Abstract: Understanding atomistic mechanisms for catalyst-assisted nanowire growth is an essential step to improve control over the properties of these versatile nanomaterials. However, in silico approaches for III-V nanowire growth have been hindered so far mainly by the limited number of interatomic potentials. Here, we present an original interatomic potential for molecular dynamics simulations of Au-catalyzed GaAs nanowire growth. Our simulations provide important insights about the atomic distribution in the nanowire catalyst and the role of As atoms during GaAs nanowire growth. We show that a stable, thin layer of As around the catalyst is essential for nanowire growth and that the composition of the region close to the solid-liquid interface is nonuniform, alternating between Ga-rich and As/Au-rich layers. These features contribute to the reservoir effect, enlarging interface widths when exchanging group III or V species for heterostructure growth. Our simulation results also provide directions for challenging in situ experiments to further probe the existence of this thin As layer on the catalyst surface, as well as for finding improved conditions to obtain sharp interfaces in nanowires with axial heterostructures.