A topological analysis of the electron localisation function of the [AuRg(4)](2+) (Rg = Ar, Kr and Xe) molecules reveals the 'closed-shell' nature of the Au-Rg binding, which is dominated by electrostatic interactions. A similar finding is obtained by means of the AIM method where the Laplacian of the electron density computed for the bond critical point of the Au-Rg linkage is positive and decreases from 0.163 [AuAr4](2+) to 0.087 [AuXe4](2+). A charge separation according to the Au+1.30[Ar+0.18](4), Au+1.19[Kr+0.20](4) and Au+0.29[Xe+0.43](4) formulas shows an increase of the positive charge on the rare-gas atom and accordingly decrease on the gold atom. The unpaired electron density is localised mainly on Au and the integrated spin density decreases from 0.28 e for [AuAr4](2+) to 0.16 e for [AuXe4](2+). (C) 2002 Published by Elsevier Science B.V.