Computer simulations are a key complement to experiments in the laboratory, providing great details of a molecular process than can be observed experimentally. For instance, ab initio molecular dynamics simulations are often key to the understanding of the mechanism, rate and yield of chemical reactions. In this talk, I will present simulations of photochemical and attochemical reactions . In the first part, I will focus on simulations of the cis-to-trans photo-isomerisation of azobenzene, after excitation to the n* and * states . A reduction of photoisomerisation quantum yield of 0.10 on exciting to the higher energy * state compared to the lower energy n* state is obtained, in close agreement with the most recent experimental values . By direct comparison of both excitations, we have found that the explanation for the decrease in quantum yield is not the same as for the reduction observed in the trans-to-cis photoisomerization (Figure 1, left). In the second part of my talk, I will discuss the dynamics induced in ethylene following ionization by an extreme ultraviolet attosecond pulse train . We have found that isotope labelling can be an efficient tool in attochemistry to identify the relevant nuclear coordinates controlling the relaxation dynamics (Figure 1, right). In the last part, I will present an example of recent experimental and theoretical results on the photo-induced dynamics of an iron photosensitizer. Coherent structural dynamics in the excited state of an iron photosensitizer was observed through oscillations in the intensity of K x-ray emission spectroscopy (XES). Using multiconfigurational wavefunction calculations, we explain the origin of the unexpected sensitivity of core-to-core transitions to structural dynamics.
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