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Dynamical Phenomena at Surfaces: The Role of Complexity
Dynamical phenomena at surfaces determine the interaction of solid bodies with their surroundings. Elementary dynamical processes occurring at surfaces and interfaces form the basis of heterogeneous catalysis, and are important to, for example, energy applications, and astrochemistry. Such processes occur in a highly complex environment, and the two central questions addressed at the workshop were: (i) how can we solve the problems associated with the complexity of the surface, and (ii) how can we take advantage of the complexity a surface inherently has, or can take on?
The workshop reported breakthroughs in electronic structure theory, in experimental tools for following dynamics in real time and in the understanding of chirality and friction. As reported by Kresse, calculations on solid-state cohesive energies can now be done with an exact method called full configuration interaction quantum Monte Carlo (FCI-QMC, see also doi:10.1038/nature11770). As discussed in one of the formal discussion meetings, FCI-QMC cannot yet be applied to molecule-surface interactions. However, as discussed by Kresse at the meeting it is foreseen that diffusion Quantum Monte Carlo method (DFMC) will find increased application to such problems, and that the DFMC method will enable description of these systems with almost chemical accuracy (1 kcal/mol) for reaction barrier heights. Another promising development concerns correlated wave function (CW) theory with embedding in DFT to molecule-surface interactions. Libisch showed that the application of this method to the O2 + Al(111) dissociative chemisorption problem reveals that the barrier to reaction originates from an abrupt charge transfer from the Al to oxygen, and that the most important experimental observations for this system could all be reconciled with the theory using the new method. A problem that still needs to be solved for the CW-embedded DFT method is the convergence of the results with the size of the subsystem treated with CW theory, with current computational resources not yet allowing computations that are converged to within a kcal/mol.
From the experimental point of view, Mansart and Wolf illustrated how atomic movement and electronic structure changes following optical excitation can be followed with time-resolved techniques on the sub-picosecond scale. They pointed out how for atomic and charge dynamics the real observation now matches the timescale previously only accessible in silicio.
In the contributions concerning chirality in two dimensions, De Feyter discussed the interplay between thermodynamics versus kinetics as determining factors for the establishment of a certain structure and pointed out the role of the solvent in self-assembly processes at the liquid/solid interface. In the discussion session lead by Kudernac and De Feyter the need for more theory / modeling was emphasized to better understand the interactions between molecules and surfaces in the presence of a solvent if a realistic picture of local and global effects has to be established. Ernst and Grill illustrated fascinating examples of molecular machines and demonstrated how molecular motion can be controlled at the nanoscale and Kudernac showed how movement at the molecular scale translates into macroscopic property changes at the surface.
Regarding the phenomenon of friction which is not only appealing as a fundamental problem to understand but also crucially important for many applications, de Wijn detailed a new theoretical approach and Frenken highlighted breakthrough developments in experiments.
We had very positive experiences with the format of the workshop, which was such that a formal, general discussion centered on 2-3 of the session themes was held every day. All participants actively participated in the discussions, which were led by scientists who are "top" in the field under discussion, and served to summarize the main challenges in these fields. We would encourage other organizers to organize formal discussions along similar lines.