Climate change is currently one of the central scientific issues in the world, and the ability to reliably forecast climate is crucial for making political decisions that affect the lives of billions of people. Aerosols remain the dominant uncertainty in predicting radiative forcing and future climate change, and also have adverse effects on human health and visibility. One of the least-well understood aerosol-related processes is nucleation: the formation of new particles from condensable vapours. While nucleation is related primarily to neutral clusters, state-of-the-art experimental methods measure only charged clusters.The main scientific objectives of this project are 1) to understand the chemical composition of charged and especially neutral atmospheric clusters from molecular to multi-nanometre scale, and explain the mechanism by which they nucleate, and 2) to direct current intense instrument development and provide theoretical tools to maximize the information on neutral clusters that can be obtained from experimental results on charged clusters.Our scientific plan consists of a multilevel computational effort to provide formation rates and properties of atmospheric clusters and particles to aerosol dynamic and climate modellers. To capture the properties of the smallest clusters, we need to perform quantum chemical calculations, combined with simulations on cluster formation kinetics. Unfortunately, these methods are computationally far too demanding to describe the entire nucleation process. Thus, we will feed quantum chemical results to classical thermodynamic models, the results of which in turn must be parameterized for efficient use in larger-scale models.