciplinary workshop is to encourage interaction and information exchange between different fields like the field of pure classical molecular dynamics simulations of large systems and the field of quantum dynamical calculations for model systems with or without dissipation. The same is, of course, true for the investigation of biological and of polymer/nanostructures substances. Therefore, from the broad range of experienced researchers working in these areas, we are inviting exceptional individuals who have already successfully made this step across traditional boundaries, in order to allow them to share their experience during the seminar with the younger participants.

Since the subject of the workshop is so interdisciplinary, also the background and scientific communities of the lecturers and participants are quite diverse. It is therefore the aim of the proposed workshop to familiarize the participants with different subjects, to encourage interdisciplinary interactions, and to share experience of different research fields with one another. In this way, we intend to foster the exchange of ideas and methods, to highlight the apparent and the hidden similarities of different systems and approaches, and to stimulate new and fruitful cooperations across subject boundaries.

Recent technological breakthroughs, such as ultra-fast pulsed lasers, high-performance computing and crystallography of macromolecules, helped to improve the experimental tools of physicists, biologists, and chemists working, in particular, in the area of chemical and biological physics. Nowadays, for instance, molecular dynamics simulations and experimental photophysical methodologies reach similar time scales, and spatial resolution is being added to many spectroscopic apparatus, turning them into microscopes. This has helped to boost, for example, the fields of femtochemistry, multi-dimensional spectroscopy [1], and molecular biophysics [2,3]. The experimental detection of long-lived coherent motion in light-harvesting systems [4,5] as well as in polymers [6] both at low and at room temperature has steered a new interest in these systems and possible origins of the phenomena [7].

When modeling large and complex molecular aggregates it is impossible to treat all degrees of freedom quantum mechanically, especially in complicated environments. In many cases one possible solution is to set up a model with a few relevant modes and to treat the rest of the system as a thermal bath [8]. For an accurate description of an experiment all modes which can be probed experimentally, e.g., via spectroscopic techniques, have to be treated explicitly. The remaining vast majority of modes are of interest only to the extent that these modes affect the dynamics of the system.

There exists a huge variety of different formalisms on how to treat such system-bath models. Because of their complexity, there is a whole scientific community treating these problems in open quantum systems. In recent years, coherent control with ultrafast lasers became also applicable to reactions in the condensed phase experimentally and to some extent theoretically. Even first experiments on laser control in biological light-harvesting complexes have been performed [9].

Many of the aforementioned system-bath theories have a model character and are most often not directly connected to an atomic-level description of the system of interest. Often atomic-level details are known from experiments in chemical and biological physics these days. For example, in biological physics these details are mainly obtained by x-ray crystallography of a crystallized version of the macromolecules. As detailed above a fully quantum-mechanical treatment of static and/or dynamical properties is out of reach for bigger molecules within an environment such as a surrounding liquid or protein structure.Therefore one often restricts calculations to classical molecular dynamics simulations for an atomic level description. Force-field-based simulations can handle large systems and are several orders of magnitude faster (and computational cheaper) than quantum-based calculations. But applications beyond the capability of classical force-field methods include such fundamental processes as electronic transitions (photon absorption, non-linear spectroscopy), electron transport phenomena and proton transfer. Therefore an extension of classical molecular-dynamics simulations is needed to treat molecules which behave quantum mechanically within the context of their protein environment which itself can be treated classically. Another route is to use classical molecular dynamics simulations at an atomic level to derive the parameter values for a more simplified system-bath model which then can be treated fully quantum mechanically [10-13].

Understanding life from its molecular foundation has become a very important and active field of research these days and even full photosynthetic vesicles can be modeled [14]. Most calculations on large chemical and/or biological molecules use classical molecular dynamics up to now. But many fundamental biological processes that involve, for example, the conversion of light into energy forms that are usable for chemical transformations are quantum mechanical in their nature. The description of quantum processes in biological systems is usually focused on specialized molecules, e.g., chlorophylls, which are embedded in a larger protein environment. The extremely challenging task of describing the dynamics of such systems involves several open problems: quantum mechanical description of large molecules, combined quantum/classical descriptions, stochastic quantum mechanics combined with molecular dynamics, etc.

Despite the apparent differences in the methods of investigation (classical, quantum mechanical, hybrid), the systems of interest are quite often very similar. Nevertheless, the research community remains strongly divided by traditional boundaries. The main goal of the proposed interdisciplinary workshop is to encourage interaction and information exchange between different fields like the field of pure classical molecular dynamics simulations of large systems and the field of quantum dynamical calculations for model systems with or without dissipation. The same is, of course, true for the investigation of biological and of polymer/nanostructures substances. Therefore, from the broad range of experienced researchers working in these areas, we are inviting exceptional individuals who have already successfully made this step across traditional boundaries, in order to allow them to share their experience during the seminar with the younger participants.Since the subject of the workshop is so interdisciplinary, also the background and scientific communities of the lecturers and participants are quite diverse. It is therefore the aim of the proposed workshop to familiarize the participants with different subjects, to encourage interdisciplinary interactions, and to share experience of different research fields with one another. In this way, we intend to foster the exchange of ideas and methods, to highlight the apparent and the hidden similarities of different systems and approaches, and to stimulate new and fruitful cooperations across subject boundaries.

## References

**Germany**

Thomas Frauenheim (University of Bremen) - Organiser

Ulrich Kleinekathöfer (Jacobs University Bremen) - Organiser

**Netherlands**

Jaspar Knoester (University Groningen) - Organiser