Geometric, energetic and electronic properties of Aun clusters, n = 2–20 are determined using DFT calculations (BB95/cc-pVTZ-PP). Global equilibrium structures were confirmed or found, and the growth mechanism was established. Au-clusters prefer 2D-geometries up to Au11 and are generated from the lowest-lying isomer of the smaller size by adding an extra gold atom. Transition from an oblate form to a pyramid is observed at Au17. Aun containing 17–20 atoms favor tetrahedral evolution by adding atoms to the Frank-Kasper 16-vertex. Binding energy per atom, second-order difference of energy, and one-step fragmentation energy show that Au6 and Au20 have high thermodynamic stability. Their valence electrons of generate magic numbers that can be understood using the phenomenological shell model. Due to Jahn-Teller effect, Au16 is characterized by oblate structure whereas the anion Au16-exhibits Frank-Kasper form.