Abstract The Au + (H 2O) complex has been studied by ab initio MO calculations using a relativistic effective core potential with a large polarized basis set. Geometry optimizations at the MP2 and the CISD levels of theory lead to a non-planar C s-symmetrical structure 1 with an AuO bond length of 2.133 Å and a hardly distorted water substructure with a wag angle θ w of 47°. In contrast to other cationic transition-metal hydrates, the corresponding planar structure 2 with C 2v symmetry is slightly higher in energy and exhibits one imaginary frequency (ν=i270 cm −1). Thus 2 is a transition structure and its further analysis reveals that the transition corresponds to the ‘umbrella’ vibration of the pyramidalized water substructure ( 1⇌ 2). The pyramidalization of 1 can be attributed to re-hydridization of the water molecule upon complexation by the Au + cation, due to the relativistically enlarged ionization energy of the gold atom. The calculated bond dissociation energies of the Au + (H 2O) complex converges to 36.0 kcal/mol at the CCSD(T) level of theory. Similar energetic and structural results are obtained using a density functional theory approach, i.e. BDE=37.0 kcal/mol, r(AuO)=2.196 Å, and θ w=49°.