The properties of clusters within the cluster-to-particle transition regime are not well understood. In the context of new particle formation, the formation of atmospheric molecular clusters has been extensively studied using quantum chemical methods. In general, it has been found that sulfuric acid (SA) and bases, such as ammonia (AM) and alky-lamines, form strongly bound clusters. The stability of SA-base clusters has been found to correlate with the basicity of the base for small cluster sizes. In addition, it has been found that SA-base clusters are most stable when they consist of an equal number of SA and base molecules, i.e., a 1:1 ratio. We refer to our recent reviews on organic and inorganic cluster formation for a comprehensive overview of studied cluster systems in the literature.