Alacranite, As8S9, has been studied by a combined approach based on micro-Raman measurements and ab initio molecular dynamics simulations, with the Car-Parrinello method. The structure of this arsenic sulfide mineral consists of an ordered packing of As4S4 and As4S5 cagelike molecules, with a topology closely resembling that found in the beta-As4S4. The presence in the crystal structure of molecular clusters with permanent dipole moment induces stronger intermolecular interactions than those observed in other arsenic sulfides, making the adoption of ab initio computational methods particularly important for a complete characterization of the structural and spectroscopic properties.