Low energy electrons (LEEs), a few eV, are abundantly produced during the irradiation of matter by ionizing radiation, without any precise knowledge of the mechanisms of their formation and relaxation in the environment. We will study by a coupled experimental/theoretical approach the photo-ionization of small molecules (diazabicyclo[2.2;2]-octane), in particular of biological interest (amino acids, DNA nucleobases), deposited on nanometric aggregates. The objective is to understand electron scattering at the molecular scale on time scales ranging from femto- to picoseconds. Experimentally, the Velocity Map Imaging (P3, ISMO, Orsay) will give access to angle and energy distribution of photoelectrons (PAED) emitted by the aggregates, allowing to characterize the elastic and inelastic scattering processes. In order to understand the role of the intensity of the interactions between the medium and the LEEs, different scattering environments will be tested (atomic Argon aggregates, molecular aggregates of H2O, NH3, CO2). These experimental data will provide a valuable test to validate new simulation approaches. To tackle the challenge of simulating the relaxation dynamics of photoelectrons within nanoscale aggregates, we will develop novel algorithms based on orbital-free DFT (OF-DFT). We will couple this approach to the Kohn-Sham DFT (P1, ICP, Orsay and P4, IDRIS, Orsay), to the DFTB (Tight Binding approach to DFT, P2, LCPQ, Toulouse) and to a polarizable MM (Molecular Mechanics) potential (P1, P2 and P4). These three methodological developments will allow complementary descriptions: a better quality description on a few trajectories with the OF-DFT approach, the exhaustive simulations necessary for the interpretation of the experiments being performed with more efficient but less precise methods (OF-DFTB and OF-MM). Once validated, the new algorithms will be made available to the scientific community in the Mon2k and MonNano codes. The experimentally studied systems will be simulated to predict their structures and dynamics. We will simulate the photo-ionization in time-dependent DFT, and the relaxation and diffusion phases will be simulated with the new algorithms. The experimentally and theoretically obtained PAEDs will be compared, which should lead to an advanced understanding of the evolution of the systems. We will provide the community with characteristic electron scattering times and mean free paths for the studied systems that can be used, for example, as parameters in Monte Carlo scattering codes. Thanks to BIRD we will have unique tools to characterize the impact of LEEs scattering in complex molecular systems in various contexts: biology (DNA lesions or other biomolecules), astrochemistry (molecule formation mechanisms in then interstellar medium or in planetary atmospheres/ionospheres) or space and nuclear industry.