The effect of atom coordination and influence of an external electrostatic field on binding and dissociation energies of molecular oxygen on a nano modified An surfaces are investigated. Weak O and O-2 adsorption are found in our models. A considerable decrease of the dissociation energy of O-2 was found when adsorbed on small gold clusters or gold pyramids. It is concluded that in the presence of an electric field no dissociation of O-2 will occur in our model. This result is in agreement with field ion microscopy (FIM) and field electron microscopy (FEM) experiments, where no dissociation of O-2 is encountered. (c) 2006 Elsevier B.V. All rights reserved.