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The limited availability of fossil fuels as primary energy sources and the concomitant emission of pollutants leading to local and global negative effects on the environment pose an enormous challenge for our future energy supply. The development of sustainable and efficient energy conversion processes is therefore of central importance for our future. How successful this challenge can be addressed will ultimately depend on the degree of understanding of basic energy conversion and storage processes based on renewable sources.
Ubiquitous and easy to transport, water is a highly appealing such source. Clearly, if a photochemical splitting of water by sunlight can be realized at the large-scale technical level, it will provide the basis for the hydrogen economy of the future. As in many other areas predictive-quality computational materials modeling could become a key contributor in this quest. However, at present such quantitative modeling from firstprinciples is still faced with severe methodological challenges. In this situation substantial impulse could come from exploiting the similarities of electrochemical and photochemical processes at solid-liquid and solid-gaseous interfaces, respectively. Method development in these two areas occurs currently rather separately, pushed by two communities with little overlap. The motivation and aim of the workshop is to bring these two communities together - to explore the similarities in general and discuss possibilities of uniting the state of-the-art approaches to the envisioned quantitative modeling of photoelectrochemical splitting of water in particular. Studying chemical reactions requires a detailed knowledge of the underlying potential-energy surfaces (PESs) that govern the nuclear motion. There is growing experimental evidence that many dynamic processes at surfaces are non-adiabatic, i.e., excited states of the system play an important role and the process can no longer be described by the Born-Oppenheimer ground state PES alone. The aim of the proposed workshop is to bring together scientists working on different approaches for excited state PESs to discuss and promote advances in this field. Applications to recent experimental results will be discussed with distinguished experimentalists.

There are now a few examples on electrocatalyst design from first principles [1-3]. However, the simulations that provide the basis all rely on binding energies of intermediates, where the electrochemical environment is only indirectly included. In energy conversion from electricity to chemical bonds selectivity is the main issue. In order to address this selectivity challenge highly accurate reaction barriers are needed. The correspondingly required explicit modeling of electrochemical reactions that take place at the electrode surface in the charged interface between the liquid electrolyte and the solid electrode remains to date a largely unaccomplished endeavor. The molecular structure of the interface, pH of the electrolyte, surface charge, field and electrochemical potential are all parameters that could be important and need to be accounted for. Different methods for modeling the charged solid/liquid interface at the atomic level are starting to appear [4-8]. Here, one of the key challenges is to investigate reactions under a given constant potential. The problem is twofold, since besides from keeping the potential constant one also needs an absolute level of the potential. In photo-catalysis many of the same issues regarding the interface are present, in particular concerning the accurate modeling of electron transfer and charge separation.
Concerning surface photochemistry one of the key challenges consists of a reliable calculation of electronically excited states of adsorbates on surfaces. The large system sizes required to model extended surfaces make many established and accurate quantum chemical approaches computationally intractable. More approximate approaches are therefore of high interest [9,10]. Their reliability to describe electronically excited states is, however, as uncertain as their reliability to model processes at a charged solid/liquid interface. Besides the principal problem of the calculation of excitation energies, surface photochemistry often turns out to be a multi-dimensional problem, which requires the construction of high-dimensional potential energy surfaces (PES) [11]. Up to now, only very few such PES of reliable accuracy have been calculated and used in subsequent quantum dynamical or classical studies [12]. Here, a reduction of the dimensionality can be the crucial factor determining the feasibility of a quantitative modeling, i.e. questions concerning the explicit or implicit consideration of degrees of freedom are as central as they are in the modeling of the complex dynamics at solid/liquid interfaces. Even with a multi-dimensional PES determined, the ensuing actual quantum (or semi-classical) dynamical simulations still represent a formidable problem as e.g. recently reviewed for the example of ultrafast molecular desorption from surfaces by Saalfrank [13]. One of the open issues concerns the description of non-adiabatic effects as the main quantity determining the lifetime of electronically excited states, which in turn finds its direct correspondence in the charge transfer and charge separation problem of electrocatalysis.