Several issues concerning the activity of the magical Au-20 cluster are addressed with the help of Density Functional Theory (DFT) simulations. First, the cluster is found to be extremely robust against distortions when supported at an oxide (MgO) surface, keeping its tetrahedral shape. Second, the activity toward CO oxidation is studied as a function of the cluster charge state. Whereas the neutral Cluster is found to be only moderately active, the anion becomes extremely active, because of the presence of a weakly bound unpaired electron. The attempt to induce cluster charging upon attachment to a charged Surface F+-center Was unsuccessful, with a cluster only slightly more active than the unsupported, neutral one. Finally, some initial steps towards the rational design of active catalysts were taken by the Study of the effect of having different impurities. Promising results are obtained by the addition of an extra Na atom, which preferentially adsorbs inside the tetrahedron. Two competing isomers with an endohedral Na atom within a Au-20 cage are found, and both of them are very active. (c) 2005 Elsevier Inc. All rights reserved.